Reactor physics aspects of plutonium burning in inert matrix fuels

Burnup calculations have been performed on fuels containing either reactor grade or weapons grade plutonium mixed in an inert matrix or mixed in a tho rium oxide matrix. At each branching during burnup, the fuel temperature co efficient, the moderator void coefficient and the boron reactivity worth ha ve 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 irradiati on 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 irradiat e plutonium reduces the fuel temperature coefficient by a factor of two to three, which is beneficial from the viewpoint of the power reactivity defec t, but disadvantageous from the viewpoint of reactivity induced accidents. Furthermore, the absence of U-238 or Th-232 in the matrix deteriorates the moderator void coefficient. Whether inert matrix fuels are feasible or not depends to a large extend on the possibilities to increase the magnitude of the moderator void coefficient and on the thermal properties of the matrix , like heat conductivity and melting point. (C) 1999 Elsevier Science B.V. All rights reserved.