FDNH - the tritium module in RODOS

Under the auspices of its RTD (Research and Technological Development) Framework Programmes, the European Commission has supported the development of the RODOS (Real-time On-line Decision Support) system for off-site emergency management. The project started in 1989 focusing on PWR/LWR type accidents and using experience from the Chernobyl accident. In 1997 it was realised that tritium should be included in the list of radionuclides, as large tritium sources exists in Europe and to allow a potential expansion of the RODOS system for application on future fusion reactor accidents. The National Institute for Physics and Nuclear Engineering (IFIN-HH) in Romania - in close co-operation with the Research Centre Karlsruhe (FZK) - was charged to develop the tritium module, based on previous experience in environmental tritium modelling and the operation of CANDU reactors in Romania (with potential tritium accidents). At present, the Food and Dose Module Hydrogen -(FDMH) - for tritium applications - is integrated and documented in the RODOS system. It calculates the time dependent tritium concentration (as tritiated water or organically bound tritium) in crops (as much as 22 different species) and up to 12 animal products, inhalation doses and ingestion dose from up to 34 diet items for various groups of the population and for up to 2520 locations around the source, following an accidental emission of tritiated water. FDMH incorporates many improved techniques in radiological assessment and makes intensively use of interdisciplinary research. It is developed in a modular structure with a variable time grid according to the physical processes. Differing from other models, using generic transfer parameters or parameters fitted on individual experiments, FDMH derives tritium transfer rates based on physical and physiological process analysis, using scientifically accepted results from interdisciplinary research on, among others, land-atmosphere interaction, water cycle in the soil-plant-atmosphere system, plant physiology, photosynthesis, growth and hydrogen metabolism in mammals. A unique feature of FDMH is the coherent modelling of tritium uptake by plant canopies and its conversion to organic matter, using a physiological plant parameter data base which can reproduce plant growth under various pedo-climatic conditions. By this approach, the difficulties of scaling from leaf to canopy are avoided and the model parameters are tested by concomitant reproduction of plant growth, using an appropriate crop growth model- developed at process level. In order to predict the tritium transfer in animal products in the absence of a complete experimental database, results from basic research on hydrogen metabolism in mammals is applied. Both forms of tritium are considered and the transfer and the conversion from tritiated water (HTO) or organics (OBT) in feed to HTO and OBT in animal products are explicitly introduced. Incorporating the environmental tritium dynamics with time steps ranging from less than one hour up to days, FDMH illustrates seasonal and diurnal effects on public dose related to the time of the accident. Due to the novel modelling approach, FDMH can be easily customised for any European site and can predict the time evolution of tritiated water or organically bound tritium in such details that it can be easily used in establishing countermeasures. The present model as integrated in the RODOS platform contains a database for Central Europe but it is not directly coupled to real-time weather prognosis data, due to external constraints. In order to increase the model flexibility and reliability some upgrades are now on going and an international, stand-alone version is in preparation. (authors)