Document Format for Quintessa - Nucleus Documents... · Web viewThe current task has required the modification of the hard wired dimensions of the capsule/disposal container/cement

Extension of the IAEA’s GSA for the Borehole Disposal Concept to include High Activity SourcesTask 4: Post-closure Safety Assessment Calculations

R H Little

QRS-1668A-TN4Version 1.0 (draft)

November 2014

Document History

Title: Extension of the IAEA’s GSA for the Borehole Disposal Concept to include High Activity Sources

Subtitle: Task 4: Post-closure Safety Assessment CalculationsClient: IAEADocument Number: QRS-1668A-TN4

Version Number: Version 1.0 (draft) Date: November 2014Notes: Draft TN for review by IAEA staffPrepared by: Richard LittleReviewed by: Richard Metcalfe

Approved by: Richard Little

QRS-1668A-TN4, Version 1.0 (draft)

Contents

1 Introduction 1

2 Modifications to the BDC Screening Tool 3

3 Modified Input Data for Post-closure Safety Assessment Calculations 6

4 Results 84.1 Reference Case Calculations 84.2 Bounding Case Calculations 8

References 9

Appendix A – Testing the BDC Screening Tool v1.1 (Long Capsule) 10


1 IntroductionThe draft IAEA publication “Generic Post-closure Safety Assessment for the Borehole Disposal of Disused Sealed Sources” (the GSA) (IAEA, 2013) considers the post-closure impacts of the disposal of disused sealed radioactive sources using the Borehole Disposal Concept (BDC). Table 17 of the GSA notes that there is no explicit consideration in the GSA of radiolysis, criticality or thermal effects, since such effects are considered to be insignificant for the typical inventories to be disposed. However, there are certain high activity sources, such as the Category 1 and 2 sources, for which it might be necessary to consider such effects. In addition, the calculations undertaken in the GSA are performed for a reference design with fixed dimensions for the capsule and disposal container (Table 5 of the GSA). It is recognised that capsules and disposal containers with alternative dimensions might be used in order to accommodate Category 1 and 2 sources. Changing the dimensions of the capsule/disposal container will impact on steel and concrete thicknesses which in turn will impact failure times. Therefore, it is also necessary to assess the impacts of alternative dimensions. The present project addresses the potential impacts of the disposal of high activity radioactive sources (specifically Co-60 and Cs-137 sources from irradiators used for sterilisation/food preservation – see Little and Thatcher, 2014) on the post-closure safety of the BDC. The results of this project will be used to determine the need, if any, for additional work. The project relates only to post-closure impacts; operational safety issues associated with the disposal of high activity radioactive sources in the BDC and the use of alternative dimensions for capsules and disposal containers is beyond the scope of the present project. This draft technical note documents work undertaken under the fourth task in the project (Perform Design and Defect Scenario Calculations) and takes account of the findings of the three previous tasks which are documented in Little and Thatcher (2014), Thatcher et al. (2014) and Little (2014), respectively. The scope of the current work is to specify, undertake, check and document the calculations for the limiting disposal system identified in the GSA (i.e., the saturated high flow rate system), including the Design (i.e., normal evolution) and Defect (i.e., alternative) Scenarios, using the alternative set of dimensions for the high activity capsule and disposal container described in Little and Thatcher (2014) and summarised in Table1-1 and Figure 1.1. Section 2 discusses the necessary modifications to be the BDC Screening Tool, Section 3 provides the modified input data for the post-closure safety assessment calculations and Section 4 provides the associated results.


Table 1-1: Capsule, Containment Barrier and Disposal Container Dimensions for the Disposal of Irradiator Sources used in

Sterilisation/Food Preservation

Waste Package

Component

Length (mm) Inside Diameter

(mm)

Outside Diameter

(mm)

Thickness (mm)

Capsule 478 47 52 2.5Containment Barrier 526 55 103 24

Disposal Container 603 103 115 6


Figure 1.1: The Capsule, Containment Barrier and Disposal Container Suitable for the Disposal of Irradiator Sources used for

Sterilisation/Food Preservation


2 Modifications to the BDC Screening ToolThe current task has required the modification of the hard wired dimensions of the capsule/disposal container/cement barrier dimensions in the IAEA’s BDC Screening Tool (Robinson et al., 2013) in order to allow stainless steel corrosion times and cement degradation times to be calculated and used as input to the AMBER model for the high activity capsule and disposal container. The parameters that define the alternative geometry are given in Table 2-2. All other parameters relating to the geometry remain the same as used in the GSA and summarised in Robinson et al. (2013).

