Simulation of a gas-cooled fluidized bed nuclear reactor – Part II: Stability of a fluidized bed reactor with mixed oxide fuels

Fertile materials can be converted by nuclear reaction into a transuranic mixture with a fissile content, mostly plutonium isotopes. Fuel which includes a limited proportion of plutonium is already used in some reactors. Use is restricted by the smaller delayed neutron yield and lower negative temperature coefficient of reactivity compared with uranium fuels.A reactor with an additional stability feedback would make possible an increased use of plutonium fuels. This feedback mechanism is present in a novel conceptual High Temperature Gas Cooled Reactor (HTGR) where the TRISO particles are fluidized by the coolant gas.Spatial and time dependent simulations using the coupled Radiation Transport and Computational Multiphase Fluid Dynamic code FETCH are applied to investigations of this stability. The reactor has been investigated using plutonium fuel of various isotopic compositions. The temperature coefficient of reactivity may not be negative, depending on the isotopic composition of the fuel. The reactor is found stable. The amplitudes of power fluctuations increase however. These cause fluctuations in fuel temperature which can be excessive unless the average power is reduced, compared with that produced from a uranium fuel.The time dependent calculations are repeated using, in the neutron transport, data representing hypothetical fuels with zero temperature coefficient and/or zero delayed neutron yield.