Draft
Section VII – Alternative Approaches
US Situation
Current civilian use of nuclear power in the United States is based on a once-through fuel cycle involving the irradiation of low-enriched uranium fuel in light-water reactors and the subsequent storage and eventual disposal of the used fuel without reprocessing. Expanded use of nuclear power globally, however, may be predicated on economic competitiveness and sustainability, which in turn may require consideration of different fuel cycles.
The May 2001 National Energy Policy Report (NEPR), developed by the National Energy Policy Development (NEPD) Group, recommended that “in the context of developing advanced nuclear fuel cycles and next-generation technologies for nuclear energy, the United States should re-examine its policies to allow research, development and deployment of fuel conditioning methods that reduce waste streams and enhance proliferation resistance.”
One outgrowth of the NEPR was the initiation of the Advanced Fuel Cycle Initiative (AFCI) within the U.S. Department of Energy (DOE). The AFCI mission is to develop fuel cycle technologies that meet the need for economic and sustained nuclear energy production while satisfying requirements for a proliferation-resistant nuclear materials management system.
In February 2006, the DOE also created the Global Nuclear Energy Partnership (GNEP). One of the main program elements focused on increasing the efficiency of managing used nuclear fuel and deferring the need for additional geologic nuclear waste repositories until the next century. GNEP proposed that commercial used fuel eventually be recycled in advanced burner reactors so that transuranic elements would be consumed, not disposed of as waste.
Almost immediately following the inauguration of President Obama in January 2009, the GNEP program was canceled, but support for the AFCI program was re-emphasized. The Obama Administration also announced that the proposed permanent repository at Yucca Mountain “was no longer an option,” and that a “Blue Ribbon Commission” would be created to evaluate alternatives to Yucca Mountain. As of the end of September 2009, the Blue Ribbon Commission has yet to be formed.
The Blue Ribbon Commission provides an opportunity for reaching consensus on how advanced nuclear fuel cycles should factor into long-term energy planning at the national level. Reprocessing and fuel cycle closure are complex topics with many competing issues – economic, technical and institutional – that need to be closely examined and properly communicated. Further, because of the substantial time and resources required to successfully demonstrate, license, and deploy advanced fuel cycle facilities, an optimum solution path is not necessarily evident.
Global Situation
Assuming that reliance on nuclear power is destined to significantly expand in the future, present and anticipated global developments of the nuclear fuel cycle are shown graphically in Figure 1. The fuel cycle is divided in four blocks: the top two [“LWR[1] Power Block” and “Managed Storage”] have been in commercial operation for several decades; the bottom one [“FBR[2] Power Block”] is receiving the greatest amount of RD&D, especially in France, Russia, India, Japan, and China. Finally, significant progress is being made related to “HLW Repository,” especially in Finland, Sweden, and France.
LWR Power Block
Over 85% of the installed nuclear capacity consists of pressurized and boiling water reactors. The head-end infrastructure (uranium mining and milling, conversion, enrichment, and fuel fabrication) is well established. The LWR technology makes very limited use of the potential energy content of natural uranium resources.
Managed Storage
The fuel discharged from LWRs is either placed in interim storage for several decades (USA, Sweden, Finland, …), or reprocessed (France, Japan, …). Interim storage of used LWR fuel has been implemented in centralized facilities (Sweden) or at the reactor sites (USA). Reprocessing uses the Plutonium and Uranium Extraction (PUREX) process and results in three main products: (1) Reprocessed Uranium; (2) Reactor-grade Plutonium; and (3) Wastes. The reprocessed uranium and the plutonium can be recycled in existing LWRs resulting in natural uranium savings of ~25%. The used fuel derived from using recycled uranium or plutonium is then placed in interim storage. Among the different waste streams, the vitrified HLW, containing the fission products and the minor actinides neptunium, americium and curium dispersed into a glass matrix, is also placed in interim storage. Therefore, the end result for both options, interim storage of used fuel or reprocessing and recycling, is interim storage. The technology and facilities to implement interim storage, including reprocessing[3] and fuel re-fabrication, is commercially available.
