Development of a RELAP5-3D three-dimensional model of a VVER-1000 Nuclear Power Plant for analysis of a large-break loss-of-coolant accident

Abstract

In order to analyze the benefits of the multi-dimensional hydrodynamic modeling capability of the RELAP5-3D system code in the VVER-1000 Nuclear Power Plant, a three-dimensional model of the core, downcomer, and lower plenum have been created to replace their one-dimensional counterparts in a complete plant model. This multi-dimensional model has been validated with plant operational data and other computer simulations of a thermal-hydraulic transient. The simulated transient considered was a large-break loss-of-coolant accident (LB LOCA). A validated, one-dimensional control of the nuclear power plant, for the study of the effects of mixed oxide (MOX) fuel, was modified to include a standard fuel loading of UO₂. The development of the three-dimensional sections of the reactor vessel consisted of ensuring geometrical fidelity with the design of the modeled plant, the Balacovo Unit 4, Nuclear Power Plant in Saratov, Russia. A stable operational steady-state was obtained and the calculated plant conditions compared well with the design values of the Balacovo Plant. Transient results verified that the simulated thermal-hydraulic conditions of the multi-dimensional model agreed well with both the control and analyses that have been performed separately from this report. It was found that the multi-dimensional model has shown a reduction in the calculated hot-spot peak-clad temperature (PCT) during the blowdown stage of a LB LOCA and an increase in PCT during the reflood stage. A preliminary uncertainty analysis of the PCT during blowdown stage was performed using a response surface method of the Code Scaling, Applicability, and Uncertainty Method and a significant number of relevant input variables. From the preliminary analysis, the PCT reduction during blowdown appears to be significant, but a further, more detailed analysis should be performed, along with an uncertainty analysis of the PCT during the reflood stage. The enhanced depiction of the flow patterns and temperature distributions in the transient situation allowed the user further understanding of the thermal-hydraulic conditions throughout the transient. The developed model proved to be suitable for analysis of the VVER-1000 plant, but to further the applicability of the model a three-dimensional kinetics model of the neutronics and three-dimensional hydrodynamic models of the horizontal steam generators should be included.