Computation studies of cladding relocation dynamics during liquid-metal fast breeder reactor transient undercooling accidents

Cladding relocation upon melting has major consequences on the sequence of events in a transient undercooling accident in a liquid-metal fast breeder reactor (LMFBR). The CMOT code developed at the Karlsruhe Nuclear Research Center is used to simulate, by computation, cladding melt-off and blockage formation without and with the possibility of sodium vapor flow diversion. The latter phenomenon is of interest in case of incoherent cladding melt-off within an LMFBR subassembly. It turns out that large waves are generated on the liquid cladding film that quickly slide over a relatively thin slowly moving film. The motion of the waves contributes considerably to the mass transport of cladding film material and to the formation of blockages. The dynamics of these waves is a very important phenomenon of the cladding relocation process. The computed results indicate that cladding blockages in the upper and lower parts of the coolant channel will be established