Characteristics of the geometry of debris beds expected to form during reflooding

The accurate description of the geometry of a severely damaged core and of debris beds expected to form during quenching as many physical models strongly depend on geometrical parameters. This relies on a good description of degradation phenomena and of debris bed formation. This paper, based on a bibliographic survey, is focussed on the geometrical characteristics of debris beds expected to form during reflooding. The consequences of a severe accident on the geometry of the reactor core will be examined in paragraph 2. The observations show that the top of the damage regions consist of debris particles of a wide range of sizes. To clarify this particle size distribution, data on fuel fragmentation under operating and LOCA conditions have been gathered and will be presented in paragraph 3. Then, in paragraph 4, we have looked for cladding failure criteria so as to determine whether the rapid cooling may lead to the collapse of the embrittled oxidized claddings. Using these data has finally enabled us to determine in paragraph 5 the extent and the composition of the debris bed formed during the reflooding of a French 900 MW PWR during a six inches LOCA