Jerry L. Stakebake

Characterization of the plutonium-water reaction II: Formation of a binary oxide containing Pu(VI)

Abstract Characterization of products formed by reacting unalloyed plutonium with water vapor at 15 Torr pressure shows that Pu 2 O 3 , PuO 2 and a previously unreported binary oxide containing Pu(VI) form on the metal between 200 and 350 °C. Although the reaction produces a uniformly adherent product layer with a thickness of about 5 μm, localized pits of plutonium hydride cover 15–20% of the oxide-metal interface beneath the layer. Analytical results show that the non-equilibrium product contains layers of three oxide phases at all temperatures. Cubic α-Pu 2 O 3 appearing at the metal interface is covered by a layer of dioxide. A third oxide formed at the gas-solid interface is identified by X-ray diffraction and X-ray photoelectron spectroscopy as the mixed-valence phase Pu(IV) 3− x Pu(VI) x O 6+ x . Diffraction data suggest that this oxide crystallizes in a fluorite-related NpH 2+δ or Sm 3 H 7 (Ca 2 YF 7 )-type structure. Formation of the higher oxide is supported by X-ray photoelectron spectra, indicating the presence of the Pu(VI) oxidation state. Evaluation of the results yields an oxide composition corresponding to a value of x near 0.5. Estimated thermodynamic data suggests that the oxide is a stable phase in the Pu-O system; experimental observations show that the product is preserved during exposure to air and ultrahigh vacuum. Although formation of PuO 2.2 is expected to be a slow process at low temperatures, reaction of PuO 2 with moisture adsorbed on its surface may generate large quantities of hydrogen if dioxide is stored for an extended period.

A mechanism for plutonium pyrophoricity

A proposed mechanism for plutonium pyrophoricity quantitatively predicts the ignition temperature of plutonium as a function of surface : mass ratio and particle size. Plutonium must exceed 475°C before self-ignition occurs. External heating of massive samples is necessary to achieve this condition, while finely divided materials can reach the ignition point by an alternative, two-step mechanism. First, the thin layer of surface PuO2 on the metal undergoes kinetically controlled reduction to Pu2O3 near 150°C. Second, the trivalent Pu2O3 reacts with gas-phase oxygen to reform PuO2. Heat generated from the second reaction is sufficient to raise the temperature of small particles or thin foils above the 475°C ignition point. Details of this mechanism are given, including a discussion of plutonium oxidation and a calculation of adiabatic temperature increase due to oxidation of the Pu2O3 surface layer. Plutonium pyrophoricity data are summarized and compared to model results.

Atmospheric oxidation of Pu-1wt.%Ga in the temperature range 150–500 °C

Abstract Oxidation of the plutonium alloy Pu-1wt.%Ga in 500 Torr dry air has been investigated over the temperature range 150–500 °C. Oxidation was found to occur in three stages. Stage I consists of the diffusion controlled growth of a dense oxide layer. Stage II is a linear reaction attributed to diffusion through an oxide of apparently constant thickness. This stage also may involve structural changes in the oxide but this could not be verified. The final Stage III is also linear and was created by the cracking and spalling of the oxide layer. Kinetics for the various processes are determined along with the activation energies involved.

Characterization of natural chabazite and 5a synthetic zeolites: Part II: Adsorption properties and porosity

Abstract Nitrogen adsorption isotherms were measured for four samples of chabazite, collected from different geographical locations, and for a Linde 5A synthetic zeolite. All isotherms were Type I and obeyed the Langmuir adsorption model. Surface areas and pore diameters were calculated from the adsorption data. The MP method was used to determine the pore volume distribution for each sample. Results indicated that the chabazite samples had nearly identical properties, and these properties were very similar to those of the Linde 5A synthetic zeolite.

Handling, Storage, and Disposition of Plutonium and Uranium

The need to address topics of handling, storage, and disposal of plutonium and uranium is driven by concern about hazards posed by the element and by the worldwide quantity of civilian and military materials. The projected inventory of separated civilian plutonium for use in fabricating mixed-oxide (MOX) reactor fuel during initial decades of this century is constant at about 120 metric tons and a comparable amount of excess military plutonium is anticipated from reductions in nuclear weapon stockpiles (IAEA Report, 1998). Although inventories of civilian material are in oxide form, Pu from weapons programs exists primarily as metal. Plutonium is a radiological toxin (Voelz, 2000); its management in a safe and secure manner is essential for protecting workers, the public, and the environment.

