Assessment of Discrete Items and Basis for WACllwrsite.com/.../2013/...R-13-10055-Discrete-Items-MASTER-07-08-13.pdf · ESC Assessment of Discrete Items LLWR/ESC/R(13)10055 Page 1

The LLWR Environmental Safety Case

Assessment of Discrete Items and Basis for WAC

T J Sumerling

LLWR/ESC/R(13)10055

August 2013

© Copyright in this document belongs to the Nuclear Decommissioning Authority

Name Signature Date

Author Trevor

Sumerling

07.08.13

Checked by Andy Baker

07.08.13

Approved by Richard

Cummings

07.08.13

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ESC Assessment of Discrete Items

LLWR/ESC/R(13)10055 Page 1 of 59

Executive Summary

The report presents an assessment of the potential radiation doses to persons who encounter discrete items of radioactive waste disposed in the LLWR. This may occur, in the long term, due their deposition on the beach following coastal erosion of the disposed waste or excavation during human intrusion. The results from the assessment provide the basis for Waste Acceptance Criteria (WAC) for disposal of discrete waste items to the LLWR’s engineered vaults.

We define a Discrete Item as a distinct item of waste that, by its characteristics, is recognisable as unusual or not of natural origin and could be a focus of interest, out of curiosity or potential for recovery and recycling/re-use of materials should the waste item be exposed after repository closure.

The report, first, sets out background to the assessment. This includes the definition of low-level radioactive waste, discussion of heterogeneity within a consignment, the importance of discrete items, and our definition of a Discrete Item.

Our approaches to assessing heterogeneity at different scales and advice from the Environment Agency on assessing discrete items are discussed. Taking account of this, we set out our approach to assessing discrete items, and to determining Discrete Item Limits, which we base on a dose-rate guide value of 20 µSv/h.

The models and data for assessment calculations and derivation of Discrete Item Limits are presented. This includes radionuclide selection and radionuclide-dependent data, the model to represent discrete items based on spheres of equivalent mass, and models to represent and calculate external and internal effective doses from model items.

Effective dose rates from encounter with model items bearing unit activity (1 GBq) of given radionuclides are then calculated and, inversely, the item activities that will lead to the chosen guide value. This leads to the grouping of radionuclides that present broadly similar potential for radiological impact if present on a discrete item.

The final section sets out proposed WAC for discrete items based on the arguments and calculations in the previous sections. This includes the Discrete Item Limits for radionuclide groups, the method to calculate compliance against the Limits, and a comparison with the consignment limits of 4 GBq/t ‘alpha’ and 12 GBq/t ‘other’.






Contents

1 Introduction ......................................................................................................... 5

2 Background ......................................................................................................... 7

2.1 Definition of low-level radioactive waste .................................................. 7

2.2 Heterogeneity within a consignment ........................................................ 7

2.3 Disruption of the LLWR by coastal erosion .............................................. 8

2.4 Accounting for waste heterogeneity......................................................... 8

2.5 What do we mean by discrete items? ...................................................... 9

3 Assessment of discrete items ............................................................................ 10

3.1 Alternative approaches to assessment .................................................. 10

3.2 Environment Agency advice .................................................................. 10

3.3 Our view and approach to assessing discrete items .............................. 11

3.4 Approach to determining Discrete Item Limits........................................ 12

4 Models and data................................................................................................ 14

4.1 Exploratory modelling ............................................................................ 14

4.2 Radionuclide selection and data............................................................ 14

4.3 Model items........................................................................................... 15

4.4 Dose factors for external radiation......................................................... 18

4.5 Dose factors for internal radiation.......................................................... 19

4.6 Children and infants............................................................................... 22

5 Derivation of Discrete Item Limits ...................................................................... 23

5.1 Summary of approach and model representation .................................. 23

5.2 Form of the Discrete Item Limit.............................................................. 24

5.3 Illustrative assessment .......................................................................... 24

5.4 Assessment and grouping of radionuclides ........................................... 27

6 WAC for Discrete Items ..................................................................................... 37

6.1 Definition of a Discrete Item................................................................... 37

6.2 Discrete Item Limits............................................................................... 37

6.3 Application of the Discrete Item Limits................................................... 39

6.4 Comparing Discrete Item Limits with 4 and 12 GBq/t............................. 39

References.............................................................................................................. 42

Appendices ............................................................................................................. 43

Appendix A: Data tables................................................................................. 44

Appendix B: Illustration of effective dose model results .................................. 47

Appendix C: Illustration of Discrete Item Limits for example radionuclides ..... 50

Appendix D: Comparison of limits for Discrete Items and low-activity sources 55

Appendix E: Allocation of radionuclides to groups .......................................... 57






1 Introduction

The Low Level Waste Repository (LLWR) submitted a fully revised Environmental Safety Case (ESC) to the Environment Agency in May 2011, referred to as the 2011 ESC [1]. The Environment Agency is reviewing the submission. In support of the review, we have engaged with the Environment Agency to answer questions and provide further clarification where required. At an appropriate stage, the LLWR will apply for a new permit for the continued operation of the site.

We are required by our permit to operate the site in accordance with the assumptions of the ESC. To this end, we have developed a process for the implementation and maintenance of the ESC as a ‘live’ safety case, including the development of waste acceptance arrangements, to ensure that the repository is operated in a safe and optimised way, consistent with the assumptions and results of the ESC.

This report presents an assessment of the potential radiation doses to persons who encounter discrete items of radioactive waste disposed in the LLWR. This may occur, in the long term, due to their deposition on the beach following coastal erosion of the disposed waste or excavation of items during human intrusion. The results from the assessment provide the basis for Waste Acceptance Criteria (WAC) for disposal of discrete waste items to the LLWR’s engineered vaults.

We define a Discrete Item as a distinct item of waste that, by its characteristics, is recognisable as unusual or not of natural origin and could be a focus of interest, out of curiosity or potential for recovery and recycling/re-use of materials should the waste item be exposed after repository closure.

The structure of the report is as follows:

• Section 2 sets out background to the assessment. This includes the definition of low-level radioactive waste (LLW), discussion of heterogeneity within a consignment, the importance of discrete items, and our definition of a Discrete Item.

• Section 3 discusses our approaches to assessment of heterogeneity at different scales, advice from the Environment Agency on assessing discrete items, our view and approach to assessing discrete items, and our approach to determining Discrete Item Limits, which we base on a dose-rate guide value of 20 µSv/h.

• Section 4 summarises findings from preliminary modelling, and sets out the models and data for assessment calculations and derivation of Discrete Item Limits. This includes the approach to radionuclide selection and radionuclide-dependent data, the model to represent discrete items based on spheres of equivalent mass, and models to represent and calculate external and internal effective doses from model items.

• Section 5 summarises the model representation and discusses the form of the Discrete Item Limit function. Effective dose rates from encounter with model items bearing unit activity (1 GBq) of given radionuclides are then calculated and, inversely, the item activities that will lead to the chosen dose-rate guide value. This leads to the grouping of radionuclides that present broadly similar potential for radiological impact if present on a discrete item.



• Section 6 sets out the proposed WAC for Discrete Items based on the arguments and calculations in the previous sections. This includes the Discrete Item Limits for radionuclide groups, the method to calculate compliance against the Limits, and a comparison with the consignment limits of 4 GBq/t ‘alpha’ and 12 GBq/t ‘other’.

Model data, additional supporting results, and a comparison of activity limits previously derived for low-activity sources, i.e. very small discrete items, are presented in appendices.



2 Background

2.1 Definition of low-level radioactive waste

The radioactive waste that can be disposed at the LLWR is defined by the terms of the LLWR Permit [2] and must be consistent with Waste Acceptance Criteria (WAC) issued by the LLWR [3]. The current Permit allows disposal of LLW by deposit on the premises, where:

"LLW" means solid radioactive waste, including any immediate package, with a

maximum concentration of 4 gigabecquerels per tonne of alpha emitting

radionuclides and 12 gigabecquerels per tonne of all other radionuclides.

This definition does not set limits on the size of package or mass over which the determination of gigabecquerels per tonne (GBq/t) should be made. However, Schedule 8 (paragraph 1) of the Permit states: The Operator shall not, by deposit on the premises:

(a) dispose of any consignment of solid waste in which the activity of alpha

emitting radionuclides exceeds 4 gigabecquerels per tonne or the activity of all

other radionuclides exceeds 12 gigabecquerels per tonne; … .

and the meaning of consignment is given in paragraph 27 of the Permit as:

an individual shipment of radioactive waste not greater in volume than 40 cubic

metres or such volume as specified in writing by the Agency.

The limitations of 4 and 12 GBq/t are taken into the LLWR’s Waste Acceptance Criteria [3], which states under paragraph L2.2 ‘Radioactivity Limits’:

The Activity of any Waste Consignment consigned for disposal as low level

waste at the Low Level Waste Repository shall not exceed the following values:

- 4 GBq/t for all alpha-emitting radionuclides;

- 12 GBq/t for all other radionuclides.

2.2 Heterogeneity within a consignment

Given the heterogeneous nature of LLW and waste items that may be disposed in a consignment, the above Permit conditions and WAC do not rule out that there may be some volumes of waste or waste items within a consignment that bear activities exceeding the 4 and 12 GBq/t levels.

The Environment Agency has been considering the definition of LLW and how it should be applied, for example considering the volume or mass over which it may be appropriate to average in order decide whether wastes may be disposed as LLW, and has asked LLWR to develop limits and arguments by which to define the range of wastes that can be safely disposed, based on assessed impacts within the Environmental Safety Case (ESC).



We have set out to determine the potential post-closure radiological impact of disposals of discrete items of waste in order to determine the levels of radionuclides that can be considered acceptable against the ESC, i.e. are unlikely to lead to impacts in excess of the guidance levels set out in the UK environment agencies’ Guidance on Requirements for Authorisation (GRA) [4].

2.3 Disruption of the LLWR by coastal erosion

Studies in support of 2011 ESC conclude that the expected evolution of the LLWR is that it will disrupted by coastal erosion within a few hundred to a few thousand years after present [5]. Modelling indicates that the most likely mode of disruption is that the repository vaults will be undercut by erosion and collapse progressively onto the beach over a further period of a few hundred to a few thousand years.

