A Comparison of (Th,Pu)O₂ and UO₂ Fuels as Waste Forms for Direct Disposal

Abstract

Issues relevant to the performance of irradiated (Th,Pu)O₂ as a waste form for geological disposal are briefly reviewed. Fuels of this type are among those being considered for burning plutonium from dismantled nuclear weapons in power reactors, including Canada deuterium uranium (CANDU) systems. The high chemical stability and low aqueous solubility of thoria make this type offuel attractive as a waste form. In contrast with UO₂ fuel, the inertness of thoria to oxidation dominates most of the chemical issues of fuel disposal. The overall performance of a thoria-based fuel waste form is likely to be determined by the “instant” release of the gap inventories of mobile fission products such as ¹²⁹I. This in turn will be controlled largely by the inreactor power history and probably also by details of fuel fabrication. Limited experience with thoria-based fuels [chiefly (Th, U)O₂] indicates that, for given power and burnup levels, gas releases can be substantially lower than with UO₂ fuels. The gap and grain-boundary inventories of fission products are expected to be correspondingly low. A fabrication route involving molecular-level mixing (e.g., sol-gelprocess) would be preferable to powder blending, because microscopic heterogeneities in the fuel might adversely affect the retention of fission products. Pilot-scale irradiation, postirradiation examination, and leaching studies are required to support this preliminary assessment. Other issues that need to be addressed include impurity specifications (to minimize formation of long-lived activation products) and criticality and safeguards issues that might influence the design of fuel-handling facilities.