Many factors need to be investigated before alternative nuclear fuel can be adapted to the service in the harsh environment in the nuclear reactor. Urania, used conventionally as a nuclear fuel, has a low thermal conductivity, which degrades with increasing stoichiometry deviation. Thoria based fuel has been considered as an alternative fuel, since it does not oxidises and has high melting point and higher thermal conductivity. The simulations show that the fuel melting observed in urania fuel rods during 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 centre of the fuel pellets. The thermal gradient also remains low and therefore the thermal stress is reduced which should improve longevity of the fuel. Thoria has also some other desirable properties as our calculation predicts a significantly higher temperature of oxygen lattice pre-melting than urania. Furthermore we found that the diffusion of fission gas, e.g. helium, is strongly affected by oxygen diffusion and therefore it is slower in thoria for the temperatures where oxygen lattice premelts in Urania, but not in thoria.