Vyshemirskyi M. Improvement of passive heat removal system from nuclear fuel of VVER reactors

In the framework of this research, an analysis of world experience in the area of passive systems at large-capacity nuclear power plants (NPP) and in small modular reactors (SRMs) was performed. Based on the results of analysis, a concept of passive safety systems for VVER-1000 and VVER-440 was developed and a computational evaluation of the developed passive system for VVER-1000 was performed. Results of the assessment confirmed the hypothesis regarding the insufficient reliability of the results of passive systems modeling by early versions of one-dimensional thermohydraulic codes used for safety analysis of Ukrainian NPPs (in particular, RELAP5/MOD3.2) due to the underestimation of heat transfer in the heat exchange equipment of the modern systems of nuclear facilities. To solve this issue computer simulation of the OSU-MASLWR experimental facility (a prototype of a light water reactor with natural circulation of the coolant and complex design of heat exchangers) was performed. Based on the results of the analysis approaches that ensure acceptable comparability of experimental data and results of passive systems modeling with application of early versions of one-dimensional thermohydraulic codes were developed. In addition, the existing methods for assessing the reliability and efficiency of passive systems were analyzed, their complexity and effort were noted, and the simplified approach to assessing of passive systems efficiency was defined, as well as a qualitative criterion was proposed that indicates reduction of the passive system efficiency.