Simulation of two loss-of-flow transients in a VVER-1000 nuclear power plant with RELAP5/MOD3.2 system code

A RELAP5/MOD3.2 model of a VVER-1000 (V-320 model) nuclear power plant was updated, improved, and validated against actual power plant data. The data included steady-state and operational transient results from unit 5 of the Kozloduy nuclear power plant in Bulgaria. The first operational transient w as initiated by a loss of flow at partial power conditions caused by the tr ip of a main coolant pump without reactor scram. The second operational tra nsient was a continuation of the first, with the trip of a second main cool ant pump. The assessment of the model has been performed in two stages: an initial va lidation against steady-state plant data and then a transient validation by comparison to operational transient data. The comparison between the plant data and the results of the calculations proved the adequacy of the model and demonstrated the capability of the code to reproduce the evolution of t he main plant parameters.