KSNP+ reactor vessel head drop analysis for a 5.5M free fall

Abstract

The KSNP+ RV closure head drop analysis was carried out to assess the reactor core coolability in case of the RV closure head drop accident during the refueling operation. The analysis consists of a number of different RV head drop scenarios as the postulated accident events during refueling that include a concentric head drop case and three different cases of laterally offset head drop cases. The analysis was initiated due to the adoption of the IHA (Integrated Head Assembly) in the KSNP+ reactor design, which increases the weight of the RV closure head assembly. Four different analysis models were developed that correspond to the RV head drop analysis scenarios. An in-house dynamic analysis code was used for the RV head drop analysis. The entire reactor internals and fuel assemblies are modeled by using lumped masses and spring elements. Because of the extreme load exerted by RV head drop, most members experience stresses that are beyond the elastic limits. A separate elastic—plastic analysis for some members was carried out and the resulting load-deflection curve was used as the stiffness of the element. The effect of water above the reactor vessel in the refueling pool was ignored for the conservative estimation of the analysis. The analysis shows that the concentric head drop is the most severe case of loading condition. It also reveals that the local deformation of some reactor internals and the fuel assemblies is occurred; however the primary membrane stresses are within the bound of allowable stress limits. Consequently the reactor core remains in coolable state.