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Researchers Address the Interrelationship of Hydrolysis and Pressure in Stored Pu

The U.S. Department of Energy/Environmental Management (DOE/EM) is responsible for the management and long-term disposition of a variety of materials located at Rocky Flats Environmental Technology Site, Hanford, Savannah River, and other DOE sites. The new plutonium storage standard, set to replace DOE 3013, requires thermal stabilization of the materials before they are packaged for storage. The Pu content of those materials can vary from ~86 weight percent, (essentially pure PuO2), down to ~30 weight percent. With such a range of compositions, plutonium dioxide can be in contact with a variety of other materials. Typically, these "impurites" include alkali metal chlorides, MgO, MgCl2, CaCl2, Fe2O3, and other materials that are not well characterized. In addition, these solid mixtures are in contact with the gas phase under which the materials were packaged; the moisture content may not be known or well controlled. The new plutonium stabilization standard does not set acceptable glove box moisture levels nor does it prescribe the time duration between calcination and packaging.

The 3013 standard contains an equation that predicts the total pressure buildup in the can over the anticipated storage time of fifty years. This equation was meant to model a worst-case scenario to insure pressures would not exceed the strength of the container at the end of 50 years. As a result, concerns about pressure generation in the storage cans, both absolute values and rates, have been raised with regard to rupture and dispersal of nuclear materials. Similar issues have been raised about the transportation of these materials around the complex.

Figure 1. To provide a stronger technical basis for the new DOE plutonium stabilization standard for storage, NMT-16 researchers measured recombination rates of hydrogen/oxygen mixtures in contact with pure and impure plutonium oxides.

The technical basis for the pressure equation given in the 3013 standard has not been fully established. The pressure equation contains two major assumptions, (1) that hydrogen and oxygen generated from radiolysis of organic materials do not react to form water and (2) that the oxygen generated by radiolysis reacts with the oxide material and does not contribute to the pressure in the container. With regard to the first assumption, if the formation of water from hydrogen and oxygen is significant, then the calculated pressures would be dramatically reduced. The formation of water is thermodynamically favored (more stable) over the splitting of water into hydrogen and oxygen. In addition, the corporate knowledge from shelf-life programs around the complex is that most containers do not show signs of extreme pressurization.

In order to provide a stronger technical basis for the standard, we measured the recombination rates of hydrogen/oxygen mixtures in contact with pure and impure plutonium oxides. The goal of these experiments was to determine whether the rate of recombination is faster than the rate of water radiolysis under controlled conditions. We used a calibrated pressure-volume-temperature apparatus to measure the recombination rates, in a fixed volume, as a gas mixture was brought into contact with oxide powders whose temperatures ranged from 50°C to 300°C.

These conditions were selected in order to bracket the temperature conditions expected in a typical storage can. The gas mixture used in these studies was composed of 2% hydrogen, 19% oxygen, and 79% nitrogen. This 2% H2/air mixture encompasses scenarios in which actual storage cans were sealed in air, and over time various amounts of hydrogen are formed. This gas composition is below the explosion limit.

Figure 2. Simulating conditions of stored plutonium, researchers obtained pressure-time curves and mass spectrometric results for pure and impure plutonium oxides over the 50°C­250°C temperature range in a 2% H2/air mixture.

The recombination of hydrogen and oxygen has been studied over the 50°C­250°C temperature range in a 2% H2/air mixture. We obtained pressure-time curves and mass spectrometric results for pure and impure plutonium oxides exposed to the gas mixture. The pure oxide was obtained from oxidation of alpha metal. The impure oxide was obtained through the Defense Nuclear Facilities Safety Board 94-1 R&D program's Materials Identification and Surveillance project and selected for its low plutonium content, 29 weight percent, and its high chloride content. Analysis by x-ray powder diffraction shows that the impure oxide is a mixture of plutonium dioxide, sodium chloride and potassium chloride. (These materials contain galium in amounts undetectable by x-ray diffraction.)

The measured kinetic data, which consist of monitoring the total system pressure as a function of time, were collected for the pure and impure oxide samples. These kinetic data show that the rate of the pressure drop at 100°C, 200°C, and 300°C for the pure oxide are equal based on the initial slopes of the pressure-time curves. Only the data collected at 50°C show deviation from this behavior. The data for the 100°C run for the pure oxide clearly show a sharp break in the slope approximately 100 minutes into the experiment. The pressure-time curves for the impure oxide experiments yield rates that are lower than those for the pure oxide counterparts.

Surface effects were investigated. The pure oxide, without any pretreatment, was allowed to react with the starting gas mixture. When that phase of the experiment was complete, that same sample was then baked at 350°C under dynamic vacuum for 24 hours and then allowed to react with the starting gas mixture again. Treating the sample by heating under vacuum increased the initial slope of the pressure-time curve by approximately a factor of four. Long-term experiments were also conducted on the pure oxide. After an initial, rapid pressure drop, the total pressure over the pure oxide sample remains constant out to 20 days.

In each experiment, the total pressure decreased rapidly after the reactants were combined. The mass spectrometric results show an overall depletion of stoichiometric amounts of hydrogen and oxygen. The mass balances for the mass spectrometric results are in excellent agreement with the assertion that hydrogen and oxygen recombine quickly to form water. The kinetic results indicate that the surface of the plutonium oxide powder plays a role in the recombination of hydrogen and oxygen. The data collected at 200°C before and after the thermal/vacuum treatment show a marked difference in the pressure-time curves and therefore in the rate of recombination. This difference is a clear indication of surface effects. In addition, the abrupt change in the slope of the pressure-time curve and therefore the rates of recombination at 100°C suggest that the surface of the oxide plays a role in the recombination of hydrogen and oxygen. For those data the slope, and hence the rate, of the pressure-time curve changes abruptly within 100 minutes. In contrast, the data collected at 200°C and 300°C show a smooth decrease in the pressure with time out to approximately 800 minutes when essentially all the hydrogen had been depleted.

Above 100°C, the reaction is second-order overall. At and below 100°C, the process may be third- or fourth-order. It is important to note that the change in the reaction order across the 100°C temperature boundary is significant. The results of the long-term experiments indicate that steady-state gas compositions are reached, suggesting that the rates of recombination and water radiolysis become equal under these experimental conditions.

These data show that when this gas mixture is exposed to pure plutonium oxide heated above 100°C, the recombination rate is dramatically faster than the radiolysis of water initially. Above 100°C enough thermal energy is provided to remove water or hydroxide and to maintain a larger fraction of the active sites available for catalysis. Below 100°C, the recombination rate seems to be governed by the number of available sites and the rate at which they are filled. These observations are more consistent with a chemical reaction of H2 and O2 at catalytic sites on the plutonium oxide, as opposed to radiolytic formation of radicals in the gas phase. One would expect the latter process to be temperature-independent and surface-insensitive. Presently, we do not know to what extent the alpha radiation may initiate or enhance the surface reaction. To resolve this issue, further work is needed in which the isotopic composition of the oxide and hence the flux of alpha particles is changed (239Pu vs. 242Pu).

The results of the long-term experiments suggest that an equilibrium gas composition is reached, indicating that the rates of recombination and water radiolysis become equal. Based on these preliminary results, extreme pressurization of sealed containers as a result of the radiolysis of water adsorbed on the oxide is not likely. Surface-catalyzed hydrogen/oxygen recombination is a primary process responsible for pressure reduction and/or hydrogen removal from the system. The kinetic data collected thus far indicate this interplay between surface-catalyzed recombination and water radiolysis. The rate of recombination serves to limit the potential pressure in the container from water radiolysis.

This article was contributed by Luis Morales. NMT-16.


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