Particle-type Burnable Poisons for Thorium-based Fuel in HTGR

Thorium-based fuels have been proposed for use as alternative nuclear fuels in a 100-MWt prismatic high temperature gas-cooled reactor (HTGR) to overcome the limitation of natural uranium resources. Safety design features have been addressed by using particle-type burnable poison to prevent a prompt supercritical accident in the reactor. The present study compared the proposed system with the use of 235U-238U conventional nuclear fuel in an HTGR. The use of thorium-based fuel showed usefulness in terms of neutron economics, but from the safety standpoint, the thorium fuel reversed the inherent safety characteristics of a negative temperature coefficient. Introducing the appropriate composition of particle-type burnable poison can be useful for reactivity control in long-term operation, but it cannot guarantee the reactor will be free from the prompt supercritical accidents. The use of particle-type burnable poisons did sufficiently minimize the reactivity and flatten the reactivity swing to decrease the burden on the control rods mechanism, and reduce the positive temperature coefficient or even make it negative. These results did improve the safety features of the reactor.