The Nuclear Renaissance: Producing ... - ACS Publications

Chapter 1

Downloaded by 81.252.202.201 on September 16, 2015 | http://pubs.acs.org Publication Date (Web): October 15, 2010 | doi: 10.1021/bk-2010-1046.ch001

The Nuclear Renaissance: Producing Environmentally Sustainable Nuclear Power Bruce J. Mincher* Aqueous Separations and Radiochemistry Department, Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-6180 USA *[email protected]

A renewed global interest in nuclear power, the so-called nuclear renaissance, is underway. Energy demand continues to rise, and it is now recognized that nuclear energy will be required to meet this demand. The long-term environmental sustainability of expanded nuclear power production will require more efficient processes for the conversion of uranium to energy. Thus, for purposes of increased efficiency of energy production and to reduce the amount of waste interred in a repository it is likely that the reprocessing of spent nuclear fuel, or “closed” fuel cycle, will be more widely adopted in the future. This will be a major component of the development of environmentally sustainable nuclear power. This chapter introduces the symposium book documenting the latest research from around the world with a goal of creating an environmentally sustainable nuclear power industry. Held 16-20 August, 2009 in Washington DC, USA, the symposium hosted scientists from the fuel cycle countries of Canada, China, Germany, Sweden, France, Japan and the USA. The scientists in attendance presented plans and progress for the aqueous separation of fission products and the minor actinides to improve the efficiency of power generation and to minimize the amount of material requiring geological disposal.

© 2010 American Chemical Society In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Introduction A renewed global interest in nuclear power, the so-called nuclear renaissance, is underway. As developed and developing countries continue to expand their populations and economies, energy demand continues to rise, and it is now recognized that nuclear energy will be required to meet this demand. The ascendance of nuclear power is being driven by both political and environmental concerns. The sources of fossil fuels are unreliable. Political turmoil frequently interrupts supply resulting in unpredictable prices. These facts were recognized early by France, which became the first nation to make a major commitment to nuclear electricity generation following the oil embargo of the 1970s. France now generates nearly 80% of its power domestically from a fleet of nearly 60 nuclear reactors. More recently, natural gas supplies to Europe have also proven to be unreliable, and while no other European country has yet announced an expansion in nuclear power plant construction that result seems to be inevitable. Meanwhile, Japan, China, India and the USA are expanding their commitments to nuclear power as a domestic source of energy. In addition to problematic supplies are concerns about the possible contribution of fossil fuels to climate change. A final product of the burning of oil, coal and natural gas is CO2, which is a greenhouse gas. The concentration of CO2 in Earth’s atmosphere is about 380 ppmv, with a pre-industrial revolution value of 280 ppmv being estimated. Although there is not a consensus (1), many believe that the increase in CO2 concentration is raising the average global temperature. Some governments are considering nuclear power as a carbon-free source of energy in an attempt to meet CO2 emission reduction goals. Nuclear power is already an important source of safe, domestic, carbon-free energy. For example about 20% of electricity consumed in the USA and 30% in Europe is produced in nuclear reactors. The percentage as base-load electricity is even higher. However, concerns about waste disposal and weapons proliferation have until now made its expanded adoption politically untenable. Proposals for the new nuclear fuel cycle are designed to address these concerns. The back end of the fuel cycle, where uranium is recycled and long-lived radioactive waste is treated has received special attention. The long-term environmental sustainability of expanded nuclear power production will require more efficient processes for the conversion of uranium to energy. Like fossil fuels, the earth contains a finite supply of uranium. This uranium must be mined, refined and enriched at significant cost prior to fuel fabrication. The light-water reactors in common use today recover only ~1% of the energy potential of the uranium that is used to construct a fuel element (2). The once-through, or “open” fuel cycle, where used reactor fuel is simply disposed in a geological repository is clearly not sensible given this low efficiency of energy extraction. Recovery of unfissioned uranium will significantly extend the reactor fuel inventory into the future. For example, it has been estimated that the recovery of uranium and/or plutonium results in 15% (2) to 30% (3) natural uranium savings. There are additional cost savings in reduced needs for uranium enrichment. 4 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


