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 was initiated by a loss of flow at partial power conditions caused by the trip of a main coolant pump without reactor scram. The second operational transient was a continuation of the first, with the trip of a second main coolant pump.

The assessment of the model has been performed in two stages: an initial validation 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 the main plant parameters.