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The need to increase the predictive power of risk assessment for large tritium releases implies a process level aproach for model development. Tritium transfer for atmosphere to plant and the conversion in organically bound tritium depend strongly on plant characteristics, season, and meteorological conditions. In order to cope with this large variability and to avoid also, expensive calibration experiments, we developped a model using knowledge of plant physiology, agrometeorology, soil sciences, hydrology, and climatology. The transfer of tritiated water to plant is modelled with resistance approach including sparce canopy. The canopy resistance is modelled using Jarvis-Calvet approach modified in order to directly use the canopy photosynthesis rate. The crop growth model WOFOST is used for photosynthesis rate both for canopy resistence and formation of organically bound tritium, also. Using this formalism, the tritium transfer parameters are directely linked to known processes and parameters from agricultural sciences. The model predictions for tritium in wheat are closed to a factor two to experimental data without any calibration. The model also is tested for rice and soybean and can be applied for various plants and environmental conditions. For sparce canopy the model uses coupled equations between soil and plants. There are three phases in the dynamics of tritium in SVAT (soil-vegetation- atmosphere transport). The first one treats the period of active deposition when the cloud of tritiated water travels the study area and the atmospheric concentration is the driving source of tritium. The last one starts few days after cloud passage, when the soil water tritium is the driving force. The middle stage takes care of the reemission of HTO from vegetation and surface soil into the atmosphere, a fast process immediately after cloud passage, slowing down later. The active and transition phase are sensitive to actual meteorological parameters (sunshine, humidity, temperature, and rainfall) as were as on plant physiological characteristics and growth stage. In the later stage the processes to be considered are movement of tritiated water in rooth soil, depth distribution of roots, evapotranspiration and plant photosynthesis. This can be modeled with a slow dynamics, using climatic data and approximate dynamics of some plant parameters. After an atmospheric dry deposition episode the tritiaded water concentration in plant is decresing fast, while the OBT concentration in whole plant will decrease very slow but will be translocated to storage plant parts. For crops harvested one time in the year, most of tritium in the harvested plant will be in form of OBT, while for continously harvestd plants as grass and leafy vegetable, in the first few days after the accident, the concetration of HTO is high. An operational model must include both situations under various agrometeorological conditions. The well-known model UFOTRI was built to cope with these requirements and senitivity and uncertainty has been analysed (1) showing reliability for maximum exposed individual (worse case) as required for a facil design and licencing. This study and discussions in the IAEA-EMRAS tritium group

A versatile model for tritium transfer from atmosphere to plant and soil