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Nuclear Wastes: Technologies for Separations and Transmutation APPENDIX I HEALTH AND SAFETY INTRODUCTION When discussing health and safety issues, it is important to be precise about the meaning of the terms involved. Three terms used extensively throughout this report are "safety," "risk," and "hazard." In general usage, they are used with slightly different meanings. Safety is the catchall descriptor having the meaning found generally in most dictionaries, i.e., the condition of being safe from undergoing or causing hurt, injury, or loss. That is, it prevents a negative consequence directly related to human health and well being. The word "hazard'' is also used in the sense of its usual dictionary definition, that is, as a "source of danger." A hazard may produce a negative consequence under certain scenarios, but the presence of a hazard is not necessarily a risk to the public in and of itself. The problem is that the word "risk" is often used to mean hazard and safety when, in fact, in the nuclear business in particular, it has quite a different meaning. For this report, the committee has adopted the triplet meaning of risk (Kaplan and Garrick, 1981) that is widely used in the probabilistic risk-assessment community. In particular, risk assessment answers three basic questions: What can go wrong? How likely are things to go wrong? What are the consequences? The first question is answered by structuring scenarios (sequences of events) of the different ways that a facility can get into trouble. This step of risk analysis requires a clear understanding of how the facility operates, the hazards involved, those events that can initiate an accident scenario, and how the facility can fail to respond safely. The question of likelihood is often a matter needing detailed analysis, since, in many instances, there are too few data to assign statistically based frequencies. The most common approach is to develop frequencies and probabilities based on all available evidence, including detailed logic models that decompose the facility into components for which there are data and increased knowledge, and to do so in such a way as to recognize the uncertainties involved. Finally, the consequences are defined in terms of injuries, fatalities, facility damage, environmental damage, etc., or combinations thereof. There may be interest in a variety of types of consequences. Thus, a risk assessment is a structured set of scenarios, their likelihoods, and their consequences. The results not only provide information on the likelihood of different levels of severity but also convey the analysts' confidence (uncertainty limits) in the results. In this report, when the discussion is about scenarios, likelihoods, and consequences, generally the term "risk" is used. The term "hazard" most often refers to the nature of the
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Nuclear Wastes: Technologies for Separations and Transmutation materials and inventories involved, including their toxicity, but not to likelihoods and consequences of upsets and accidents that get the materials into the biosphere. Safety is used in a qualitative and much more general sense. When applied to the radioactive waste problem, environmental, health, and safety issues are closely intertwined. The health effects of concern are the negative consequences of being exposed to ionizing radiation. By far the major concerns are latent cancer and genetic effects from low doses, although accidents during the transmutation, separation, and handling phases have some potential for producing somatic effects. The physical principle involved is that a person will receive a dose only when radioactive material is in relatively close proximity and unshielded. Environmental impacts are generally associated with contamination of soil or ground water that could result in an eventual transport of radioactive material and exposure of the population, unless there is some type of isolation (resulting in loss of land use) or an extensive cleanup effort to prevent the gradual spread of the contaminants. Safety concerns refer to near-term scenarios that breach the engineered facilities to produce the resultant environmental contamination or more direct (e.g., airborne) pathways of radioactive materials to workers or the population. The likelihood that a set of conditions will actually occur that could produce a dose that results in health effects can be estimated with a risk assessment of the scenarios that produce those conditions. This section highlights the risks and safety issues associated with the various concepts being proposed for the reduction or alteration of radioactive waste. The information presented is based on work performed by the proponents of the various concepts. While there is no attempt to perform an independent analysis to verify the details of the claims of the proponents, there is an attempt to provide a limited, objective review of the risk and safety work performed to date. The approach taken is to develop risk and safety summaries of each of the radioactive waste treatment concepts based on studies and reports made available to the Committee on Separations and Transmutation Systems. These summaries are at the end of this appendix. The risk and safety summaries all have the same format to facilitate comparisons of the different concepts and serve as the centerpiece for the discussion and conclusions that follow. The radioactive waste treatment concepts are identified to correspond with the major concepts described in this report and generally are as follows: reactor-based advanced liquid-metal reactor (ALMR)/integral fast reactor (IFR), light-water reactor (LWR), particle bed reactor (PBR); accelerator-based Los Alamos National Laboratory (LANL) accelerator transmutation of radioactive waste (ATW), Brookhaven National Laboratory (BNL) Phoenix;
