Validation of the thermal-hydraulic thunder code and application for PWR core in transient conditions

Abstract

The THUNDER computer code allows performing thermal-hydraulic core analysis of pressurized water reactors aiming at the design and safety analysis of power and naval reactors in steady state and transient conditions. It uses the subchannel and control volume methods to perform a tridimensional analysis of the nuclear reactor core. The conservation equations are solved simultaneously for each control volume based on geometric and operation conditions. The Integral Momentum Method is used for solving the conservation equations decoupling the momentum equation from the mass and energy conservation equations. The drift-flux model is adopted to treat the two-phase flow condition. The analysis is performed in two-stages: first, a macroscopic analysis is conducted with subchannel equivalent to a fuel assembly (bundle), and second, with a subchannel equivalent to an individual coolant channel. Only the hottest fuel assembly is analyzed with detail. To validate the THUNDER code we used the experimental data from the NUPEC PWR benchmark (subchannel and bundle tests), in which we verify the accuracy of the coolant temperature at steady-state in 4 different void fraction conditions and the accuracy of the void fraction at transients initiated by four different conditions: core power increase, reduction of inlet mass flow rate, increase of inlet coolant temperature and depressurization of the inlet coolant. At steady-state conditions, the mean relative discrepancy for the coolant temperature was 0.09. At transient conditions, the void fraction mean absolute discrepancy for power increase was 0.045, for mass flow reduction was 0.079, for inlet coolant depressurization was 0.032 and for temperature increase was 0.092. The code was applied for the Brazilian 1350 MW nuclear power plant ANGRA 2 for evaluating the thermal-hydraulic safety parameters at steady-state, as well as the operational transients of loss of one or several primary coolant pumps. The THUNDER code results showed that the safety limits were not exceeded in concordance to the Final Safety Analysis Report of the plant. These results indicate that the THUNDER code is sufficiently accurate to perform thermal-hydraulic analyses for PWR power plants at steady-state and transient conditions.