Numerical Simulation of the Natural Circulation Experiment in the PSB-VVER Test Facility Using the SOCRAT Code. Part 1. Analysis Model Development and Verification

The purpose of the study is to numerically simulate, using the SOCRAT computer code, the experiment on investigation of different natural circulation modes in the large-scale integral test facility PSB-VVER. Single-phase and two-phase natural circulation modes of the coolant are the main mechanism of decay heat removal from the core to the steam generators during accidents involving primary coolant leaks from the VVER reactor plant. The efficiency of natural circulation under the conditions of a partially filled primary coolant system is of crucial importance for the operation of passive safety systems during accidents accompanied by prolonged station blackout. For evaluating the SOCRAT code ability to model various natural circulation mechanisms and adequately predict the transition from one mode to another, the PSB-VVER test facility nodalization scheme was developed, which includes a reactor model, four circulation loops, a pressurizer, and steam generators. The coolant volume distribution along the vertical axis in the developed nodalization scheme and in the experimental facility has been presented as the analysis model verification. The obtained results are necessary for validating the SOCRAT code as applied to safety analysis of VVER-type reactor plants.