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Nuclear Wastes: Technologies for Separations and Transmutation Clean Use of Reactor Energy (CURE; complementary mix of concepts); and base case (LWR once-through cycle). Seven issues are presented in the attached risk and safety summaries to highlight factors that would assist in the selection of a potential mix of facilities from a safety and health risk point of view. The first group of issues summarizes the status of evidence regarding the overall concept presented by each proponent. The issues include the systems technology maturity, quantitative safety assessments, and the experience demonstrating feasibility. As a general rule, the greater the uncertainty regarding the system design, interfaces, and performance under actual operating conditions, the higher the likelihood of unforeseen functional and phenomenological events, that is, the higher the risk. The second group of issues lists the selected design features in terms of applicability to waste objectives, unique risk reduction features, unique safety concerns, and major sources of uncertainties. These issues highlight the ability of the technology to address the risks associated with permanent geologic disposal, the advantages and disadvantages of the design with respect to risk, and unresolved issues to ensure safety. Because of the great differences in the nature of the radioactive hazard from step to step in the fuel cycle, the seven issues are addressed for each of three phases common to all concepts: transmutation, separation, and storage. During the transmutation process, the safety concerns are very similar to those normally associated with the operation of nuclear reactors. Except for the one accelerator concept involving a homogeneous blanket, the radioactive material is contained in either solid fuel or target material. For the LANL ATW concepts, the fuel is already in a liquid and mobile state in the event of any kind of containment failure. Similar to nuclear reactors, the accident pathway of greatest concern is a release into the atmosphere. This is primarily because of the power levels, temperatures, and decay heat involved. Pathways other than atmospheric dispersion are believed to be less important in this phase because the mobility of molten material (corium), once it leaves the immediate region of the core, will be severely restricted as it solidifies within the confines of the facility, where it could be recovered. As with commercial nuclear reactors using the once-through fuel cycle, the basic approach to safety is defense-in-depth with both active and passive safety systems. Unlike commercial nuclear reactors, there is a lack of operating experience with any of the advanced reactor or accelerator-based transmutation concepts. The separation phase presents a different set of challenges. Although the high power levels and temperatures of the transmutation phase no longer are present, the material is expected to be sufficiently radioactive to generate significant quantities of heat. Moreover, to facilitate separation, the material will be converted into liquid form during appropriate parts of the separation process and transported throughout a more dispersed facility. Thus, although the driving force for airborne dispersal has declined, the physical form of the materials is more conducive to widespread contamination. Finally, since the material will be actively handled over a larger facility, maintaining tight control and defense-in-depth at all times will be more difficult. For the disposal phase, the concern is the eventual degradation of the engineered barriers and the uncertainty in the ability of the geologic formation to keep the longer lived isotopes isolated from the biosphere. Once the material is packaged and placed in the repository, and
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Nuclear Wastes: Technologies for Separations and Transmutation once the repository is sealed, there is a loss of active control over its mobility. The acceptability of the repository then hinges on society's confidence that enough has been done to maintain isolation of the waste long enough to prevent health and environmental effects. DISCUSSION OF CONCEPTS All of the proposed concepts have some health and safety characteristics in common. First, as their objective is to reduce the requirements for a geologic repository, they all produce a reduction in long-term risk. Second, as they all require some form of spent-fuel processing, the concepts all present a short-term risk during the separation phase that is currently bypassed by the once-through fuel cycle. Conversely, to the extent that they produce a net positive energy output through the use of recycled actinides, the concepts all have the potential to reduce the short-term risk due to uranium mining and milling operations. Third, all transmutation technologies have the objective of providing some form of a passive safety feature not employed in currently licensed LWRs to reduce the likelihood of the release of radioactive material into the environment. Consequently, the proponents claim at this conceptual stage that an integrated separations and transmutation (S&T) system will produce a net decrease in health and safety risk to the public. As discussed later, there is insufficient evidence to support this claim at this time. Before addressing the impact of an integrated S&T system on risk due to high-level waste, it is reasonable to compare it with the front end of the cycle to determine if resources are being focused on that aspect of the problem that would provide the greatest risk reduction. The mining and milling operations disturb