Analysis of the loss of heat sink transients in the secondary circuit of a VVER-1000 using RELAP5/MOD3.2

A major group of accidents related to the secondary loop of a VVER-1000 nuclear reactor are the events that lead to the reduction of heat removal from the secondary side. A reduction in the steam flow rate [due to turbine-generator (TG) load reduction] and spurious closure of a main steam isolation valve (MSIV) are two cases considered in this study. In case 1, the reduction of the TG load by 70% of nominal value is investigated. The transient condition leads to an increase in the reactor core pressure up to nearly 17.3 MPa, followed by negative reactivity feedback. After about 1800 s into the incident, the power level is stabilized at about 90% of its nominal value. In case 2, MSIV closure due to a control system malfunction or an operator's error could be the causes of the initiating event. The condition leads to deterioration of heat removal from the secondary loop and the primary pressure increases up to about 16.3 MPa as a consequence. The cool-down of the reactor plant (RP) is carried out through control rods and safety valves mechanisms. After 100 s from the initiating event, the reactor power decreases to 70% nominal value and after about 500 s into the incident complete stabilization of RP parameters is reached. Qualitative examination of the results indicates that the VVER-1000 RELAP5 model has the ability to predict the main thermo-hydraulic parameters and their behavior during the design-base accidents. Using the Fast Fourier Transform Best Estimate Method (FFTBM) analysis, the quantitative analysis of the results also demonstrates that our RELAP5 model of VVER-1000 is reliable for other transient analyzes. (C) 2012 Elsevier Ltd. All rights reserved.