In order to improve the economical aspects of MOX pellet fabrication and to extend the fabrication pro­cess to MOX pellets containing MAs, various R&D programs have been started especially in France, Germany, Japan, and Russia.

In France, several coconversion processes have been developed and combined with the development of reprocessing processes. One typical coconversion process, called the CO-EXtraction (COEX) process,

Figure 23 Flow sheet for the short process.

has been developed at the ATALANTE (Atelier Alpha et Laboratoires d’Analyses des Transuraniens et d’Etudes de Retraitement)61,69 In this, a mixture of uranyl and plutonium nitrate solutions containing MAs is coconverted to MOX powder following the oxalate precipitation method. According to the results of COEX pellet fabrication tests in the MELOX test chain, MOX pellets produced by the COEX pro­cess have mean grain size larger than 6 pm. These are compatible with current MOX manufacturing values obtained in the MELOX.61 In parallel with the above development, fuel fabrication processes have also been developed in the ATALANTE and LEFCA (Laboratoire d’Etudes et de Fabrications Experimen — tales de Combustibles Nucleaires Avances).

In Germany, basic R&D concerning fabrication processes for MOX fuel bearing MAs have been carried out at the Institute for Transuranium Ele­ments (ITU).70 One of the fuel irradiation test pro­grams carried out by ITU was the SUPERFACT experiment. In this experiment, SUPERFACT fuels bearing Np or Am were fabricated by the sol-gel method and they were irradiated in various fast

70,71

reactors.

In Japan, a simplified MOX pellet fabrication pro­cess, the short process, has been developed on the
basis of the MH method, for the above purposes. The flow sheet for this process is shown in Figure 23. A 300 g scale laboratory test of the short process has been successfully completed.72

In the short process, three different solutions, ura — nyl nitrate, plutonium nitrate, and a nitrate solution in which rejected MOX pellets are dissolved, are mixed to obtain the desired plutonium content in the final mixed solution. Then, the mixed solution is converted to the MH-MOX powder with desired plutonium content by the MH method. This con­verted MH-MOX powder is tumbling-granulated after adding an adequate amount of water as a binder to improve its flowability. The tumbling-granulated MH-MOX powder is calcined at 750 °C in air and reduced to MH-MOX powder at 750 °C under an atmosphere of N2 + 5% Ar mixed gas. The MH-MOX powder so obtained is directly pressed into green annular pellets using a die-wall lubrication method. These are then sintered without heat treat­ment in the dewaxing furnace because the amount of organic compounds contained in the green pellets is controlled at a lower value than that in pellets prepared by the conventional MOX fuel fabrication process. Sintered MOX pellets are ground by a cen­terless grinder, and subsequently, the geometrical

density and appearance of each pellet are inspected. The MOX pellets rejected at the inspections are dis­solved in nitric acid and used as part of the final blending solution as shown in Figure 23. Figure 24 shows a photograph of annular MOX pellets prepared by the short process. A ceramograph of a pellet prepared by the short process is shown in Figure 25.

The MOX pellets manufactured by the short pro­cess have a larger mean grain size than those manufac­tured by the other processes such as the SBR, MIMAS, JAEA, and COEX processes. The development of a series of small scale (kg scale) test devices was started in 2007.73 In parallel with this work, JAEA has an irradiation test program for MOX pellets bearing MAs, to understand their irradiation behavior. In this program, MOX pellets bearing the MAs, Am, and Np were prepared by the JAEA process and irradiated in the JOYO. These irradiated pellets were subjected to postirradiation examinations and the results obtained have been reported in Maeda et a/.74,75

In Russia, RIAR (Research Institute of Atomic Reactors) has proposed the demonstration program concept DOVITA (Dry reprocessing, Oxide fuel, Vibropac, Integral, Transmutation of actinides) and many R&D activities related to them have been car­ried out. From this program, vibro-packing technol­ogy has been applied to load MOX granules into a cladding tube.76,77