About 10,500 t of used fuel are already discharged yearly from more than 400 nuclear reactors around the world (4). The rate of spent fuel generation could reach 15,000 t by 2050 (5) and this spent fuel contains appreciable quantities of actinides and fission products, some with very long half-lives. For example, 1 t of UO2 fuel, after 30 GW days of burn-up, contains 950 kg U, 9 kg Pu, 75 g Np, 140 g Am, 47 g Cm and 31 kg fission products (6). With half-lives ranging from 433 years for 241Am to 2.4 million years for 237Np, the α-emitting actinide nuclides determine the radiotoxicity of the waste for periods of time on a geological scale. The partitioning of these radionuclides and their transmutation to short-lived isotopes will significantly reduce the long-term hazard (7). Thus, for purposes of increased efficiency of energy production and to reduce the amount of waste interred in a repository it is likely that the reprocessing of spent nuclear fuel, or “closed” fuel cycle, will be more widely adopted in the future than currently. Thus, the development of the future closed fuel cycle will be a major component of the development of environmentally sustainable nuclear power. This symposium book documents the latest research from around the world with a goal of creating an environmentally sustainable nuclear power industry. Held 16-20 August, 2009 in Washington DC, USA, the symposium hosted scientists from the fuel cycle countries of Canada, China, Germany, Sweden, France, Japan and the USA. The scientists and students in attendance presented plans and progress for the aqueous separation of fission products and the minor actinides to improve the efficiency of power generation and to minimize the amount of material requiring geological disposal. Although much progress has been made, significant challenges remain. These challenges are being overcome by the science-based approaches of dedicated researchers, as revealed in the symposium. This includes not only the development of new separations processes, some being nonaqueous, but also responsible disposition of the final waste-form following separations, as well as development of the analytical techniques necessary for both. The remediation of previously contaminated sites, mainly contaminated by cold-war activities rather than by power generation, is also a concern in the demonstration to a suspicious public that nuclear power is environmentally sustainable.

Alternatives to PUREX Aqueous reprocessing currently begins with the chopping and dissolution of the used reactor fuel in nitric acid. The resulting acidic and highly radioactive solution is then contacted with an appropriate organic solvent to selectively recover desired metal ions. Although no longer practiced in the USA, the PUREX (Plutonium Uranium Refining by EXtraction) solvent extraction process for the recycle of light-water reactor fuels has been in use for decades. Consisting of 30% tributyl phosphate (TBP) in an alkane diluent, the process can either partition uranium separately or co-extract uranium, neptunium, and plutonium depending on how the valence states of the latter metals are set during pre-treatment. The history of its development and use over six decades has been reviewed by McKay (8). The metal-loaded solvent is then stripped to recover the uranium and/or 5 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


neptunium and plutonium and recycled for further contacts with newly dissolved fuel. Both aqueous and non-aqueous alternative extraction processes are under development. Among aqueous processes under development are the use of alkylamides which provide good separation factors for the partitioning of U and Pu from fission products and therefore have been proposed as replacement compounds for TBP (9). The most mature example is the European DIAMEX (DIAmide EXtraction) process, based upon the malonamide dimethyl dioctyl hexylethoxymalonamide (DMDOHEMA) (10). A related diglycolamide extractant N, N, N′,N′-tetraoctyl-3-oxapentane-1,5-diamide (TODGA) has received study in Europe and Japan (11). Unlike TBP, the amides do not contain phosphorous. This may be an important advantage in that they are incinerable, simplifying the waste disposal of spent solvents. This is referred to as the CHON principal, in which ligand design favors molecules containing only C, H, O and N atoms. Other benefits include the production of relatively benign radiolysis products. The potential advantages and uses of amides as actinide extractants were reviewed by Gasparini and Grossi (12). This symposium contains several papers discussing the use of amidic compounds as actinide extractants. Several research groups reported the development of non-aqueous separation strategies. The demonstration of the direct dissolution of oxides by ligand assisted SFE, containing the CO2-soluble TBP•HNO3 complex, opens up novel opportunities for fuel reprocessing while minimizing secondary waste generation. The separation of uranium from lanthanide fission products is possible directly from their oxides by proper selection of operating temperature and pressure. Supercritical CO2 and ionic liquids are green separations reagents because they minimize secondary waste and rely on relatively harmless diluents for their metal complexing agents. Their use in reprocessing and in uranium recovery from waste is reviewed here.

