Deep-burn spent fuel. Graphite as a possible high level waste matrix

The behavior of Deep-Burn Spent Fuel (DBSF) is investigated in two repository environments, a welded tuff repository, and a granitic repository, with respect to the mass release rate (mol/yr) of radionuclides released from the graphite waste matrix and transported downstream through fracture networks to distances of 10m and 1km from the failed waste packages. Overall mass release rates of radionuclides in the granite repository are seen to be between three to four orders of magnitude less than in the welded tuff repository. A number of factors contribute to this observed reduction including lower radionuclide solubility and a more favorable geology in the granitic environment with respect to the welded tuff environment. The effects of waste matrix durability were also investigated in both repository environments. A reduction of one order of magnitude in the mass release rates is observed for every order of magnitude increase in the waste matrix lifetime. This highlights the importance of the durability of materials which are selected as waste matrices for the disposal of high-level nuclear waste. DBSF, with a minimum lifetime of 3 x 106 years highlights the possibility of graphite's utility as a potential waste matrix for the disposal of high-level waste. (author)