the geologic formations that retain naturally occurring radioisotopes and allow some public exposure. Moreover, the depleted uranium tails can produce a continuous source of radon if reasonable disposal methods are not employed. In the long run, the main concerns are 226Ra and 210Pb (from the decay of 238U) and 231Pa, a daughter product of 235U. Michaels (1992, and private communication, 1993) of the Oak Ridge National Laboratory has presented summaries of potential health risks (effects) among the total U.S. population for the current once-through LWR fuel cycle and for an ALMR fuel cycle. The potential for radiation exposure from mining and milling operations is estimated to be approximately 50% of the total exposure from the existing nuclear fuel cycle, and the waste management exposure potential is about 1% of the total exposure potential. Comparison of the risk implications of selected design features of the various S&T concepts is difficult primarily because of the differences of the specific objectives of the individual technologies. Two of the concepts, the accelerator reactor combinations and the PBR, have been conceived specifically to transmute wastes. Therefore, they have some revolutionary design features that provide unique opportunities to reduce the risk of long-term high-level waste disposal. However, these concepts introduce near-term operational safety concerns that have not yet been resolved. There remains the problem of the low-level waste (LLW) and its risks, which could be significant by comparison. Except for the Phoenix concept, all provide a mix of reactors and reprocessing facilities to make effective use of the energy-producing capabilities of the actinides, with the important by-product of also reducing the quantity of high-level wastes (HLW) that must be geologically isolated. These concepts tend
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Nuclear Wastes: Technologies for Separations and Transmutation to be more evolutionary; they do not eliminate the requirement for disposal but simply reduce the magnitude of the disposal. Based on the information in the summaries and discussions at the end of this section, it can be seen that the accelerator concepts are still in a very preliminary conceptual stage of development, with little or no actual experience yet available to support the development process. The PBR has some basis in existing high-temperature gas-cooled reactor (HTGR) technology, but the proposed design parameters for the PBR concept go far beyond the HTGR experience. The fast reactors have progressed further into the development process, and it appears that many of the technologies have been demonstrated, at least experimentally or in a prototype. LWRs are by far the most developed of any reactor type, and many of the recent designs can be modified to become efficient plutonium burners. The ALMRs offer even more advantages in terms of passive safety systems. The uncertainties with the use of LWRs center around the more stringent reactivity control requirements of mixed actinide fuels. The CURE concept is not tied to any one design philosophy, and so it cannot be compared directly with specific reactor types. Rather, it selects a mix of the above technologies and gradually integrates them over a long development process to optimize both energy production and reduction of long-term waste disposal requirements. The CURE concept, if properly followed, would tend to reduce the risks associated with problems that arise from a commitment to one technology about which there is little information. ATW The proponents of ATW as a waste treatment concept point to several safety advantages of the approach. Most notable are that ATWs operate below nuclear criticality, reduce the source term of radioactive material by continuous processing of the liquid fuel, and can transport liquid fuel to safe geometries for decay-heat removal. The proponents state that the ATW accelerator controls can rapidly shut down on any system malfunctions. When combined with continuous fission product removal, this feature will reduce the heat load on the safety systems following an initiating event. Among the safety concerns are the ability to maintain system integrity at the beam/target/blanket interface, the reduction in defense-in-depth with the fuel already in a liquid state, and the fact that the accelerator's localized particle beam can initiate a variety of events. Activation of replaceable components, such as beam targets, may even increase short-term storage requirements. It should be noted that continuous processing does not totally solve the safety problem with respect to fission product inventory. The processing rates will not be sufficiently rapid to mitigate the most severe shutdown heating transient on loss of normal cooling. However, the short delay times between transmutation and separation will result in much higher activities of short-lived fission products in fuel processing equipment. Reactivity swings are another issue that will have to be studied. Fluid fuel reactors typically have very large negative temperature coefficients of reactivity. Therefore, a large reactivity swing can be expected in going from hot
