engleski

Analysis of In-Vessel Severe Accident Phenomena in NPP Krško

A hypothetical severe accident (SA) with substantial core melt-down can have serious consequences regarding the public safety. Integrity of the containment and the release of fission products in the environment following the containment failure depend strongly on in-vessel core degradation processes. The knowledge of in-vessel melt relocation processes is also important with respect to cooling recovery actions (flooding of the core) and the reactor pressure vessel (RPV) failure analysis. The early in-vessel SA scenario includes cladding oxidation, melting and liquefaction of core materials and formation of a molten pool inside the core. The late in-vessel phase includes molten pool relocation to the lower head, formation of a crust surrounding the molten pool, thermal attack on the vessel wall and, finally, the vessel failure. The accident analyzed in the paper was a station blackout (SBO) with a leakage from the reactor coolant system (RCS) through reactor coolant pump (RCP) seals following their degradation. It was assumed that both off-site and on-site (emergency diesel generators) AC power was unavailable, therefore the primary system coolant inventory was decreasing due to the unavailability of the high pressure (HPSI) and the low pressure safety injection (LPSI) flow. Water was only injected from the accumulators because their operation did not depend on the availability of electrical power. RELAP5/SCDAPSIM computer code was used in the analysis. One of the goals of the analysis was to determine the time of the structural failure (creep rupture) of the vessel wall in the lower plenum following the relocation of the molten corium to the lower head. For this purpose COUPLE model of the RPV lower plenum of NPP Krško was used. Additionally, one more scenario was analyzed with diesel generators available which would provide power for the HPSI and LPSI pumps. The purpose of that additional case was to evaluate the influence of on-site power availability on the accident progression.

Severe Accident; NPP Krško; SCDAP/RELAP

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