In Belgium, the micronized master blend (MIMAS) process was developed by Belgonucleaire (BN) in the early 1980s based on the experiences acquired in the reference fabrication process developed earlier and commercially used in the 1970s at BN’s Dessel plant.2 The reference process consisted of a single blending of PuO2 powder with free-flowing UO2 powder and this blending resulted in a blend with adequate flow — ability to feed the pelletizing press.6 As MOX pellets fabricated by the reference process could not satisfy the preprocessor’s new requirement, which was that MOX pellets had to be soluble in a nitric acid solu­tion, BN had to improve the solubility of MOX pel­lets in the nitric acid solution. In order to improve their solubility, the MIMAS process was introduced in the Dessel plant. Figure 13 shows the flow sheet for the MIMAS process.

In the MIMAS process, suitable amounts of PuO2 powder, UO2 powder, and dry recycled scrap powder are prepared to get a 60 kg MOX master blend pow­der with 30% plutonium concentration. The master blend powder is ball milled to obtain a homogeneous distribution of plutonium. In the second blending, force-sieved (i. e., micronized) master blend powder is diluted with the free-flowing UO2 powder and

additional dry recycled scrap to form 80 kg of the final blended MOX powder with the desired pluto­nium concentration.6 In this step, it is very important to obtain uniform distribution of master blend in free-flowing UO2 powder. This final blended MOX powder is pelletized into green pellets using a pressing machine with multiple punches and a reciprocating mechanism. Approximately 10-12 green pellets can be pressed simultaneously. These green pellets are sintered at about 1700 °C under a reduced atmosphere of Ar + H2 mixed gas, after dewaxing. Not only does the intimate contact between the comicronized UO2 and PuO2 powders provide adequate interdiffusion during sintering and therefore enhanced solubility, but also the larger contact area between the more abundant fine powder and the free-flowing UO2 pow­der results in a more heterogeneous MOX structure than in the earlier reference process. This is apparent in measurements such as the a-autoradiograph of a transverse section of a MOX pellet prepared by the MIMAS process, given by Lippens et al5

During the 1990s, the Dessel plant accounted for over 60% of the world’s production of MOX fuel.49 However, MOX fuel fabrication was terminated in 2006. Now, this plant is undergoing preparative work for its decommissioning.