Table 2-2: Geometry Parameters in the Original and Revised Version of the BDC Screening Tool

Parameter Original Value

Revised Value

Capsule Length 110 mm 478 mm

Capsule Outer Diameter 21 mm 52 mm

Capsule Thickness 3 mm 2.5 mm

Containment Barrier Length 186 mm 526 mm

Disposal Container Length 250 mm 603 mm

In addition to the changes to geometry, as discussed in Little and Thatcher (2014), the capsule material has been changed from the Type 304 considered in the GSA to 316L stainless steel. This is the same material as used for the disposal container, with the relevant parameters summarised in the Robinson et al. (2013).Previous versions of the BDC Screening tool assumed that the capsule fails once 80% of the thickness has been corroded. For the updated version, consistent with the discussions with experts from the Nuclear Energy Corporation of South Africa, it has been assumed that the capsule fails when the corrosion depth reaches the thickness of the weld, which is 1.7 mm. The model for the failure of the disposal container remains the same as before, i.e., it fails once 80% of the thickness has been corroded.


These modifications have been made to the tool and a modified version (“BDC Screening Tool v1.1 (Long Capsule)”) has been produced. The testing of this version of the BDC Screening Tool is described in Appendix A.The modified BDC Screening Tool has been used to calculate failure times for the high activity capsule and disposal container, and the associated cement degradation times for the containment barrier and backfill in the disposal zone. Calculations have been undertaken for the saturated high flow rate system using GSA groundwater #5 (i.e. the limiting disposal system identified in the GSA). The resulting times are given in Table 2-3, in which the equivalent times are also given for the GSA reference capsule and disposal container.

Table 2-3: Capsule/Disposal Container Failure Times and Cement Degradation Times for the Saturated High Flow Rate System

Parameter

Times for High Activity Capsule and Container (y)

Times for Reference

Capsule and Container (y)

Failure TimesDisposal Container 4.8E+5 4.8E+5

Capsule 6.5E+5(1) 7.2E+5

Cement Degradation Times: Backfill

Length of time in Stage 1 513 513



Cement Degradation Times: Containment Barrier

Length of time in Stage 1 188(2) 242



Notes:1. More rapid corrosion due to thinner capsule thickness (2.5 mm vs. 3 mm)

and modified corrosion model (see text).2. Faster cement degradation due to smaller thickness of the containment

barrier (24 mm vs. 41 mm).

Little (2014) noted that, whilst the temperature increase and radiolysis effects associated with the high activity sources considered in the current study might cause some increase the rate of general corrosion and cement degradation during the initial 50 to 200 year period following disposal of the


sources, the primary impact is likely to be on the potential for localised corrosion. Although the initial generation of temperatures in excess of the local boiling point calculated in Section 3.2 of Thatcher et al. (2014) will promote the initial drying of the cement surrounding the disposal container and so limit corrosion, the subsequent cooling will result in water condensation and the potential for localised corrosion due to high concentrations of electrolytes on the surface of the capsule and disposal container. However, as the temperature falls further, the concentration of electrolytes on the outside of the disposal container is likely to reduce due to the diluting effect of groundwater and so the rate of localised corrosion will decrease. In addition, the potential for localised corrosion will be further limited by the high pH conditions in the borehole. To model the behaviour of the system described above would require the use of a complex THMC (thermal, hydraulic, mechanical and chemical) coupled model that also took into account impact of radiolysis. The development of such a model could not be justified especially in the absence of site-specific and engineered barrier-specific information (e.g., exact porewater composition, groundwater flow rate and cement grout composition). Therefore, it is beyond the scope of the current project to modify the corrosion and degradation models used in the GSA to account for these processes. So the same models as used in IAEA (2013) have been used, although account is taken for the differing near-field dimensions (as discussed above). In order to account for the potential detrimental impacts of increased temperature and radiolysis on the corrosion and degradation rates, results for bounding calculations are provided in Section 4 (as well as for the reference cases, i.e., the cases based on the corrosion and degradation models described in IAEA (2013)).


