IAEA’s Programme on Environmental Modelling for RAdiation Safety

(EMRAS II)WG7 “Tritium” Working Group

IAEA Scientific SecretaryVolodymyr Berkovskyy, IAEA

Working Group LeaderDan Galeriu, IFIN-HH,

Romania

IntroductionWorkshop

28-29 September 2009EDF, Chatou, France

TritiumHydrogen is ubiquitous in the environment and is part of many chemical compounds,

including water and most organic materials. As an isotope of hydrogen, tritium enters freely into these compounds and its movement through the environment can be inferred from the cycling of hydrogen. As a result, tritium behaves differently in a number of respects from other radionuclides:

• In aqueous systems, tritium moves as a non-reactive, non-absorbed constituent with the bulk water flow. Accordingly, the environmental transport of tritium is governed in large part by local and global hydrologic cycles.

• As a gas, tritium moves in response to its vapor pressure gradient and can, under some circumstances, move against the water vapor flux.

• Tritium deposited from the atmosphere to soil and plants is readily recycled back to the atmosphere via evapotranspiration, forming a secondary airborne plume.

• The processes responsible for tritium transfer have time scales as short as minutes. Tritium can be rapidly taken up by organisms but just as rapidly lost. As a result, tritium transfer is highly dependent upon the environmental conditions prevailing at the time and place of release and the time of measurement.

• Tritium can be effectively incorporated into biological systems, including the human body, as organically bound tritium (OBT). Many environmental pathways to humans.

• OBT has long biological half-life in humans and biota• Tritiated hydrogen, which is biologically inert, can be oxidized in soil to tritiated water

vapor, which is about 20,000 times more radiotoxic.• Although tritium is substantially heavier than other hydrogen isotopes, it is usually

incorporated into larger molecules. Therefore, isotopic effects, although present, are not important in environmental tritium transport, except in OBT formation. BUT for OBT: Very short range, so damage depends on where in cell, eg close to DNA

PAST RESULTS Scenario V1.05 in the Tritium WG (BIOMOVS II 1995) Farmland was exposed with 1E10 Bq/m3 of HTO in air for one hour starting at midnight in

one case and at 10 a.m. in the other, 30 days before the harvest of the various crops.In most cases the predicted concentrations among the models agreed within one order of magnitude and for some endpoints within two orders of magnitude. The higher discrepancy occurred after the night. Some processes are highlighted that may need further experimental work to improve the model performance:• HTO in soil:1. deposition beneath plant canopies and re-emission from soil, particularly in stable air

and low wind speeds;2. number and thickness of soil layers needed to describe vertical movement in soil and

between soil surfaces and atmosphere.• HTO in vegetation:1. deposition from the atmosphere particularly at night when leaf stomata are closed or

partly closed;2. effective rooting depth of different species.• OBT in vegetation:1. rates of OBT formation, particularly at night;2. translocation of HTO and OBT to plant storage tissues, grain, tubers and roots;3. effect of stage of development of grain when release occurs.• HTO and OBT in animal products:1. rates of OBT formation in animals;2. rates of loss of OBT from milk and meat;3. effect of time elapsed between release and slaughter on concentration in beef.

PAST RESULTS Scenario V3.0 in the Tritium WG (BIOMOVS II 1995)• The importance of better understanding of the uptake of HTO and conversion

to OBT in dark conditions was pointed in the scenario V3.0 (BIOMOVS II 1995), where modelers were asked to predict HTO in leaves and OBT in grains and to compare with experimental data from FZK.

• The primary purpose of the experiment was to find out how much HTO enters the leaves when stomata close at night and, if HTO is present in leaves, whether it can become incorporated into OBT in the dark. The results clearly show that HTO enters plants at night and is converted to OBT in the dark. From the experimental results some transfer parameters were extracted. The velocity of deposition was estimated to range from 16 to 23 mm s-1 in light and from 3 to 4 mm s-1 in the dark. The loss rate factor was estimated to be about 0.95 h-1 in light and about 0.15 h-1in the dark. The rate of OBT formation was estimated to range from 6 E-4 to 1 E-3 h-1 in light and between 2 E-4 and 3 E-4 h-1 in the dark. This rate is defined as the ratio between the OBT concentration at harvest and the time integrated HTO concentration in air moisture.

• The deposition velocity in the dark implies a canopy resistance of the order of 150-200 s m-1 (using the experimental data on wind in the growth chamber) which suggests a stomata resistance of less than 1000 s m-1. If the stomata are completely closed, this would a smaller value than expected for the cuticular resistance. For daytime exposure, the estimated deposition velocity is 3-4 times higher than the model predictions, and consequently the canopy resistance seems to be overpredicted by most models. These assertions must be considered with precaution because the finite size of the growth chamber can influence the results if comparing with large canopies in the field.