The calculated rate of coastal recession and hence the time of commencement of erosion of the LLWR vaults depends on the rate of global sea-level rise over the next centuries and millennia, which is uncertain but constrained by global physical properties. In the 2011 ESC, we concluded that, based on site information and modelling of coastal erosion, it is very unlikely that erosion of the vaults could begin before about 300 years after present. To assess radiological impacts, we calculated radiation doses for cases in which erosion of the vaults commenced at 300, 1000 and 3000 years after present, which we consider to bound the uncertainty [6].

Given the conclusion that disruption of the repository by coastal erosion is the expected natural evolution scenario for the LLWR, we have developed models to assess the radiological impacts of erosion that represent the key processes and uncertainties at a level of detail commensurate with that conclusion. In particular, we recognise that it is necessary to understand the heterogeneity of wastes within the LLWR at various scales, and the implications that this has for the assessment of radiological impacts during the erosion of the facility.

2.4 Accounting for waste heterogeneity

The effect of large-scale heterogeneity (at vault scale) is represented in long-term radiological assessments in support of the LLWR [6]. The impact of heterogeneity at particulate scale has been raised as an issue by the Agency, and has been assessed in LLWR/ESC memos [7,8]. The present report is concerned with an intermediate scale, specifically the radiation exposures due to items of radioactive waste recognisable at the time of disposal that might remain and be encountered as discrete objects after erosion or excavation from the repository.

Our standard assessments of annual effective dose and risk from erosion of the facility assume potentially exposed groups (PEGs) that make use of the beach, for leisure or other uses, oblivious to the presence of the eroded waste [6]. Hence, over an annual period, it is reasonable to assume they will be exposed to some average of radionuclides present in the eroding wastes or distributed on the beach and foreshore, as they walk across different areas of the beach. It is also assumed that the waste containers, grout and wastes are degraded to some extent and will, over time, break up on the beach and hence be dispersed on the beach and foreshore.

Some wastes by their nature may remain largely intact and encourage investigation either for curiosity or with intent to recover. This could be especially so for discrete



items that remain recognisable as human artefacts, e.g. durable metal items, in a range of sizes. We are not concerned here with volumes of waste that will be dispersed following deposition on the beach, since results from the standard assessment models apply to these wastes.

Similarly, our assessments of human intrusion into the disposal facility assume exposure to the average activity of the wastes that are excavated [6]. Depending on the mode of intrusion, however, it is possible that intact items could be recovered that would attract special attention from the intruder or from a subsequent site occupier if the items are left exposed at the ground surface.

2.5 What do we mean by discrete items?

Here, we define a Discrete Item as a distinct item of waste that, by its characteristics, is recognisable as unusual or not of natural origin and could be a focus of interest, out of curiosity or potential for recovery and recycling/re-use of materials should the waste item be exposed after repository closure.

Examples of Discrete Items are:

• hand tools, engineered items and equipment of durable materials (such as may be disposed with other wastes in drums for grouting or high-force compaction, or directly to a Disposal Container);

• grouted drums of waste or pucks from high-force compaction;

• large metal items, e.g. steel beams and plates, pipework, shielding, heavy equipment and flasks (but not general scrap metal) such as may be disposed directly to a Disposal Container.



3 Assessment of discrete items

3.1 Alternative approaches to assessment

A key feature in the assessment of a discrete item is that the item has the potential to modify the behaviour of a person that encounters it, i.e. it is visible and therefore an individual may deliberately go towards and inspect or (if small enough) pick up the item. This is different from the standard assessment calculations in which the beach user carries out activities on the beach without regard to the presence of the waste or the radioactive hazard it may pose (see above). It is also different from the case of exposure to radioactive particles, in which case the individual is not aware of a radioactive particle and hence behaviour is not modified.

In principle, since coastal erosion is part of the expected long-term natural evolution of the LLWR facility, we could assess the calculated exposure of the beach and foreshore user against the 10-6 (one in one million) annual risk guidance value set in Requirement R6 of the GRA [4]. However, as noted above, there is uncertainty about the timing of erosion and, following discussion with the Environment Agency, the impact parameter that we actually calculate is the annual effective dose over the period of erosion for alternative erosion sequences [6]. We then compare the peak (in time) annual effective dose with a value of 20 µSv, which is the annual dose corresponding an annual risk guidance value of 10-6 where the probability of receiving the dose is one (paragraph 6.3.17 of the GRA [4]).

To assess exposure to radioactive particles, we explicitly calculate the risk at the time at which particles may be present on the beach and foreshore and compare with the risk guidance value. The risk is calculated as the product of effective dose from (inadvertent) encounter with a particle multiplied by the probability of encounter treated as a random event. The probability can be bounded from the cautiously estimated maximum frequency or number of such particles on the beach and foreshore [7].

To assess exposure to discrete items, we might assess against a risk guidance value in the same way as for radioactive particles, but this would involve estimating the probability of encounter with the item. This is problematic because, as discussed above, the item can modify the behaviour of a person encountering it, so encounters are not random events. Thus, an alternative method is required.

3.2 Environment Agency advice

We have discussed issues related to the assessment of radioactive particles and discrete items with the Environment Agency in support of their review of our ESC. Stemming from these discussions and additional assessment calculations since the 2011 ESC, the Environment Agency has issued ‘Advice to Environment Agency Assessors’ on these matters [9]. The ’Advice’ and our discussion with the Agency staff indicates, amongst other things, that:



• our approach to estimating risk from radioactive particles may be acceptable, although we must also assess the maximum doses from encounter with radioactive particles as may be present, and seek to reduce the potential for radioactive particles in future disposals;

• the Environment Agency would accept an assessment of encounter with discrete ‘visually identifiable objects’ against a significance test of an effective dose guidance level in the range of around 3 mSv/year to around 20 mSv/year.

The latter is by analogy with the GRA Requirement R7 for assessment of human intrusion, which is to account for situations that ‘cannot reliably be assessed in terms of a numerical value of probability’ (paragraph 6.3.38 of the GRA [4]).

3.3 Our view and approach to assessing discrete items

We appreciate the Environment Agency ‘Advice to Assessors’ and associated discussions. Taking account of this advice and discussions, our view is as follows.

We have considered in some detail the various probabilities that might be invoked around the occurrence and encounter with a discrete item within our assessment of low-activity sources, documented in reference [10]. Therein, we concluded that it was difficult to argue that an encounter with a low-activity source container would not occur at some time and that it was cautious to assess the impact to an individual that actually interacted with a container. (That is, the potentially exposed group (PEG) consists of the individuals that interact with such containers, and this is a subset of the larger group of individuals that make use of the beach and foreshore.) Therefore, in reference [10], we assessed the encounter with a low-activity source container against an annual effective dose of 20 µSv, corresponding with the annual risk guidance value.

Consistent with the idea of assessing the encounter as an event that would happen (probability = 1 per year), we parameterised the encounter as representative of a casual encounter of limited duration. We discussed and excluded from quantitative assessment the case of individuals who deliberately sought out source containers or attempted to open and disrupt them, noting that higher doses could be calculated for such a case, but it would not be appropriate to assess the case against an effective dose corresponding to the risk guidance value. On the other hand, the significance test of an effective dose guidance level in the range of around 3 mSv/year to around 20 mSv/year, proposed in the Environment Agency ‘Advice to Assessors’, would seem an appropriate criterion for such activities.

We conclude that it is cautious to assess casual and short duration encounters with discrete items against an effective dose of 20 µSv and this will be protective of the average beach user that also happens to encounter and briefly examine unusual items on the beach. The individual may examine or remain in close proximity with a number of such items over the period of a year, but only one or a few will bear radionuclides at higher levels, which will be limited by the Discrete Items Limits that we propose to introduce.

On the other hand, the case of individuals deliberately seeking out, collecting, taking away or attempting to disrupt discrete items should be judged against the effective dose guidance range of around 3 mSv/year to around 20 mSv/year. The 3 mSv/year would apply to an individual that was carrying out such activities over a number of



years and 20 mSv/year would apply to cases where the activity extended over about a year or less. This last could apply to items that were very infrequently disposed in the LLWR, such that it would be unlikely for more than one such item to ever be encountered.

For this criterion to become limiting, would require contact times of two to three orders of magnitude longer than we consider for the casual and short duration encounters with discrete items against an effective dose of 20 µSv, Thus, we expect the exposure from the casual encounter on the beach to be limiting. Moreover, concerted collecting or recovering requires a motive, related to the perceived use or recovery value of the items. That is, collectors may be drawn selectively to specific types of item and their actions depend on the intended use or recovery purpose of the item. Thus, it is more difficult to assign generalised parameter values to the encounter and subsequent recovery.

In the case of discrete items excavated during human intrusion, the effective dose guidance range of around 3 mSv/year to around 20 mSv/year in any case applies. As above, contact times of two to three orders of magnitude greater than we consider for the casual and short duration beach encounter would be required for exposure to items excavated during human intrusion to be limiting. In addition, the number of discrete items that could at any time be exposed on the beach and foreshore is much greater than could be excavated through any of the assessed human intrusion events. Thus, the likelihood (or frequency) of encounter with a discrete item bearing activity near to the Discrete Item Limit during human intrusion is much lower than following coastal erosion. Therefore, we consider it is not necessary to assess cases of human intrusion as part of the determination of Discrete Item Limits.

If assessed at a given time of occurrence, the effective dose rate to a human intruder that examined an excavated object would be similar to the effective dose rate to a person examining the same object freshly deposited on the beach by coastal erosion. The protection criterion to apply, around 3 to 20 mSv/year, is two to three orders of magnitude less stringent, but a human intrusion event might occur earlier than the earliest assessed time for onset of coastal erosion of the facility. Sensitivity of calculated dose to time of occurrence is assessed in Subsection 5.4.

3.4 Approach to determining Discrete Item Limits

In view of the above, we focus on the local beach user that also happens to encounter and examine unusual items on the beach. We assess a proximate encounter (i.e. very close to and touching or handling an item) of one-hour duration, and compare the calculated effective dose with an effective dose of 20 µSv, which corresponds with the annual risk guidance value set in Requirement 6 of the GRA.

Thus, we calculate Discrete Item Limits as the level of radionuclides on an item that will give rise to an effective dose rate during encounter of 20 µSv/h. The assessed dose rates vary depending on the object dimensions, form of contamination and radionuclides present.