FBR Power Block
By recovering the plutonium available in used fuel, utilization of U-238 can be fully enabled in fast reactors: the plutonium is consumed and regenerated from the U-238. The leading design is the sodium-cooled, fast reactor operating in the near-breeder or breeder mode. Reprocessing of used FBR fuel and re-fabrication are required. Depending on the reprocessing scheme, separation and transmutation of some long-lived fission products and minor actinides can be contemplated. The largest operating fast reactor is presently the Russian BN-600 (1470 MWth) fueled with enriched uranium and operating since 1980. An advanced design, the BN-800 fueled with mixed uranium and plutonium oxide, is scheduled for operation in 2016. First criticality of the 65-MWth China Experimental Reactor (CEFR) is expected this year. Re-start of the 714-MWth Monju reactor in Japan is scheduled by the end of March 2010. Initial criticality of a 500-MWe Prototype Fast Breeder Reactor (PFBR) in India is scheduled by the end of next year.
Advanced reprocessing technologies based on the PUREX process are being developed in several countries. Two main options are pursued: (1) Selective separation of minor actinides for heterogeneous[4] recycling in fast reactors (DIAMEX-SANEX in France, TALSPEAK in US, TOGDA in Japan); and (2) Group actinide separation intended for homogeneous[5] recycling in fast reactors (GANEX in France, UREX+ in US, NEXT in Japan). Also, innovative methods based on pyro-chemistry are also being developed as integral parts of the refueling/waste management system of specific types of fast reactors. They allow for the treatment of different types of highly radioactive fuels with high plutonium content. Commercial deployment of these technologies is not likely for several decades. The French program anticipates commercial deployment of a fast reactor fleet possibly as early as 2040; a preliminary proposal integrating fast reactor, reprocessing, and waste management technologies is scheduled for 2012, with an operating fast reactor prototype in 2020.
Geologic Repository
All options eventually require a geologic repository. There is broad agreement among the technical community that deep geological disposal constitutes a safe option for the relatively small volumes of HLW (including used fuel) generated by the nuclear power plants. The safety case for a HLW repository requires extensive R&D (site suitability, waste packaging, etc.) as the final selection of a site and disposal concept will be challenged from every possible angle. However, technical issues are generally not the limiting timing factors. Societal and political acceptance of these systems is currently the limiting factor for implementation in most countries. In this regard, the way Finland sought and obtained public support for its program is widely regarded as a good model [Reference: “Timing of High-Level Waste Disposal,” OECD 2008, NEA No. 6244 (2008)]. Currently, Finland, Sweden and France appear to be on a robust path to have an operating geologic repository by 2025, or sooner.
Discussion
Natural Resources Utilization
Worldwide, the primary driver for the deployment of nuclear fuel cycle facilities, as shown in Figure 1, is the promise to cost-effectively unlock the energy content of U-238, should the price of natural and enriched uranium progressively escalates to levels that would make the technology non-competitive. In this case, the economic potential of plutonium (and more specifically Pu-239) becomes very high, and fast reactors would operate in near-breeding or breeding mode. Plutonium (associated with depleted uranium) is an effective fuel for fast breeder reactors. Plutonium can be recycled indefinitely and produces low levels of Pu-241 and Pu-242, which, in turn, results in small amounts of americium and curium. Therefore, multi-recycling of plutonium in fast reactors has also benefits with regard to minimizing the production of minor actinides.
There are several alternatives to the sodium-cooled fast reactors. Among the most prominent ones are those being the subjects of cooperative R&D in the context of the GEN IV International Forum [Reference: GEN IV International Forum – 2008 Annual Report]. These include:
- Lead-cooled fast reactors
- Gas-cooled fast reactors
- Molten salt thermal and fast reactors
Recent developments in breeder reactor designs also include innovative designs such as Hitachi’s Resource-Renewable Boiling Water Reactor, a fast reactor relying on existing thermal BWR technology, and TerraPower’s traveling wave reactor (TWR), which belongs to a class of reactors that promotes maximum fuel utilization in fast reactors without chemical reprocessing.
For completeness, the fissile Pu-239 – fertile U-238 scheme can be augmented by a fissile U-233 – fertile Th-232 scheme. In all reactor-based cases, however, U-235 is needed to create a sufficiently large inventory of either Pu-239 or U-233 before breeding becomes possible.
It should also be mentioned that accelerator-driven systems (ADS), which will be presented in the following section for their potential benefits to waste management, can also be envisioned for breeding of fissile material, independently of the need to first rely on U-235.
When natural resource utilization is not the main concern as a result of abundant uranium supply compared to the potential demand, other fuel cycle schemes have been investigated for reduction of the waste management burden, for proliferation resistance and physical protection, or both.