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Nuclear Wastes: Technologies for Separations and Transmutation operating conditions to cold shutdown. The nonaqueous ATW that is fueled partly or completely with thorium will be subject to a reactivity transient safety issue at high neutron flux. There are questions about the homogeneity and chemical consistency of the blanket that could impact fuel mobility along its flow path and produce the possibility of hot spots. There are a number of questions that have to be answered about system behavior during both abnormal and transient conditions. The ATW approach to the treatment of radioactive waste is handicapped by a lack of experience and therefore knowledge about safety issues. No significant and quantitative risk assessments have been made on any phase of the ATW concept, much less the integrated system. Thus, the state of knowledge about ATW safety is not such that an independent confirmation can be made of the safety claims of its proponents. Accelerator (Phoenix) The Phoenix accelerator concept has some of the same safety advantages as ATW, most notable is the subcritical mode of operation. It employs a large linear proton accelerator with solid oxide targets in a configuration resembling the core of the Fast Flux Test Facility (FFTF) reactor, so separation can build on established processing technology. However, the pre-and postirradiation processing schemes have not been developed. The specific transmutation targets are neptunium, americium, curium, and much of the iodine produced by LWRs. The direct impingement on the target material, together with the fact that it would operate with a fast neutron spectrum, allows use of some known technologies. There are a number of uncertainties with this concept. One is the behavior of the target material under combined proton and neutron irradiation. There may be safety issues associated with a failed beam raster, for example, burning holes in targets with the attendant radioactivity release. And decay heat is an issue even with a rapid removal of the proton source. The most significant shortcomings of the concept are the absence of a design and the lack of any in-depth safety analysis. There is insufficient information to reach definitive favorable conclusions on the safety of the Phoenix concept. ALMR/IFR The ALMR/IFR approach to the reduction of actinides has been under consideration for many years. The technology of fast reactors is reasonably well established, at least much more so than for waste reduction concepts based on the use of accelerators. EBR-II has been operating since 1963, and numerous other prototype fast reactors, including the FFTF, have accumulated a substantial experience base. The result is a considerable amount of knowledge on the safety of fast reactors. Some of the safety advantages are the large heat capacity of the reactor coolant system and the possibility of achieving safe shutdown independent of active systems.
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Nuclear Wastes: Technologies for Separations and Transmutation Even with the substantial amount of fast reactor experience, there is still little experience on a commercial scale. The fast reactor experience to date has been largely with experimental and prototype special-purpose reactors. While EBR-II and FFTF experiences have been favorable, there have been difficulties with some fast reactors. Specific examples are fuel damage in the Fermi reactor and the EBR-1, cracking of primary system components in a U.K. fast reactor, and steam generator leaks in France's Phoenix reactor and the former Soviet Union's BN-350. Unlike the accelerator concepts, there have been considerable safety analyses performed on fast reactors. For example, a probabilistic risk assessment (PRA) performed on EBR-II has produced results that are more favorable than most LWRs. There has also been a PRA performed on the ALMR with favorable safety results. However, these risk models have not been subject to the intense reviews that are typical of licensed commercial nuclear power plants. Furthermore, the scopes of the PRAs of fast reactors and LWRs do not appear to be comparable. For example, the PRA results of LWR indicate the extreme importance of the dependencies of safety functions on support systems and site-and plant-specific considerations. Contemporary LWR PRAs also include extensive consideration of human reliability and detailed studies of long-term accident recovery capability. Before a favorable conclusion on the risk of ALMR/IFR concepts could be reached, scope comparisons would have to be made of the different PRAs. PRAs that have been performed on fast reactors have not been scoped against a specific design that has evolved for the explicit purpose of transmuting radioactive waste. Separation technologies to support ALMR fuel cycles are in a less mature stage. it is anticipated that pyroprocessing will satisfy this need, and pilot-plant studies have been initiated. However, it has not yet been demonstrated at a scale required to support an ALMR fuel cycle. While ALMR/IFR technology and its associated safety assessment are much further developed than for accelerator-based concepts, uncertainties remain on the safety of waste reduction processes using fast reactors. LWR The safety of LWRs has the least uncertainty of any reactor type. The present generation of nuclear power plants is dependent primarily on active systems to mitigate accidents, should they occur. The reliability of those systems has been analyzed extensively, and their performance characteristics are well understood. In addition, newer evolutionary designs, including the ALMRs, enable safe cold shutdown with passive systems. These designs are being subjected to PRAs as part of their certification process. However, the large experience base with LWRs, at least for power applications, is with low enriched 235U fuel. There are safety issues, such as more stringent reactivity control requirements, that would have to be studied when using mixed actinide fuels in LWRs. There is a high state of knowledge on fuel reprocessing from experiences gathered during the initial years of nuclear fuel-cycle development, at least in terms of the plutonium and uranium recovery by extraction (PUREX) process, in spite of the fact that reprocessing of commercial fuel in the United States ceased many years ago.