The encounter of one-hour duration can be considered representative of several shorter duration encounters within one year, where a range of items is examined but only one or a few are contaminated with levels of radionuclides approaching the Discrete Item Limits.



Radionuclide contamination on the surface of an item may be transferred to the hands or clothing of the individual. This gives potential for absorbed doses to the skin from beta radiation as well as effective doses from ingestion of inadvertently transferred contamination. The method for assessing the absorbed dose to skin would be to compare the absorbed dose with the threshold for deterministic effects or with the skin dose limit for members of the public of 50 mSv/y. Our assessments for radioactive particles, however, show that ingestion will to be the limiting impact [7]. Therefore, we do not assess skin doses but rather include an ingestion pathway representing the inadvertent ingestion of contamination from the item.

We thus consider that application of Discrete Item Limits based on assessment of effective dose (external and internal radiation) from an encounter of one-hour duration will be protective for the local beach user encountering and examining unusual items on the beach.



4 Models and data

4.1 Exploratory modelling

Preliminary calculations were undertaken to explore the problem and the results were presented to and discussed with the Environment Agency. Three example items were modelled as spheres of equivalent volume with activity distributed either uniformly throughout the volume of the sphere or uniformly over the surface of the sphere. Calculations were made for 18 radionuclides (and their progeny) commonly important in ESC assessments, with radionuclides present individually and in combinations based on radionuclide ratios representative of ratios found in Vault 8 and projected for Vault 14.

The work indicated that:

• It is possible to define groups of radionuclides that present broadly similar levels of effective dose, therefore it is not necessary to determine the activity of every single radionuclide on an item, rather acceptability against a limit can be demonstrated by determining activity of certain key and representative radionuclides that dominate the total activity in each group.

• Over the size range investigated a limit based on total activity, rather than specific activity, could be more appropriate.

• There is a large difference between the calculated effective dose from surface- and volume-contaminated items (which is to be expected), but this can be considered as an indicator of the uncertainty arising from the range of item geometries and distribution of contamination within or on the item.

A key criticism, however, was that the results did not give confidence over the full size range that should be considered, in particular the assessment of smaller items such as could be easily picked up and handled. It was also recognised that a single limit based on total activity could not be appropriate over the whole size range, as this would imply allowing very high concentrations of radionuclides on smaller items.

4.2 Radionuclide selection and data

A somewhat expanded list of 26 radionuclides (with progeny) is considered in the main assessment calculations in this report. This includes all the radionuclides that contribute significantly to the assessed post-closure radiological impacts for coastal erosion and/or for human intrusion in the 2011 ESC. To this have been added other radionuclides important in the inventory and ESC. This includes radionuclides with and without potential to contribute to doses from exposure to discrete items; this is to illustrate the wide range of effective doses that may be calculated for different radionuclides. Consistent with the WAC, the radionuclides are identified as alpha-emitting radionuclides (alpha) and radionuclides not decaying by alpha emission (other).

The following radionuclides are included:



• Other: H-3, C-14, Cl-36, Ca-41 Co-60, Ni-63, Sr-90*, Nb-94, Tc-99, Ag-108m, I-129, Cs-137*, Pb-210*, Pu-241**;

• Alpha: Ra-226*, Pa-231*, Th-232*, U-234, U-235*, U-238*, Np-237*, Pu-238, Pu-239, Pu-240, Am-241, Cm-244**.

* Asterisks indicate radionuclides with shorter-lived progeny assumed to be present in equilibrium.

** Double asterisks indicate radionuclides that decay to longer-lived progeny that are important in the time interval of interest.

Appendix A, Table A-1, shows half-life and included progeny for the selected radionuclides. Other radionuclide-dependent data that are required are also presented:

– effective dose rates from an infinite plane surface,

– effective dose rates from a semi-infinite slab, and

– committed effective dose from ingestion.

The 26 radionuclides, listed above, are sufficient to examine the sensitivity of assessed dose rates to key variables and the method of radionuclide grouping, as presented in Section 5 of this report. Subsequently, calculations have been made for additional radionuclides, as presented in Appendix E of this report. This includes all radionuclides that are expected to contribute significantly to total assessed dose rate from discrete items. The basis for the list is discussed in Appendix E.

4.3 Model items

For ESC assessments, a method of calculating external radiation doses from radionuclide-bearing spheres of different radii and at different distances has been devised (see reference [11] and Appendix B in reference [12]). The method multiplies the radionuclide-dependent effective dose rate above a semi-infinite slab or infinite plane [13], by radionuclide-independent scale factors that take account of the geometric differences and have been worked out analytically. Further information on the method and its accuracy is given in Subsection 4.4.

For assessment calculations in this report, we opt to assess discrete items as spheres of equivalent mass, and consider spheres bearing activity:

– distributed uniformly throughout the sphere volume, and

– distributed uniformly over the sphere surface.

The concept is illustrated for three example items in Figure 4-1 and Table 4-1.



Figure 4-1: Three examples of discrete item represented by spheres of equivalent mass. Spheres are shown at correct diameter relative to each item but the items are not to a common scale. Data on the items and representative spheres are given in Table 4-1.



Table 4-1: Data for example discrete items and spheres of equivalent mass

Steel hammer

Shaft 30 cm

Head 12 cm

Density 7850 kg/m2

Mass 1.4 kg

Surface area 300 cm2

200 L drum grouted waste

External diameter 0.6 m

External height 0.88 m

Density 2500 kg/m2

Mass 580 kg

Surface area 1.6 m2

Thick wall steel pipe

External diameter 30 cm

Wall thickness 1 cm

Length 2 m

Density 7850 kg/m2

Mass 140 kg

Surface area 3.6 m2

(Internal 1.9 m2 + external 1.8 m

2)

Represented as

Sphere, radius 5.5 cm

Density 2000 kg/m3

Mass 1.4 kg

Surface area 380 cm2

Represented as

Sphere, radius 0.41 m

Density 2000 kg/m3

Mass 580 kg

Surface area 2.1 m2

Represented as

Sphere, radius 0.26 m

Density 2000 kg/m3

Mass 140 kg

Surface area 1.9 m2

Model / item surface area

1.27


1.32


External 0.44

Internal 0.47

Total 0.23

A model sphere of equivalent mass, rather than of equivalent volume, is selected for two reasons:

• First, this leads to Discrete Item Limits in terms of activity on items of given mass (or weight), which, for irregular items, may be easier for consigners to measure or estimate than volume.

• Second, it should provide a more accurate estimate of effective dose from volume-contaminated items since the equivalent mass sphere presents the same mass for shielding.

Surface contamination is liable to be ‘patchy’ or heterogeneous over or within an item. Modelling as a sphere minimises the surface area for an item of given volume or mass, thus maximises the concentration of calculated surface contamination (assumed to be uniformly distributed). The choice of sphere density of 2000 kg/m3 is a matter of judgment and is selected so as to provide a sphere of very roughly similar surface area to irregular objects such as might arise as discrete items. This is illustrated by items (a) and (c) in Figure 4-1 and Table 4-1.

In the case of item (a), the steel hammer, the choice of 2000 kg/m3 leads to a model surface area somewhat higher than the item surface area, hence the model surface activity is somewhat less than the item surface activity (assuming both are uniformly distributed), although not by a large amount compared with the uncertainty about the actual distribution of activity on an item. On the other hand, for item (c), the steel pipe, the model surface area is only about one quarter of the item surface area, hence the model surface activity will be about four times the item surface activity, although it will also be relevant whether the surface activity is present on internal, external surfaces or both. Overall, we judge the assumption of 2000 kg/m3 leads to broadly reasonable results for smaller metal items, and will tend to be cautious for larger or complex geometry items.



The item (b), the 200 litre drum with assumed density 2500 kg/m3, is included to illustrate that for similar density and regular objects the model and ‘true’ surface areas are similar. In this particular case, however, it would be expected that activity is distributed throughout the volume, not on the surface of the item.

To derive Discrete Item Limits, we consider items with mass between 10 g and 10 tonnes. 10 g is possibly lower than any item of interest, but is useful because it is desirable to show that the allowed activity on such a small item is consistent with the activity limit for low activity sources, already examined in reference [10]. 10 tonnes is selected as an upper limit since it is unlikely that an item above this weight would be placed in a disposal container. If consigned for direct grouting into the vaults the item(s) would, in any case, require a variation request to be submitted; in such cases the items would be assessed taking account of the actual item characteristics and estimated radionuclide burden.

Appendix A, Table A-2, shows calculated volume, radius and surface area for model spheres with mass between 10 g and 10 tonnes and density 2000 kg/m3. Total item activity, specific activity and surface activity are calculated for model spheres bearing 10 GBq/t and 1 GBq total.

4.4 Dose factors for external radiation

As introduced in Subsection 4.3, a method of calculating external radiation doses from radionuclide-bearing spheres of different radii and at different distances has been devised (reference [11] and Appendix B in reference [12]). This relies on a demonstration of a consistent ratio of dose rates close to a large (2 m radius) sphere and above a semi-infinite slab or infinite plane each containing unit concentrations of radionuclides, and the mathematical derivation of dose rates at different distances from spheres of smaller radii. The relation to the semi-infinite slab and or infinite plane is useful because effective doses have been systematically and correctly worked out for these, taking account of source and receptor geometry and attenuation [13].

It has been shown that the method provides a good approximation to results of Microshield calculations for the 15 litre source container (paint tin) considered in the assessment of low-activity sources [10]. It is also indicated, in reference [11], that the method is accurate for radionuclides with photon emission above about 100 keV but provides a cautious representation for radionuclides emitting photons only below that energy.

References [11] and [12] provide scale factors for spherical sources at distances of 0.3, 1.0 and 3 m from spheres of radii between 0.1 and 2 m. However, model spheres with mass between 10 g and 10 tonnes and density 2000 kg/m3 have radii between about 0.01 and 1.06 m (see Table A-2). Values for scale factors for the model items in Table A-3 have been worked out by extrapolating using the equations set out in references [12] and interpolating. This yields scale factors for items of given mass as shown in Appendix A, Table A-3.

The behaviour of the calculated effective dose according to the scale factors has been checked by calculations considering volume-contaminated and surface-contaminated spheres at each mass with 10 GBq/t and with 1 GBq total of Cs-137. The results show that the calculated effective dose behaves as expected from geometric and physical principles. It is also shown for an item bearing 1 GBq total



that, as mass decreases, the results tend towards effective dose from a point source. Results from the calculations are shown in Appendix B.

