Transient cooling of shallow debris beds in the first two FARO-LWR experiments

Abstract

The investigation of the early behaviour of reactor core debris beds in high pressure water and the heating of the bottom plate on which the debris rests are important aspects of the FARO-LWR out-of-pile tests. In the first two tests 18 and 44 kg of molten U0 2-ZrO 2 fell through 1 m of water and led to the fragmentation of 2 3 of the melt in both tests and to the formation of shallow debris beds of 3 and 7 cm in height. The unfragmented melt ended up as frozen cakes on the bottom plate. The latter got heated rather little in comparison with the heating of the overlying water by the debris bed. From global energy balances of these tests, heat fluxes from the transient debris beds to the water could be evaluated. They turned out to be between 7.8 and 10.5 MW m −2 during the first 10 s and about 3.9 MW m- −2 from 10 to 24 s. In particular the first values are considerably higher than dryout heat fluxes measured in experiments with steady state debris beds. This implies that the shallow and initially very hot FARO debris beds must have cooled down rapidly and that they did not dry out significantly. Probably some fluidization occurred in the early life of these beds.