In France, the Complexe de Fabrication de Cadar — ache (CFCa) started operation in 1962, on a pilot scale, for developing FBR fuel. The Cobroyage

(co-milling) Cadarache (COCA) process was devel­oped there in the 1970s to fabricate MOX pellets for FBRs using two fuel fabrication lines.

Figure 14 shows the flow sheet for the COCA process. It utilizes an optimized ball mill as a blender and involves the forced extraction of the lubricated micronized powder through a sieve. This results in free-flowing granules which are suit­able for feeding at the pelletizing step.49 In the COCA process, the lubricant and the porogen, which is a pore former to control pellet density, are added to the force-sieved powder.51 One of the two FBR fuel fabrication lines in CFCa was switched to a LWR fuel fabrication line which intro­duced the LWR fuel fabrication technology devel­oped by BN. This LWR fuel fabrication line started producing PWR fuel in 1990.6 MOX fabrication at CFCa was stopped in 2005 because of seismic safety issues and the facility is now undergoing preparative work for its decommissioning.

In 1985, the construction of the MELOX plant at Marcoule was started; it had an annual production capability of 100 tons of heavy metal (tHM) for PWR fuel which was decided on the basis of operational experiences with the MIMAS process obtained at CFCa and it started MOX fuel production in 1995. Gradually, its licensed annual production capability was expanded and it reached 195 tHM as of April 2007; MOX fuel fabrication for BWRs was also cov­ered during this expansion. The process adopted in the MELOX plant is called the advanced MIMAS process and its flow sheet is shown in Figure 15. The accumulated MOX fuel production at the MELOX plant reached 1426 tHM at the end of 2008. The features of this process are given below.

In order to utilize up to 50% of dry recycled scrap powder in the master blend powder and to achieve excellent homogeneity and uniformity of PuO2 as well, a new ball mill was developed for the first blend­ing step.6 This mill uses three-dimensional movement and U—Ti alloy balls. For the second blending, a high capacity (640 kg) blender consisting of a conical screw mixer with a double envelope cooling system was adopted.6,49 In order to achieve MOX fuel production on a large scale, complete automation was implemen­ted in the production line. Similar to the original MIMAS process invented in BN, three kinds of feed powders, PuO2 powder, UO2 powder, and dry recycled scrap powder, are ball milled to obtain the master blend powder with about 30% plutonium

concentration. The force-sieved master blend powder is diluted with the free-flowing UO2 powder, pre­pared by the ADU process or the AUC process and additional dry recycled scrap powder using the high capacity conical screw mixer. This free-flowing diluted power is pelletized into green pellets using a pressing machine with multiple punches and a reci­procating mechanism. Approximately 10-14 green pellets can be pressed simultaneously. The green pel­lets are sintered in a continuous-type sintering fur­nace consisting of a dewaxing part and a sintering part. After dry centerless grinding of sintered pellets, the exterior of all pellets are inspected.

A mapping image of plutonium, acquired by X-ray microanalysis of a transverse section of a MOX pellet prepared by the advanced MIMAS process, was reported by Oudinet et al5 In the MIMAS process, a two-step blending method is utilized to obtain the desired plutonium content in the pellets, as described above. This results in the presence of two or three phases in the transverse section of a sintered pellet. The MOX pellets prepared with UO2 powder from the ADU process show three phases, plutonium rich clusters, a coating phase and a UO2 phase on their transverse sections while those prepared with UO2 powder from the AUC process show two phases, plutonium rich clusters and a UO2 phase.58,59 The MOX pellets manufactured by the short binderless route (SBR) and Japan Atomic Energy Agency (JAEA) processes in which a one-step blending method is adopted to obtain the desired plutonium concentration of pellets show a single homogeneous phase on their transverse sections, and are different from pellets fabricated by the MIMAS process.51,60 The MOX pellets currently manufactured in the MELOX plant are reported to have a mean grain size of 5.8 pm.61