A key observation is that the calculated doses at 0.3 m are about an order of magnitude higher than calculated doses at 1.0 m for items up to about 10 kg (sphere radius ~0.1 m). This is a result of the inverse square relation for a point source. As the spheres become larger, the geometry tends towards that of a plane or semi-infinite slab and the calculated doses at 0.3 and 1.0 m converge. At 1 tonne (sphere radius ~0.5 m) the difference between the 0.3 and 1.0 m results are about a factor of 4.1; at 10 tonne (sphere radius ~1.1 m) the difference is about a factor of 2.7.

In the preliminary modelling (Subsection 4.1), we calculated external exposure at a distance of 1 metre for 10 hours, representing an individual happening to spend time over one year in the vicinity of an object, but not directly interacting with the object for all that time. Following the arguments set out in Section 3, we conclude that the appropriate basis for deriving Discrete Items Limits is to assess a proximate encounter (i.e. very close to and touching or handling an item) of one-hour duration. This is better represented by the calculated effective dose at 0.3 m. We note, however, that the two representations (10 hours at 1 m and 1 hour at 0.3 m) yield similar effective dose rates from external radiation.

4.5 Dose factors for internal radiation

Comparison of inhalation and ingestion

Internal radiation doses may arise from inhalation and from ingestion of radionuclides. However, assessment calculations in support of the 2011 ESC show that, in most post-closure assessment situations, ingestion is usually more important than inhalation [6]. The reason for this is as follows.

To assess the internal effective dose to an individual in a contaminated environment, we calculate:

• intakes of radionuclide by inhalation as concentration of dust in air, multiplied by an inhalation rate, multiplied by the concentration of radionuclides on dust in air;

• intakes of radionuclide by ingestion as a rate of inadvertent ingestion multiplied by the concentration of radionuclides in soil, sediment or other local substrate.

Inadvertent ingestion includes inadvertent transfer from hand to mouth, and mouth and nose breathing of larger (non-respirable) particles.

For the user of the beach and foreshore reference parameter values for the intake modes are as follows.

Inhalation: Concentration of dust in air 0.1 mg/m3

Inhalation rate (light exercise) 1.375 m3/h

Yields 0.14 mg/h

Ingestion: Inadvertent ingestion rate 5 mg/h

Thus, assuming the local dust in air carries the same average specific activity as the local substrates, the calculated intake of a radionuclide by ingestion is about 30 to 40



times that by inhalation. Thus, inhalation is only important for radionuclides for which the dose per unit intake by inhalation is at least 30 times the dose per unit intake by ingestion. This only occurs for isotopes of elements with very low uptake from the human gastrointestinal tract, such as thorium and plutonium [12].

For the situation of examining discrete items on the beach and foreshore, however, the local respirable dust will arise almost entirely from the general environment, not from the discrete item. On the basis of this second argument, it is reasonable to neglect inhalation.

Ingestion from surface-contaminated items

We are seeking to represent the inadvertent ingestion due to handling or close inspection of an item and transfer of small particles/dusts to hands and thence to mouth.

Ingestion of removable surface contamination after transfer to hands, foods or other items entering the mouth is referred to as secondary ingestion. This pathway is important in assessing doses to workers engaged in building renovation or demolition where radioactive contamination is present, and in the event of contamination of urban areas from nuclear accidents, and hence has been well studied. Contractors for the US NRC carried out a literature survey of estimates for secondary ingestion in support of estimating impacts from residual radioactive contamination remaining after decommissioning of licensed facilities [14]. The literature surveyed included data and models related to the estimation of doses from radioactive contamination and other sources, notably data on the quantities of lead contamination that can be ingested by adults and children.

Secondary ingestion is expressed as mg/hour ingested per mg/m2 of loose contamination present hence m2/h. This formulation avoids the need to define the nature of the contamination that is removed but, rather, relates ingestion directly to the loose contamination per unit area. The authors of reference [14] arrived a reference value of secondary ingestion by adult workers of 10-4 m2/h of loose contamination, although secondary ingestion by young children could be an order of magnitude higher. The value of 10-4 m2/h was derived for circumstances in which all surfaces are contaminated, thus it is cautious to apply to the circumstances of a contaminated eroded or excavated items where other surfaces are present, and where some areas of the contamination may be contained within the item or covered by grout or soil.

The additional factor to be estimated is the fraction of contamination that is both still present on the item and yet could be loose or removable by contact or handling.

Over several hundred years in the repository, many of the surfaces on which contamination could have been present at the time of disposal will have decomposed, degraded or corroded such that activity that was on these surfaces will be taken up in surrounding grout or debris. Then, on erosion or excavation, these will be separated from the item and be taken up in the general bulk of waste materials and natural materials with which it becomes mixed during erosion or excavation.

If activities remain on the surfaces, e.g. corrosion products firmly attached to metal objects, then further corrosion of the item will lead to some of this activity becoming removable, but consequently it will also be removed by sand, wind and wave action



on the beach and thence widely dispersed. Thus, it could be envisaged that for a contaminated item, the potential for exposure to removable surface-contamination is at a maximum shortly after it is deposited on the beach and then declines as the contamination on accessible surfaces is corroded and sand-abraded away.

The standard assumption in occupational radiological protection practice is that 10% of surface contamination is removable. For discrete items on the beach that were originally surface-contaminated, we consider that a further order of magnitude reduction is justified related to the above-mentioned factors of surface activity loss both before and after erosion. Indeed, for items that have been exposed on the beach for more than a few weeks, the reduction to 1% is probably still cautious.

In summary, we consider a secondary ingestion coefficient of 10-4 m2/h of removable contamination, and assume that at any one time only 1% of the surface activity present at the time of disposal is removable on contact. Hence, a secondary ingestion coefficient of 10-6 m2/h of the surface activity originally present is indicated.

Committed effective doses from ingestion are calculated by multiplying the calculated intake by the radionuclide dose-per-unit ingestion coefficients, as tabulated in Appendix A, Table A-1 (derived from reference [12]).

Ingestion from volume-contaminated items

Volume-contaminated discrete items include irradiated metal items, in which the contamination consists of activation products within the item, and drums containing grouted solid waste items or encapsulated sludges and resins. In this case, if deposited intact on the beach, there is initially minimal surface contamination, but as the items corrode or break down contamination is revealed at exposed surfaces: in corrosion products, on cement surfaces or on revealed small solid items.

The standard rate of inadvertent ingestion that we would apply to contaminated material dispersed in soils and sediments is 5 mg/h (see above). There are good reasons to judge this is overly cautious for the assessment of exposure to discrete items, as follows:

• The generic figures for inadvertent ingestion (10-4 m2/h and 5 mg/h) relate to situations in which all surfaces are contaminated and all dust or dirt that may be transferred to mouth bears activity related to that general contamination. Applied to discrete items, this assumes that all dust or dirt that is ingested comes from the item, which may be unduly cautious especially for smaller items.

• For grouted items or wastes, after the order of 300 years or more in vault conditions, it is likely that more mobile radionuclides will have migrated to some degree and be retained on cement surfaces. In this form, they will be considerably less well taken up in the GI tract than the radionuclide forms assumed in ICRP dose per unit ingestion calculations.

• For irradiated metal items, the activation products will be contained in corrosion products generated as the item corrodes on the beach. Again, in this form, they will be less well taken up in the GI tract than the radionuclide forms assumed in ICRP dose per unit ingestion calculations.

For these reasons – ingested dust and dirt comes also from other sources and uptake from the GI tract is liable to be less than assumed in ICRP ingestion dose



factors – we judge that a reduction factor of ten is appropriate. Thus, to assess volume-contaminated discrete items we adopt an inadvertent ingestion rate of 0.5 mg/h.

Committed effective doses from ingestion are calculated by multiplying the calculated intake by the radionuclide dose-per-unit ingestion coefficients, as tabulated in Appendix A, Table A-1 (derived from reference [12]).

4.6 Children and infants

The above models and data are based on consideration of adult behaviours and dosimetry.

For older (teenage) children there is no reason to alter any of the above assumptions, models and data.

Data on beach use habits used by the HPA in their assessment of beach particles [15] indicate younger children spend less time on the beaches than adults. Younger (e.g. 10-year old) childrens’ play could be focussed around identifiable objects, but a one-hour encounter still seems a reasonable time to consider for play around relatively freshly eroded objects. In this case, their external radiation dose would be very similar to adults. Their potential for inadvertent ingestion could be higher due to less scrupulous hand to mouth habits, but we consider this is outweighed by the cautions already factored into the adopted assessment models and data. In addition, play during repeated visits is more likely to be focused around larger objects that can remain on the beach longer and also therefore are likely to be cleaned of accessible contamination by sand, wind and wave action.

It is unlikely that infants (e.g. one-year old children), who would be supervised, would be allowed to play around objects of uncertain origin and appearance, and if doing so the factors mentioned above for the ten-year old apply.

We conclude that, although different encounter characteristics could be postulated for children and infants, and hence different and possibly higher effective doses calculated, it is not appropriate to attempt to characterise such encounters for a generic item. Rather, as discussed in Subsections 3.3 and 3.4, we might consider assessing the cases of children and infant for items for which approval was sought to consign to LLWR under a variation, but consider the one-hour encounter by an adult as the appropriate basis for deriving Discrete Item Limits.



5 Derivation of Discrete Item Limits

5.1 Summary of approach and model representation

Summarising from Sections 3 and 4:

• We assess a proximate encounter (i.e. very close to and touching or handling an item) of one-hour duration, and compare the calculated effective dose with an effective dose of 20 µSv, which corresponds with the annual risk guidance value set in Requirement 6 of the GRA. Thus, we calculate Discrete Item Limits as the level of radionuclides on an item that will give rise to an effective dose rate during encounter of 20 µSv/h.

• We consider that application of Discrete Item Limits based on assessment of the effective dose (external and internal radiation) from a proximate encounter of one-hour duration is appropriate, and will protect the local beach user who encounters and examines unusual items on the beach.

• We model discrete items as spheres of equivalent mass with a density of 2000 kg/m3.

• We assess spheres with radionuclide activity distributed uniformly over the surface and uniformly throughout the volume of the sphere. We consider that these two cases bound the uncertainty about the actual distribution of activity within or on a discrete item.

