Hydration of plutonium oxide and process salts, NaCl, KCl, CaCl2, MgCl2: effect of calcination on residual water and rehydration

The Department of Energy has issued a standard (DOE-STD-3013-96) that establishes the criteria for packaging of plutonium metal and stabilized oxides for an anticipated storage period of 50 years. The basis for the standard comes from data collected from relatively pure plutonium metal and oxide. However, much of the material in the Department of Energy complex currently awaiting packaging for long-term storage is not pure metal or oxide but also contains chloride salts, including NaCl, KCl, CaCl2, and MgCl2, which must be taken into consideration when making decisions regarding pre-packaging processing. The DOE-STD-3013-96 standard is currently undergoing revisions to include these considerations, specifically a lowering of the temperature of pre-packaging calcination from 950 to 800°C. Calcining impure plutonium residues at 800°C may or may not be sufficient to reduce the water content of the residue to below 0.5 wt%, a criteria deemed important to prevent the over-pressurization of long-term storage containers. This paper summarizes the literature describing the dehydration and rehydration of PuO2, NaCl, KCl, CaCl2, and MgCl2 and experimental results from impure oxides containing chloride salts, and applies this information to the direct problem of stabilizing impure plutonium oxide residues for long-term storage. Overall, the literature data available is not sufficient to unequivocally guarantee the stabilization processing and packaging conditions necessary to meet the 0.5 wt% water content criteria of DOE-STD-3013-96. It can be concluded that studies and measurements performed on individual components are not an adequate indication of overall hydration behavior of mixtures. In order to minimize the amount of water adsorbed for samples containing chloride salts, we recommend salt residues be calcined at 850°C for at least 2 h, they be sealed in an approved storage container as soon as possible after calcination, and the moisture content of the glovebox atmosphere not exceed 35% relative humidity.