Comparative RELAP5-3D analysis in support of the NPP DBA analysis in Ukraine

In the Ukrainian in-depth safety assessment (ISA) projects the computer code RELAP5/Mod3.2 with point kinetics approximation is being used in the deterministic safety analysis of VVERs. It is generally accepted that the use of this approximation, with the proper modeling assumptions, results in conservative results. However, only coupled three-dimensional codes are capable to estimate the real localized feedback effects for such VVER specific transients as control rod ejection or main steam line break. Some results of the comparative RELAP5-3D analysis for the scenarios, that present strong local reactivity effects, are discussed in this paper. The goal of this RELAP5-3D analysis is to examine the differences in results obtained by the three-dimensional approach and the methodology that was used in Ukrainian ISA projects.