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STATUS AND MAIN UNCERTAINTIES IN LEADING SEVERE ACCIDENT ANALYSIS CODES

DOI: 10.1615/ICHMT.1995.RadTransfProcHeatMassTransfSevNuclReactAcc.490

U. Brockmeier
Ruhr-University of Bochum, 44780 Bochum, Germany

Resumo

The SFD (Severe Fuel Damage) codes ATHLET/CD, ICARE2, MELCOR and SCDAP/RELAP5 underlay intensive developmental and verification activities. At the same time, the experimental data base is broadened particularly as regards late phase core melt progression and damage (e.g. the Sandia MP- and French PHEBUS-FP-experiments). Having access to an extending data base, model development focuses on late phase phenomena and on the transition between early and late phase; e.g. ceramic material dissolution, relocation and blockage formation. Further activities focus on a quench induced hydrogen production, non fuel eutectics and specifics of Eastern European type PWR bundles (CORA-W1, -W2). Thermal hydraulics and numerical methods are enhanced with view on new reactor designs with natural convection driven heat removal systems. After roughly 25 years of code development, the codes have advanced to high quality powerful tools for the prediction of core damage phenomena in the course of severe accidents. Still existing uncertainties are only to a certain extent due to modelling deficiencies. User effects, e.g. differences in the nodalization of a given problem and unreasonable choice of user defined parameters appear to be a major source for the remaining uncertainty bands particularly identified in comparative assessment activities as for instance the International Standard Problems.
Nevertheless, modelling needs still can be identified particularly with regard to late phase phenomena and the transition between early and late phase core damage progression, e.g. the formation of in-core-debris, oxidation of melts and complex mixtures and the effect of quenching on clad oxidation, embrittlement and hydrogen generation. In turn, typical early phase phenomena up to the relocation of basically metallic melts seem to be modelled fairly adequate, although there is still a lack in reliable thermophysical material property data. Thus generally, further improvement of existing models is to a high degree dependent on the availability of realistic material properties.

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