Analysis of a severe accident management procedure at VVER-1000 reactor units: effect on the degradation of the containment ground slab and on the magnitude of the source term

The procedure involves opening of the two doors closing the tunnel between the downcomer and the neighbouring room GA302 in a time before failure of the reactor vessel bottom. It is assumed that the progress of concrete degradation by action of the molten material will be appreciably slower in the case of an extensive and shallow melt basin, providing certain time for additional remedial provisions capable of preventing containment damage or, at least, mitigating radiological consequences of the accident. The aims of the study was: (a) investigate if melt spreading will bring about slower melt penetration through the concrete, and if so, to what extent, and (b) ascertain how the source term will be thereby affected. (P.A.)