Minor Actinide Separations Traditionally, the remaining fission products such as Cs, Sr and the lanthanides, as well as the minor actinides Np, Am and Cm have been disposed as high-level waste following the PUREX extraction. Most current plans call for vitrification of the waste, and storage of the glass inside metal containers disposed in a geological repository. However, these minor actinides are the major contributors to the radiotoxicity of the high-level waste after 1000 years of decay time. Their presence has provoked requirements that propose to guarantee the integrity of the waste forms over unimaginably long periods of time. Current fuel cycle proposals call for the removal and fission of the minor actinides in fast reactors. Their conversion into short-lived fission products, in the process known as partitioning and transmutation, would eliminate the risks associated with their long-term underground storage. Neptunium can be partitioned using the PUREX process given careful attention to valence control; however, trivalent Am and Cm require the development of new approaches. Their separation from the fission-product 6 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


lanthanides is one of the more formidable challenges associated with the design of the advanced fuel cycle. The lanthanides represent about a third of the total fission product inventory and they have very similar chemical properties. They also have large neutron capture cross sections and poor metal alloy properties and thus they can not be incorporated into fast reactor fuel. A separation amenable to currently existing aqueous solvent extraction processes is therefore desired, and the current approaches being studied around the world were presented in this symposium. Among them, the TALSPEAK (Trivalent Actinide Lanthanide Separation by Phosphorous reagent Extraction from Aqueous Komplexes) process is favored in the USA. It is, however, a complicated process and although it has undergone flow-sheet testing much remains to be elucidated about the mechanism by which the lanthanides are complexed and extracted by diethylhexylphosphoric acid (HDEHP) while the actinides are complexed and retained in the aqueous phase by diethylenetriaminepentaacetic acid (DTPA) in the presence of molar amounts of lactic acid (13). The lactic acid buffer allows for αAm/Eu separation factors of ~ 90, with the use of most other buffer systems being somewhat less selective. A novel approach to combine TALSPEAK and TRUEX (TRansUranic EXtraction) into one step to simplify the currently-proposed fuel cycle has been proposed and is discussed here. Simplification of current proposals is considered to be necessary prior to practical implementation. In European work, current investigation into trivalent actinide/trivalent lanthanide partitioning is focused on a process called SANEX (Selective ActiNide EXtraction). SANEX adheres to the CHON principal by using heterocyclic nitrogen donor ligands in combination with diamides to selectively extract the trivalent actinides from an actinide/lanthanide acidic solution. Kolarik (14) and Ekberg et al (15) have reviewed the history of the development of these heterocyclic nitrogen donor ligands including attempts to improve their radiolytic and hydrolytic stability. Among the most promising of these compounds are various derivatives of the 2,6-bistriazinylpyridines (BTPs). Separation factors as high as αAm/Eu of ~ 120 for extractions from 1 M HNO3 have been achieved (16).. Other techniques for separation of the trivalent actinides from the trivalent lanthanides are also being considered. Among them are the use of higher oxidation states of americium (17), and the use of soft-donor-S ligands such as dithiophosphinic acids, and column chromatographic techniques based on the BTPs as discussed in this symposium. Each of these techniques is less mature than TALSPEAK or SANEX but nonetheless show promise in achieving this difficult separation.

