Mixed oxide or MOX fuel as used in present light water reactors means a mix of uranium and plutonium oxides. Thorium can be a constituent of mixed oxide fuel as well, but is presently not used on a commercial basis.

The pellet fabrication steps are similar to those of UO2 fuel, but the powder preparation must necessarily differ. The entire fuel powder, pellet and rod fabrication has to occur in shielded facilities. By and large, two methods are in use for powder preparation and mixing. The technology developed by former BNFL (Weston et al., 2001) is called the ‘Short Binderless Route’ (SBR). The UO2 and PuO2 are mixed in the right quantities to obtain the desired Pu content of the final product in one step. The blend is attrition-milled to break down agglomerates and to produce an intimately mixed and micronised powder feed for pellet pressing.

Another technology is the ‘Micronised Master Blend’ (MIMAS) (Vliet et al., 1996). In this case, a master blend with a plutonium content of about 30% is produced and micronised by attrition milling. This blend is then diluted with UO2 powder to the desired plutonium content and homogenised. The process is used

by AREVA (Arslan and Krellmann, 2006), has been selected for the US ex-weapons plutonium disposition, and is being implemented in Japan for recycling of spent Japanese fuel.