Bulk and rough tap densities of two granular fuel lots

Bulk density and an estimate of tap density are needed for granular 238PuO2 fuel, in order to estimate how large a fuel charge can fit in an unwelded heat source body without fuel protruding into the weld region. Knowing the range of fuel volumes that can be expected from current manufacturing methods is necessary in order to safely load fuels of varying power densities. If low power density fuel must be used, a large fuel volume may be required. The small quantity of fuel available, the requirement that no fuel be lost, and the demanding glovebox environment preclude use of a commercial tap density apparatus. An estimate of the tap density was made by hand-tapping according to ASTM formulae. The method provides an adequate degree of repeatability and achieves saturation of compaction. For the two fuel lots the average values of bulk density are 4.6 ± 0.2 g/cm3 and 4.16 ± 0.07 g/cm3. Compaction was observed to be small; 19.7% and 6.42% for the two fuel lots. Differences between the behaviors of the two fuel lots are consistent with known variation in the processing histories of the two fuel lots. A priori error in tap density is large, between 8% and 34% depending on the degree of compaction achievable. The measured values for bulk density of this fuel are lower than expected for the least dense possible regular packing for spheres of a single diameter. Spheres having the crystal density of PuO2 would exhibit a minimum powder density of 6.02 g/cm3, much higher than observed. The measured densities suggest that the individual particles making up the oxide are themselves quite porous, with a bulk density of between 7.95 g/cm3 and 8.84 g/cm3 (between 31% porous and 23% porous.) Results are examined using classical Kawakita equation, which describes compaction of particles in terms of two adjustable parameters at and bt that are related to total volume change and particle strength. Relative values of the Kawakita parameters for various fuel lots are useful in justifying numerical bounds on safe operational fuel loadings.