3 Modified Input Data for Post-closure Safety Assessment Calculations

The use of the high activity capsule and disposal container described in Little and Thatcher (2014) requires the modification of the following input data used in the post-closure safety assessment calculations performed in AMBER for the BDC:▲ failure times for the capsule and disposal container (given in Table 3-4

for the references cases (i.e., the cases based on the corrosion and degradation models described in IAEA (2013));

▲ degradation times for the containment barrier and backfill (given in Table 3-4 for the references cases);

▲ dimensions for flow and transport in the borehole (Table 3-5); and▲ the radius of the seal source which is taken to be 5.5 mm (IAEA (2007)

notes that irradiator sources are mostly 11 mm in diameter).

Table 3-4: Times for the Failure of the Performance of the Near-field Components for the Design and Defect Scenarios for the Associated

Reference Cases

Component Start of Failure (y)

Totally Failed (y)

Backfill cement grout 5133 5170Failed stainless steel disposal container (Defect Scenario only)

0 480,001

Intact stainless steel disposal containers 480,000 480,001Containment barrier in failed disposal container (Defect Scenario only)

18,800 19,000

Containment barrier in intact disposal containers

481,884 481,897

Failed stainless steel capsule in failed disposal container (Defect Scenario only)

0 170,000

Stainless steel capsule in intact disposal container

650,000 650,001


Table 3-5: Dimensions for Flow and Transport in the Borehole

Component Length in Direction of

Flow(m)

Area Perpendicular

to Flow(m2)

Diffusion Length

(m)

Area for Diffusion

(m2)

Capsule (containing source container)

2.35E-2 1.24E+0 2.58E-2 3.90E+0

Containment Barrier 2.40E-2 2.71E+0 1.90E-2 9.50E+0

Disposal Zone (horizontally adjacent to capsule)

1.25E-2 7.00E+0 3.13E-2 2.51E+1

Disturbed Zone (Backfill)

5.00E-2 1.30E+1 2.49E-1 4.08E+1


4 Results4.1 Reference Case CalculationsIt can be seen from Table 3-4 that, for the Design Scenario’s reference case (i.e. the cases based on the corrosion and degradation models described in IAEA (2013)), the capsules do not fail until 650,000 years. By this time, the Co-60 (half-life around 5 years) and Cs-137 (half-life 30 years) inventories will have decayed. So there will be no release of Co-60 or Cs-137 for the Design Scenario’s reference case. The limiting Defect Scenario for Co-60 and Cs-137 disposal is Defect Scenario D4, i.e., all welds are satisfactory except for the closure weld in one waste container and one capsule, with the faulty capsule being in the faulty disposal container. All other near-field barriers perform as in the Design Scenario. Calculations show that there is no effective limit on the activity of Co-60 that can be disposed from a post-closure safety perspective. For the longer lived Cs-137, the limiting activity in a capsule is 5,700 TBq which is more than an order of magnitude higher than the 470 TBq that results from the disposal of ten Cs-137 sources in one capsule.

4.2 Bounding Case CalculationsAs noted in Section 2, in order to account for the potential detrimental impacts of increased temperature and radiolysis on the corrosion and degradation rates, calculations for a bounding case have been undertaken. They conservatively assume that both the capsule and disposal container fail after 50 years due to localised corrosion and that cement degradation is an order of magnitude more rapid than for the reference case (Table 4-6).

Table 4-6: Times for the Failure of the Performance of the Near-field Components for the Bounding Case

Component Start of Failure (y)

Totally Failed (y)

Backfill cement grout 513 517Stainless steel disposal container 50 480,001Containment barrier in disposal container 1,880 1,900Stainless steel capsule in disposal container

50 170,000

These conservative calculations show that there is no effective limit on the activity of Co-60 that can be disposed from a post-closure safety


perspective. For the longer lived Cs-137, the limiting activity in a capsule is about 490 TBq which is marginally higher than the 470 TBq that results from the disposal of ten Cs-137 sources in one capsule.


