Particle size of plutonium-238 oxide fuel as a function of processing history and age

Fuel pellets prepared from Pu-238 oxide powder provide heat and electrical power for planetary exploration. The integrity of the fuel pellet is key to successful construction of heat sources for use in space missions. Several processing steps are needed to prepare fuel for pressing into pellets, and each step is deliberately designed to produce a particular particle size distribution, as subsequent processing steps often depend on particle size. Particle size distributions have never been monitored throughout the several steps of the production process, as the particle sizes are too large for the laser diffraction method employed until recently. Moreover, particle size distributions are known to change with time, even within 24 hours, occasionally requiring reprocessing before subsequent process steps can proceed. An example of fuel is characterized for several of the fuel processing steps. Our goal is to monitor the particle size for each processing step for a sufficient number of runs to establish the range of sizes typical of feed, and characterize the range of range of sizes routinely produced by each processing step. Pellet cohesiveness has been anecdotally observed to depend on the time interval between fuel preparation and pellet pressing. Increases in particle sizes have previously been measured, indicating agglomeration over the course of one to ten days. Particle sizes have been qualitatively observed to decrease by fracture under auto-radiolysis, on a timescale of months to years. The limited data set presented here is the first step toward improved process control, to optimize process efficiency and reduce waste and the necessity of reprocessing. An improved understanding of fuel evolution will allow adjustment of the fuel preparation process as required to accommodate new sources of Pu-238 fuel, and to allow process modernization.