• We calculate effective dose from external radiation according to a model based on the effective dose rate from a uniformly contaminated infinite plane and a uniformly contaminated semi-infinite slab, multiplied by scale factors that take account of geometric relations between the infinite plane and semi-infinite slab and model spheres.

• We calculate the effective dose rate from external radiation at a distance of 0.3 m from the sphere surface, i.e. handling a small item or very close to a large item.

• We calculate inadvertent ingestion as 10-6 m2/h of the surface activity as calculated from the activity of the item as disposed (allowing for radioactive decay), and as 0.5 mg/h of the calculated specific activity of the item as disposed, (allowing for radioactive decay). These values include allowance for the fraction of activity that may be removable, fraction that may have already been removed, and fraction bound in insoluble matrices.

• We use ICRP committed effective dose per unit ingestion factors for an adult as compiled in reference [12], which are appropriate to environmentally dispersed forms of radionuclides.

• For both external and internal radiation calculations, we assume shorter-lived progeny are in equilibrium with longer-lived parents, and take account of in-growth of longer-lived progeny from shorter-lived parents where needed.



Finally, based on current scientific understanding of global temperature and sea-level changes, and site-specific modelling of erosion, we consider it is unlikely that the erosion of the LLWR repository could commence earlier than 300 years after present (see Subsection 2.3 and reference [5]). Therefore, we calculate effective dose rates from external radiation, and committed effective doses rate from ingestion, at 300 years after consignment (although sensitivity to this value is also evaluated).

5.2 Form of the Discrete Item Limit

We need to derive Discrete Item Limits that apply to a wide range of diverse items, examples are given in Subsection 2.5. At the same time, the limits need to be relatively easy to apply.

Discrete Item Limits based on specific activity have been considered, but the range from 1 kg to several tonnes is too large, i.e. widely different effective doses are calculated for encounter with items of 1 kg and items of 1 tonne each bearing the same specific activity. Thus, Discrete Item Limits based on specific activity would be incautious for large items and overly restrictive for small items.

A Discrete Item Limit based on total activity of an item has attractions. It is simple. It will also tend to be increasingly cautious for larger items. This is reasonable as larger items (by their size) are more likely to be objects for attention by future beach and shore users and (by their weight) are likely to remain on the beach longer, i.e. of potentially greater concern. An exceedance of the Discrete Item Limits will mean that a consigner would need to seek approval to dispose under variation, which will trigger a more detailed assessment of the item taking account of its specific characteristics including radionuclide burden and form of contamination, possible behaviour on the shore, and possible attraction to future beach users or others.

On the other hand, a Discrete Item Limit based on total activity may be incautious for small items and would imply activity concentrations on an item that would be unacceptably high relative to the definition of LLW. Rather, we envisage that Discrete Item Limits for smaller items should be consistent with WAC and limits previously worked out for low-activity sources and source containers [10] and already commented on by the Environment Agency.

This indicates that the preferred form of the Discrete Item Limits is a total activity limit for the heavier (larger) items, a different and lower total activity limit for lighter (smaller) items, and a transitional function between these total activity limits.

5.3 Illustrative assessment

This subsection illustrates the method, in principle, of arriving at numerical values for the Discrete Item Limit function.

The illustration is given for Cs-137, which is a familiar radionuclide with simple, well-known characteristics. It also chosen because the calculated total dose from Cs-137 contaminated model items is contributed to by both external and internal radiation, which aids understanding of the dependency of the calculated doses from each exposure mode. The calculations are performed for model items of different mass each bearing 1 GBq of Cs-137 with Ba-137m in equilibrium (which we denote by Cs-137*). Radioactive decay is not included.



(a)

(b)

Note: The vertical scale is six plus logarithm (base 10) of the dose rate in mSv/h, i.e. 6.0 corresponds to 1 mSv/h and 3.0 corresponds to 1 µSv/h.

Figure 5-1: Illustrative case: Cs-137* with no decay: Calculated effective dose rate from model items bearing 1 GBq (a) surface contaminated item (b) volume contaminated item



Note: The vertical scale is six plus logarithm (base 10) of the item activity in GBq, i.e. 6.0 corresponds to 1 GBq and 3.0 corresponds to 1 MBq.

Figure 5-2: Illustrative case: Item activity giving rise to 0.02 mSv/h (20 µSv/h) assuming volume and surface contamination and possible location for item limit

Figure 5-1 shows the calculated effective dose rate as a function of mass from model items bearing a 1 GBq total distributed uniformly (a) over the surface and (b) throughout the volume of the item. Both figures show a ‘cross over’ between total dose due primarily to ingestion and total dose due primarily to external radiation. The total dose is compared with a dose rate of 20 µSv/h, which corresponds to the risk guidance value assuming an event duration of one-hour.

Figure 5-2 shows the inverse relation for total dose via both ingestion and external radiation. That is, it shows the item activity that gives rise to 20 µSv/h assuming that the activity is distributed uniformly over the surface (red line) and throughout the volume (blue line) of the item.

In principle, the item limit (green line) is placed so that it falls between the item activity leading to 20 µSv/h for the volume- and surface-contaminated items (blue and red line). In this case, it could be argued that the item limit for lighter items is unnecessarily cautious. However, for radionuclides with no, or only weak, photon emissions, so that ingestion dominates over the whole range, the function of item activity vs item mass is steeper.

Examining a range of radionuclides, it is found that the general form of a single value limit for items of 100 kg and above, a two order of magnitude lower limit for items of 1 kg and below and a linear transition between these values is a reasonable fit, or is cautious, across the range of radionuclides. This is illustrated in Appendix C.



5.4 Assessment and grouping of radionuclides

An assessment of the selected radionuclides (Subsection 4.2) is made for a reference case of one-hour encounter with surface- and volume-contaminated model spheres of:

• 1 tonne bearing 1 GBq of each radionuclide and allowing 300 years radioactive decay (reference case).

Sensitivity to key variables is explored by calculations for model spheres of:

• 1 kg bearing 1 GBq of each radionuclide and allowing 300 years radioactive decay;

• 1 tonne bearing 1 GBq of each radionuclide and allowing 100 years radioactive decay.

Reference case: 1 tonne sphere and 300 years decay

Figure 5-3 shows the calculated effective dose1 for the reference case showing the results for surface- and volume-contaminated model spheres and the geometric median of these results. The calculated effective doses (horizontal bars) are shown on logarithmic scale on which 6.0 corresponds to 1 mSv/h. Radionuclides are ordered by atomic number and atomic weight.

Plotted in this form the results are not very instructive other than to observe the very large range of calculated effective doses, which spans about eighteen orders of magnitude. This is largely due to the inclusion of Co-60 (half-life ~5 years) and H-3 (half-life ~12 years and low energy per disintegration), which have both decayed to trivial levels at 300 years.

Figure 5-4 shows an alternative presentation of the same data in which dose rates less than 1 pSv/h (0.001 µSv/h) are not shown and results are rank ordered according to the geometric mean. This shows the radionuclides in order of potency in determining total effective dose from the model sphere of given mass and decay time.

The figure indicates that for prolific photon emitters (e.g. Th-232*, Ra-226*, Nb-94, Ag-108m, Cs-137*) the calculated dose from the surface-contaminated sphere is only about an order of magnitude higher than from the volume-contaminated sphere. Radionuclides with no or only very weak photon emissions (e.g. Pu-239, Pu-240, I-129, Cl-36, Pu-238) show a wider difference: the calculated dose from the surface-contaminated sphere is about three orders of magnitude higher than from the volume-contaminated sphere.

1 Strictly, the calculated effective dose from external radiation during the encounter plus

the calculated committed effective dose from inadvertent ingestion of radionuclides from the encounter.



Note: The horizontal scale is six plus logarithm (base 10) of the dose rate in mSv/h, i.e. 6.0 corresponds to 1 mSv/h and 0.0 corresponds to 1 pSv/h. Radionuclides are ordered by atomic number and atomic weight.

Figure 5-3: Calculated effective dose for surface- and volume-contaminated model spheres (Reference case: 1 tonne spheres bearing 1 GBq of each radionuclide and allowing 300 years radioactive decay)



Note: The horizontal scale is six plus logarithm (base 10) of the dose rate in mSv/h, i.e. 6.0 corresponds to 1 mSv/h and 0.0 corresponds to 1 pSv/h. Results are rank ordered according to the geometric mean.

Figure 5-4: Calculated effective dose for surface- and volume-contaminated model spheres, rank ordered (Reference case: 1 tonne spheres bearing 1 GBq of each radionuclide and allowing 300 years radioactive decay)



Note: Results are reverse rank ordered according to the geometric mean.

Figure 5-5: Item activity of given radionuclide calculated to give rise to an effective dose rate of 20 µSv/h and indicated Discrete Item Limits (Reference case: 1 tonne spheres allowing 300 y radioactive decay)



Reference case (continued)

Finally, Figure 5-5 (above) shows the inverse results, i.e. the item activity of a given radionuclide calculated to give rise to an effective dose rate of 20 µSv/h from the model sphere. The radionuclides are then separated into groups of radionuclides that generate roughly similar radiological impact, i.e. similar activities are required to yield a calculated effective dose rate of 20 µSv/h, and a Discrete Item Limit assigned to each group 2.

As discussed in Subsection 5.3, in principle, it is reasonable to place the item limit so that it falls between the item activity leading to 20 µSv/h for the volume- and surface-contaminated items, that is falls between the maxima of the green and blue horizontal bars in Figure 5-5. The further that horizontal bars progress to the right of the Discrete Item Limit, the more cautious will be the application of that limit.

For the most potent radionuclides (Group A), the Discrete Item Limit is placed at an activity that we consider can be justified, lying between the results for volume- and surface-contaminated spheres. For Groups B1, B2 and C, the Discrete Item Limit is stepped upwards, increasing by an order of magnitude at each step. This has the effect of making Discrete Item Limits increasingly more cautious for each group from A to C. In principle, based only on the radiological impact from discrete items and other long-term radiological impacts in the ESC, it would be ‘safe’ to dispose of amounts of Group C radionuclides which were many orders of magnitude greater than the Discrete Item Limit indicated in Figure 5-5. This, however, would allow items to be disposed that we consider are not consistent with the role of LLWR as stated in out Permit, which is to dispose of LLW.

