Section 3. Reactor physics studies Reactor physics aspects of plutonium burning in inert matrix fuels

Abstract Burnup calculations have been performed on fuels containing either reactor grade or weapons grade plutonium mixed in an inert matrix or mixed in a thorium oxide matrix. At each branching during burnup, the fuel temperature coecient, the moderator void coecient and the boron reactivity worth have been calculated. From the reactor physics point of view, use of thorium oxide as a matrix compares best with irradiation of plutonium in`ordinary' (U, Pu) mixed oxide fuel. Because the thermal properties and the irradiation resistance of thorium oxide are generally better than of uranium oxide, the irradiation of plutonium (either reactor or weapons grade) in a thorium oxide matrix seems without problem. The use of an inert matrix to irradiate plutonium reduces the fuel temperature coecient by a factor of two to three, which is bene®cial from the viewpoint of the power reactivity defect, but disadvantageous from the viewpoint of reactivity induced accidents. Furthermore, the absence of 238 U or 232 Th in the matrix deteriorates the moderator void coecient. Whether inert matrix fuels are feasible or not depends to a large extend on the possibilities to increase the magnitude of the moderator void coecient and on the thermal properties of the matrix, like heat conductivity and melting point.