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Nuclear Wastes: Technologies for Separations and Transmutation With respect to safety, the biggest difference in the safety area between LWR-based waste reduction applications and other proposed concepts is that there is very strong and quantitative evidence on the safety of LWRs and little to no evidence on the other concepts. In the end, it may be possible to demonstrate others to be more safe but not on the basis of the evidence currently available. PBR The particle bed reactor has been proposed by the BNL as a transmutor for the minor actinides, some of the plutonium, and long-lived fission products from LWR spent fuel. Little is known about the safety of the PBR, as the entire reactor concept is very preliminary. There are some safety concerns. The very high power density (5 MW/liter) make it possible to transmute a significant fraction of actinides, thus eliminating the need for geologic disposal. However, this may also make it difficult to ensure that, on loss of coolant flow, an adequate means of core cooling can be provided in time to prevent excessive heating of core materials. The delivery of high-speed helium coolant (275 m/s) at a reasonable pressure drop remains to be validated at steady state. Also, the effects of burn-up reactivity swings on the operations of such a reactor remain to be evaluated, together with the power distribution issues characteristic of high thermal-flux systems. Even though no significant safety studies have been performed, the proponents of the PBR believe that many key technologies have been demonstrated by the experience of the HTGRs, despite the fact that power densities for the current graphite reactors are more than two orders of magnitude lower than those proposed for the PBR. Finally, the technology for reprocessing the PBR fuel has not yet been demonstrated. There does not yet exist a technical basis for concluding that a PBR-based waste reduction facility could be operated safely. CURE CURE is a concept for mixing existing thermal and fast reactor technologies in some economical way to achieve a complex of both power-producing and waste-reducing nuclear facilities. The result is that the risk and safety issues are very dependent on the mix of reactors actually selected. However, it is reasonable to conclude that more is known about the safety of the CURE approach than about accelerator-based approaches, that is, there is a great deal more known about the safety of the reactor types involved in the CURE concept than about the safety of the accelerator systems being proposed. Base Case The base case is the current open fuel cycle of permanently storing the spent nuclear fuel and other radioactive wastes in geologic repositories and allowing them to decay at their natural
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Nuclear Wastes: Technologies for Separations and Transmutation half-life. From a risk and safety standpoint, there is a minimum of waste handling and treatment prior to permanent disposal. Consequently, the base case represents less near-term risk than any of the S&T systems, all of which require active handling of the radioactive waste in the short term. However, if the once-through cycle is used exclusively in nuclear power plants, the base case does present greater long-term risk during the permanent disposal phase, as well as more risk due to mining and milling operations. There is considerable evidence that the health consequences from permanent disposal are extremely small, although the uncertainty of such postulated consequences is relatively high because of the long time periods involved. CONCLUSIONS Risk assessment is a matter of quantifying our state of knowledge about the threat to society of a particular system, engineered or otherwise. Generally, systems (that exist only on paper) involving hazardous materials and then only in conceptual form are on the high end of the risk spectrum due to a poor state of knowledge about possible accidents and consequences. The lack of definition of the system and the attendant poor state of knowledge (high uncertainty) of the health and safety threats (risk) involved make it difficult to be confident that the potential problems could be understood. The result is a penalty to those concepts on paper as opposed to those that have evolved to definitive designs. Of course, those systems that have operating experience and a good safety record have an even greater advantage. What is generally involved with the various waste reduction concepts under consideration is a trade-off of greater short-term risk (for example, less than 100 years) to achieve a reduction of long-term risk (for example, in the thousands of years). There