In the United Kingdom, over the past 25 years, exten­sive work has been carried out on the manufacture of MOX fuel under the support of the UK Fast Reactor Development Program.51

Based on these experiences, the SBR process was developed by the British Nuclear Fuels plc (BNFL) to fabricate MOX pellets for LWRs. The process was originally developed in the 1980s by BNFL-UKAEA (United Kingdom Atomic Energy Authority). Figure 22 shows the flow sheet for the SBR process.

In the SBR process, three kinds of feed materials, PuO2 powder prepared by the oxalate precipitation method, UO2 powder prepared by the ADU process, and dry recycled scrap powder are prepared to get the desired plutonium concentration in the initially mixed powder. These powders are milled completely using an attritor mill (a photograph is shown in MacLeod and Yates51), an off-the-shelf mill widely used in the pharmaceutical industry. The attritor mill provides good blends with a homogenized plutonium distribution in a short blending time and can be operated continuously.6 The milled MOX powder must be granulated in order to provide a free-flowing, dust-free feed to the pelletizing press to ensure uniform die filling and good compaction.51 In the milling step, the lubricant and Compo pore former are added in order to control the pellet density and obtain character­istics similar to those of the UO2 pellets produced by BNFL from IDR UO2 powder.66 In order to condition the milled MOX powder to form granules prior to pelletizing and sintering, a spheroidizer is introduced instead of the precompaction granulation equipment commonly used.6 The spheroidizer is used in a powder agglomeration process and was invented by SCK’CEN (Studiecentrum voor Kernenergie — Centre d’Etude de l’energie Nucleaire) in the 1970s to fabricate a fuel kernel, the pit of coated particles fuelling high temper­ature reactors.6

In the SBR process, the binder that is commonly used in the conventional MOX fuel manufacturing process is not used. As a result, the dewaxing step of the green pellets prior to sintering is not needed and the process is similar to the current UO2 fuel fabrica­tion process in this respect. The processing time is short and the equipment can be stacked so that the powder can be discharged by gravity from the feed dispensing and dosing glove box through the proces­sing equipment into the hopper of the pelletizing press. The simple sequence of one attritor mill and one spheroidizer, utilized in the Manufacturing Demonstration Facility, was made more sophisticated for the Sellafield MOX Plant by the addition of one homogenizer and one more attritor mill.68 This expansion allowed the size of the powder lot to be increased from 50 kg MOX to 150 kg MOX with additional benefits such as reducing the number of quality control points and operating with a larger quantity of fuel with uniform plutonium isotopic composition.6 After conditioning in the spheroidizer, the powder is pelletized into green pellets using a hydraulic multipunch press, and then green pellets are sintered at temperatures of up to 1750 °C under an atmosphere of Ar + 4% H2 mixture gas without heat treatment in a dewaxing furnace.67 An automatic pellet inspection system is adopted for monitoring each pellet diameter, pellet surface, and end surfaces after centerless grinding.51 The MOX pellets pro­duced by the SBR process have a mean grain size of about 7.4 pm with a standard deviation of 0.6 pm, and mean pore diameter is about 5 pm.68