Different techniques for reducing DLOFC fuel temperatures in a PBMR-DPP-400 core

Abstract

This article addresses the topic of modifying the fuel cycles of Pebble Bed Reactors in order to reduce their maximum fuel temperatures during Depressurised Loss of Forced Coolant (DLOFC) accidents, in order to prevent unacceptable levels of release of radioactive fission products from the fuel into the environment. The principle strategies used for reducing the maximum DLOFC temperatures were (a) flattening the peaks in the axial power density profile, in order to increase the surface areas over which effective evacuation of decay heat takes place. This reduces the resulting maximum heat fluxes and temperatures in the hotspots; and (b) “pushing” the radial profiles of the equilibrium power density outward towards the external reflector, thereby decreasing the distance, and thus the thermal resistance, over which the decay heat has to be evacuated towards the external reflector. Easier radial evacuation of decay heat reduces the maximum DLOFC temperatures, which always occur in the inner layers of the fuel core. These strategies were applied for both 6-pass recirculation fuelling schemes and Once Through Then Out (OTTO) fuelling schemes by (a) flattening of the peaks in the axial profiles of the equilibrium power density by adding thorium to the LEU fuel and (b) placing purposely-designed distributions of neutron poison in the central reflector. This strategy was further implemented by creating asymmetric cores in which the enrichment of the fuel in the outer fuel flow channels was higher than in the inner ones. The result was a reduction in the maximum DLOFC temperatures from 1536 °C to 1298 °C for the multi-pass and from 2273 °C to 1448 °C for the OTTO. The use of neutron poison in the central reflector to flatten the peaks in the axial profiles of the maximum DLOFC temperatures reduced the maximum DLOFC temperature much more effectively than any of the other techniques