is great uncertainty about long-term risk even though there is considerable evidence that the health consequences are small. There are considerable uncertainties about the exact magnitude of that small consequence. There is difficulty in having confidence about intervening events thousands of years in the future, and therefore there is high uncertainty. On the other hand, much more is known about the operations involved in the short term resulting in greater confidence in the risk results. Taking a purely state-of-knowledge approach to risk and considering each phase of operation as mutually exclusive, the following qualitative risk rankings are offered within each phase in order of increasing risk: For transmutation, the lowest to highest risk alternatives appear to be light-water reactors, fast reactors, and accelerators. For separations, the lowest to highest risk alternatives appear to be PUREX, pyrometallurgy, TRUEX, and
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Nuclear Wastes: Technologies for Separations and Transmutation on-line processing of liquid fuels. For disposal, the lowest to highest risk alternatives appear to be geologic repository and surface. The primary basis for the above qualitative rankings is that risk analyses have been performed on such operations as nuclear reactors; aqueous reprocessing, and, to some extent, geologic repositories. On the other hand, potential safety concerns have been identified, and little or no definitive risk assessment work has been performed on accelerator-based systems, non-PUREX separation methods, and long-term surface storage facilities. To the extent that each of the various proposed approaches makes use of some mix of these technologies, the safety risk incurred must reflect the uncertainties associated with the individual technologies. One of the arguments presented by the proponents of S&T technologies is that they have the potential to reduce the radiological risks of the once-through cycle by making more effective use of fissionable resources to make energy. However, published studies indicate that a major source of radiological risk to the public in the once-through cycle is due to releases generated by mining and milling operations. This suggests that spending money on transmutation and separation may not provide the most effective allocation of resources for reducing radiological risk to the public. There is a strong case for concentrating on the front end of the cycle now, while supporting advanced reactor technology research. This would leave open options to take maximum advantage of evolving technologies in order to better use spent fuel as an energy resource in the future. RISK AND SAFETY SUMMARIES: RADIOACTIVE WASTE TREATMENT CONCEPTS Concept: Accelerator Transmutation of Radioactive Waste (ATW) Transmutation Phase Status of Evidence Systems Technology Maturity A detailed conceptual design of an ATW concept does not exist. Many elements of the concept have not been demonstrated. Quantitative Safety Assessments
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Nuclear Wastes: Technologies for Separations and Transmutation No significant and quantitative risk assessment exists for the ATW, Experience Demonstrating Feasibility Design is still in the preliminary concept phase. Selected Design Features Applicability to Waste Objectives Designed specifically to eliminate both actinides and long-lived fission products. Unique Risk-Reduction Features Operates below criticality and has rapid system response times. Continuous processing of liquid fuel reduces source term and decay heat load. Fuel can be drained to subcritical holding facilities with decay heat removal. Unique Safety Concerns Must demonstrate ability to maintain system integrity at the beam/ target/blanket interface. Liquid fuel eliminates solid lattice and cladding barriers to releases. High, localized beam power can initiate a variety of events. The high power density in the blanket (150% greater than in a PWR) creates a shutdown cooling problem due to decay heat. Processing rates probably will not be sufficiently rapid to mitigate the most severe shutdown heating transient with loss of normal cooling. Large reactivity swings are expected in going from cold shutdown to hot operating conditions. ATW concepts appear to be vulnerable to xenon oscillations. The nonaqueous ATW fueled partly or completely with thorium will be subject to a reactivity transient safety issue at high neutron flux. Uncertainties Beam power of an ATW system would be on the order of 100 times greater than the Los Alamos Meson Physics Facility. Behavior of liquid fuel in blanket and during transport through system is not well known. Nonequilibrium operations not yet addressed.
