Disposition of fission products in inert matrix fuels for plutonium burning

The use of inert matrix fuels (IMFs) for plutonium burning in nuclear reactors is discussed. Major desirable features of IMFs are high melting points, good thermal conductivities, and resistance to swelling from fissiogenic gas and radiation-induced lattice defects. While there is now an extensive body of literature on the use of inert matrix fuels as plutonium burners, including reactor test data, minimal attention has been given to the fate of the fission products generated and the disposition of the spent IMF fuel. If the fuel composition was such that the fission products formed nearly-insoluble phases within the fuel, subsequent geological disposal would be considerably facilitated corresponding to a directly disposable spent fuel concept. First we review the physical and chemical changes to UO2 fuel in standard power reactors, with emphasis on the disposition of the fission products. While synroc-C can accommodate > 30 wt% of fission products and transuranics, its relatively low melting point and thermal conductivity are disadvantages, not to mention its possible sensitivity to fission gas swelling. However a spinel-bearing synroc-D formulation is more favourable. The use of a candidate UO2 + magnetoplumbite + spinel + corundum fuel, suggested by Japanese workers, is then discussed. Other possible alternative ceramic phases such as sodium zirconium phosphate-structured Ca0.5(Zr/Ti)2(PO4)3 which can act as near-umversal solvents for fission products and transuranic ions are also discussed in the light of the other requirements for IMFs. Some comments are made also on the related possibility of using ceramic matrices for fast-reactor transmutation of fission products and actinides to yield short-lived isotopes.