Simulation of alpha dose for predicting radiolytic species at the surface of spent nuclear fuel pellets

In many countries, spent nuclear fuel is considered as a waste form to be disposed of in underground disposal. Under deep host rock conditions, a reducing environment prevails. In the case of water contact, long-term radionuclide release from the fuel depends on dissolution processes of the UO2 matrix. The dissolution rate of irradiated UO2 is controlled by oxidizing processes facilitated by dissolved species formed by alpharadiolysis of water in contact with spent nuclear fuel. To understand the effect of the radiation, the information of the dose rate at the surface of the fuel and its proximity is needed. α particles contribute strongly due to their high linear energy transfer. However, their dose rate and the energy deposition at the fuel surface are difficult to measure. Cylindrical fuel pellets as used in fuel rods show specific features, such as the rim zone, where a higher Pu concentration and a different porosity of the fuel matrix is present. The a particle dose rate was determined by simulations with the code MCNPX with focus on the rim zone of a pellet. As a result a 40% increased dose level in the rim zone exists in comparison to the center of a pellet. The potential dominant and inhomogeneous α-dose distribution is supposed to have a strong impact on radiolysis phenomena and in turn on an inhomogeneous dissolution of elements over the surface.