Long-term behaviour of a thorium-based fuel

The radiological impact of a Th-based fuel disposed off in a deep granite repository has been assessed from the simulation code MELODIE developed by IPSN. A normal evolution scenario has been considered with a possible release of the radionuclides after 1000 years (container failure) and their transport from the disposal to some outlets of the geosphere over periods of time as long as 106 years. The solubility of crystallized ThO2, synthesized by calcination of thorium oxalate up to 1600°C, has been measured under various conditions of temperature and water composition; an upper limit value of 10−8 mol l−1 has been chosen for the calculations. As for a direct disposal of U-based fuels, the radiological impact of a Th-based fuel is essentially due to the labil fraction (2±1% of the total inventory) of the fission products, 129I and 135Cs. The contribution of the transuranium elements, 231Pa, 229Th and 226Ra, mainly bounded to the ThO2 matrix, increases over time, but remains lower up to 6×105 years.