Preparation of fully-active Synroc and its radiation stability. Final report

Samples of Synroc have been made from fully-active waste with a composition chemically equivalent to that which will arise from the Thermal Oxide Reprocessing Plant (THORP) at Sellafield. The samples have an activity of 20 GBq.g/sup -1/ and leach-rates, for Cs-137 and Sr-90, measured at 90/sup 0/C, were about 20 and 100 times lower than for equivalent fully-active glass samples. The actinide leach rates were significantly higher than the nearest equivalent samples made at Australian Nuclear Science and Technology Organisation (ANSTO), by a factor of 3 for Cm-244 and by a factor of 200 for Pu-239. The major cause is thought to be due to premature neutralisation of the waste solution during manufacture, leading to incomplete actinide incorporation in the Synroc phases. The actinide leach rates for the fully-active samples were nevertheless 15 to 450 times less than those for fully active glass samples. Synroc samples doped with Pu-238 and Cm-244 have also been made to test the material's radiation stability. Some Pu-238 doped samples made earlier from less well characterised materials, but with the correct phase composition, have decreased in density by about 6% after a dose equivalent to 300,000 years for real waste.