The behaviour of nitride fuel is very similar to that of carbide. The thermal conductivity of UN is shown in Figure 2.24, showing that, as in carbide, high linear heat ratings are possible in principle. The conduct­ivity of PuN is about half that of UN and for mixtures containing 20 to 35% Pu values in the range of 20 Wm-1 K-1 are possible (depending on the porosity). There are fewer complications due to variation in stoichiometry because higher nitrides of uranium are unstable and can be dissociated to UN by heating to 1400 °C, and plutonium forms only PuN. The melting point of UN is about 2740 °C and PuN dissociates at about 2570 °C.

Nitride can be produced, like carbide, by carbothermic reduction of oxide, but in this case in a nitrogen atmosphere. The disadvantage of this method is that the product contains carbon and oxygen as impurities, and these pose some risk of carburisation of the cladding. Nitride is not pyrophoric.

Under irradiation in general nitride behaves similarly to carbide, but it swells less and releases less fission-product gas. It is also more prone to cracking.

The principal disadvantage of nitride fuel made with atmospheric nitrogen is the reaction 14N(n, p)14C. The 14C produced during irradi­ation is a hazard during reprocessing and is an unwanted additional component of the radioactive waste that has to be disposed of. It could in principle be reduced or eliminated by enriching the nitrogen from which the fuel is made in 15N (which constitutes 0.36% of natural nitro­gen) but this would be expensive, and it would be difficult to conserve it during reprocessing.