Nuclear Technology / Volume 147 / Number 1 / July 2004 / Pages 157-170
Technical Paper / Thoria-Urania NERI
The aqueous dissolution of irradiated and unirradiated uranium-thorium dioxide, (U,Th)O2, fuel pellets in Yucca Mountain well water has been investigated. Whole and crushed pellets were reacted at 25 and 90°C for periods of up to 195 days. The fuel dissolution was measured by analyzing the concentrations of soluble uranium, thorium, and important fission products (137Cs, 99Tc, 237Np, 239Pu, 240Pu, and 241Am) in the well water. The surface-area-normalized fractional uranium release rates for unirradiated crushed uranium dioxide (UO2) pellets were 10 to 40 times higher than the values for (U,Th)O2 fuel. Similarly, the dissolution rates of irradiated (U,Th)O2 pellets with compositions ranging from 2.0 to 5.2% UO2 were at least two orders of magnitude lower than reported literature values for pure UO2. These results demonstrate an advantage of (U,Th)O2 over UO2 in terms of matrix dissolution in groundwater and suggest that (U,Th)O2 fuel is a more stable long-term waste form than conventional UO2 fuel.