Waste Transmutation: The Nuclear Alchemy Gamble

By Annie Makhijani and Hisham Zerriffi

"Research on partitioning and transmutation is rather seductive to all of us. It requires new reprocessing techniques, new fuel developments, additional nuclear data, new reactors and irradiation facilities, new waste treatment and disposal concepts, and specific safety studies. The global nuclear scientific and engineering community is challenged by this opportunity." "Everybody realizes however that this voyage to the promised land will pass a desert with a lot of mountains and that we are not so sure that the horizon will be as bright as one can hope." ---Paul Govaerts, SCK-CEN (Belgian Nuclear Research Center). "Welcome Address to the Fifth International Information and Exchange Meeting on Actinide and Fission Product Partitioning and Transmutation," Mol, Belgium, 25-27 November 1998. "The [transmutation] program is expected to serve to revitalize the nuclear R&D in general, and also to attract capable young researchers dedicated to bringing the nuclear option into the 21st century in a healthy state." ---"OMEGA Programme: Partitioning and Transmutation R&D Programme of Japan," in Organisation for Economic Co-Operation and Development/Nuclear Energy Agency, Actinide and Fission Product Partitioning and Transmutation: Status and Assessment Report, Paris: OECD/NEA 1999, page 253. One of the biggest obstacles facing the nuclear industry is what to do with the nuclear waste generated in the form of spent fuel discharged from commercial reactors or in the form of high-level waste originating from the extraction of plutonium from spent fuel. Most countries' preferred option for the isolation of nuclear waste from the public and the environment is to bury it underground in a deep geological repository. However, because the spent fuel and the high-level waste contain a number of radionuclides that have very long half-lives (thousands of years to millions of years) it is admitted that it is impossible to ensure the isolation of the waste for such long periods of time. Besides the likelihood of leakage of some long-lived radionuclides, it is also impossible to guarantee against human intrusion (intentional or inadvertent). Table 1 shows the main long-lived radionuclides of concern. Table 1: Main Long-lived Radionuclides of Concern Radionuclide (half-life in years, to two significant digits) Type Impact Transmutation Potential Transmutation Problems Strontium-90 (29) Medium-lived Fission Product Contributes to initial heat of waste. Determines repository capacity. Intrusion scenario dose. Behaves like calcium in the body None Cannot be transmuted due to small neutron cross-section. Forms a large part of the heat of spent fuel and high level waste and therefore limits increase in repository capacity from transmutation.. Cesium-137 (30) Same Same except behaves like potassium in the body. Also radiation barrier to proliferation. None Same. Also, separation from fissile materials eliminates radiation shielding for proliferation prevention. Tin-126 (100,000) Long-Lived Fission Product Groundwater release Difficult Difficult to separate from spent fuel/HLW. Long time to transmute. Lower isotopes result in new production of radionuclide Selenium-79 (60,000) Same Same None Same Cesium-135 (2.3 million) Same Same None Formation of more Cs-135 from Cs-133. Isotopic separation difficult due to presence of Cs-137 Zirconium-93 (1.5 million) Activation Product Groundwater release None Presence of stable Zr isotopes would produce more Zr-93. Would require expensive isotopic separation. Carbon-14 (5,700) Activation Product Groundwater release and/or air release as CO2; incorporation into living matter None Small neutron capture cross-section. Often released as gas from reprocessing operations Chlorine-36 (300,000) Activation Product Groundwater None Presence of natural Cl-35 would generate more Cl-36 Technetium-99 (210,000) Long-Lived Fission Product Groundwater Release. Affects thyroid Yes. Requires slow neutrons Would require several transmutation cycles Iodine-129 (16 million) Long-Lived Fission Product Same Yes. Requires slow neutrons Same. Also, difficulty in capturing during separation. Difficulty in fabricating targets. Could pose corrosion problems Uranium (mainly U-238, 4.5 billion) Actinide source material Forms bulk of spent fuel (~94 percent by weight). Has higher radioactivity than TRU waste slated for geologic disposal None. Would be separated and disposed of as LLW or used like depleted uranium U-238 transmutation would result in the generation of more Pu-239 defeating the purpose of transmutation as a waste management strategy. Would essentially create a breeder reactor economy. Americium-241 (430) Actinide Gamma-emitter. Human intrusion. Groundwater release (parent of U-233). Radiotoxicity Preferably in fast reactors Would require multiple separation and irradiation cycles. Would result in creation of curium which would make subsequent cycles more difficult Neptunium-237 (2.1 million) Actinide Groundwater release Preferably in fast reactor Formation of more radioactive shorter-lived Pu-238 Curium-244 (18) Actinide Highly radioactive alpha and gamma emitter. Contributes to heat of spent fuel. Difficult. Requires fast reactor Difficult to separate from other actinides in HLW due to handling and chemistry problems. Would require multi-recycling along with other actinides. Could require storage of decades or even a century. More Cm-244 and other Cm isotopes created in irradiation of lower actinides (Pu and Am). Plutonium (mainly Pu-239, 24,000) Actinide Pu-239 Fissile. Radiotoxicity. Goes to bones Fast reactor required for non-fissile isotopes. Neutron capture forms higher isotopes and higher actinides (e.g. Am and Cm). Table is adapted and expanded from Organisation for Economic Co-Operation and Development/Nuclear Energy Agency, Actinide and Fission Product Partitioning and Transmutation: Proceedings of the Fifth International Information Exchange Meeting. Mol, Belgium. 