EVALUATION OF HYDROGEN GENERATION MODEL IN DOE STANDARD 3013 FOR LONG-TERM STORAGE OF PLUTONIUM-BEARING MATERIALS

Ambient room-temperature equilibrium constants have been determined for the physical and chemical reactions within the PuO2-H2O-H2-O2 system characteristic of stored plutonium oxides, from data for nine Hanford items studied in the materials identification and surveillance (MIS) program at Los Alamos National Laboratory. Our analysis led to the conclusion that under the low temperature and water content conditions for the investigation, the vapor pressure of water, or chemical activity of the adsorbed water in equilibrium with the vapor, would be very low for pure and impure plutonium oxide that had been stabilized for transportation or storage. The pure or impure plutonium oxide would getter any free oxygen in a strong equilibrium to reduce its concentration to minor levels in the storage package, and, together with the very weak equilibrium for the dissociation of water bound to the solid, it would prevent both further oxidation of the oxide material and any significant buildup of hydrogen. These findings imply that for the low temperatures investigated, it would not be necessary to render inert packages of plutonium oxides that had been stabilized for transportation and storage. These are benign results compared with the much higher hydrogen pressures over plutonium oxides that would be determined from the model given in the Appendix of DOE-STD-3013. For various estimated upper-limit temperatures that depend on the power density within the plutonium oxide, the DOE-STD- 3013 model assumes hydrogen pressures derived by extrapolation from results of experiments by Haschke, Allen and Morales at temperatures between 25 and 350°C that did not achieve equilibrium. Therefore, further analysis in the ongoing MIS program could be directed toward determination of the upper-limit temperatures for plutonium oxides with higher power densities and the equilibrium constants associated with those temperatures.