Variant: 1 kg spheres and 300 years decay

The Discrete Item Limits are to apply to items the mass (weight) that will vary over a wide range. The reference case (above) considers items of 1 tonne, representative of larger items. This variant considers items of 1 kg, representative of smaller items.

Figure 5-6 and Figure 5-7 show results for this variant that are equivalent to Figure 5-4 and Figure 5-5, respectively, for the reference case. Comparison of Figure 5-6 with Figure 5-4 shows two notable features:

• The calculated effective doses for the 1 kg sphere are higher than from the 1 tonne sphere, typically by about two orders of magnitude. Roughly speaking, this is because both spheres bear the same total activity but the surface area of a 1 kg sphere is two orders of magnitude less than that of a 1 te sphere, and activity on or near the sphere surface of the spheres contributes most to the assessed pathways.

• Radionuclides with higher doses per unit ingestion tend to move up the rank order compared with radionuclides for which the impact is also contributed to by photon emissions. For example: although both are prolific photon emitters, Ra-226* swops places with Th-232* on account of the high dose per unit ingestion from Pb-210 and Po-210 also assumed to be present; and Pu-239 and Pu-240 move up relative Np-237, Am-241 and U-enr* and U-nat*.

2 The group ‘names’ A, B1, B2 and C are chosen to make a link with radionuclide groups

worked out previously for low-activity sources [10]. Consistency between the groups for low activity sources and for discrete items is discussed in Appendix D.




Figure 5-6: Calculated effective dose for surface- and volume-contaminated model spheres, rank ordered (Variant case: 1 kg spheres bearing 1 GBq of each radionuclide and allowing 300 years radioactive decay)




Figure 5-7: Item activity of given radionuclide calculated to give rise to an effective dose rate of 20 µSv/h and indicated Discrete Item Limits (Variant case: 1 kg spheres allowing 300 y radioactive decay)



Variant: 1 kg spheres and 300 years decay (continued)

Comparison of Figure 5-7 with Figure 5-5 shows, however, that the changes described to above do not lead to large changes in the grouping of radionuclides.

Applying the same principle as in the reference case, that the Group A Discrete Item Limit is placed at an activity that we consider can be justified, the Group A limit is placed at 0.001 GBq, two orders of magnitude lower than for the 1 tonne item. For Groups B1, B2 and C, the Discrete Item Limit is stepped upwards, increasing by an order of magnitude at each step. As a result, there is reordering but limited movement between the groups:

– three radionuclides (Nb-94, Ag-108m and Np-237) move down from Group

A to Group B1;

– one radionuclide (Pu-238) moves up from Group B2 to Group B1, although

it is arguable as to which group Pu-238 should be assigned to based on

Figure 5-7.

Therefore, it is cautious to retain the same radionuclide groups for the 1 kg item as defined for the 1 tonne item, with the possible adjustment that Pu-238 (half-life ~88 years) is moved up to Group B1 to join the longer-lived plutonium isotopes (Pu-239 and Pu-240).

Variant: 1 tonne spheres and 100 years decay

The Discrete Item Limits are to apply to waste items such as may be deposited on the beach during coastal erosion of the repository vaults. There is no suggestion that such erosion could occur before about 300 years after present. On the other hand, it is reasonable to ask if erosion at an earlier time would lead to significantly different impacts? Or, equivalently, it can be asked if erosion does progress at the fastest rate envisaged in the ESC, and the LLWR continues to operate, might there come a time when limits based on expectation of at least 300 years to erosion were no longer cautious?

Figure 5-8 and Figure 5-9 show results for this variant that are equivalent to Figure 5-4 and Figure 5-5, respectively, for the reference case.

Results are intuitively obvious: only radionuclides with shorter effective3 half-lives are affected. Even calculating for an encounter 200 years earlier than in the reference case, only Eu-152 (half-life 13.5 y) and Cs-137* (30.2 y) move up between groups. Sr-90* (28.8 y) and Pb-210* (22.2 y) move up markedly in the rank order but, in both cases, remain within the Group B2. Simple calculations show that, assessed with 200 years radioactive decay, and for the same group boundaries, Cs-137* would remain within Group B2 and Eu-152 remain in Group C.

Cm-244** (18.1 y) moves down the rank order in the 100 years decay variant only because Sr-90* and Pb-210* have moved up; the calculated dose has only changed by about 2% (see footnote 3).

3 Pu-241 (half-life 14.4 y) decays to Am-241 (432 y), hence radiological impact at about

100 years and beyond is controlled by Am-241. Similarly, Cm-244 (18.1 y) decays to Pu-240 (6560 y), hence impact at about 100 years and beyond is controlled by Pu-240.




Figure 5-8: Calculated effective dose for surface- and volume-contaminated model spheres, rank ordered (Variant case: 1 tonne spheres bearing 1 GBq of each radionuclide and allowing 100 years radioactive decay)




Figure 5-9: Item activity of given radionuclide calculated to give rise to an effective dose rate of 20 µSv/h and indicated Discrete Item Limits (Variant case: 1 tonne spheres allowing 100 y radioactive decay)



6 WAC for Discrete Items

6.1 Definition of a Discrete Item

A Discrete Item is a distinct item of waste that, by its characteristics, is recognisable as unusual or not of natural origin and could be a focus of interest, out of curiosity or potential for recovery and recycling/re-use of materials should the waste item be exposed after repository closure.

6.2 Discrete Item Limits

Based on the approach and model representation summarised in Subsection 5.1, and the understanding and assessment calculations set out in Subsections 5.2 to 5.4, we derive radionuclide activity limits to apply to Discrete Items as follows.

Effective doses are assessed for a proximate encounter4 with surface- and volume- contaminated model items for a range of radionuclides, and radionuclides are divided into groups according to the total activity on an item that would lead to a calculated effective dose rate of 20 µSv/h. The reasons why we consider that this is an appropriate test are set out in Subsections 3.3 and 3.4.

This yields Discrete Item Limits as set out in Table 6-1.

Table 6-1: Discrete Item Limits

Radionuclide Weight 1 kg or less Weight between 1

and 100 kg Weight 100 kg or

greater

Group A 0.001 GBq 1 GBq per t 0.1 GBq

Group B1 0.01 GBq 10 GBq per t 1 GBq

Group B2 0.1 GBq 100 GBq per t 10 GBq

Group C 1 GBq 1000 GBq per t 100 GBq

Radionuclides that fall into each group are given in Table 6-2. This is based on calculations of effective dose for 51 radionuclides and their progeny as presented in Appendix E. This is believed to include all radionuclides that are liable to contribute to the total effective dose from a discrete item assessed at 300 years after disposal.

Radionuclides are assigned to groups based on the calculated effective dose rates from the defined encounter, see Figure E-1 in Appendix E. Characteristics of members of each group are given in Table 6-3, but these are not defining characteristics of the group.

4 Very close to and touching or handling the item.



Table 6-2: Radionuclide groups for limiting Discrete Items

Group A Nb-94 Ag-108m Sn-126

Ra-226

Th-229 Th-230 Th-232

Pa-231 Np-237 Am-243

Cm-247 Cm-248 Cf-251

Group B1 I-129

Pu-238 (1)

Pu-239 Pu-240 Pu-242

Am-241 Am-242m

U-235 U-238 U-nat U-enr (2)

Cm-245 Cm-246

Group B1 C-14 Cl-36 Ca-41 Sr-90

Zr-93 Mo-93 Tc-99 Cs-135 Cs-137

Pb-210 Ac-227

U-233 U-234 U-236

Pu-241 Cm-243 Cm-244 Cf-250

Group C H-3 Co-60 Ni-59 Ni-63 Nb-93m Sm-151 Eu-152

All radionuclides with half-life shorter than 10 years.

Most radionuclides with half-life shorter than 20 years (3)

.

(1) Pu-238 is placed in Group B1 for reasons discussed in the main text.

(2) U-natural* and U-enriched* means total uranium alpha activity (U-238+U-235+U-238) where isotopes are in the ratios as in natural uranium (0.7% U-235 by mass, 2.2% by activity) and as in uranium enriched to about 4-5% U-235 by mass (11% by activity).

(3) The exceptions are radionuclides that have half-life shorter than 20 years but decay to moderately long-lived alpha emitters, notably Pu-241, Cm-244 and Ca-250.

Table 6-3: Characteristics of members each group (not defining characteristics of the group)

Group Characteristics of members each group (not defining characteristics of the group)

Group A Radionuclides with half-life of more than about 100 years that emit, or decay to short-lived progeny that emit, significant photon emissions.

Group B1 Alpha radionuclides, or other radionuclides with higher dose per unit ingestion, with half-life of more than about 100 years that do not emit, or decay to short-lived progeny that emit, significant photon emissions.

Group B2 Non-alpha radionuclides with half-life of more than about 100 years that do not emit, or decay to short-lived progeny that emit, significant photon emissions.

Non-alpha radionuclides with half-life less than about 100 years that emit, or decay to short-lived progeny that emit, significant photon emissions.

Alpha radionuclides with half-life less than about 100 years or with relatively low dose per unit ingestion.

Group C Other radionuclides with half-life less than about 100 years that do not emit, or decay to short-lived progeny that emit, significant photon emissions.

Most radionuclides with half-life less than about 20 years.



6.3 Application of the Discrete Item Limits

The method to determine acceptability against the Discrete Item Limits is as follows.

The activity, at time of consignment, of each radionuclide that could be present on or within the item should be estimated. This may be from measurements on the item or at its source, use of waste stream information, information on the origin or history of the item, or combination of these.

If a radionuclide cannot contribute more than about 1% to the total activity of all radionuclides within the same group present on the item, then this will not affect the result and the radionuclides present in amounts less than 1% of the group total may be neglected.

Thus, for example, if the total activity of an item is dominated by Cs-137, it is not necessary to determine the presence or activity of other Group B2 radionuclides at levels less than about 1% of the estimated Cs-137 activity. It is necessary to determine the activity of radionuclides in other groups at the level necessary to determine total activity in each of these other groups.

If a radionuclide known to be present on an item is not listed in Table 6-2 and has a half-life longer than 20 years, i.e. undefined group, then it can be cautiously assigned to Group A. If it contributes less than 1% to the total activity of Group A radionuclides, or otherwise makes no difference as to whether to the sum of fractions inequality (see below) is met, this is acceptable. Otherwise advice on which group to assign to the radionuclide should be sought from the ESC team through Waste Services.

