2.15.2.1.2.1 Flowability

In pellet fabrication, powder flowability is one of the most important characteristics that determine the productivity of the fabrication process. It is well known that blended powders have very poor powder flowability, just after milling.5 Therefore, the milled powder is granulated or mixed with a powder having good flowability to ensure uniform die filling and good compaction behavior.5-7 Carr indices are a well-known method to evaluate powder flowability of dry solids.8,9 The powder flowabilities of micro­wave heating denitrated MOX (MH-MOX) powder and ammonium diuranate (ADU) powder have been evaluated on the basis of Carr indices both before and after granulation.1 ,

2.15.2.1.2.2 Effective thermal conductivity

The temperature of MOX powder increases by self heat generation of plutonium by a-decay when the powder is kept in the fuel fabrication process. In a MOX fuel fabrication plant, the temperature increase in MOX powder should be prevented because the excessive temperature increase of MOX powder may possibly cause changes in powder characteristics (e. g., O/M ratio variation), degradation of additives (e. g., lubricant agents), and overheating of equipment in the fabri­cation process. An example of a preventive measure against the temperature increase of MOX powder is the use of a storage vessel that has radiator plates.

The effective thermal conductivity ofMOX powder is important for estimating its temperature distribution. The effective thermal conductivity of a powder can be defined as the combination of thermal conductivities of powder particles and the atmospheric gas because the volume fraction of the atmosphere gas in the total volume is large. In addition, particle shapes, mean particle size, specific surface area, and O/M ratio of powder particles influence the effective thermal con­ductivity of the powder.12 Figure 1 shows the effective thermal conductivities of various MOX

under a controlled atmosphere to improve their mechanical strength. The powder compact is com­posed of individual grains separated by 35—50 vol.% porosity. During sintering, the following major changes commonly occur: an increase in grain size, and changes in pore shape, pore size, and pore num­ber. In the early stages of sintering, the powder par­ticles begin to mutually bond. In the middle stage, grain growth, disappearance of pores, and formation of closed pores occur. The pellet densification pro­ceeds according to the shape change from a point contact to a face contact between grains. In the last stage, disappearance of the closed pores occurs. The diffusion of uranium, plutonium, and oxygen, the evaporation-condensation process of their com­pounds, the grain growth process, the pore migration process, and the pore disappearance processes are important for understanding the process of sintering. To obtain pellets with high mechanical strength and density, it is desirable to eliminate as much porosity as possible.

Diffusion coefficients ofthese elements are needed for evaluating the sintering behavior (e. g., volume shrinkage in the fuel fabrication technology). Section 9.1.6.1 shows that the oxygen self diffusion coeffi­cients of actinide oxides increase with increasing deviation from stoichiometry near the stoichiometric region and that the diffusion coefficients of cations in hyperstoichiometric actinide oxides increase dras­tically with deviation from stoichiometry. It was shown that the diffusion coefficient of plutonium in (U0.8 Pu0.2)O2±x has the lowest value near the stoi­chiometric region and it increases significantly with an increase in deviation from stoichiometry (see Figure 2).

Vapor species of oxide fuel and its vapor pressure are required to assess the redistribution of elements, pore migration, and fuel restructuring. The O/M ratio dependencies of vapor pressures in the vapor species of uranium oxide, plutonium oxide, and MOX are shown in Figures 26 and 27 of Section 9.1.5. The vapor pressures of each of these species have a large dependency on the O/M ratio and their behavior is different in each vapor species.

Temperatures used during dewaxing and sintering are very important factors in the fabrication process. The Huttig and Tamman temperatures, which are defined as the start temperatures for surface diffusion and volume diffusion of powder particles, respec­tively, are provided for establishing temperatures for dewaxing and sintering. These temperatures can be easily calculated using melting point temperature.