ReferencesIAEA (2007). Identification of Radioactive Sources and Devices. International Atomic Energy Agency Nuclear Security Series No. 5, IAEA, Vienna, Austria.IAEA (2013). Generic Post-closure Safety Assessment for Borehole Disposal of Disused Sealed Sources. International Atomic Energy Agency Draft TECDOC, Vienna, Austria.Little, R.H. (2014). Extension of the IAEA’s GSA for the Borehole Disposal Concept to Include High Activity Sources. Task 3: Screening of GSA FEPs. Quintessa Report to IAEA QRS-1668A-TN3, Version 1.0 (draft).Little, R.H., and Thatcher, K.E. (2014). Extension of the IAEA’s GSA for the Borehole Disposal Concept to Include High Activity Sources. Task 1: Collation and Documentation of Relevant Data. Quintessa Report to IAEA QRS-1668A-TN1, Version 1.0.Robinson, P.C., Watson, C.E. and Little, R.H. (2013). The Borehole Disposal Concept Screening Tool v1.0: User Guide. Quintessa Report to IAEA QRS-3038A-2 Version 1.0.Thatcher, K.E., Metcalfe, M. and Emery, P. (2014). Extension of the IAEA’s GSA for the Borehole Disposal Concept to Include High Activity Sources. Task 2: Thermal and Other Calculations. Quintessa Report to IAEA QRS-1668A-TN2, Version 1.0 (draft).


Appendix A – Testing the BDC Screening Tool v1.1 (Long Capsule) Because the changes to the source code are localised and trivial, a full testing programme has not been carried out. To ensure that the changes were made as intended the same case was run in the original and branched versions of the software and the results compared. The test file SatLF(11Jun13).bdc was used for this purpose.Figure A.1 shows the output from the tool that shows what values for the geometry were used in the calculation. The values shown for the “long” capsule case (the bottom figure) have been compared against those given in Table 2-2 and are correct.Figure A.2 shows the detailed output from the tool giving the failure and degradation times for each component. It can be seen that the failure time for the disposal container is the same in both cases, which is expected because there were no changes to the dimensions of this component, or to those of the surrounding backfill. Similarly the backfill degradation times are the same in both cases.The barrier degradation times are shorter for the “long” capsule case, with each stage taking approximately 78% of the time of the original case. A quick calculation determines that this reduction is also as expected, given that the time per flush is proportional to the ratio of the barrier volume to the surface area. The time per flush is defined as (Robinson et al., 2013):

τ=θVqχ

where θ is the barrier porosity, V is the volume, q is the flow rate and χ is the surface area. Given that the porosity and flow rate are the same for both cases, the ratio of the times is simply the ratio of volumes to surface areas:

τ L

τ 0=V

L / χL

V 0/ χ0

where the superscript L denotes the value for the “long” capsule and 0 denotes the value for the original capsule. Substituting in the formulae for the volumes and surface areas and cancelling out common factors gives the expression

τ L

τ 0=LCAPL ( (dCONL )2−(dCAPL )2 )LCAP0 dCON

0

LCAP0 ( (dCON0 )2−(dCAP0 )2 )LCAPL dCON

L


where LCAP and dCAP are the length and diameter of the capsule and dCON is the diameter of the containment barrier. Substituting in the values from Table 2-2 gives a value of 0.78, agreeing with the degradation times reported by the tool.Figure A.2 also shows that the capsule failure time is shorter for the “long” capsule case (86,615 years) than for the original case (94,339 years); this is partly due to the cement barrier being thinner and, as discussed above, degrading more quickly. However it is also compounded by the assumption that the capsule fails when the corrosion depth reaches the weld thickness (1.7 mm), rather than 80% of the wall thickness (0.8*3 = 2.4 mm).


Figure A.1: The Input Data tab for the original (top) and large capsule (bottom) versions of the tool, showing the alternative

geometry parameters used.


Figure A.2: The Detailed Information tab for the original (top) and large capsule (bottom) versions of the tool, showing the failure and

degradation times for each component.