Improvement of Molten Core Cooling Strategy in a Severe Accident Management Guideline

Weaknesses of the current Severe Accident Management Guideline (SAMG) in handling the cooling of a molten core are discussed, and three improvements for the SAMG are presented. It is suggested that instrumentation to detect either a breach of the reactor vessel or a discharge of corium into the reactor cavity is essential to effectively perform the SAMG. A detailed analysis for a specific plant is necessary to make a decision as to whether preflooding or postflooding should be initiated for effective molten core cooling. Also, an optimal choice of depressurization capacity not only would significantly delay failure of the reactor vessel but also would increase the coolability margin of the molten corium in a reactor cavity. Analyses using the MELCOR computer code were performed for the Ulchin Units 1 and 2 and Kori Unit 1 nuclear power plants to illustrate the effectiveness of the proposed improvements in cooling of the molten core in the reactor cavity, where in-vessel retention of molten corium by preflooding is not feasible.