It has been proposed recently that high-level radioactive wastes from spent nuclear reactor fuels be converted into a crystalline phosphate form prior to their disposal in geological repositories. This approach is based on the known durability in nature of the mineral monazite, (Ce,La)P0, which contains appreciable concentrations of thorium and uranium. Of significance is the demonstrated resistance of monazite to excessive metamictization, in spite of the fact that the mineral has been subjected since its formation to radiation from the radioactive disintegration of U, Th and their many intermediate decay products. Nevertheless, radiation effects on the leaching behavior of the material in possible groundwater environments must be studied in the development and testing of monazite-like phases for the long-term disposal of fission product and actinide wastes.