Two MOX pellet fabrication processes were devel­oped in Germany, the Optimized CO-Milling (OCOM) process and the AUPuC process.7,62

The OCOM process was developed by Alkem and uses UO2 powder, PuO2 powder, and recycled scrap powder as feed materials. The manufactured MOX pellets are made fully soluble in nitric acid by opti­mizing the co-milling of the three powders. In the OCOM process, two different MOX pellet fabrica­tion routes can be taken as shown in Figure 16.

In the first route (left half of Figure 16), three powders are prepared to achieve specified plutonium concentrations required for the fuel to be used in

FBRs and LWRs. The powders are co-milled to obtain a homogeneous distribution of plutonium, and the milled powder is pressed into green pellets after granulation. The second route (right half of Figure 16) is used to fabricate MOX pellets for LWRs; it effectively introduces the master blend concept into the process for better economy.7 This means that a mixture containing ^30% plutonium is made from UO2 powder and PuO2 powder, and this mixture is then milled using the OCOM milling process. The MOX powder that results from the milling process is no longer free-flowing. By mixing this master blend with the eight — to tenfold amount of free-flowing UO2 powder to obtain the required plu­tonium content for LWR MOX fuel, a feed powder is obtained with sufficient flowability for direct pelle­tizing. An issue requiring special attention for this route is the homogeneity of the plutonium distribu­tion; two powders of very different physical proper­ties have to be mixed together to obtain the desired plutonium content. One powder is the master blend of PuO2 and UO2, which after milling consists of a powder with very fine nonflowing grains and having a high tendency to self-agglomerate, while the second part is the free-flowing UO2 powder prepared by the AUC process with its rather coarse grains.7 The mix­ing of the two powder components and preventing their segregation during further processing steps

require special attention and expertise. The green pellets prepared by the two routes are sintered in a reducing atmosphere after dewaxing. A typical a-autoradiograph of a transverse section of an LWR pellet manufactured by the OCOM process has been reported by Roepennack et a/.62 The density and appearance of sintered pellets are inspected after centerless grinding.

The AUPuC process (Figure 177) was developed as a coprecipitation process based on the AUC pro­cess. The AUPuC process uses plutonium in the form of a nitrate solution. NH3 and CO2 gases are introduced into a mixed solution ofplutonium nitrate and uranyl nitrate with a concentration of about 400 gl-1 of heavy metal at first, and then tetra — ammonium tricarbonate dioxo urinate/plutonate is precipitated by the following reaction.7

(U, Pu)O2(NOb)2 + 6NH3 + 3CO2 + 3H2O! (NH4)4[(U, Pu)O2(CO3)3] +NH4NO3

The precipitated AUPuC is filtered and directly reduced at ~-750 °C in an atmosphere of hydrogen gas. The obtained MOX powder with about 30% plutonium concentration is utilized as the master blend and is the same as in the OCOM process. The homogeneity of plutonium in the master blend is much better in the AUPuC process than in the

OCOM process because solid solutions have already formed during precipitation in the AUPuC process. This coconverted powder is also diluted like the master blend by the free-flowing UO2 prepared by the AUC process and recycled MOX powder so that the final blended MOX powder has the desired plu­tonium concentration. This final blended MOX pow­der flows easily, just as in the OCOM process, and it is pressed into green pellets by a rotary pressing machine without granulation.43 The steps after pel­letizing are the same as those in the OCOM process. A typical a-autoradiograph of a transverse section of a LWR pellet manufactured by the AUPuC process has also been reported by Krellmann.7

On the basis of the above processes, Siemens constructed the MOX fuel fabrication facility in Hanau as a dual purpose (FBR and LWR) facility and started operation in 1972. After reaching an effective capacity of 20-25 tHM per year of LWR fuel in the 1987-1991 period, it was shut down, as a result of a contamination incident in 1991.6 This plant was subsequently decommissioned. On the same site, Siemens constructed a larger plant with an annual capacity of 120 tHM for LWRs.7 However, this plant was abandoned before starting operation
because Siemens never received an operating license from the local government.