VVER-1000 Three-Dimensional RELAP-3D Model Assessment

Detailed thermal-hydraulic analysis of commercial power reactors requires modeling of complex multidimensional thermal, fluids, and neutronic behavior. One code that has three-dimensional (3-D) thermal-hydraulic and neutronic capabilities is RELAP-3D. A comparison of RELAP-3D predictions to experimental data obtained during start-up of the Kozloduy VVER-1000 nuclear power plant in Bulgaria is presented here. The experiment has distinguishable 3-D hydraulics effects that allow for code model verification and reasonable agreement with the experimental results obtained. The transient investigated was conducted at 29% reactor power, and it was initiated from a steady state where three out of four reactor coolant pumps were operating. The transient consisted of the start-up of the nonoperating pump. Simulation results were compared to both temperature and pump data from the experiment. Temperature predictions compared reasonably well to the experimental data; however, discrepancies existed between predicted and experimental pump head values.