The kinetics and oxygen pressure dependence of the high temperature oxidation of Pu-1wt.%Ga

Abstract Oxidation of Pu-1wtAGa was measured between 150 and 500 °C in oxygen pressures of 0.004 – 500 Torr. Three stages of oxidation were identified beyond the initial oxide nucleation. The effect of temperature on oxidation rates was determined at an oxygen pressure of 500 Torr. A discontinuity was observed between 300 and 370 °C that resulted in a change in the activation energy for the Stage II and III processes. Oxygen pressure effects were measured at 200, 300 and 400 °C. Both the parabolic rates for Stage I and the linear rates for Stage II were independent of pressure below 60 Torr and directly proportional to pressure above 60 Torr. Stage III was an interface reaction created by cracking and spalling of the oxide. This reaction was controlled by oxygen adsorption and was directly proportional to P 1 4 or P 1 depending on the temperature at pressures above 37 Torr.

Characterization of natural chabazite and 5A synthetic zeolites

Abstract Samples of natural chabazite were obtained from four geographical locations and separated in bulk from a basalt matrix. Thermal properties and dehydration characteristics were investigated for each sample and compared with the results obtained from a Type 5A synthetic zeolite. Both DTA and DTG spectra for the chabazite and the 5A zeolite compare with those reported in the literature. The presence of calcium carbonate as an impurity in one chabazite sample was detected by DTA and DTG analysis and identified by X-ray diffraction, mass spectroscopy, and temperature-programmed desportion (TPD). TPD was used to characterize the physical adsorption of water on both chabazite and 5A zeolites. One bonding mechanism was indicated for chabazite with an activation energy, E d , for desorption of 9.6 kcal/mole. Two bonding mechanisms for physical adsorption were detected for the 5A zeolite; one with an E d of 13.7 kcal/mole and the second with an E d of 15.0 kcal/mole.

Reaction kinetics for the high temperature oxidation of Pu-1wt.%Ga in water vapor

Abstract Oxidation of Pu-1wt.%Ga by water vapor was investigated between 100 and 540 °C at pressures ranging from 0.1 to 15 Torr. Three kinetic stages were identified for the reaction. Stages I and II were controlled by a diffusion process with an activation energy of 15 kcal mol −1 . The pressure dependence for the parabolic stage I process changed from P 0 to P 1 6 at pressures between 1.5 and 15 Torr. Stage II was linear and had a pressure dependence of P 1 4 . The type of pressure behavior for both stages suggests that the diffusing species is an oxygen ion instead of a hydroxyl ion. Stage III was an interface reaction with a linear rate proportional to P 1 and an activation energy of 2.7 kcal mol −1 . This stage is believed to be adsorption controlled.

Characterization of the thermal, surface, and oxygen adsorption properties of type 4A and 5A synthetic zeolites

Abstract Thermal, surface, and adsorption properties of a Davison 4A zeolite were investigated using DTA, TGA, and oxygen adsorption at −196°C. Surface properties of a Linde 5A zeolite were also evaluated using oxygen adsorption and compared with similar properties determined by nitrogen adsorption. Temperature-programmed desorption analyses were performed using the TGA data and showed the activation energy for water desorption to be 7.8 kcal/mole. Adsorbed water was found to inhibit oxygen adsorption, but this inhibiting influence was reduced by increasing the outgassing temperature and eliminated by outgassing at 200°C or higher. Pore volumes and diameters were determined using the MP method. The total pore volume was 0.225 cm3/g for the 4A and 0.261 cm3/g for the 5A. Pore diameters were calculated to be 12 and 12.8 A for the 4A and 5A, respectively. These values agreed well with those calculated for the alpha cavity using other methods.

The physical adsorption of 1,1,2-trichloro-1,2,2-trifluoroethane on uranium dioxide

Abstract The interaction between 1,1,2-trichloro-1,2,2-trifluoroethane (Freon TF) and uranium dioxide was investigated from a series of gravimetric adsorption measurements in the temperature range −15 to 28°C. All of the isotherms were Type II and completely reversible obeying the BET adsorption equation. Adsorption data also could be fit to the Freundlich equation which suggested a heterogeneous surface. The isosteric heat of adsorption varied from 8.6 to 5.8 kcal/mole as coverage increased and reached a minimum at a coverage of 1.5 molecular layers. The cross-sectional area of the adsorbed molecule was calculated from BET data to be 40A 2 . Adsorption of Freon TF was compared to the adsorption of other chlorinated solvents using the potential theory of adsorption.