NMT Studies Fuel Fabrication Methods to Advance Efforts in Plutonium Disposition

The disposition of plutonium is an area of significant importance to the national security of the United States. In 1994 the Committee for International Security and Arms Control (CISAC), a standing committee of the National Academy of Sciences, conducted a study on the management and disposition of excess weapons plutonium. The committee concluded that the use of plutonium as fuel in existing or modified reactors with no subsequent reprocessing of the spent fuel is the leading contender for the long-term disposition of weapons plutonium. In addition to the inventory of excess weapons plutonium, the growing inventory of unseparated plutonium in spent nuclear fuel is a concern for maintaining peace and security on a global basis. Using chemical separation methods, the plutonium in spent fuel can be recovered, as is done routinely in France and the United Kingdom, and subsequently used in nuclear reactors. Nuclear Materials Technology (NMT) Division personnel are studying two different processes for the eventual fabrication of plutonium fuel sources to power nuclear reactors and reduce the nation's inventory of plutonium at the same time.

Approximately two years ago, the Department of Energy (DOE) formed the Office of Fissile Material Disposition (FMD), whose charter is to develop plans and technologies for the disposition of excess fissile material from the U.S. nuclear weapons program. The FMD is considering the option of converting weapons plutonium into mixed uranium-plutonium oxide (MOX) fuel for use in domestic or Canadian water reactors. Recently NMT Division did pioneer work on that option when they dissembled a pit from a nuclear weapon, separated the plutonium by the hydride-dehydride process, oxidized the plutonium, blended the PuO₂ with UO₂, and pressed and sintered a MOX fuel pellet.

Figure 1: On the left, plutonium dioxide produced from a dismantled weapon by the hydride/oxidation process. On the right, first MOX fuel pellet produced using the plutonium.

Another fuel under investigation is "nonfertile" fuel, which does not produce more fissile material than is consumed when it is burned in a reactor. Nonfertile fuel has the potential for allowing new or existing water reactors to become net consumers of plutonium instead of net breeders. The balance of this article discusses NMT's study of nonfertile fuel.

At the request of the CISAC, Idaho National Engineering Laboratory (INEL) personnel investigated the feasibility of using a nonfertile fuel form for near-total destruction of weapons plutonium in existing or advanced light-water reactors. Neutronic performance results show the nonfertile fuel containing weapons plutonium to be a potential fuel for use in a pressurized-water reactor. INEL evaluated the neutronic performance of a PuO₂ -ZrO₂ -CaO-Er₂ O₃ fuel form suitable for use in a commercial boiling-water reactor. Plutonium oxide derived from weapons plutonium, calcia-stabilized zirconium oxide, and erbium oxide serve as the fuel, fuel diluent, and depletable neutron absorber, respectively. The results show this fuel form to be suitable for potential use in such a reactor.

Los Alamos Studies
The Los Alamos study of nonfertile fuel fabrication is supported with Laboratory Directed Research and Development Office funds. One goal of the study is to develop fuel fabrication methods that would allow weapons plutonium to be used as fuel in water reactors. Specifically, we have chosen the PuO₂ -ZrO₂ -CaO-Er₂ O₃ evaluated by INEL as the fuel composition for our initial fuel fabrication study.

The first phase was the fabrication of a surrogate CeO₂ -ZrO₂ -CaO fuel. The purpose of the surrogate study was to 1) evaluate the feasibility of preparing the fuel by the solid-state reaction method using reagent-grade calcia (CaO), zirconia (ZrO₂), and ceria (CeO₂) as oxide precursors, 2) develop a powder comminution (pulverizing) method acceptable to glove box operations, 3) evaluate the behavior of PuO₂ in the fuel diluent using CeO₂ as the actinide surrogate, and 4) determine the specifications for a sintering furnace design and operation. The surrogate fuel enabled us to use a nonradioactive environment to study the effect of ball milling, green pellet formation, and sintering conditions on the micro-structural development of a pellet of nonfertile fuel.

The equivalent spherical diameter of the precursor powders was determined using laser diffraction analysis. The particle size and morphology of the precursor powders was characterized using scanning electron microscopy (SEM). Sintered pellets were ground in an agate mortar and subsequently analyzed for crystalline phase content using x-ray diffractometry (XRD). Pellets were formed as follows: Reagent-grade ZrO₂ (87.19 wt%), CaO (10.12 wt%), CeO₂ (2.69 wt%), stearic acid (1 wt%), and polyethylene glycol (1 wt%) were dry ball milled for 24 hours. As shown in Figure 2, large (greater than 500 µm) agglomerates were formed as a result of ball milling the ZrO₂, CaO, and CeO₂ precursor powders for 24 hours. The scanning electron micrographs show a broad particle-size distribution. Submicron particles are visible on the surface of the agglomerates. The equivalent spherical diameter of the ball milled powder was determined to be 87.3 µm. The milled powder was uniaxially pressed into pellets at 310 MPa. The green pellets were sintered for 5 hours at 1200 °C, 1400 °C, and 1700 °C in an atmosphere consisting of 80% N₂ and 20% O₂. The bulk density and volume percent of open porosity were determined using the immersion density technique. Grain and pore structure including average size and size distribution were determined using optical microscopy and SEM analysis. As shown in Figure 3, significant increases in the bulk density of the surrogate fuel pellet occurred between the sintering temperatures of 1400 °C and 1700 °C. The XRD data indicate that a sintering temperature of between 1400 °C and 1700 °C is required to form a solid solution of the precursor CeO₂ in calcia-stabilized zirconia.

Figure 2: Scanning electron micrographs of ball milled powder: particle agglomerates and surface of agglomerate.

Figure 3: Optical micrographs of surrogate fuel pellets sintered at 1400 °C and 1700°C.

Summary and Conclusions
Dry ball milling of the precursor powders did not produce a highly reactive powder for pellet fabrication (i.e., pressing and sintering). Future work will examine the feasibility of using vibratory and/or attrition milling methods to produce reactive precursor powders for the solid-state reaction synthesis. A significant increase in the bulk density of the surrogate fuel pellet occurred between the sintering temperatures of 1200 °C and 1400 °C. A significant decrease in open porosity (vol%) of the surrogate fuel pellet occurred between the sintering temperatures of 1400 °C and 1700 °C. Formation of the calcia-stabilized zirconia occurred as a result of sintering the fuel pellets at 1200 °C, complete solid solution formation between the surrogate (CeO₂) and the stabilized zirconia occurred as a result of sintering the fuel pellets at 1700 °C, and significant grain growth occurred as a result of increasing the sintering temperature from 1400 °C to 1700 °C.

The principal developers of this project are Kevin B. Ramsey and H. Thomas Blair of NMT-9, Actinide Ceramics and Fabrication.

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