Transient analysis of dissolution of a reactor bottom head into a melt pool

The dissolution of the bottom head of a heavy-water reactor into a pool of molten fuel under severe accident conditions is investigated using a distributed-parameter model. The main objectives were to determine the rate of dissolution-front propagation and to estimate the extent to which the bottom head is thinned owing to dissolution. The model consists mainly of partitioning the bottom head into a number of rings and analyzing the transient dissolution of each ring with a localized lumped-parameter model. For each of the rings, the dissolution is modeled using a mass transfer coefficient, the temperature distribution is considered to be one-dimensional and quasi-steady and the heat flux across the melt-bottom head interface is modeled using a heat transfer coefficient. The distribution of the heat transfer coefficients is considered to be quasi-steady and is based on the heat transfer calculation results obtained using the ACCORD code. The model thus takes into account both the variation of heat fluxes over the melt pool-bottom head interface and the variations of interface mass transfer with time and with position along the interface. The basic equations and their solution method for the distributed-parameter model are described. Comparisons of calculation results with those obtained previously using the overall lumped-parameter model are presented.