25-27 November 1998. Paris: OECD/NEA 1999, p. 470, and Organisation for Economic Co-Operation and Development/Nuclear Energy Agency, Actinide and Fission Product Partitioning and Transmutation: Status and Assessment Report, Paris: OECD/NEA 1999. The extremely difficult questions regarding ensuring isolation of waste to a degree sufficient to prevent severe contamination of resources, notably water resources, has made the siting of repositories a contentious scientific and policy issue and has been at the center of much of the public concern and opposition to repositories. Further, the political expediency that has frequently accompanied the selection of sites for study has intensified this opposition. While programs for siting repositories for spent fuel and high level waste are in various stages in different parts of the world, these still face immense scientific hurdles and intense public opposition. In the United States, which has a target date for opening a repository that could be as early as 2010, there are still no final environmental standards for the protection of the health of future generations and the environment from the proposed repository at Yucca Mountain.1 The difficulties and questions associated with repository siting, notably the extremely long periods of isolation required, have caused some to view the transmutation of long-lived radionuclides into short-lived ones as a potential solution to the problem of radioactive waste management. Transmutation is done by inducing nuclear reactions of various types in the nuclei of long-lived radionuclides. The theory is that a transmutation program would transform the problem of long-term isolation into a far less difficult one of storage for several decades or a few hundred years. The theoretical promise has led proponents of transmutation to claim that it would greatly decrease the problems associated with long-term management. Occasionally, they have even claimed that it might eliminate the need for a repository, though such claims have tended to recede as investigations into the practicalities of transmutation have progressed. At the same time, environmental, waste management, cost, and proliferation concerns have risen. IEER has evaluated the merits and problems associated with transmutation as a waste management concept. This article summarizes our findings and recommendations.2 Transmutation basics Transmutation is the transformation of a radionuclide into another radionuclide, or into two or more radionuclides. Transmutation involves nuclear reactions that would occur in some form of nuclear reactor. A variety of reactor schemes have been proposed, but they all possess a common characteristic: a substantial amount of energy must be delivered to the nucleus of a long-lived radionuclide in order to induce a nuclear reaction that would convert it into a short-lived radionuclide or a stable element. The figure above shows the main components of an idealized transmutation system. A reprocessing plant is needed to sort out the candidate radionuclides slated for transmutation by separating certain long-lived radionuclides from the others. (In the context of transmutation, reprocessing is also called "separation" or "partitioning.") This allows the selective conversion of long-lived radionuclides into short-lived ones when they are irradiated in a reactor. Without reprocessing, the opposite kind of nuclear reactions would cause a counterproductive conversion of short-lived radionuclides into long-lived ones. The fabrication facility manufactures the long-lived radionuclides into fuel and/or targets that are then sent to the transmutation facility, which may consist of a reactor, or a combination of an accelerator, heavy metal target, and sub-critical reactor. The neutron induced reactions in the reactor transmute the long-lived fission products into short-lived ones; they also fission the actinides, such as plutonium, creating new fission products. Most of these fission products are short-lived, but new long-lived fission products are also created (see below). The actinides can also absorb neutrons, resulting in the creation of higher-mass actinides (see below). Further, not all actinides can be transmuted before the nuclear reactor becomes very inefficient. Hence, a number of passes through the reprocessing, fuel fabrication, and reactor facilities are needed in order to transmute most long-lived radionuclides. But even elaborate schemes cannot practically convert all long-lived radionuclides into short-lived ones. Transmutation of separated uranium, which constitutes about 94 percent of the weight of light water reactor spent fuel and which is very long-lived and generally contaminated with some fission products, would be counterproductive since the main transmutation route for almost all the uranium would be to convert uranium-238 into plutonium-239. Other long-lived fission products as well as residual transuranic actinides would also need disposal. Hence, a repository, as well as other waste management and storage facilities would still be an essential part of transmutation schemes. The merits of