Reducing the Use of Highly Enriched Uranium in Civilian Research Reactors (2016) / Chapter Skim
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4 Technical Obstacles to Conversion
4 Technical Obstacles to Conversion
Pages 55-96
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From page 55... ...
BASIC STEPS TO CONVERSION Altering the fuel of an existing operating research reactor (to LEU or other types) requires many steps and consideration of both technical and nontechnical factors (e.g., reactor performance, economics, licensing)
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Obtain license to manufacture convert 6 months 9) Conversion FIGURE 4.1 General steps to reactor conversion to LEU fuel.
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• Commercially available fuel assembly -- an LEU fuel assembly that is available for purchase from a commercial manufacturer. • Suitable fuel assembly -- a fuel assembly that satisfies criteria for LEU conversion of a specific reactor, including the following: -- Fuel service lifetime comparable to current HEU fuel (e.g., number of fuel assemblies used per year is equal to or fewer than with HEU fuel)
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From page 58... ...
18) : In assessing the practical feasibility of utilizing lower enriched fuel in exist ing research reactors, the agreed criteria are that safety margins and fuel reliability should not be lower than for the current design based on highly enriched uranium and that neither any loss in reactor performance, e.g., flux-per-unit power, nor any increase in operating costs should be more than marginal.
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HEU fuel assembly designs for high performance research reactors (HPRRs) rely on high coolant velocities through narrow channels to remove the heat generated by the high-power core.
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Assuring that the performance requirements are met necessitates detailed neutronic analyses of proposed LEU core designs that meet the geometric and thermal and fast flux constraints. Finally, the operational costs of using LEU fuel should not be substantially greater than those for HEU fuel.
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. Many medium-power research reactors have successfully converted using high-density LEU fuel such as silicides (U3Si2 and U3Si)
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(b) Dispersion fuel Monolithic fuel Fuel particle Fuel meat Fuel foil Al matrix Al -alloy Al -alloy cladding cladding FIGURE B4.1 Schematic cross sections of (a)
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High Performance Research Reactors. In the figure, the MIT reactor is referred to as simply "MIT," whereas throughout the report it is referred to as "MITR-II." SOURCE: Courtesy of Argonne National Laboratory (Yacout, 2015)
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From page 64... ...
domestic reactors as well as U.S. Government enriched uranium supply agreements and commitments of 19.75 percent supply for foreign research reactors or medical isotope production predominately pull material from the 1994 surplus HEU declaration, and those supplies are expected to be practically exhausted by around 2025.
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From page 65... ...
.12 Consequently, several countries have launched efforts to develop and qualify higher-density LEU fuel that can meet the performance, safety, and operational constraints of these reactors. Early Fuel Development Efforts Three main fuels based on HEU were in use when the RERTR Program began in 1978: • UAlx powder dispersed in an aluminum matrix (UAlx-Al dispersion fuel)
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The United States, the main driving force behind the conversion requirements and an 13 RERTR's original mission was limited to reducing HEU exports by converting U.S.supplied research reactors to LEU fuel. See Chapter 2.
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From page 67... ...
For U3Si2, the value 6.5 gU/cm3 is associated with a volume fraction of particles Figure 4-5 which exceeds the technological limit of 60 percent, commonly adopted. UMo dispersion fuels are currently being tested for use in the Bitmapped European HPRRs at 5, 7, and k31 weight percent Mo (U-5Mo, U-7Mo, and U-9Mo)
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From page 68... ...
This allows fuel designers to tailor individual fuel plates to their operating environment. Additionally, UMo dispersion fuel is structurally similar to existing fuels, giving it the advantage of a well-understood fuel fabrication process.
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From page 69... ...
of the uranium in the fuel meat once the UMo alloy composition has been chosen, fuel designers must rely on changing the UMo foil thickness to achieve the same flexibility in uranium content as with dispersion fuels. A rule of thumb developed by fuel manufacturers allows for a maximum of 50 to 55 percent volume fraction for UMo dispersion fuel; thus, the UMo foil thickness in monolithic fuel needs to be only about one-half as thick as the fuel meat in UMo dispersion fuel to achieve the same uranium content.
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From page 70... ...
Fuel type Primary: UMo dispersion fuel UMo monolithic fuel only Secondary: UMo monolithic (for FRM-II) and high-density U3Si2 as a backup Fuel fabricators CERCA, a subsidiary of AREVAa BWXT, private company funded by DOE to make research reactor fuel (a small percentage of overall BWXT uranium fuel business)
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From page 71... ...
if no specific frame density LEU research reactor fuels. CERCA sells research reactor fuels to many of the HPRRs in Europe, and, as with operators worldwide, there is concern over the uncertainty in the pricing for the high-density LEU fuels.
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Notably, many of the irradiation tests performed on the new fuel are performed in the research reactors slated for conver sion after the fuel has been qualified (e.g., Belgian Reactor 2 [BR2]
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monolithic fuel test)
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Furthermore, the success of the IRIS-1 test, a full-sized experiment, is attributed to two factors: • A lower-power density (lower than IRIS-2, FUTURE, and the RERTR-4 mini-plate experiments)
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are full-sized plate irradiation experiments. Four of these test campaigns (all but the FUTURE-MONO-I and -II monolithic fuel tests)
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Fission Density (fissions/cm3) FIGURE 4.7 Results of irradiation tests performed in BR2 on U-7Mo dispersion fuel: Fuel swelling from the FUTURE, E-FUTURE (plates with 4 and 6 weight by percent silicon in the matrix)
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From page 77... ...
