Improving the Scientific Basis for Managing DOE's Excess Nuclear Materials and Spent Nuclear Fuel (2003) / Chapter Skim
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3 Plutonium-239
3 Plutonium-239
Pages 28-42
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From page 28... ...
. About half of this production has been declared as surplus.2 The surplus inventory includes clean metal, mainly from disassembly of weapons; oxide; and plutonium combined with a variety of other materials in reactor fuels, targets, and miscellaneous forms (see Figure 3.1)
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From page 29... ...
Some sites, such as Rocky Flats, are being "deinventoried" of their plutonium, which is being shipped to consolidate the inventory at a limited number of sites, such as Savannah River. A key element in DOE's strategy for eventual disposal of its surplus inventory is the conversion of as much of the excess Pu-239 as is technically and economically feasible into mixed oxide (MOX)
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From page 30... ...
be used to generate electricity at commercial nuclear reactors.4 The spent MOX fuel would be disposed with other DOE and commercial spent fuels, for example, in the planned Yucca Mountain repository. Most of the surplus plutonium is in the form of pits,5 clean metal, and oxides for which conversion to MOX fuel should be straightforward, based on French experience (Johnson and Brabazon, 2002~.
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From page 31... ...
waste in the Waste Isolation Pilot Plant (WIPP) ; and · disposal along with high-level waste and spent fuels, for pie, in the planned Yucca Mountain repository.
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From page 32... ...
MOX Fuel Fabrication For eventual disposition of the inventory, as much of the excess material as possible including al I materials currently in pit form wi 11 be processed and fabricated into MOX fuel for once-through use in commercial nuclear reactors. The DOE has solicited specifications from European nuclear fuel manufacturers who routinely perform MOX recycling.
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From page 33... ...
The variety of materials, expressed as a function of plutonium content versus number of items, is from an inventory of about 900 items containing approximately 0.8 metric tons of plutonium with a total weight of 1.1 metric tons. Of note are the 79 items with plutonium contents less than 50 Figure 3.3 An inventory of impure plutonium oxides stored at Hanford shows that some materials may be too poor in plutonium for weight percent.
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From page 34... ...
For these materials, DOE could revert to its earlier option of co-disposing some of this plutonium with defense high-level waste and spent fuels (NAS,1994~.6 In the earlier option, incorporating the plutonium into a ceramic was proposed to meet waste acceptance criteria and provide resistance to theft (NAS, 2000~. The principal challenge in formulating crystalline ceramic hosts is radiation-induced transformation from a crystalline to an amorphous state (amorphization)
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From page 35... ...
to support plutonium storage and fabrication into MOX fuel. Ongoing projects in other offices could be extended to improve scientific understanding of long-term corrosion, gas generation, and moisture analysis relevant for storage of plutonium-bearing materials, process analytical chemistry and materials characterization for MOX fuel fabrication, and nondestructive assay technique improvements for use in high-radiation environments.
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From page 36... ...
For material not destined for MOX fuel or WIPP7 there are opportunities for research toward developing highly durable forms for co-disposal with high-level waste and spent fuel. Finally, there is potential for crosscutting research toward stabilization of both spent fuel and plutonium residues for storage, as well as for eventual disposal.
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From page 37... ...
Furnaces and furnace components frequently exhibit spectacular and debilitating corrosion effects that seriously affect processing timelines, and secondarily, site closure schedules. These issues have been identified in disposition planning documents driven by specific site concerns (WSRC, 2002)
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From page 38... ...
In addition, accurate determination of impurity levels in the highly radioactive environment of the plutonium matrix poses unique challenges for the analytical chemist. Current means of determining impurity levels of interest at sub-ppm levels require arduous dissolution procedures followed by atomic emission spectroscopy techniques.
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From page 39... ...
Crosscutting research could also provide scientific support for disposing of spent reactor fuels, discussed in Chapter 4, and other actinide elements, discussed in Chapter 7 Ceramic Matrices Research into immobilizing plutonium from difficult-to-process materials classes is warranted from the standpoint of long-term stable disposition and may include development of novel material types and technologies. A comprehensive set of research recommendations for evaluating alternative crystalline wasteforms for plutonium disposition with respect to radiation stability emerged from an expert panel convened under the auspices of the DOE Council on Materials Science (Weber et al., 1998~.
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From page 40... ...
Because pyrochlores are fluorite structure derivatives, disordering the cation sublattices yields an average defect fluorite structure; the fluorite structure class is the most resistant to amorphization because of the huge redundancy in structural constraints (Hobbs et al., 1 996, 1 999~. The closer the initial pyrochlore structure is to the fluorite structure, the more stable the cation-disordered structure is toward subsequent amorphization: this observation is more pronounced for the pyrochlore lanthanide zirconates than for the lanthanide titanates, and particularly so for actinide substitutions.
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From page 41... ...
While the product appears to meet waste acceptance product specification criteria in leach and durability tests as produced, at short times or with low activity surrogates, it may not do so in the long term with the radiation field of Pu-239, particularly if the chlorides are retained as alkali halide compounds and do not dissolve into the zeolite or glass matrices. Alkali halides are perhaps the most radiation sensitive of all materials and undergo efficient radiolytic decomposition in an ionizing radiation field; substantial decomposition would be expected in repository conditions in several hundred years for a 10 weight percent plutonium loading.
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From page 42... ...
Measurements of internal stress developing with irradiation in accelerated tests with short-lived isotopes, for example using X-ray diffractionbased or Raman-based stress measurements used for assessing stresses developing in oxidation scales, need to be carried out. Processing methods for producing fine-grained polycrystalline assemblages of host phases incorporating appreciable plutonium or other actinides need also to be researched.
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