Objective
The four main contractors have fully achieved the initially defined objectives of their projects. The laboratory and small-scale field experiments on tritium reemission from bare soils have answered the question of the extent of independence of tritium reemission from water evaporation. The importance of this problem had appeared, as tritium reemission is often modeled in terms of water evaporation because of lack of better alternatives, while theoretical considerations and some experimental results had indicated substantial differences between the two processes. The detailed investigations have shown that reemission is largely independent from evaporation and a new separate model has been developed (ZSR). Furthermore, an existing model based on the exchange-velocity concept has proved to reproduce the main physical mechanisms of the process (CEA). One outstanding result is the finding of high initial reemission rates after the deposition of HTO. This has initiated modifications of the broadly accepted UFOTRI code. If those unexpectedly high reemission rates found in the open-air experiments for bare soil also apply for vegetated surfaces, the consequences would be a considerable shift in the dose contributions from inhalation and ingestion. Therefore, ZSR and FZK are planning to continue reemission experiments including vegetation. Upon requests from the modelers, also experiments under extreme climatic conditions are envisaged, to ensure coverage of the whole realistic range of the meteorological parameters involved in the process.
The investigations on tritium fractionation processes in plants and soils have identified a variety of effects resulting from isotopic differences of the two atoms, H and T (TUM), which are usually neglected in natural systems, but more importantly also in the analytical procedures of environmental samples. The dose-relevancy of these studies is introduced by the difference in the radiotoxicity of tritium whether appearing in tissue, ie, tissue water tritium (TWT), or in organic molecules (OBT). This leads to the question, if the value of the specific activity ratio (R-value) of organically bound tritium to tissue water tritium may exceed unity. One important result of this study is the finding that reported values of R>1 may also be a consequence of erroneous analyses of the OBT contents of biological samples, as the analytical procedure itself is subject to isotopic effects, which may increase the error of the total R-value up to 400% depending upon the analytical parameters.
In the course of the investigations on the production and translocation mechanisms of OBT into diet-relevant plant organs no indication was found for elevated T/H-ratios in OBT-compartments compared to those in the preceding compartments (FZK), thus contradicting the theoretical derivation. The detailed understanding and the high level of modeling capability achieved during these studies is demonstrated, eg, by the good agreement between modeled and measured time courses of the OBT-concentrations in wheat even under water stress conditions. Validation of this model will be a focal point for future work. It became apparent that this high degree of sophistication is necessary to reliably predict the tritium uptake into plants and the subsequent OBT formation and translocation.
In view of the tritium release potentials of fission fuel reprocessing plants, but especially of experimental and projected fusion reactors, it has been decided (eg, OECD/IEA Tritium workshops) that the accuracy of dose predictions has to be improved considerably by more detailed investigations of a number of environmental tritium processes including retention and reemission as well as transfer and bio-chemical conversion processes along to the food chain. Within this joint research project two essential topics were selected for intensive studies:
- Parameterzation of tritium reemission from soil after HT and HTO deposition and
- Natural variations and the dynamics of tritium uptake and OBT formation in food-chain relevant plants.
Tritium reemission from soils after HT releases was found to dominate the inhalation and skin absorption dose and to determine the prompt appearence of tritium in plants. It is expected to depend on a number of meteorological and soil physical parameters. Research will focus on single-parameter dependencies of the reemission rate extracted from laboratory experiments under controlled conditions (NIR). The derived model will be validated against the results of small-scale field experiments using a complementary method to evaluate reemission rates (DPEI/SERGD).
Because of the higher radiotoxicity of organically-bound tritium (OBT) and its different transfer behaviour compared to tissue water tritium (TWT) its correct determination in samples of the food chain is of essential dose-relevancy. Therefore, a new analytical method will be developed eliminating artifacts caused by physical and biochemical alterations during the analytical procedures applied so far. The method will be employed to evaluate OBT/TWT ratios under steady state exposure of biological systems (RCM-TUM) and it will be used to parameterize the dynamics of HTO uptake, OBT formation and translocation into diet-relevant Plant organs under short-term exposure conditions (Kfk).
The resulting submodels are meant to be incorporated into existing tritium codes to improve the accuracy of dose predictions. They will also serve as a basis for the planning of effective counter-measures in case of accidental releases.
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30167 HANNOVER
Germany