Interface between thermal-hydraulics and fission product transport in severe accident analysis

Studies of the effectiveness of the containment following hypothetical severe accidents in LWRs have traditionally been split into two separate disciplines. The first is the thermal-hydraulics of the containment atmosphere, leading to an estimate of the magnitudes of the threats to the integrity of the containment building. The second is the study of the transport of the fission products released from the core within and from the building, the ultimate output of which, given a building damage state, is a radiological source term to the environment. In fact, the transport of fission products is strongly influenced by the prevailing thermal-hydraulic conditions, and there are some ways in which the fission products have an influence back on the thermal-hydraulics. The paper describes work funded and co-ordinated by the Commission of the European Communities (CEC) to investigate how the two sorts of calculation, the thermal-hydraulic and the fission product transport, can be made more responsive to the needs of each other. Recent theoretical and experimental work has indicated that the deposition of fission product aerosols is markedly enhanced by the condensation of steam on the particles. The rate of such condensation is a sensitive function of the thermodynamic conditions in the atmosphere, and the release of latent heat because of condensation can in turn significantly change these conditions. The rates of condensation are increased if the particles contain chemical components which are water soluble. To model this effect one requires considerable knowledge of the thermodynamic properties of the relevant aqueous solutions. Recent experience has shown that considerable differences occur between the predictions of different codes. In order to resolve these discrepancies, the CEC has organised a code comparison exercise based on the LACE LA-4 experiment and is currently organising another code comparison based on one of the experiments in the DEMONA series. (author). 22 refs