The reporting of radionuclide activity on Discrete Items is an additional reporting requirement. It in no way reduces or changes the requirements for reporting of radionuclide activity within a consignment, and the activity on items within a consignment will have to be determined for, or included within the determination of, the full range of radionuclides present with sufficient accuracy to satisfy for the criteria for waste consignments.

The total activity of radionuclides in each group is then compared with the Discrete Item Limit for that group. An item complies if the following sum of fractions is satisfied:

QA/DILA + QB1/DILB1 + QB2/DILB2 + QC/DILC ≤ 1

where QN is the total activity of group N radionuclides and DILN is the Discrete Item

Limit for that group.

These WAC apply to items within a consignment that meets the 4 and 12 GBq/t limits. They do not apply to items disposed directly in the vaults. Such a consignment will require approval under the Waste Consignment Variation process.

6.4 Comparing Discrete Item Limits with 4 and 12 GBq/t

As discussed in Subsection 2.2, the requirement that the specific activity averaged over a consignment should not exceed 4 GBq/t alpha and 12 GBq/t does not rule out



that there may be some volumes of waste or waste items within a consignment that bear activities exceeding the 4 and 12 GBq/t levels. Part of the motivation for this work has been to determine the extent to which it may be acceptable to dispose of discrete items that bear radionuclide activities above the values of 4 and 12 GBq/t, judged against the ESC and requirements of the GRA [4].

Figure 6-1 plots the relation between the Discrete Items Limits set out in Figure 6-1 and the 4 GBq/t ‘alpha’ and 12 GBq/t ‘other’ values.

Figure 6-1: Relation of Discrete Item Limits with the 4 GBq/t ‘alpha’ and 12 GBq/t ‘other’ values

The figure illustrates (confirmed by values in Table 6-1) that for Group A radionuclides and items above 1 kg weight the Discrete Item Limit is more restrictive than 4 GBq/t (by a factor of 4 up to 100 kg and more above 100 kg). Below 1 kg it is set at 0.001 GBq (1 MBq), which is the limit given in conditions agreed with the Environment Agency for acceptance of low activity sources [16], also see Appendix D.

Between 1 and 100 kg, the Discrete Item Limit for Group B1 radionuclides, which are mainly alpha-emitting radionuclides, is 2.5 times 4 GBq/t, i.e. marginally less restrictive, but the Discrete Item Limit is more restrictive for items above 250 kg.

For Groups B2 and C, which are mainly ‘others’ (plus some uranium isotopes and shorter-lived actinides), the Discrete Item Limit between 1 and 100 kg is eight and eighty times, respectively, higher than 12 GBq/t.

Overall, the Discrete Item Limits defined in Table 6-1 provide a margin for disposal of items that may bear activities of most radionuclide at levels somewhat above the 4



and 12 GBq/t levels, depending on radionuclide up to about 4, 8 and 80 times over the range 1 to 100 kg. Some radionuclides (Group A) are controlled at levels below 4 GBq/t.

Heavier items are (relatively) more tightly controlled, and lighter items are (relatively) less tightly controlled. We consider it appropriate that larger (heavier) are more tightly controlled, as discussed in Subsection 5.2.



References

1 LLWR, The 2011 Environmental Safety Case, LLWR/ESC/R(11)10016, May 2011.

2 Environment Agency, Low Level Waste Repository, Permit number EPR/YP3293SA, Effective date 01/01/2011.

3 LLWR, Waste Services Contract: Waste Acceptance Criteria – Low Level Waste Disposal, WSC-WAC-LOW – Version 1.0 – April 2010.

4 Environment Agency, Northern Ireland Environment Agency and Scottish Environment Protection Agency, Near-surface Disposal Facilities on Land for Solid Radioactive Wastes: Guidance on Requirements for Authorisation, February 2009.

5 LLWR, The 2011 ESC: Site Evolution, LLWR/ESC/R(11)10023, May 2011.

6 LLWR, The 2011 ESC: Assessment of Long-term Radiological Impacts, LLWR/ESC/R(11)10028, May 2011.

7 Mobbs SF and Sumerling T, Assessment of Impact of Heterogeneity at the Particulate Scale, LLWR/ESC/Mem(12)146, 2012.

8 Sumerling T and Mobbs S, Inventory Heterogeneity and Events in Human Intrusion Dose Assessment, LLWR/ESC/Mem(12)148, 2012.

9 Environment Agency, Advice to Environment Agency Assessors on the Disposal of Discrete Items, Specific to the Low Level Waste Repository, Near Drigg, Cumbria, Version 6a, R E Smith, 1 May 2013.

10 LLWR, Assessment of the Disposal of Low-activity Sources at the LLWR, LLWR/ESC/R(11)10037, Version 1, March 2011.

11 Thorne M, External memorandum: Comparison of Sphere and Slab Dose Factors, Mike Thorne and Associates Ltd, 15 March 2010.

12 Thorne M, Balding D, Egan M and Paulley A, LLWR Radiological Handbook, LLWR/ESC/R(10)10033 Issue 1.4, July 2012.

13 USEPA, 2002, Federal Guidance Report 13, Cancer Risk Coefficients for Environmental Exposure to Radionuclides: CD Supplement, EPA-402-C-99-001, Rev.1, US Environmental Protection Agency, Washington, DC.

14 Kennedy WE and Strenge DL, Residual Radioactive Contamination from Decommissioning: Technical Basis for Translating Contamination Levels to Annual Total Effective Dose Equivalent: Final Report, NUREG/CR-5512, PNL-7994, Volume 1, 1992.

15 Brown J and G Etherington G, Health Risks from Radioactive Objects on Beaches in the Vicinity of the Sellafield Site, HPA-CRCE-018, HPA, April 2011 and supporting document HPA-CRCE-018 (supplement).

16 Letters from R. Scott, British Nuclear Group, to Ian Streatfield, Environment Agency, and from I. Streatfield to D. Mason, Director EHS&Q, British Nuclear Group, 13 September to 18 October 2005.



Appendices



Appendix A: Data tables

Table A-1: Radionuclide half-life, included progeny, and dose factors for selected radionuclides

See notes at foot of table.

Committed effective dose per unit intake Effective dose rate from

Radio-nuclide

Included progeny

Half-life

years

Ingestion

Sv/Bq

Inhalation

Sv/Bq

Infinite surface

Sv/s per Bq/m

2

Semi-infinite slab

Sv/s per Bq/m

3

H-3 1.23E+01 4.19E-11 not 0.00E+00 0.00E+00

C-14 5.70E+03 5.81E-10 required 1.28E-20 5.90E-23

Cl-36 3.01E+05 9.29E-10 1.12E-17 1.33E-20

Ca-41 1.02E+05 1.93E-10 0.00E+00 0.00E+00

Co-60 5.27E+00 3.42E-09 2.30E-15 8.24E-17

Ni-63 1.00E+02 1.52E-10 0.00E+00 0.00E+00

Sr-90 Y-90 2.88E+01 3.04E-08 1.12E-16 2.18E-19

Nb-94 2.03E+04 1.74E-09 1.49E-15 4.88E-17

Tc-99 2.11E+05 6.42E-10 6.49E-20 5.81E-22

Ag-108m 4.18E+02 2.36E-09 1.54E-15 4.83E-17

I-129 1.57E+07 1.06E-07 1.96E-17 5.14E-20

Cs-137 Ba-137m 3.02E+01 1.36E-08 5.49E-16 1.71E-17

Eu-152 1.35E+01 1.37E-09 1.07E-15 3.54E-17

Ra-226 to Po-214 1.60E+03 2.80E-07 1.69E-15 5.67E-17

Pb-210 Po-210 to stable 2.22E+01 1.91E-06 3.72E-17 4.01E-20

Ra-226* to stable 1.60E+03 2.19E-06 1.73E-15 5.67E-17

Pa-231 3.28E+04 4.79E-07 3.78E-17 9.44E-19

Ac-227 Th-227, Ra-223 2.18E+01 4.36E-07 4.65E-16 9.98E-18

Pa-231* to stable 3.28E+04 9.15E-07 5.03E-16 1.09E-17

Th-232 1.41E+10 2.31E-07 4.55E-19 2.44E-21

Ra-228 Ac-228 5.75E+00 6.97E-07 9.38E-16 3.03E-17

Th-228 to stable 1.91E+00 1.43E-07 1.44E-15 5.18E-17

Th-232* to stable 1.41E+10 1.07E-06 2.38E-15 8.21E-17



Committed effective dose per unit intake Effective dose rate from

Radio-nuclide

Included progeny

Half-life

years

Ingestion

Sv/Bq

Inhalation

Sv/Bq

Infinite surface

Sv/s per Bq/m

2

Semi-infinite slab

Sv/s per Bq/m

3

U-234 2.46E+05 4.95E-08 5.86E-19 1.84E-21

U-235 Th-231 7.04E+08 4.70E-08 1.56E-16 3.70E-18

U-238 Th-234, Pa-234m 4.47E+09 4.79E-08 1.16E-16 6.42E-19

U-nat* effective 4.47E+09 4.87E-08 6.01E-17 3.94E-19

U-enr* effective 4.47E+09 4.85E-08 6.85E-17 6.90E-19

Np-237 Pa-233 2.14E+06 1.08E-07 2.11E-16 5.41E-18

Pu-238 8.77E+01 2.28E-07 6.26E-19 6.25E-22

Pu-239 2.41E+04 2.51E-07 2.84E-19 1.41E-21

Pu-240 6.56E+03 2.51E-07 6.01E-19 6.03E-22

Pu-241 ** Am-241 4.32E+02 6.77E-09 7.74E-19 6.61E-21

Am-241 4.32E+02 2.04E-07 2.33E-17 1.99E-19

Cm-244 ** Pu-240 6.56E+03 6.92E-10 1.66E-21 1.66E-24

Notes:

Data from LLWR (ESC) Radiological Handbook, Issue 1.4 [12].

* Asterisks indicate radionuclides with shorter-lived progeny assumed to be present in equilibrium at the time of assessment.

** Double asterisks indicate radionuclides that decay to longer-lived progeny that dominate impact in the time interval of interest.