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Nuclear Wastes: Technologies for Separations and Transmutation Performance with low-enriched fuels well established through actual operating experience. Unique Safety Concerns Larger concentration of transuranics in the fuel than in once-through cycles requires verification of reactivity control. Safety implications of actinide-containing nuclear fuels need to be established. Active systems are required to successfully mitigate accidents. Uncertainties Commercial experience has not involved actinide-containing fuels. Predominant commercial experience limited to slightly enriched uranium. Separation Phase (PUREX process) Status of Evidence Systems Technology Maturity Reprocessing of oxide fuels well established. Quantitative Safety Assessments Risk assessments available for PUREX proces. Experience Demonstrating Feasibility Good experience base for standard reprocessing. Selected Design Features Applicability to Waste Objectives Creates separate actinide and fission product streams. Unique Risk-Reduction Features High state of knowledge of separation processes. Unique Safety Concerns
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Nuclear Wastes: Technologies for Separations and Transmutation Disposition of the nonfissile actinides. Spontaneous fissioning of some actinides increases radiological hazards during reprocessing. Uncertainties Long-term operation of closed fuel cycle. Storage Phase Status of Evidence Same body of information applies to storage waste from all concepts. Selected Design Features Applicability to Waste Objectives Reduction of storage requirements only as part of a broader concept. Unique Risk-Reduction Features Thermal neutron spectrum is the most effective transmutor for many fission products, thus reducing storage requirements for some long-lived isotopes. Unique Safety Concerns None identified. Concept: PBR Transmutation Phase Status of Evidence Systems Technology Maturity Concept feasibility not yet demonstrated. Quantitative Safety Assessments Safety analysis not yet undertaken.
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Nuclear Wastes: Technologies for Separations and Transmutation Experience Demonstrating Feasibility No experience at the power densities (5 MW/liter) and gas flow rates (275 m/s) envisioned. Brookhaven National Laboratory believes that many key technologies have been demonstrated in the military and HTGR experience. Selected Design Features Applicability to Waste Objectives Designed to burn plutonium, MAs, and long-lived fission products. Unique Risk-Reduction Features Low inventories of plutonium, MA, and fission products in reactor. Unique Safety Concerns The effects of burn-up reactivity swings on the operations of such a reactor remain to be evaluated, together with the power distribution issues characteristic of high-thermal-flux systems. Very high power density (5 MW/liter) creates a safety concern. The very high gas-coolant velocity (275 m/s) must be evaluated in terms of emergency cooling requirements. Agglomeration of larger particles could lead to hot spots and containment problems. Uncertainties Entire reactor concept is very preliminary. Separation Phase Status of Evidence Systems Technology Maturity Technology not demonstrated on a total system level. Technology supported by HTGRs and weapons-related reactors. Quantitative Safety Assessments
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Nuclear Wastes: Technologies for Separations and Transmutation No published integrated system safety studies available on either the reactor or the proposed processing facilities. Experience Demonstrating Feasibility Reprocessing of PBR fuel not yet demonstrated. Selected Design Features Applicability to Waste Objectives Burn plutonium, MAs, and long-lived fission products. Store cesium and strontium in an monitored retrievable storage, recycle uranium in LWRs, and recycle unburned plutonium into LWRs as MOX fuel. Dispose of short-lived fission products as low-level waste after decay. Unique Risk-Reduction Features Long-term risk greatly reduced at the expense of greater short-term risk associated with operations. Unique Safety Concerns Safety of reprocessing PBR fuel is not understood. Uncertainties Highly dependent on processes not yet demonstrated (TRUEX, fluoride volatility, electrolysis, etc.). Storage Phase Status of Evidence Same body of information applies to storage waste from all concepts. Selected Design Features Applicability to Waste Objectives The PBR aims to eliminate the need for geologic disposal of high-level wastes. Unique Risk-Reduction Features
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Nuclear Wastes: Technologies for Separations and Transmutation Thermal neutron spectrum is the most effective transmutor for many fission products, thus reducing storage requirements for some long-lived isotopes. Unique Safety Concerns None identified. Uncertainties Ability to eliminate geologic disposal depends on transmutation and separation efficiencies. Concept: Clean Use of Reactor Energy (CURE) Transmutation Phase Status of Evidence Systems Technology Maturity Considers both ALMRs and LWRs, but its emphasis is on ALMRs. Risks are dependent on mix chosen and evaluation of individual systems. Integrated system concepts in early stage of development. Quantitative Safety Assessments Estimated risk will be dependent on evaluation of individual reactor types. Experience Demonstrating Feasibility See experience for individual reactor types. Selected Design Features Applicability to Waste Objectives Mix of fast and thermal reactors can reduce both actinides and long-lived fission products. Unique Risk-Reduction Features See advantages for individual reactor types.
