Dynamics aspects of plutonium burning in an inert matrix

Burnup calculations have been performed on a mini fuel assembly containing 21 fuel rods and four water holes at the corners. The fuel rod positions were filled with 4% enriched UO₂ fuel and with either reactor grade or weapons grade plutonium mixed in an inert matrix. The ratio between the UO₂ and the IMF rods was varied to investigate the influence of the UO₂ fuel on the dynamics of the assembly. From a simple reactor model with one delayed neutron group and first-order fuel and temperature feedback mechanisms, the linear transfer function from reactivity to reactor power was calculated that was subsequently used in a root-locus analysis. From this, it is concluded that only 20% of the fuel rods need to be made of UO₂ to have a fuel that is linearly stable up to 1000 days of irradiation.