Experiments and the development of apparatus relating to the removal of oxide film and the attempts to prepare an oxide-free surface on uranium are described. Polishing the metal in vacua of the order of 10 -7 tort appears to be the most successful method. UI4~ and UO~ were observed to form rapidly on the metal in such vacua. PuO~ was found to form on plutonium in vacua at pressures of less than 1 x 10 -~ tort. The possibility that plutonium hydride also formed could not be investigated, because of the similarity in crystal structure and lattice parameter between PuO~ and PuH~.The oxide film that commonly occurs on uranium when it is exposed to aqueous or gaseous media has been identified as UO~ in electron diffraction studies by Hickman (1), by Hart (2), and by Flint, Polling, and Charlesby (3). The presence of UO~ in films that are too thin to give rise to interference colors can also be established by standard x-ray technique. Although the detection of thin films by x-ray diffraction is not possible for many metals, it is successful for uranium, because of the high scattering power of the uranium atom. Hart (2) and Flint et al. (3) list additional lineswhich do not correspond to the diffraction pattern of uranium dioxide. Hart (2) interprets them as due to a mixture of U20~ and U~O~. In our work, no evidence for these lines was obtained. Wilman (4) suggested that the lines observed by Flint et al. should be attributed to diaspore, which had been embedded in the metal surface during polishing. This explanation, however, is not applicable to Hart's work, because he employed diamond dust as an abrasive.In the investigations (1-3) cited above, as well as in a number of the experiments described herein, the metal was polished in air before being admitted to the specimen chamber of the electron diffraction unit. Some of the experiments described below show that it is difficult to eliminate traces of UO~ from the surface. A completely oxide-free surface on uranium (with which to study the formation of the oxide film without the influence of previously existing oxide) was not achieved; our efforts were directed toward improving the ultimate vacuum in which the polishing is conducted, so that the oxide-free portions of the surface would persist long enough to be studied.
This study is in response to a request by the Reactor Panel Subcommittee of the National Academy of Sciences (NAS) Committee on International Security and Arms Control (CISAC) to evaluate the feasibility of using plutonium fuels (without uranium) for disposal in existing conventional or advanced light water reactor (LWR) designs and in low temperature/pressure LWR designs that might be developed for plutonium disposal. Three plutonium-based fuel forms (oxides, aluminum metallics, and carbides) are evaluated for neutronic performance, fabrication technology, and material and compatibility issues. For the carbides, only the fabrication technologies are addressed. Viable plutonium oxide fuels for conventional or advanced LWRs include plutonium-zirconium-calcium oxide (PuO2-ZrO2-CaO) with the addition of thorium oxide (ThO2) or a burnable poison such as erbium oxide (Er203) or europium oxide (Eu203) to achieve acceptable neutronic performance. Thorium will breed fissile uranium that may be unacceptable from a proliferation standpoint. Fabrication of uranium and mixed uranium-plutonium oxide fuels is well established; however, fabrication of plutonium-based oxide fuels will require further development and performance qualification. Viable aluminum-plutonium metallic fuels for a low temperature/pressure LWR include plutonium aluminide in an aluminum matrix (PuAI4-AI) with the addition of a burnable poison such as erbium (Er) or europium (Eu). Fabrication of low-enriched plutonium in aluminum-plutonium metallic fuel rods was initially established 30 years ago and will require development to recapture and adapt the technology to meet current environmental and safety regulations. Fabrication of high-enriched uranium plate fuel by the picture-frame process is a well established process, but the use of plutonium would require the process to be upgraded in the United States to conform with current regulations and minimize the waste streams. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process,or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
PoFtions of this document mag be illegible in electronic image produck h a g m are produced f h m the best a-le AbstractAs a result of dismantling the bomb, there is about 100 MT of excess weapons grade plutonium in the United States and about 150 MT in the Commonwealth of Independent States. In addition, there is another 1000 MT of plutonium in commercial spent fuel that may be used as degraded weapons material. One means to disposition weapons grade plutonium is by irradiating the fuel in light water reactors (LWRs) using a non-fertile fuel based on plutonia dispersed in an oxide mixture of zirconia stabilized with calcia or yttria as a solid solution. Plutonium dispersed in a zirconia matrix offers the potential to achieve very high burnups while maintaining mechanical integrity.A potential improvement is based on replacing the calcia With yttria and reducing the yttria content to form a partially stabilized zirconia that may have a higher resistance to crackmg. A rare-earth element is also added as a burnable poison to enhance the lifetime of the fuel and the neutronic characteristics. Yttria lowers the tetragonal to monoclinic transformation temperature to 275 "C compared with 850 "C for calcia. In addition, yttria does not significantly reduce the melting point of the solid soluton from that of zirconia near 2700 "C. However, if the mechanical properties of partially stabilized zirconia with yttria proves favorable, the yttria content may be reduced resulting in higher thermal conductivity of the ternary fuel.The use of urania-zirconia-calcia fuels with high uranium concentration in three reactors during the 1970s and 1980s established a basis for the development of a plutonia-zirconia-calcia fuel and a urania-zirconia-calcia fuel with lower concentrations of uranium than that previously used. The existing data on phase equilibria of the plutonia-zirconia-calcia or yttria and the urania-zirconiacalcia or -yttria systems were obtained and evaluated for feasible fissile concentrations and concentrations of the matrix materials. Based on the urania ternary fuels and neutronic calculations, the composition of the ternary fuel containing plutonia is 8.3 wt% plutonia, 80.4 wt% zirconia, 9.7 wt% calcia, and 1.6% erbia. For yttria replacing calcia, the same fissile and erbia contents are retained, and the yttria content varies from 9.9 wt% to 30.4 wt% with the zirconia varying from 80.2 to 59.7 wt%.Since the melting point of plutonia (about 2350 "C) is lower than that of 2 0 2 , the addition of zirconia increases the melting point of the fuel mixture about 125 "C. The ternary phase diagram for either the urania or plutonia based fuels using yttria in place of calcia could not be derived since the urania-yttria and the plutonia-yttria phase diagrams are needed and are not available. In addition, the plutonia-zirconia-calcia phase diagram could not be derived because the plutoniacalcia phase diagram is not available. Experimental determinations would have to be performed to obtain these phase diagrams. Although the ...
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