Reducing the fuel temperature for pressure-tube supercritical-water-cooled reactors and the effect of fuel burnup

Abstract

The Pressure-Tube Supercritical-Water-Cooled Reactor (PT-SCWR) is one of the concepts under investigation by the Generation IV International Forum for its promise to deliver higher thermal efficiency than nuclear reactors currently in operation. The high coolant temperature (>625K) and high linear power density employed by the PT-SCWR cause the fuel temperature to be fairly high, leading to a reduced margin to fuel melting, thus increasing the risk of actual melting during accident scenarios. It is therefore desirable to come up with a fuel design that lowers the fuel temperature while preserving the high linear power ratio and high coolant temperature. One possible solution is to separate the fertile (ThO2) and fissile (PuO2) fuel materials into different radial regions in each fuel pin. Previously-reported work found that by locating the fertile material at the centre and the fissile material at the periphery of the fuel pin, the fuel centreline temperature can be reduced by ∼650K for fresh fuel compared to the case of a homogeneous (Th–Pu)O2 mixture for the same coolant temperature and linear power density. This work provides a justification for the observed reduction in fuel centreline temperature and suggests a systematic approach to lower the fuel temperature. It also extends the analysis to the dependence of the radial temperature profile on fuel burnup. The radial temperature profile is determined from the analytical solution of the steady-state heat conduction equation in the fuel pin, ignoring azimuthal dependence and axial heat flow. The temperature dependence of the thermal conductivity is accounted for, as is the radial dependence of the volumetric power density, which is determined from detailed lattice-level transport calculations performed using the lattice code DRAGON. The reduction in the centreline fuel temperature is shown to be caused partly by a reduction in the heat flux in the case of the two-region fuel and partly by the higher thermal conductivity of both pure ThO2 and pure PuO2 compared to that of a ThO2–PuO2 mixture. The centreline temperature of the two-region fuel is shown to be higher for irradiated fuel than for fresh fuel, a fact explained by the depletion of the fissile material in the peripheral region and the buildup of fissile material in the central region.