PROPERTIES AND BEHAVIOR OF 238PU RELEVANT TO DECONTAMINATION OF BUILDING 235-F

This report was prepared to document the physical, chemical and radiological properties of plutonium oxide materials that were processed in the Plutonium Fuel Form Facility (PuFF) in building 235-F at the Savannah River Plant (now known as the Savannah River Site) in the late 1970s and early 1980s. An understanding of these properties is needed to support current project planning for the safe and effective decontamination and deactivation (D&D) of PuFF. The PuFF mission was production of heat sources to power Radioisotope Thermoelectric Generators (RTGs) used in space craft. The specification for the PuO{sub 2} used to fabricate the heat sources required that the isotopic content of the plutonium be 83 {+-} 1% Pu-238 due to its high decay heat of 0.57 W/g. The high specific activity of Pu-238 (17.1 Ci/g) due to alpha decay makes this material very difficult to manage. The production process produced micron-sized particles which proved difficult to contain during operations, creating personnel contamination concerns and resulting in the expenditure of significant resources to decontaminate spaces after loss of material containment. This report examines high {sup 238}Pu-content material properties relevant to the D&D of PuFF. These relevant properties are those that contribute to the mobility of the material. Physical properties which produce or maintain small particle size work to increase particle mobility. Early workers with {sup 238}PuO{sub 2} felt that, unlike most small particles, Pu-238 oxide particles would not naturally agglomerate to form larger, less mobile particles. It was thought that the heat generated by the particles would prevent water molecules from binding to the particle surface. Particles covered with bound water tend to agglomerate more easily. However, it is now understood that the self-heating effect is not sufficient to prevent adsorption of water on particle surfaces and thus would not prevent agglomeration of particles. Operational experience at PuFF indicates that the Pu-238 contamination was observed to move along surfaces and through High Efficiency Particulate Air (HEPA) filters over time. Recent research into the phenomenon known as alpha recoil offers a potential explanation for this observed behavior. Momentum is conserved when an alpha particle is ejected from a Pu-238 atom due to radioactive decay. Consequently, the entire particle of which that Pu-238 atom is a constituent experiences a movement similar to the recoil of a gun when a bullet is ejected. Furthermore, the particle often fractures in response to Pu-238 atom disintegration (yielding an alpha particle), with a small particle fragment also being ejected in order to conserve momentum. This process results in the continuous size reduction and transport of particles containing Pu-238 atoms, thus explaining movement of contamination along surfaces and through HEPA filters. A better understanding of the thermal behavior of {sup 238}PuO{sub 2} particles is needed to inform the planning process for the PuFF D&D project at the 235-F facility. There has been a concern that the surface temperature of individual particles may be high enough to cause problems with decontamination equipment and materials as a result of heat generation due to radioactive decay. A calculation under conservative assumptions shows that the surface temperature of particles less than about 100 {micro}m diameter is not appreciably above ambient. Since most particles in PuFF are on order of 1 {micro}m in diameter, the effect of particle surface temperature on decontamination equipment and materials is expected to be minimal. The result of this calculation also indicates that thermal imaging, which has been under consideration as a method to monitor the progress of system decontamination efforts would not likely be effective. The use of strippable coating was suggested as a possible alternative to other decontamination techniques. One particular system (i.e., Decon Gel 1101) may offer significant advantages over conventional liquid decontamination solutions. Previous trials are discussed and have been used successfully at SRS on various surfaces. Some areas showed minimal improvement, while others presented complete (100%) decontamination. However, small scale testing is recommended prior to the full scale use in the cells of PuFF in building 235-F.