Therefore, conversions of research reactors would occur several years after the conclusion of the testing program, assuming it is successful and there are no further delays. In summary, the results of current irradiation experiments provide confidence that UMo dispersion fuels could be qualified for burnup below fission densities of 4.5 × 1021 fissions/cm3 and up to power densities on the order of 40 kW/cm3.
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2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 78 Comprehension Phase EMPIrE D M I CS PIE SEMPER FIDELIS D M I CS PIE E-FUTURE 3 D M I CS PIE MIXED ELEMENT D M I CS PIE FUTURE-MONO- 1 D M I CS PIE FUTURE-MONO- 2 D M I C PIE MIXED ELEMENT D M I C PIE CERCA Powder Atomization Development CERCA Powder Coating Development CERCA Dispersion Plates Manufacturing Development CERCA Monolithic Plates Manufacturing Development 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 FIGURE 4.8 The HERACLES time line for UMo dispersion fuel testing (status as of Quarter 4 of 2014)
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. Previous studies have shown that conversion of the MIR.M1 reactor with medium-density LEU fuels (U-9Mo dispersion, 5 gU/cm3)
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.28 The U.S. program has divided its fuel development and qualification effort into "base fuel" and "complex fuel" programs, both of which will use the same basic UMo monolithic fuel.
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UMo monolithic fuel development, qualification, and USHPRR conversion, last updated in June 2015. Test campaigns are split into base fuel and complex fuel testing.
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The base fuel fabrication process cannot be used for the manufacture of the HFIR fuel assembly because the fuel plate is curved in a noncylindrical shape,29 the fuel meat within the plate has variable thickness, and burnable poisons are included (see Figure 4.10)
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If successful, then this will be the first monolithic fuel qualified for use in research reactors worldwide. It is clear, however, that manufacturing this fuel will be considerably more complicated, and presumably more expensive, than the base monolithic fuel.
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.30 The high volume of fuel utilization allows the fixed costs of USHPRR fuel manufacture to be spread over a larger number of total fuel plates than would be possible in the absence of these two reactors.31 In short, the future demand for LEU fuel assemblies for HFIR and ATR are expected to drive the economics of LEU fuel assemblies for the remaining USHPRRs. Without a qualified fuel for HFIR and ATR, the cost per fuel plate for the remaining three USHPRRs would be expected to increase dramatically.
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From page 85... ...
ASSESSMENT OF PROGRESS TO DEVELOP LEU FUEL The previous sections have discussed worldwide fuel development programs, including UMo dispersion and monolithic LEU fuels. Table 4.3 summarizes the committee's conclusions regarding the progress toward delivery of a variety of high-density LEU fuels, including the key steps of
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Fuel qualification: Has the LEU fuel been qualified under irradiation? Y N a N N Y = yes, N = no Manufacturing qualification: Has the manufacturing process for the LEU fuel Y b N N N been qualified?
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, the committee has separated this fuel from the "high" power density UMo dispersion fuel, which satisfies the usual HPRR power density-burnup envelope. This assessment leads to the following findings: Finding 6: Most of the technical challenges to converting the remaining research reactors to low enriched uranium (LEU)
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From page 88... ...
fuel, as-yet-unqualified higher-density U3Si2 fuel, and UMo dispersion fuel were considered for each USHPRR, either as a path to conversion earlier than currently scheduled or as a backup option to mitigate the risks in monolithic fuel development and fabrication. 32 USHPRR Roadmap, provided to the committee by DOE/NNSA in January 2015 and (updated)
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0.45 M ( 0.93 ) 0.20 In other words, a device based on 45 percent enriched uranium requires ap proximately 3.4 times more material, and a device based on 20 percent enriched uranium requires approximately 13.6 times more material when compared to a device using W-HEU.
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to compare proliferation risk attributes of different fuel enrichment levels: 45 percent enriched fuel offers a 40 percent reduction, and 20 percent enriched fuel offers a 70 percent reduction, compared to weapon-grade fuel (93 percent U235)
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. 3 2 In addition, consideration was given to the possibility that dispersion and/ or monolithic fuel could be qualified for reduced performance envelopes appropriate for some of the USHPRRs.
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For other European HPRRs, the successful completion of the entire UMo dispersion fuel development program is necessary. Even then, such fuel would only be suitable for NBSR and possibly MITR-II in the United States, with no, or perhaps only marginal, acceleration of the conversion time line possible over what is projected with monolithic fuel.
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From page 93... ...
that would be shipped while awaiting the development of suitable UMo LEU fuel (i.e., monolithic UMo fuel for the USHPRRs and FRM-II and dispersion UMo fuel for the European HPRRs)
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research reactors. Even without reduced time lines, the dispersion fuels being developed by South Korea and the European consortium known as HERACLES can be used to mitigate the technical risk that remains in the current U.S.
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monolithic fuel. Recommendation 4: To achieve the goal of using as little highly enriched uranium as possible during the many years that it will take to design and qualify appropriate low enriched uranium (LEU)
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. Therefore, it is important that the high-density LEU fuel development and qualification proceed in parallel with the stepwise conversion.
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