In the beginning stages of R&D for MOX fuel pro­duction, many kinds of manufacturing techniques were investigated. In the 1960s, the pellet route was adopted for all the pilot plants in Belgium, France, Germany, the United Kingdom, and Japan.2, 8 The two types of MOX fuel for LWRs and FBRs have quite different characteristics, affecting both the fabrication process and the quality requirements. These characteristics are summarized in the following points6:

• The plutonium content of FBR fuel is several times higher than that of LWR fuel.

• The smear density of FBR fuel has to be lower than that of LWR fuel because the former has to be used at higher temperature and for higher burn-up.

• The higher plasticity of FBR fuel, resulting from the higher irradiation temperature, justifies less

restrictive specification tolerances and quality requirements, than for LWR fuel.

• The uniformity in plutonium isotopic composition within a batch of fuel assemblies is a key perfor­mance-related quality for LWR fuel, while it is rather unimportant for FBR fuel.

On the basis of these points, various kinds of processes were developed to fabricate MOX pellets for FBRs and LWRs. The MOX pellet fabrication processes that have been adopted in several countries are described below.