Environmental and Radiological Impact of

Accidental Tritium Release

Philippe Guétat, Luc Patryl CEA - France

8th International Conference on Tritium Science and Technology September 16-21, 2007 Rochester, New York

Review of past conclusions

Conclusions for Scientistes

General features are known but :• Are we able to do better than a factor 10 ?• What fundamental parameters should be

known in the vicinity of a tritium plant ?• Some experiments to realize :

– deposition velocity;– Air-plant exchanges during the night;– Parts coming from air and from soil.

Review of past conclusions

TRITIUMTRITIUM and the ENVIRONMENTand the ENVIRONMENT

SOURCESSOURCES

MEASUREMENT MEASUREMENT

and TRANSFERand TRANSFERPh GUETATPh GUETAT, , CEACEA

Thanks for their help to C Douche, JC Hubinois, N. Baglan , Thanks for their help to C Douche, JC Hubinois, N. Baglan ,

D Galeriu, Ph. Davis, W RaskobD Galeriu, Ph. Davis, W Raskob

CEA/DAM/VA UE scientific seminar emerging issues on tritium 13/11/2007

Review of past conclusions

Conclusions for environment: R&D• Tritium does not concentrate in food chainAbout models• Variability remains very large in case of accident especially in rain and night

cases. • Modification needed for wheat modelling - realistic approachAbout experimental Data• Translocation of organic matter from leaves to edible part of the vegetable.• Case of the night for experimental data.• What about Tritiated particulates ?About modelers• The present Tritium scientific community is very small,• have to synthesize what is absolutely needed in models for acute release. • This community could disappear from EU in the few next years.

Review of past conclusions

TRITIUM RADIOECOLOGY AND DOSIMETRY - TODAY AND TOMORROW D. Galeriu*, P. Davis†, W. Raskob‡, A. Melintescu*

*IFIN-HH Romania †AECL Canada ‡IKET GermanyInvited lecture

8th International Conference on Tritium Science and Technology September 16-21, 2007 Rochester, New York

Review of past conclusions

The 1990 Aiken list was amended in 1997 by W. Raskob and P. Barry. Sensitivities and hence ‘importance’ in this list vary with both inputs and end points. Site- and task-specific analyses must be done to identify the most important processes in a given application.

Areas Requiring Further Work:• plant uptake of HTO at night;• rates of OBT formation in plants, particularly at night;• dispersion in soil; • reemission from soil and plants;• rates of OBT formation and loss in animals;• translocation to fruits and roots;• tritium behavior in winter;• HTO concentrations in the environment following an HT release.

TOWARD CONCLUSIONSReview of past conclusions

No emergency action beyond 800 m

No delayed action at any time beyond 3 km

No long-term action (longer than 1 year) beyond 800 m

Restriction on the consumption of foodstuff and crops limited in terms of timescale and ground area

Limited economic impact

With present knowledge, it can be argued that the expected dose to members of the public from routine tritium releases is unlikely to be higher than 30 μSv/y for today’s nuclear facilities. Accidental releases of HT or aquatic HTO releases have much lower radiological impact than an accidental atmospheric release of HTO. EU guidance on response to accidental releases is as follows:

The next generation of models for accidental HTO releases must be improved to decrease uncertainties and to cope with tighter regulatory requirements.

Review of past conclusions

Requirements for the Next Generation of Dynamic HTO Models

• Reliable atmospheric transport and dispersion codes (particle models) with good representation of reemission and inclusion of turbulence, topography etc;

• Changing environmental conditions must be taken into account;• Several sub-models are needed to describe the behaviour of tritium in soil

and crops;• The crop sub-model is most important and here the plant physiological

parameters must be considered;• Conversion processes from HT to HTO and further to OBT have to be

modelled;• Sub-models have to be based on physical approaches; knowledge from

other disciplines should be used to derive general dependencies based on data for other substances than tritium;

• Site-specific information on land use, soil types and crop genotypes should be applied, together with realistic habits for the maximally exposed individual.

