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 we re filled with 4% enriched UO2 fuel and with either reactor grade or weapon s grade plutonium mixed in an inert matrix. The ratio between the UO2 and t he 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 neut ron group and first-order fuel and temperature feedback mechanisms, the lin ear 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 i s linearly stable up to 1000 days of irradiation. (C) 2001 Elsevier Science Ltd. All rights reserved.