As explained in section 1.4.1 isotopes of americium and curium are pro­duced in all uranium-fuelled reactors, in greater quantities the higher the burnup of the fuel. They constitute hazardous waste products and there may be an incentive to eliminate them. Because they are fission­able this can be done in principle by incorporating them as fuel in a fast reactor. However if the objective is to reduce their quantity the reactor should not generate more of them, and therefore, as in the case of a plutonium-consumer reactor, the fuel should contain little or no uranium and would have to be diluted.

Higher-actinide ceramic fuel would probably be reprocessed in dedicated facilities, separate from the mainstream, using either the Purex process or a modification of it (see section 2.3.3) with lower separation efficiency and a product stream matched to the fuel-cycle requirements. For this reason the fuel and any diluent materials must be soluble in nitric acid.

In addition the higher actinide fuel would have to be compatible with the coolant in the sense that, in the event of a small cladding failure, in the sense thatgross swelling or any other reaction that would cause the release of significant quantities of fuel material or fission products to the coolant would have to be impossible. This limits the choice of fuel material for a sodium-cooled reactor, but not necessarily for reactors cooled with gas or lead. In principle the fuel material could be a pure compound, a solid solution of fuel and a diluent compound, or a two-phase mixture of fuel and a diluent compound. The two-phase mixture could be either a cermet or a cercer.

Information for most of the candidate materials is incomplete. Physical property data on thermal conductivity or melting point are sometimes known or can be inferred, but there is usually nothing on properties such as thermal creep. There is little irradiation experience but some aspects of the behaviour under irradiation can be deduced from the phase diagram. (If there are phase changes in the operating temperature range the structure is unlikely to be stable.) There is often no empirical information on the compatibility with cladding or coolant, although theoretical inferences can sometimes be made. Information on curium compounds is scarce so the selections are mostly based on americium data.

The most obvious choices for pure compound fuel materials are summarised in Table 2.5. The most important considerations are that

carbides are pyrophoric, AmO2 has a high oxygen potential, and Am2 Оз has several phase changes.

Table 2.6 summarises the main candidate diluent materials for cermets or cercers. Nitrides might require enrichment in 15N to avoid production of 14C. In addition to the materials listed in the table Si3N4, TiN, YN, and AlN are also candidates.