Analysis of Natural Convection Capability in NuScale Primary Cooling Systems with Mixture of Uranium and Plutonium (Mixed Oxide) Fuel Using RELAP5-3D

摘要

NuScale is an integral UO_2 fueled PWR that operates with natural circulation. In this study thermal-hydraulic analysis was carried out on the NuScale primary system to observe the natural convection phenomenon when the fuel was changed to mixed oxide (MOX). The use of MOX could increase the neutron advantage, but it was offset by enlargement of the core diameter. This modification may cause the thermal properties of the fuel and the flow distribution in the core changes. It is thus necessary to analyze the effect of such changes to ensure that NuScale's natural convection capability is maintained and the reactor remains safe. This research was carried out using the RELAP5-3D thermal-hydraulics code. Four thermal-hydraulics models were analyzed, based on the properties of high bum-up fuel. Each type of fuel was simulated under end-of-cycle (EOC) and beginning-of-cycle (BOC) conditions. BOC simulations were used to test reactor safety when operating with a large power peaking factor (PPF), while the fuel properties were maintained in high bum-up conditions. The results showed that the ability of natural circulation in each model remained able to be maintained based on differences in cooling density. There were no significant differences in the coolant temperature, cooling flow rate, and void fraction of each model. The enlargement of the core diameter, however, causes an increase in the fuel channel void fraction due to the reduction of cooling flow within. A significant impact occurred at the pellet temperature, where the highest pellet temperature occurred at MOX because MOX conductivity was lower than UO_2, but the pellet peak temperature was below the melting temperature of MOX. There are no operating parameters that exceed the safety limit, so the reactor can still maintain its natural convection capability, and MOX is suitable for use at the NuScale reactor.