Simulation of operational transients in a VVER-1000 nuclear power plant using the RELAP5/MOD3.2 computer program

Abstract

A RELAP5/MOD3.2 nodalization model of a VVER-1OOO (V-320) nuclear power plant was updated, improved and validated against available experimental data. The data included integrated test results obtained from actual power plant testing. The steady state and the operational transients test data describe the behavior of the Unit 5 of Kozloduy NPP (Bulgaria). The operational transients consisted of a loss of flow caused by the successive trip of two main coolant pumps without reactor scram. A validation process of the developed model has been performed in two stages comprising an initial and a transient validation. The comparison between experimental data and calculation results proved the adequacy of the model and also the code capacity to reproduce main plant parameter evolutions. The plant model was also used for a preliminary analysis of a large break loss of coolant accident (LB LOCA) which is the design basis accident (DBA) for the VVER-1000 plants. Due to the limitations of the utilized code version (unavailability of the redwood model), only the first stage (blowdown) of the accident was investigated. The results have been compared with similar calculations obtained by the Russian specialists with an indigenous thermal-hydraulic code (TECH-M). The comparison showed a good agreement. For the most important calculated parameter (hot spot cladding temperature) an uncertainty analysis using the response surface method was performed. The nodalization model seems to be adequate for the class of transients and accidents investigated, but the inclusion of the reactor specific point kinetics parameters, emergency headwater system model and updating some of the component parameters (e.g. main coolant pump friction) will increase its area of applicability.