The most stringent requirements are those for selecting fuel for a ded­icated consumer reactor fuelled with minor actinides alone, mainly because the linear rating of the fuel has to be as high as in a conven­tional reactor. Both the thermal conductivity and the melting point are lower for minor actinide oxides than for UO2 or PuO2. This rules out single-component oxide fuel. Similarly the thermal conductivity of americium-bearing oxide cercers is so low that realistic linear ratings cannot be achieved. The situation is better for oxide cermets as far as linear rating is concerned, but unless the metal fraction in the cermet is to be kept around 50 v/o or more it cannot be expected to seal the fuel, so in the event of cladding failure there is a risk of swelling on contact with Na. Thus oxide in any form is effectively ruled out for a dedicated

sodium-cooled consumer, although oxide cermets might be possible in consumers cooled by lead or gas. The remaining possibilities are

• Pure compound (Am, Cm)N

• Solid solutions (Am, Cm, Zr)N or (An, Cm, Y)N

• Cercers (Am, Cm)N+TiN or (Am, Cm)N+AlN

• Cermets (Am, Cm)N+W

An alternative to a reactor fuelled entirely with minor actinides is a “normal” plutonium-fuelled reactor with special fuel subassemblies containing the minor actinides in or around the core. The power rating of these “target” assemblies would not have to be so high, especially if

they were placed round the periphery of the core, so they could make use of oxide fuel.

Pure minor actinide oxides are ruled out because they are unlikely to be stable under irradiation, but it may be possible to stabilise them in solid solution with Zr or Y to give a fuel suitable for targets. Similarly the low thermal conductivity of Am-bearing oxide cercers may be acceptable. The lower rating of the targets might also make cladding failure sufficiently unlikely that swelling of the fuel material on contact with sodium is less important, allowing oxide cermets or cercers to be considered. The possibilities for target fuel include all those identified above and in addition the following:

• Solid solutions (Am, Cm, Zr)O2 or (Am, Cm, Y)2O3

• Cercers (Am, Cm)O2+MgO

• Cermets (Am, Cm)O2+W, Cr, V or steel