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Nuclear Wastes: Technologies for Separations and Transmutation Unique Safety Concerns See concerns for individual reactor types. Uncertainties CURE is simply a concept for mixing other proposed technologies. Uncertainties are dependent on both the choice of technology and the mix. Largest uncertainty is a method for transmutation of actinides in the near term. Separation Phase Status of Evidence Systems Technology Maturity Advocates aqueous reprocessing and separations technologies. Well adapted to continuous operation and simple process control. The stage for separating cesium, strontium, and targeted isotopes for storage and geologic disposal requires significant technology development. Quantitative Safety Assessment No published integrated system safety assessments. Experience Demonstrating Feasibility Considerable experience with several stages of the CURE concept. Selected Design Features Applicability to Waste Objectives Mix of fast and thermal reactors with modern aqueous reprocessing technology forms basis for a long-range development program to minimize waste. Unique Risk-Reduction Features Proponents claim CURE with transmutation substantially reduces repository risks for both intrusive and leakage scenarios. Unique Safety Concerns
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Nuclear Wastes: Technologies for Separations and Transmutation The risk of multifaceted operations, including some not yet developed, still needs to be examined. Uncertainties Little is known about the step to recover the residual transuranics plus technetium. A large uncertainty with the CURE-type philosophy is a method for transmutation of the MAs in the near term. Storage Phase Status of Evidence Same body of information applies to storage waste from all concepts. Selected Design Features Applicability to Waste Objectives CURE aims to make both the liquid reprocessing stream and the solid waste stream qualify for a low-level waste category. Unique Risk-Reduction Features Concept advocates a mix of facilities that would minimize geologic storage requirements. Unique Safety Concerns None identified. Uncertainties None uniquely applicable to CURE. Concept: Base Case (civilian/military) Transmutation Phase Status of Evidence
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Nuclear Wastes: Technologies for Separations and Transmutation Systems Technology Maturity No transmutation. Quantitative Safety Assessments No transmutation. Experience Demonstrating Feasibility No transmutation. Selected Design Features Applicability to Waste Objectives No transmutation. Unique Risk-Reduction Features No transmutation. Unique Safety Concerns No transmutation. Uncertainties No transmutation. Separation Phase Status of Evidence Systems Technology Maturity No back-end separation required. Quantitative Safety Assessments No back-end separation required. Experience Demonstrating Feasibility
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Nuclear Wastes: Technologies for Separations and Transmutation No back-end separation required. Selected Design Features Applicability to Waste Objectives Permanent storage of unprocessed waste. Unique Risk-Reduction Features Once-through fuel cycle eliminates requirement for separation processes involving high-level waste. Unique Safety Concerns Requires continued mining, milling, conversion, and enrichment, which is a contributor to the radiological hazards of nuclear power. Storage Phase Status of Evidence Same body of information applies to storage waste from all concepts. Selected Design Features Applicability to Waste Objectives All actinides and fission products remain to decay naturally. Unique Risk-Reduction Features None identified Unique Safety Concerns Magnitude of geologic storage requirements is a prime motivation for this evaluation. Uncertainties Uncertainty regarding geological stability and isotopic transport is a prime motivation for this evaluation.
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Nuclear Wastes: Technologies for Separations and Transmutation REFERENCES Kaplan, S., and B. J. Garrick. 1981. On the quantitative definition of risk. Risk Analysis 1(1):11-27. Michaels, G.E. 1992. Impact of Actinide Recycle on Nuclear Fuel Cycle Health Risks . ORNLM-1947. Oak Ridge, Tenn.: Oak Ridge National Laboratory.
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