Enhanced Accident Tolerance of Thoria-Based Nuclear Fuels

Many factors need to be investigated before alternative nuclear fuel can be adapted for service in the harsh environment of a nuclear reactor. Urania, used conventionally as a nuclear fuel, has a low thermal conductivity, which degrades with increasing stoichiometric deviation. Thoria-based fuel has been considered as an alternative fuel, since it does not oxidize and has a high melting point and higher thermal conductivity. Simulations have shown that the fuel melting observed in urania fuel rods during an accident with steam ingress should not be observed (or will be delayed) in thoria as its thermal conductivity remains high enough to dissipate excessive heat in the center of the fuel pellets. The thermal gradient also remains low and therefore thermal stress is reduced, which should improve the longevity of the fuel. Thoria also has some other desirable properties as our calculations predict a significantly higher temperature of oxygen lattice premelting than urania. Furthermore, we found that the diffusion of fission gas, e.g., helium, is strongly affected by oxygen diffusion and therefore is slower in thoria for the temperatures where the oxygen lattice premelts in urania, but not in thoria.