transmutation schemes and the difficulties associated with them become clearer if we understand some basics about the physics of transmutation. The physics of transmutation For nuclear waste management there are two transmutation reactions which are important: neutron capture and fission.3 The goal is that long-lived radionuclides be transformed into short-lived radionuclides. The absorption of a neutron by iodine-129 and by cesium-135 are two such reactions (with half-lives shown in parentheses, rounded to two significant figures):4 I-129 (1.6x107 years) + n ® I-130m (9 minutes)® I-130 (12 hours) ® Xe-130 (stable) + e Cs-135 (2.3x106 years) + n ® Cs-136m (19 seconds) ® Cs-136 (13 days) ® Ba-136m (0.3 seconds) + e ® Ba-136 (stable) However, neutron capture can also result in the creation of long-lived radionuclides, defeating the purpose of transmutation, as would be the case with Cs-133: Cs-133 (stable) + n ® Cs-134 (2.1 years) + n ® Cs-135 (2.3x106 years) The cesium in spent fuel is a mixture of both Cs-133 and Cs-135 isotopes which cannot feasibly be separated, in part because the presence of the very radioactive Cs-137 isotope makes the handling and processing of the cesium extremely difficult, expensive, and dangerous. Thus, it is easy to see that the benefit of transmuting Cs-135 would be negated by the production of more Cs-135 from the neutron capture of Cs-133. The following example (with half-lives shown in parentheses) shows how plutonium-239 would be transmuted by two successive reactions: Pu-239 (24,000 years) + n ® Pu-240 (6,500 years) + n ® Pu-241 (14 years) However, further neutron capture would give Pu-242, which has a long half-life: Pu-241 (14.4 years) + n ® Pu-242 (380,000 years) This illustrates that transmutation nuclear reactions would need to be closely controlled so that there is an overall change from long-lived to short-lived radionuclides without a build up of new long-lived radionuclides. Note also that neutron capture by plutonium-239 and -240 would not solve the problem of eliminating long-lived radionuclides even if all the plutonium were converted to short-lived plutonium-241. This is because plutonium-241 has an entire decay chain associated with it. It decays into americium-241, which has a half-live of 430 years. Amercium-241 in turn decays into neptunium-237, which has a half life of over 2 million years. Hence, significant reduction of long-lived actinides, such as plutonium, generally necessitates fission of the nuclei. Fission transmutation reactions produce mostly short-lived fission products that decay into stable elements, but some of these short-lived fission products can also decay into long-lived ones. The example below shows the production of two short-lived fission products, tellurium and molybdenum. They both undergo a series of beta decays. The decay chain of molybdenum-102 consists of short lived radionuclides until it reaches stable (non-radioactive) ruthenium-102. Tellurium decays into long-lived cesium-135. Pu-239 + n ® Pu-240 ® Te-135 (19 seconds) + Mo-102 (11 minutes) + 3 n ¯¯ I-135 (6.6 hours) + e Tc-102m (4.4 minutes) + e ¯ ¯ Xe-135 m (15 minutes) + e Tc-102 (5.3 seconds) ¯ ¯ Xe-135 (9.1 hours) Ru-102 (stable) + e ¯ Cs-135m (53 minutes) + e ¯ Cs-135 (2.3x106 years) Proposed transmutation schemes Various schemes have been proposed for transmutation. Three types of reactors (light water reactors, fast reactors, and sub-critical reactors) and two types of reprocessing have been proposed. Table 2 shows the type or types of reprocessing associated with each type of reactor and the radionuclides that would be candidates for transmutation. Most transmutation schemes would use a combination of reactors and associated reprocessing technologies. For example, in one scheme, light water reactors would be fueled with mixed oxide (MOX) fuel - that is, fuel made with plutonium extracted from low-enriched uranium spent fuel. The MOX spent fuel then would be reprocessed and the transuranic actinides would be extracted to fuel a fast neutron reactor (commonly called a breeder reactor). The fast reactor fuel would, in turn, be reprocessed and the remaining actinides would fuel a sub-critical accelerator driven reactor. Table 2: Transmutation schemes Reactors and neutron sources Reprocessing and radionuclides Comments Light water reactors (LWRs) (the most common type of commercial nuclear reactor) The reactor is critical and fueled with either low-enriched uranium or mixed oxide uranium-plutonium fuel. Reprocessing: aqueous Radionuclides: Primarily plutonium, Tc-99, I-129. Creates high proportion of higher mass actinides with associated severe radiation hazards Reprocessing creates large amounts of liquid radioactive waste Issues of reactor safety Cannot fission most actinides Heavy transuranic build-up, creating waste management problems Fast reactors: The reactor is critical and can be fueled with plutonium, uranium or, potentially, fuel containing some minor actinides. Reprocessing: mostly dry in advanced schemes. Radionuclides: Plutonium and possibly minor actinides. Tc-99 and I-129 may be possible but only in moderated targets outside the reactor core. The development of fast reactors has been crippled by persistent problems Fission products are not efficiently transmuted Heavy transuranic build-up though to a lesser extent than with LWRs Issues of reactor safety Sub-critical reactors: an accelerator-target system provides fast neutrons to a sub-critical reactor Reprocessing: the reprocessing can be all aqueous or all dry or a combination of the two Radionuclides: plutonium