2.6.1 Carbide

Extensive work has been done on mixed carbide fuel in India in con­nection with the thorium-based breeder cycle. To initiate this cycle reactors fuelled with 239Pu but with 232Th rather than 238U as the fertile material are needed. In such reactors uranium is essentially redund­ant, so mixed oxide, which as indicated in section 2.3.3 is limited to about 40% plutonium, or ternary metal alloy with a large uranium

content, are not attractive options. In several respects mixed carbide has superior properties, particularly its high density and high thermal conductivity.

The properties of mixed carbide depend strongly on the stoi­chiometry. Mixtures of UC and PuC at any ratio form solid solutions. The melting points of UC and PuC are around 2780 °C and 1875 °C respectively, but those of the sesquicarbides U2C3 and Pu2C3, which are present to a varying degree in a hyperstoichiometric mixture, are 2100 °C and 2285 °C respectively. As a result the melting point varies over a wide range depending on the contents of the mixture.

Figure 2.23 shows the conductivity of UC and its integral. Both are much higher than the corresponding properties of oxide fuel (see Figure 2.1). However the conductivity of PuC is much lower, rising from 10 Wm-1 K-1 at 700 °C to 20 Wm-1 K-1 at 1100 °C, and the influence of stoichiometry and impurities such as oxygen on the con­ductivity is complex. As a result the effective conductivity of a partic­ular fuel material is much lower than that of pure UC, and values of about 7 Wm-1 K-1 at 500 °C rising to 11 Wm-1 K-1 at 1000 °C have been reported for (U1-aPua)C fuel with a in the range 0.55-0.7. Nev­ertheless the conductivity is still much higher than that of oxide, and with a surface temperature of 1000 °C (assuming the gap between fuel and cladding is filled with gas) and a linear heat rating q of 50 kWm-1 carbide fuel would have a central temperature of around 1380 °C, well below the melting point. Thus higher values of q without central melt­ing are possible in principle. (It may, however, be that this potential cannot be exploited because of limitations to the rate at which heat can be transported out of the reactor core — see section 3.2.3.)

A disadvantage of carbide fuel is that it carries the risk of carbur­isation of the steel cladding. The risk depends on the stoichiometry because the carbon activity of (U, Pu)2C3 is high. Carbide is chemic­ally compatible with sodium so it is possible to fill the gap between fuel and cladding with sodium to provide a good thermal bond. The sodium however tends to transport carbon to the cladding, so helium bonding is usually preferred.

Carbide fuel has a higher density than oxide (because each heavy atom is accompanied by only one light atom rather than two), so it swells more on irradiation. As with metal fuel the swelling is accom­modated mainly by a low smear density. At the start of irradiation the fuel pellets form extensive cracks. Swelling closes the gap between fuel and cladding and eventually eliminates the as-manufactured poros­ity in the pellets. The stress on the cladding from further swelling is relieved, to some extent, by creep of the fuel material. This is particu­larly important for high-plutonium mixtures, which are softer because the melting temperature is lower.

Because the temperatures are so much lower carbide suffers much less restructuring than oxide (see section 2.4.1) and a smaller fraction of the fission-product gas is released.

Carbide is usually produced by carbothermic reduction of oxide. Oxide and carbon powders are mixed, formed into pellets and heated, and the oxygen is driven off in the form of carbon monoxide. Further heating in a hydrogen atmosphere can control the stoichiometry and remove residual oxide and other impurities. A significant disadvantage of carbide is that it is pyrophoric, so the manufacturing process and all fuel-handling activities have to be carried out in an inert atmosphere.