Simulation of a gas-cooled fluidized bed nuclear reactor. Part I: Mixed oxide fuels

The worldwide use of enriched uranium has resulted over several decades in a stockpile of 238U. Fertile 238U can be converted by nuclear reaction into a transuranic mixture with a fissile content. Fuel which includes a limited proportion of this converted material 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 fuels containing fissile transuranic isotopes. 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 (RT) and Computational Multiphase Fluid Dynamic (CMFD) code FETCH go some way towards demonstrating this. FETCH couples together the two Finite Element Method (FEM) based codes EVENT (RT) and FLUIDITY (CMFD). In previous research FETCH has been applied to simulate fluidized bed reactors fuelled with uranium kernel TRISO particles. In this paper the stability of the reactor is investigated using plutonium fuel of various isotopic compositions. The FETCH model suggests that the plutonium fuelled reactor will remain stable for continuous operation. However, the amplitudes of power fluctuations increase compared with those for uranium fuel.