and minor actinides. Tc-99 and I-129 may be possible but only in moderated targets outside the reactor core. Sub-critical reactors are only at the R&D stage Cost is projected to be high. Reactor safety still an issue Fission products are not efficiently transmuted None of these schemes can, for either fundamental physical reasons or practical reasons, transmute uranium, cesium-135, carbon-14, or some other radionuclides. Table 1 shows the various radionuclides of concern from the point of view of long-term management and their status with respect to various transmutation schemes. Residual Waste Transmutation does not eliminate the need for a repository for high-level waste and spent fuel. The theoretical schemes shown above cannot be translated into a practical reality that would eliminate almost all long-lived radionuclides. First, no transmutation scheme is able to deal with all of the radionuclides of concern since many cannot be transmuted for practical purposes (see example of Cs-133 and Cs-135, above). Second, transmutation of Tc-99 and I-129 is not 100% effective, even with multiple passes through the reactor, and new long-lived fission products are created from the fission of the actinides. Third, fissioning of the actinides is not 100% effective. For instance, in the best estimate of any proposed scheme, transmuting 906 metric tons of transuranics (anticipated to be produced by US nuclear reactors during their licensed lifetimes) would leave a residual of 2.4 metric tons. The composition of the residual transuranic waste would be shifted towards higher isotope actinides and the waste would thus be more radioactive. This would pose greater radiological risks and complicate disposal. Finally, since cesium-137 will be disposed of in the repository with cesium-135, the large amount of heat generated by it would mean that the space requirements for disposal could be considerable.5 Only storage of long-lived wastes for hundreds of years, with its attendant uncertainties, risks, and costs, would alleviate this repository capacity problem. Besides failing to deal with the uranium, which accounts for about 94 percent of the weight of radioactive material in spent fuel, and with significant amounts of long-lived transuranic radionuclides and fission products, transmutation would create significant quantities of additional waste, particularly if aqueous reprocessing is used. (See data on waste generation from once-through LEU and MOX fuel cycles page 9-11). It would also shift some material from geologic disposal to low-level waste disposal, particularly if, as has been inappropriately proposed, the uranium is managed as "low-level" waste. This could result in an even greater overall radiological risk to the public, compared to disposal of all spent fuel in an appropriately selected and engineered repository. Transmutation, even in the context of a phase-out of nuclear power, would also require decades to implement and possibly centuries to complete.6 This may require institutional control over the waste for time periods much longer than is feasible or desirable. Implications of Transmutation The implementation of any of the transmutation schemes discussed above would also have a number of implications for nuclear proliferation, the environment and human health, safety, cost, and the future of nuclear power. Proliferation. All transmutation schemes require reprocessing of transuranic radionuclides. While these schemes may not yield materials attractive to weapons designers in nuclear weapons states, they can be used to make nuclear weapons and would pose significant proliferation risks in that non-state groups or non-weapons states might seek to acquire and use them. Even the reprocessing methods that are labeled as proliferation resistant, such as pyroprocessing, can be easily modified to allow for the extraction of plutonium pure enough to make weapons. These types of facilities may in fact increase proliferation risks due to their compact size and potential problems in developing adequate safeguards. Furthermore, promotion of transmutation as a waste management tool may result in the widespread transfer of this technology. The separation of isotopes like neptunium-237 and americium-241 would also increase proliferation risks, since both of these radionuclides can also be used to make nuclear weapons. Creating and implementing schemes that greatly increase separation of weapons-usable material will considerably increase the risks of proliferation. Environment and Health. Reprocessing, which is required by all transmutation schemes, is one the most damaging components of the fuel cycle. It results in large volumes of waste and radioactive emissions to air and water. Its health impacts on workers, off-site residents, and even far away populations are well documented. For instance, health and environmental concerns are the basis of the demands of Ireland, Norway, Iceland and other countries that Britain and France eliminate their so-called "low-level" radioactive waste discharges into the seas. Because fuel fabrication does not involve the production of liquid waste, its effects are mainly restricted to workers and are on the same order as for workers in the reprocessing sector. The increased radiological risk of handling fuel that has been repeatedly irradiated is cause for serious concern. Finally, the increased transportation of high level waste required under a number of transmutation schemes would increase the probability of a transportation accident with its attendant effects. Reactor Safety. Transmutation