Review of past conclusions

Modelling the transfer of 3H and 14C into the environment - lessons learnt from IAEA’s EMRAS project

A. Melintescu and D. Galeriu

“Horia Hulubei” National Institute for Physics and Nuclear Engineering, Bucharest-Magurele, ROMANIA

International Conference on RADIOECOLOGY & ENVIRONMENTAL RADIOACTIVITY 15 – 20 June 2008, Bergen, Norway

Review of past conclusions

Suggestions for improving H-3 and C-14 accidental release models

• Models must include more reliable atmospheric transport and dispersion with turbulence data and topography, as well as improved area source for re-emission;

• HT conversion into HTO in soils must be analysed starting from basic science and modelled accordingly with local soil properties;

• The influence of environmental condition on the transfer of tritium to plants must be included and generic models must separate wet, dry, and hot or cold situations;

• Knowledge from agricultural science must be incorporated, including physiologically based crop growth modelling (photosynthesis, partition of newly formed dry matter, genotype influence, evapotranspiration);

• For OBT production at night it must develop an improved model based on a deeper analysis of plant processes;

• Translocation in fruits and roots must be modelled using knowledge in agricultural research; tests with experimental data are needed;

• Robust operational models based on energy metabolism are needed for transfer in animals;

• The predictions for contamination of eggs or broilers must be experimentally checked;

• For cold climate, tritium behaviour in winter, including washout by snow, dry deposition to snow and the fate of tritium in the snow pack must be studied;

• A further reduction of uncertainties must be based on the ability to use site-specific information on land use, local soil properties and predominant crop genotype characteristics, together with realistic assumptions concerning habits of the maximally exposed individual.

Review of past conclusions

WG7- PARIS• The Working Group focuses on the development of a dynamic reference

model that allows the estimation exposure to tritium subsequent to accidental releases. For this purpose, the processes involved in the transfer of tritium in the environment will be analyzed in dependence on the environmental conditions, season and time of the day. A main issue is the integration of actual weather data to enable reliable estimation of the tritium behavior.

• Our meeting must:- discuss and harmonize the views of participants concerning the approaches

for developing the conceptual model for tritium accidents (atmospheric and aquatic);

- agree on the structure and scope of the conceptual model;- identify potential gaps in knowledge and expertise, which should be

addressed during the model development;- define the structure of the technical document and share tasks according to

the expertise of each participant and the interests of his/her organization or institute;

- elaborate the work plan for developing the conceptual model; and- distribute specific tasks to be accomplished and reported at the next

EMRAS II Technical Meeting (25–29 January 2010).

Task groups• Task Group I covering

-    Tritium washout-    HT/HTO deposition-reemission

-    Actual evaporation and transpiration and connected HTO concentration dynamics

-    HTO uptake and retention in plant in rain condition -    Movement of HTO to deeper soil layers-    Winter case (particularly deposition on snow and how

to deal with snow)

Task Group II covering -    Use of growth models - define the minimal needs-    OBT formation in night -    Translocation of OBT from leaves to edible plant parts

Task Group III -    Modelling the transfer in aquatic food chain

Questionnaire• 11 members of WG7 responded to the questionnaire. From their replies, some

preliminary conclusions can be reached:

• There is interest in both liquid and atmospheric releases.

• About half of the respondents have an interest in HT emissions.

• Plants of interest include: pasture, lucerne, vegetables (leafy and root vegetables), rice, wheat, corn, tomatoes, potatoes, apples and citrus fruits, grapes. For Cernavoda add sunflower and sugar beat

• Animals of interest include cow (+ milk), sheep (+ milk), beef, goat, pork, chicken, fish, boars.

• All agree that the local climate and soils have a large influence.

• Some prefer compartmental models with site-specific parameters. There is an increased interest in process level modeling of minimal complexity.

• • To be conservative is the requirement, but with no details on how to control

the robustness.

Expert view (IAEA)• “ It is especially important to focus on the uncertainties and sensitivities that

are involved in modeling the behavior of tritium in the environment after accidents.

• Although we know much about the behavior of water in the environment, the reliable prediction of tritium concentrations in environmental  media subsequent to an accident is the result of the complex interaction of a number of factors that are subject to hourly, daily and annual fluctuations. Due to these large uncertainties related to the environmental conditions at the time of the accidental release, predictions are unavoidably associated with considerable uncertainties.

• However, these inherent problems in tritium modeling are not clear to everybody. Therefore, it would be very important for the work to:

• to identify the main contributors to uncertainty;• to identify the critical periods during the year in relation to resulting

exposures to tritium;• to identify the important and sensitive parameters, having in mind hourly,

daily and annual variations  in parameters/processes;• to explore the practical  possibilities in determining those parameters;  • to get an idea about the achievable reliability of tritium modelling under

practical, this means under accidental field conditions; • to get a clear idea for which phases of the tritium accident the application of

a tritium model is desirable and useful.”