– Pu-241 (14.4y) decays 100% by beta to Am-241 (432y). Pu-241 is 'pre-

decayed' to Am-241, hence 1 Bq Pu-241 is treated as 14.4/432=0.0333 Bq

of Am-241

– Cm-244 (18.1y) decays 100% by alpha to Pu-240 (6560y). Hence 1 Bq

Cm-244 is treated as 18.1/6560=2.76e-3 Bq of Pu-240.

– Check with Bateman Equations for a two-member chain confirms that at

300 years this is accurate to within 3% for Pu-241 -> Am-241 and within

0.3% for Cm-244 -> Pu-240. At 100 years it is accurate to within 2.4 and

2% respectively.

U-natural* and U-enriched* means total uranium alpha activity (U-238+U-235+U-238) where isotopes are in the ratios as in natural uranium (0.7% U-235 by mass, 2.2% by activity) and as in uranium enriched to about 4-5% U-235 by mass (11% by activity).



Table A-3: Volume, radius and surface area of model spheres with mass between 10 g and 10 tonnes. Total item activity, specific activity and surface activity are calculated for model spheres bearing 10 GBq/t and 1 GBq total.

Table A-4: Scale factors to calculate effective dose rates at 0.3 m and 1 m from the surface of the model spheres defined above.



Appendix B: Illustration of effective dose model results

The behaviour of the calculated effective dose rate has been checked by calculations considering spheres at masses between 10 g and 10 tonnes contaminated with 10 GBq/t and with 1 GBq total of Cs-137 (with Ba-137m in equilibrium).

Figures B-1 to B-3 shows the behaviour of the calculated effective dose according to the spheres of equivalent mass scale factor model.

Figures B-1 and B-2 show that the calculated effective dose behaves as expected from geometric and physical principles. For example, the calculated doses at 0.3 m are about an order of magnitude lower than calculated doses at 1.0 m for items up to about 10 kg (sphere radius ~0.1 m). This is a result of the inverse square relation for a point source. Figure B-2 also shows, for an item bearing 1 GBq total, that as mass decreases the results tend towards effective dose from a point source. Figure B-3 compares results for 1 GBq/t and 10 GBq at a single distance.

Figure B-4 shows the simpler relation of committed effective dose from ingestion for surface- and volume-contaminated model spheres bearing 10 GBq/t and bearing 1 GBq. In this case, the committed effective dose is proportional to the surface activity (GBq/m2) for the surface-contaminated sphere, and to the specific activity (GBq/t) for the volume-contaminated sphere.



Figure B1: Effective dose rate from surface- and volume-contaminated model spheres each bearing 10 GBq/t at 0.3 and 1.0 metres to surface of the spheres

Figure B2: Effective dose rate from surface- and volume-contaminated model spheres each bearing 1 GBq at 0.3 and 1.0 metres to surface of the spheres and from a 1 GBq point source



Figure B3: Effective dose rate from surface- and volume-contaminated model spheres bearing 10 GBq/t and bearing 1 GBq at 0.3 metres to surface of the spheres

Figure B4: Committed effective dose from ingestion for surface- and volume-contaminated model spheres bearing 10 GBq/t and bearing 1 GBq



Appendix C: Illustration of Discrete Item Limits for example radionuclides

Figures C-1 to C-4 shows the activity for surface- and volume-contaminated model spheres calculated to lead to an effective dose rate 20 µSv/h (including the effective dose from intakes of radionuclides from the encounter), allowing for 300 years radioactive decay.

In each case, this is overlaid by the Discrete Item Limit for the relevant group. Each figure shows two example radionuclides from a given group, thus:

Figure C-1: Group A: Ag-108m and Ra-226*

Figure C-2: Group B1: U-enr* and Pu-239

Figure C-1: Group A: Tc-99 and Cs-137*

Figure C-1: Group A: H-3 and Co-60

The progressive increase in caution of the Discrete Item Limit for Groups B1, B2 and C can be observed.



Ag-108m

Ra-226*


Figure C-1 Model item activity at 300 years giving rise to 0.02 mSv/h for two examples of Group A radionuclides



U-enr*

Pu-239


Figure C-2: Model item activity at 300 years giving rise to 0.02 mSv/h for two examples of Group B1 radionuclides



Tc-99

Cs-137*


Figure C-3: Model item activity at 300 years giving rise to 0.02 mSv/h for two examples of Group B2 radionuclides



H-3

Co-60


Figure C-4: Model item activity at 300 years giving rise to 0.02 mSv/h for two examples of Group C radionuclides



Appendix D: Comparison of limits for Discrete Items and low-activity sources

The current LLWR WAC allow for the disposal of sealed sources under a set of conditions including that ‘Each individual source to be disposed of must not, in its raw state, exceed 1 MBq total alpha and non-alpha activity’.

The conditions under which low-activity sources might be disposed and activities that could be acceptable assessed against assumptions of the ESC and the requirements of the GRA [4] are examined in a previous study report [10]. Therein, it is concluded that for many radionuclides sources of higher activity could be safely disposed.

This is summarised in Tables 10 and 12 in that report, which are reproduced below.

It is also shown in reference [10] that, based on the calculations, much higher activity limits could be derived, especially for shorter-lived radionuclides, i.e. Group C2.

The group identifiers adopted in the present report are chosen to link to the identifiers adopted for low-activity sources in reference [10], since there is clearly a connection between the activity that might be allowed on small discrete items as assessed in the present report and for a low-activity source, i.e. a very small discrete item.



There is commonality, but some discrepancies, between radionuclides allocated to each group in the two reports. The discrepancies arise because in the assessment of low-activity sources an ingestion pathway was not considered realistic, although skin dose due to photons and beta radiation was cautiously assessed 5. This is why long-lived beta emitters fall in Group B1 for low activity sources but in Group B2 for discrete items. However, for low-activity sources a single limit of 10 MBq was allowed for both Group B1 and B2 (see above), so the distinction carried no practical importance.

The key feature to notice is the alignment of the Discrete Item Limits and limits for low activity sources. Depending on form, low activity sources as typically produced for instrument calibration and testing, have mass a few grammes or less. The agreement between the limits for Discrete Items and for low-activity sources is illustrated in Figure D-1, in which a point has been added, nominally at 1 g, to Figure 6-1 as given in the body of the present report.

Figure D-1: Comparison of limits for Discrete Items and for low-activity sources (LAS)

5 As discussed in Subsection 3.4 of the present report, we consider that the neglect of

beta dose to skin and addition of inadvertent ingestion is appropriate given the larger items that that we intend to represent in the present report.



Appendix E: Allocation of radionuclides to groups

The list of 26 radionuclides that are assessed in Section 5 of this report are sufficient to illustrate the sensitivity of results to key parameters and illustrate the process of allocating radionuclides to groups. For WAC, a somewhat wider list of radionuclides is desirable, sufficient to include all radionuclides that might contribute to the total assessed effective dose from encounter with a discrete item.

The list of 46 radionuclides identified in the 2011 ESC as liable to contribute to total dose by any pathway [6] provides a useful starting point. This list was compared with the 146 radionuclides present in the 2011 ESC inventory 6 and 83 radionuclides retained after inventory and half-life screening in the Radiological Handbook. Any radionuclide with inventory greater than 1 TBq and half-life longer than ten years for alpha-emitter or 20 years for ‘other’, and/or with copious photon emissions was added to the list. This led to list of 53 radionuclides, including for example Ni-59, Sn 126, Sm-151, Cf-250 and Cf-251 being added to the list. Four of the radionuclides, however, (Ra-228, Th-228, Pu-244, Cm-244) are only of interest as progeny so that the list includes 51 parent radionuclides.

Figure E-1 shows results from analysis and allocation to radionuclide group for the list of 51 parent radionuclides. This includes radionuclides requiring explicit modelling of radionuclide chains, notably Th-230, Am-242m, and isotopes of curium and californium. For several of these radionuclides, the calculated effective dose rate is contributed to by the primary nuclide and also by progeny that grow in during the assessed period of 300 years.

The radionuclides modelled and allocation to radionuclide group is given in Table E-1. For radionuclides not found in the table, the following procedure can be applied:

• All radionuclide with a half-life less than ten years can be assigned to Group C.

• Most radionuclides with half-life less than 20 year can also be assigned to Group C. The exceptions are nuclides that decay to an alpha daughter.with half-life a few tens to hundreds of times the parent half-life, in which case the ingrown progeny are liable to determine the calculated impact at 300 years 7.

For any radionuclide not included in the Table E-1, and not assessable by half-life or having very similar radiological properties to a radionuclide that is included, then it will be cautious to assign to Group A.

6 LLWR, The 2011 ESC: Inventory, LLWR/ESC/R(11)10019, May 2011.

7 Examples are Pu-241/Am-241, Cm-244/Pu-240 and Cf-250/Cm-246, which are

assessed as Group B2.



Table E-1: Radionuclide groups for limiting Discrete Items

Group A Nb-94 Ag-108m Sn-126

Ra-226

Th-229 Th-230 Th-232

Pa-231 Np-237 Am-243

Cm-247 Cm-248 Cf-251

Group B1 I-129

Pu-238 (1)

Pu-239 Pu-240 Pu-242

Am-241 Am-242m

U-235 U-238 U-nat U-enr (2)

Cm-245 Cm-246

Group B1 C-14 Cl-36 Ca-41 Sr-90

Zr-93 Mo-93 Tc-99 Cs-135 Cs-137

Pb-210 Ac-227

U-233 U-234 U-236

Pu-241 Cm-243 Cm-244 Cf-250

Group C H-3 Co-60 Ni-59 Ni-63 Nb-93m Sm-151 Eu-152

All radionuclides with half-life shorter than 10 years.

Most radionuclides with half-life shorter than 20 years (3)

.

(1) Pu-238 is moved from Group B2 to B1 for reasons discussed in the main text.

(2) U-natural* and U-enriched* means total uranium alpha activity (U-238+U-235+U-238) where isotopes are in the ratios as in natural uranium (0.7% U-235 by mass, 2.2% by activity) and as in uranium enriched to about 4-5% U-235 by mass (11% by activity).

(3) The exceptions are radionuclides that have half-life shorter than 20 years but decay to moderately long-lived alpha emitters, notably Pu-241, Cm-244 and Ca-250.



Figure E-1: Item activity of given radionuclide calculated to give rise to an effective dose rate of 20 µSv/h and indicated Discrete Item Limits (Reference case: 1 tonne spheres allowing 300 y radioactive decay)

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