Sensitivity calculations of PRZ water level during the Natural Circulation test at Unit 6 of Kozloduy NPP

Abstract: This paper presents the results of thermal-hydraulic sensitivity calculation of the “Natural Circulation” test at Unit 6, Kozloduy NPP. The RELAP5/MOD3.2 computer code has been used to simulate the Natural Circulation in a VVER-1000 Nuclear Power Plant (NPP) model. The results are an important part of the validation of the RELAP5/MOD3.2 code. A model of VVER-1000 based on Unit 6 of Kozloduy NPP has been developed for the systems thermal-hydraulics code RELAP5/MOD3.2. This model was developed at the Institute for Nuclear Research and Nuclear Energy – Bulgarian Academy of Sciences (INRNE-BAS), Sofia and was used for simulation of the behavior of the real VVER-1000 NPP during the Natural Circulation test. The data included steady-state and operational transient results. The purpose of the experiment, which was done according to the “Program for investigation of primary side Natural Circulation of coolant on NPP”, was to explicitly establish the response of the plant under primary loop Natural Circulation conditions. The possibility to remove approximately 150MW using the Natural Circulation was also demonstrated during the test. The paper discusses the influence of Pressurizer nodalization on calculation of Pressurizer water level. The comparisons between the RELAP5 results and the test data indicate good general agreement. The report also contains a brief summary of the main NPP systems included in the RELAP5 VVER model and the NC transient sequences. This report was possible through the participation of leading specialists from Kozloduy NPP and with the assistance of Argonne National Laboratory, under the International Nuclear Safety Program (INSP) of the United States Department of Energy. [Copyright &y& Elsevier]