Using thorium to decrease maximum DLOFC temperatures and increase total power output in Pebble-Bed High Temperature Reactors

Abstract

This is a study on 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. It therefore reviews the literature on the subject, with a special focus on possible solutions to the identified problems. It discusses the lessons learnt in the preceding papers, identifies gaps in them and adds new research in order to fill these gaps. The principle strategies used for reducing the maximum DLOFC temperatures were (a) flattening the peaks in the axial profiles of the maximum DLOFC temperature, which increases the surface areas over which effective evacuation of decay heat takes place and thus reduces the resulting heat fluxes and temperatures 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. These strategies were applied for both 6-pass recirculation fuelling schemes and Once Through Then Out (OTTO) fuelling schemes. The techniques used for flattening the peaks in the axial profiles of the maximum DLOFC temperature were (a) flattening of the peaks in the axial profiles of the equilibrium power density by adding thorium to the Low Enriched Uranium (LEU) fuel in order to improve the breeding and conversion ratios, which slowed the depletion of the enrichment of the fuel with increasing burn-up, and (b) placing purposely designed distributions of neutron poison in the central reflector in order to supress the standard peaks in the axial profiles of the equilibrium power density. The poison in the central reflector simultaneously served the purpose of pushing the power densities outward from the central towards the external reflector. This strategy was further targeted by creating asymmetric cores in which the enrichment of the fuel in the outer fuel flow channels are higher than in the inner ones, which automatically shifts the power out to these higher enriched outer fuel zones. The result was a substantial reduction in the maximum DLOFC temperatures from 1536 °C to 1488 °C for the multi-pass. This is small compared to the 1298 °C achieved in a different study with the same reactor. Our strategy was more effective with a large reduction in the maximum DLOFC temperature from 2273 °C to 1448 °C for the OTTO. Using 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. The effectiveness of pushing the power out by creating asymmetric cores was disappointingly low. As this technique adds design complexity which might create new accident risks and thus licencing risks, it was recommended that this technique should not be pursued in commercial Pebble-Bed Reactors, although it might be more useful in Prismatic Block Reactors.