The preceding part of the chapter has been written with breeder react­ors in mind. Different fuel is needed for a reactor designed to consume plutonium rather than breed it, the principal difference being that it should not contain uranium or at least that it should have a minimum of it. There is little irradiation experience with high-Pu fuels so in what follows it is possible only to indicate what seems most likely to be of use in practice.

If plutonium is to be consumed then it is clearly desirable that it should be consumed as quickly as possible. Plutonium is consumed by fissioning it, which produces energy. The power of a plutonium­consuming reactor should therefore be high if it is to be effective. A 2500 MW (heat) reactor with an 80% load factor, fuelled with pure plutonium with no breeding, would consume about 800 kg of plutonium per year.

The constraints of heat transport (see section 3.2) mean that a 2500 MW core would have to have a volume of about 3 m3 and would need some 1700 kg of plutonium to make it critical. If the plutonium were in fuel elements generating an average of 32 kWm-1 (i. e. a max­imum rating of 50 kWm-1 at the centre of the core) the fuel elements would have to have a total length of 78 km. If the fuel were PuO2 with an 80% smear density the radius of the pellets would be about 1 mm, and if it were metal or some other ceramic the radius would be even smaller! Clearly this is impossibly small for practical fuel elements: they have to be larger, and in practice a diameter of about 6 mm is the minimum that is economically and structurally possible. Thus the plutonium has to be diluted with some inert material, which in effect is present to replace the uranium that makes up such a large part of the fuel of a breeder reactor.

There are two ways in which a diluent material can be incorporated in a ceramic fuel. It can either be a solid solution in the ceramic, or a second component in a two-component mixture. Because of the difficulty of dissolving PuO2 in nitric acid for Purex reprocessing, and also because it reacts chemically with sodium, plutonium oxide solid solutions such as (Pu, Zr)O2 are not attractive. The most promising ceramic solid solution is (Pu, Zr)N. There are few data on its physical properties but theoretical estimates suggest that, as in the case of Pu-Zr alloy, it would perform well in a 50 kWm-1 fuel element.

In the 1960s, when a solution to the problem of fuel swelling was being sought, “cermet” fuels were investigated. A cermet is a sintered mixture of ceramic and metal powders. The idea was that the metal would form a strong matrix that would constrain the swelling of the fuel ceramic. Some irradiation testing was done and high burnups of UO2-stainless steel cermets were achieved. However, the project was abandoned when it was realised that if the metal fraction was high enough to prevent swelling it would absorb so many neutrons that the breeding ratio would be reduced unacceptably. This of course would not be a disadvantage in a plutonium-consuming reactor, and cermets of PuO2 with steel or other metals such as chromium, vanadium or tungsten may be attractive.

By analogy with a cermet, a sintered mixture of two mutually insol­uble ceramic powders is sometimes called a “cercer”. The range of suit­able diluent ceramics is restricted because the resulting cercer would have to be soluble in nitric acid if the fuel is to be reprocessed by the Purex process. Cercers of PuO2 and magnesia (MgO) or yttria (Y2O3) are possibilities. Ceria (CeO2) should probably be ruled out because it is incompatible with liquid sodium and would swell severely in the event of a cladding failure.

Because of the success of U-Pu-Zr alloy fuel Pu-Zr alloy appears attractive. The melting point of plutonium is very low at 640 °C but that of zirconium is 1852 °C. The solidus temperatures of Pu-Zr containing 20% and 40% Pu are about 1680 °C and 1480 °C respectively. Since their thermal conductivities (without porosity) are around 20 Wm-1 K-1, comparison with the 70U-20Pu-10Zr fuel of section 2.5.1 seems to indicate that a high-Pu metal-fuelled element would have acceptable irradiation performance at a linear rating of 50 kWm-1.