Performance of PyC, SiC, ZrC coatings in the geologic repository

The U.S. Deep-Burn Project is pursuing a fuel cycle that uses transuranic material taken from Light-Water Reactor (LWR) spent fuel in a High Temperature Reactor (HTR) to extract energy and transmute long-lived radionuclides. Burning transuranic radionuclides (TRU) from LWR spent fuel in a Deep-Burn High Temperature Reactor (DB-HTR) without addition of fertile material is an efficient and cost-effective way to process spent LWR fuel for disposal. With one coated particle fuel fabrication and one pass through the core, the HTR uses 95% of the energy from the fissile plutonium, and reduces the inventory of long-lived radionuclides by up to 60% by transmutation and fission. Public protection equivalent to that achieved by multi-recycle in a fast spectrum reactor can be achieved by disposing of the spent Deep-Burn fuel without further reprocessing – relying on the corrosion resistant TRISO or TRIZO coatings to contain the long-lived radioisotopes until they decay to low levels in the geologic repository. Coated particle behavior in a geologic repository is predicted here using coating corrosion data from a new, more comprehensive compilation in the one-dimensional, finite element fuel performance code PISA.Because of uncertainties associated with the properties of irradiated coatings and the mechanisms and rates of corrosion of coatings in repositories, the predictions made in this analysis are not quantitative but may be useful indicators of the behavior of the particle coatings.The uncertainties notwithstanding, our results indicate that integrity and radionuclide retention by coated particles are much longer than for other waste forms. SiC coated particles corrode slowly in dry and moist air and can survive for long times. If protected by the outer pyrocarbon coatings SiC can be an effective radionuclide barrier in aqueous media for long times; exposure to aqueous media substantially shortens the life of SiC coatings. ZrC coatings oxidize readily in repository air and provide less protection than SiC.