Waste Management
Plutonium dominates used fuel radiotoxicity. However, radiotoxicity is a figure-of-merit of limited value, because it does not take into account the mobility of the nuclide in the geologic environment. Performance assessments show that soluble species such as I-129, Tc-99, Se-79, or Cl-36 generally dominate long-term dose assessments. The latter species do not lend themselves readily to separation and transmutation. An alternative to transmutation of troublesome fission products is disposal into deep boreholes.[6]
Reduction of the waste burden generally translates into minimizing waste decay heat to minimize the size of the geologic repository. Because HLW can be allowed to cool in interim storage, decay heat after 100 years is a better figure-of-merit to quantify waste burden, rather than radiotoxicity. After 100 years, the main contributors to decay heat are the actinides, especially Am-241. Potential benefits of reducing the size of a repository, or the total number of repositories needed for a given amount of energy production, should be considered both in the context of public acceptance, as noted above, and in the context of comparisons of overall cost and safety among the different strategies under consideration.
Minor actinide management is a complex topic, especially in the context of transmutation in fast reactors. The topic has been addressed in details in several studies in the early 1990’s by the National Academy of Sciences’ Committee on Separations Technology and Transmutation Systems (STATS) [Reference: Nuclear Wastes – Technologies for Separations and Transmutation, National Academy Press, Washington, DC, 1996] and by the Electric Power Research Institute [An Evaluation of the Concept of Transuranic Burning Using Liquid Metal Reactors, NP-7261, Palo Alto, CA, 1991] at the request of the U.S. Department of Energy.
Fast Reactors
There was general agreement in the early 1990’s that it made no sense to develop and deploy liquid metal reactors (LMRs) solely for actinide burning. The fraction of the actinide inventory that could be consumed depends upon the decontamination factor, reactor fuel-cycle parameters, and length of time of LMR operation. Transuranic actinide inventory reduction is enhanced by a low breeding ratio (so that the LMR creates fewer new actinides) and by a high decontamination factor (the inverse of which measures the fraction of actinides lost to waste in the reprocessing cycle). By reference to Figure 2 extracted from the NAS report, it can be seen that the time required to reach an inventory reduction factor of 10, equivalent to burning 90% of the actinides, would be more than 100 years.
Thermal Reactors
The thermal flux of a LWR could be used to transmute the transuranic elements (TRUs). The following concepts are the topics of active research programs:
Use of inert matrix fuel (IMF) [Reference: “Viability of inert matrix fuel in reducing plutonium amounts in reactors,” IAEA-TECDOC-1516 (2006)]
IMF is a type of nuclear reactor fuel that consists of a neutron-transparent matrix and a fissile phase that is either dissolved in the matrix or incorporated as macroscopic inclusions. The matrix dilutes the fissile phase to the volumetric concentrations required by reactor control considerations, the same role U-238 plays in conventional low enriched fuel. The key difference is that replacing fertile U-238 with a neutron-transparent matrix eliminates plutonium formation as a result of neutron capture.
The use of inert matrix fuel in the current generation of reactors would provide a means for reducing plutonium inventories. Another application of IMF would be the reduction of minor actinide content, with or without plutonium. As an example, the combined non-fertile and UO2 (CONFU) fuel concept is an advanced PWR assembly that is designed to allow multi-recycling of TRUs in existing PWRs [Reference: Implications of Alternative Strategies for Transition to Sustainable Fuel Cycles, A. Romano et al., Nuclear Science & Engineering, 154, 1-27 (2006)].
Finally, IMF materials are also being considered for GEN IV reactors.
“Deep Burn” in Modular Helium-Cooled Reactors (DB-MHR) [Reference: “Gas Turbine-Modular Helium Reactor (GTMHR) Conceptual Design Description Report,” Potter, and A. Shenoy, GA Report 910720, Revision 1, General Atomics, July 1996]
The Deep-Burn, Modular Helium-cooled Reactor (DB-MHR) concept has been proposed by General Atomics (GA) for the purpose of incinerating plutonium, neptunium, and americium nuclides, based on the technologies of the graphite moderated Gas-Turbine, Modular Helium-cooled Reactor (GT-MHR). The essential feature of this transmutation concept is the use of the coated fuel particles (TRISO) that are considered strong and highly resistant to irradiation. The TRU fuel formed into TRISO particles can be irradiated for a long time in a thermal system, and thereby a very high TRU consumption (in particular fissile nuclides) can be expected. This is referred to as a “deep burn.”
Accelerator-Driven Systems [Reference: Accelerator-driven Systems (ADS) and Fast Reactors in Advanced Fuel Cycles, OECD (2002)]