Radiation Chemistry The design of successful separations for the nuclear fuel cycle relies on the use of reagents that are stable in an acidic radioactive environment. Hydrolysis and radiolysis of ligands, diluents and solvent modifiers can have deleterious effects on metal distribution ratios, separation factors and solvent physical properties (18, 19). For example, the introduction of solvent reconditioning steps to the PUREX process was necessary to remove TBP degradation products which 7 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


interfered with separation factors for uranium and fission products, which adds to the cost and complexity of process design. For the new fuel cycle increased attention is being given to these issues, and the evolution of the ligands in the DIAMEX and SANEX processes shows a series of structural modifications that have evolved meant to address them. The radiolytic production of transient reactive radical species from irradiated solution such as •OH and •NO3 and their reactions with solution constituents are now recognized to be major sources of radiation damage. The increased attention being devoted to radiation chemistry in solvent design is reflected in the fact that an entire session of the symposium was devoted to this phenomenon. Overviews of the chemistry of irradiated nitric acid, and for radiation chemistry in solvent extraction are provided here. Research findings related to specific solvent systems being developed for separation of the trivalent actinides from the trivalent lanthanides are also presented. A common source of undesirable products in irradiated nitric acid is due to nitration reactions and the mechanism of radiolytic nitration is given special attention where it is shown that under acidic conditions much nitration is due to the radiolytic production of nitrous acid. By far, most radiation chemical research has been performed with γ-sources, despite that a great deal of α-activity is to be expected in the fuel dissolution. Given the much higher linear energy transfer of the α-particle it might be expected that its effects would be different. A comparison of both radiation sources in the radiolytic degradation of DMDOHEMA interestingly reports that α-effects were less severe. A recent Japanese report has come to a similar conclusion for TODGA radiolysis (20). Radiation effects are not a black box, and research into the mechanisms of solvent system radiolysis should help to design more robust systems with less demanding solvent reconditioning needs.

Chemistry in the Repository Following the separations described above the fuel dissolution contains primarily the lanthanides and possibly other fission products such as Cs and Sr, depending on whether these latter activities have been removed in a dedicated step (21). In the absence of the actinides the amount of time required for decay to low levels is greatly decreased. Most scenarios call for the vitrification of such radioactive waste as a glass, to be contained in metal canisters destined for deep disposal in a geological repository. Low-level waste might simply be grouted prior to disposal. In countries devoted to the once-through, open fuel cycle, used fuel elements are proposed to be containerized and then disposed in a deep repository. Canada, for example, uses natural uranium in its reactors and will not likely need to reprocess to recover unfissioned 235U. The corrosion of UO2 under repository conditions is discussed here. Multiple engineered barriers will provide containment of the radioactive materials in isolation from the environment. Among them are the non-leachable waste forms themselves, being glass or metal-clad oxide fuels. These are stored in appropriate canisters, which are embedded in absorbent clays such as bentonite. Repository locations will be selected in the most appropriate available geologically stable locations. 8 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Despite these precautions it must be assumed that after many thousands of years the containment canisters will be breached. Additionally, many surface sites remain radioactively contaminated primarily due to weapons production and testing associated with the cold war and the testing of experimental reactors. For these reasons major research efforts continue to be devoted to the behavior of radionuclides, especially the actinides, in the environment and the remediation of contaminated sites. Once released to the environment, the mobility and transport of the actinides to sites remote from that release will be limited by their speciation. The form in which they occur will be determined by chemical and biological processes. Among important chemical process are carbonate formation, redox cycling or hydrolysis as discussed here.

Conclusion This symposium brought together many of the scientists and engineers working globally to ensure a continued supply of clean nuclear energy for growing economies and populations. Environmentally sustainable nuclear energy production requires the recycling of uranium from used light water reactor fuels, and the partitioning and transmutation, rather than the geological disposition, of the minor actinides. Several approaches are being investigated world-wide. Some amount of shorter-lived radioactive waste will still require deep geological disposition, and the chemistry of the repository is also being investigated to provide an understanding of the best containment strategies and the behavior of radionuclides in the environment. Taken in sum, the science-based approach to used fuel recycling and waste disposal revealed in this symposium illustrates that the environmental responsibilities of future nuclear energy production are being given serious attention. Progress is being made in the difficult issues of actinide/lanthanide separations and understanding radionuclide speciation. This is necessary for the successful fulfillment of the nuclear energy renaissance now in progress.

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The Nuclear Renaissance: Producing ... - ACS Publications

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