| I. Materials > The Uranium-Plutonium Chain > Fabrication of Plutonium-based Fuel |
Mox fuel, in which the pastilles are composed of a mixture of plutonium oxide (PuO2) and uranium oxide (UO2), has always been used in fast neutron reactors (RNR); today it is used in certain light water reactors.
In classic RNR fuel, the plutonium constitutes 15 to 30 percent of the nuclear material in each assembly [RNR v-vi.97]; in PWR Mox fuel manufactured for EDF, the plutonium constitutes only about 5 to 7 percent on average of the nuclear material in each assembly. The uranium for the base can be natural, depleted, or URT.
The process of fabricating Mox is similar to that of fabricating standard fuel, but there are three important differences:
--The Mox workshop or factory receives and mixes UO2 and PuO2 powder before fabricating the pastilles and the assemblies. Two installations produce Mox in France: ATPu at Cadarache and Melox at Marcoule. The CEA used at Cadarache's ATPu the Coca process (CObroyage CAdarache), which consists of grinding together in a single operation a mixture of UO2 and PUO2 powders, respecting the final composition of the product. Cogéma, which acquired the workshop, was not satisfied. For Melox, Cogéma uses the procedure Mimas (Micronized Master Blend), developed by Belgonucléaire. Cogéma creates a mixture of UO2 and PuO2 with an elevated proportion of PuO2, grinds the mixture, and then dilutes it by adding more UO2. In 1986, Cadarache substituted Mimas for Coca for the fabrication of PWR Mox [Comag viii-x.96].
-- The fabrication of Mox is complicated by the fact that the composition of the fuel elements varies. To avoid interface problems between Mox assemblies and standard assemblies in the reactor, each Mox assembly is constituted of three different proportions of plutonium, the proportion being greater in the fuel elements at the center of the assembly than it is in those at the edge. Moreover, the isotopic composition of plutonium varies from lot to lot.
-- In a plant for fabricating Mox, there is a higher level of neutron and gamma activity around the materials and equipment, in particular because of the presence of plutonium 238 and americium 241. Production must be carried out as soon as possible after reprocessing to avoid the formation of americium to the extent possible; and rigorous measures must be taken to protect workers, the public, and the environment.
The fabrication of plutonium fuel in plaques for the Celestin and for research reactors has taken place at Cadarache, probably using a procedure similar to that for plaques of UAl and UAlx, but complicated by the necessity of having to work rapidly and to protect people and the environment under more difficult conditions.
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