Reactivity effects in the modelling of fuel melt-down

The paper gives experimental results on the reactivity effects associated with the displacement of materials in a simulation of the individual stages of a hypothetical accident involving fuel melt-down in the core. The experiments were carried out in one of the BN-600 reactor models assembled on a BFS-2 test rig. Consideration was given to various configurations of core and axial blanket materials corresponding to particular stages of the accident leading to sodium boiling and melt-down of cladding and fuel. For some configurations the influence of the damage zone volume on the magnitude of reactivity change was studied. The experimental results were analysed by various multigroup programmes. It is shown that the methods used satisfactorily describe the experiment. (author)