would require the development and implementation of new reactor technologies and/or the expanded use of existing reactors. Some of these new reactors have been described as "inherently safe." However, increases in certain safety features, in comparison with existing reactors, is countered by decreases in other safety features and the creation of new safety problems unique to the new reactor designs. For example, some feedback effects that help prevent a runaway reaction in existing reactors do not exist in some transmutation reactors. For accelerator based systems, the ability to shut off the neutron source and the fact that the reactor is ordinarily sub-critical provide certain safety advantages. On the other hand, these systems rely strongly on the ability to shut off the neutron source in an emergency. Also, it may be necessary to ensure that the external neutron source is not operating at full power when fresh fuel is in the reactor or else the reactor could become supercritical. Cost. The cost of transmutation, particularly for the advanced schemes that would be required in order to have significant reduction of actinides, is prohibitively expensive. Furthermore, while electricity would be produced to offset these costs, it is highly unlikely that these revenues will be sufficient. Transmutation would likely require tens of billions of dollars to develop, and additional large subsidies even during operations, when electric power sales are expected to generate some revenue. Continuation of Nuclear Power. Transmutation is not only considered in the context of managing the waste from the current generation of nuclear reactors (i.e. as part of a phase-out of nuclear power). Most transmutation schemes, particularly in Europe and Japan, assume an indefinite continuation of nuclear power, with transmutation as one part of a new nuclear fuel cycle. By supposedly solving some of the current problems with nuclear power, transmutation is seen by some as essential to ensuring the continued growth of nuclear power. Conclusions and Recommendations Our main finding is that transmutation schemes will not solve long-term waste management problems. Almost all the weight of the waste proposed for transmutation consists of uranium, which would, according to current official proposals, be treated as low-level radioactive waste and be disposed of in ways that will pose far greater risks than disposal in a carefully selected and engineered repository. In addition, considerable quantities of transuranic materials would remain after transmutation, along with long-lived fission products. Large quantities of new waste would be created, along with new proliferation risks and high costs. Despite these severe limitations, transmutation continues to be seen by some as a "seductive" area of research and essential for revitalizing the "nuclear option." The evaluations that have promoted transmutation as a waste management technology are seriously deficient in their analysis and have been made mainly by those who would like to see a continuation of nuclear power. In light of these conclusions, IEER's main recommendation is that, because there is no sound technical basis for proceeding, transmutation should be abandoned as a waste management technology. GLOSSARY Actinide: A group of elements high on the periodic table which includes uranium, plutonium, neptunium, and americium among others. Transuranic actinide refers to those actinides above uranium on the periodic table, primarily plutonium. Minor actinides refers to those actinides other than uranium and plutonium (primarily neptunium, americium, and curium). Elements belonging to the actinide group have broadly similar chemical properties. Aqueous separation: The use of an aqueous medium -- for example, nitric acid in water -- to enable the separation of radionuclides. Beta decay: The emission of electrons or positrons (particles identical to electrons, but with a positive electrical charge) from the nucleus of an element in the process of radioactive decay of the element. Decay chain: A series of radioactive decays leading to a stable nucleus. Dry separation: The use of electrochemical techniques to separate radionuclides Fission product: Any atom created by the fission of a heavy element. Fission products are radioactive (generally by beta decay). Neutron: An elementary particle slightly heavier than a proton, with no electric charge. The nucleus of an atom consists of protons and neutrons (the number of protons determines the element while the total number of nucleons determines the isotope). Neutron capture refers to the absorption of a neutron by a nucleus to form a new isotope. Pyroprocessing: A form of dry electrochemical separation proposed for use with metal-based Reprocessing: A generic term for the separation of elements in irradiated nuclear fuel. Sub-critical reactor: A nuclear reactor that is configured to operate with an external source of neutrons to supplement internally generated neutrons to maintain the chain reaction Supercritical: When each fission in a reactor results in more than one subsequent fission, resulting in a runaway chain reaction except in carefully controlled cases when reactor power is being increased in a controlled way by making it slightly supercritical for brief periods. Target: In the context of proton-accelerator transmutation schemes, a material which, when struck with protons from the accelerator, emits neutrons through a process called spallation. The term is also used for separated radionuclides that are formed into targets for irradiation.