Early in the 1960s, comprehensive R&D programs concerning MOX fuel were started in Japan and they resulted in the JAEA process that was adopted by the Plutonium Fuel Fabrication Facility (PFFF) which started operation in 1972. The PFFF used local control equipment to fabricate MOX fuel for the advanced thermal reactor FUGEN,63 and the experimental fast reactor JOYO on an engineering scale. Following the completion of the Plutonium Fuel Production Facility (PFPF) in 1987, MOX fuel fabrications for JOYO and the prototype FBR MONJU have been conducted in PFPF since 1988. MOX fuel fabrication for FUGEN in PFFF was completed in 2001. Now, this plant is undergoing preparative work for its decommissioning.

Figure 18 shows the flow sheet of the JAEA pro­cess utilized in the PFPF. Two kinds of plutonium, either PuO2 powder prepared by the oxalate precipi­tation or the MH-MOX powder, can be used in the JAEA process to fabricate FBR MOX pellets.

In this process, three feed powders, UO2 prepared by the ADU process, PuO2 or MH-MOX powder,

and dry recycled scrap powder, are prepared to get the plutonium concentration specified by the fuel specifications in the mixed powder. The feed powders are ball milled to get a homogeneous distribution of plutonium in the sintered MOX pellets. This mill pot has a silicon rubber lining on its inner surface to enhance the charging and discharging of powders by automated operation. About 40 kg of powder can be charged in this ball mill. A photograph of the ball mill is shown in Figure 19.

Similar to the milled powder in the SBR process (see Section 39.5.2.5), this powder must be granu­lated to provide a free-flowing property.51,52 After mixing zinc stearate (binder) and Avicel (microcrys­tal cellulose; pore former) with the milled powder, this powder mixture is roughly pressed into tablets at pressures of around 200 MPa and the tablets are then crushed into granules of sizes that make them free-flowing. These granules are pelletized into green pellets at pressures of around 500 MPa followed by the addition of zinc stearate as lubricant. Normally, these green pellets are sintered at about 1700 °C for 4 h under an atmosphere of Ar + 5% H2 mixed gas after dewaxing at about 800 °C for 2 h under the same atmo­sphere as used in the sintering.64 A ceramograph of a transverse section of a sintered MOX pellet prepared by the JAEA process is shown in Figure 20. This MOX pellet was fabricated under specifications for pellets to be loaded in the MONJU outer core.

After centerless grinding, the diameter, geometri­cal density, and appearance of each sintered pellet are inspected. An inspection device to check pellet den­sity and appearance is shown in Figure 21; it is installed in the PFPF. Details of the JAEA process

Figure 20 Ceramograph of a transverse section of a sintered mixed oxide of uranium and plutonium pellet for MONJU fuel prepared by the Japan Atomic Energy Agency process (plutonium content: 30.8 wt%, density: 84.84% theoretical density, mean grain size: 3.9 mm).

and its fuel fabrication technologies have been previ­ously reported in the literature.64,65