Setting the Stage for International Spent Nuclear Fuel Storage Facilities: International Workshop Proceedings (2008)

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14 On the Problem of Creating Regional International Storage Facilities for Spent Nuclear Fuel (Based on the Russian Example)* Nikolay P. Laverov Russian Academy of Sciences WHY IS IT NECESSARY TO ACCELERATE THE CREATION OF AN INTERNATIONAL SPENT NUCLEAR FUEL STORAGE FACILITY? Almost 170,000 metric tons of equivalent heavy metal from spent fuel from commercial reactors and more than 60,000 fuel assemblies from research reactors are currently being stored worldwide. Spent fuel is accumulating at a substan- tially faster rate than it is being reprocessed. An increasing number of countries have poorly developed industrial sectors and lack the necessary experience and personnel engaged in handling spent nuclear fuel. The world undoubtedly faces an increasing threat from radiation-related danger. Recognizing the serious potential consequences of radiation terrorism, Russia’s leaders and public have focused constant attention in recent years on the reliable long-term (50-100 years) storage of spent fuel as one of the most important elements of the fuel cycle. Important steps have been taken with re- gard to international efforts in the scientific-technical, socioeconomic, and legal sectors, including matters related to the creation of a regional international spent fuel storage facility in Russia. In our opinion, multinational agreements on the creation of a spent fuel stor- age facility in Russia could be implemented under the aegis of the International Atomic Energy Agency (IAEA). Here we are counting on the fact that creation of such a facility will entail application of the world’s best technologies for design and implementation of the storage process to ensure the safety of the population *Translated from the Russian by Kelly Robbins. 93 

94 SPENT NUCLEAR FUEL STORAGE FACILITIES and reliable physical protection of the materials, transportation, high-quality containers, methods for analyzing the condition of the fuel rods, licensing and guarantees, hiring and training of personnel, site selection, provision of account- ing and control of operating status, and possibilities for professional exchanges with other similar facilities. We proceed based on the belief that the creation of international regional spent fuel storage facilities will undoubtedly promote nonproliferation of nuclear materials and should be categorized as an antiterrorism measure. WHAT PLACE MUST AN INTERNATIONAL SPENT FUEL STORAGE FACILITY HOLD IN THE FUEL-CYCLE SYSTEM? This problem has been discussed frequently in recent years at scientific con- ferences, seminars, and councils to review problems of managing spent nuclear fuel and high-level wastes of civilian and military origin. Russia adopted and until recently operated a system in which technological wastes from major radiochemical plants were stored in liquid form deep under- ground at facilities adjoining the plants where they were produced. These wastes were stored in the water-bearing sedimentary layers bounded at the top and bottom by poorly penetrable clay covers (similar to the formations in which oil and gas deposits are found). This prevented the wastes from having a substantial impact on the biosphere. In Russia, spent nuclear fuel was never viewed as radioactive waste; there- fore, its underground disposal was not considered in plans for the development of the nuclear power industry. An insignificant portion of Russia’s spent fuel was reprocessed at the Mayak enterprise. Most spent fuel from nuclear power plants is today concentrated at the plants, whose storage potentials are already full to capacity. At Mayak, vitrified technical waste is kept in a surface facility on the grounds of the enterprise. Spent fuel planned for reprocessing is kept in a pool. In selecting a site for an international spent fuel storage facility, Federal Atomic Energy Agency (Rosatom) personnel and Russian scientists considered several options within the fuel-cycle system: • an underground storage facility for spent nuclear fuel and high-level waste not located on the premises of any of the fuel-cycle enterprises (Kola peninsula); • an underground international spent fuel facility near the Krasnoyarsk enterprise (Siberia), where there is a Russian-built surface spent fuel storage facility and an underground liquid waste repository, with construction of a spent fuel reprocessing enterprise planned; and • an underground international spent fuel storage facility on the grounds of a major natural uranium mining enterprise (Krasnokamensk, Siberia). 

CREATING REGIONAL INTERNATIONAL STORAGE FACILITIES 95 Geologists and specialists in the fields of mining and radioecology support the proposal for the Krasnokamensk site. A final decision has not yet been made. However, a number of foreign experts (especially in the United States) oppose the idea of having the reprocessing plant and the international spent fuel storage facility located at the same site. International efforts under the patronage of the IAEA to develop optimal regional systems for spent fuel storage seem very much needed. Longstanding relations among countries that store spent fuel and those that supply it must be taken into account, as well as guarantees, controls, systems for managing the stor- age process, resolution of joint technological questions common to the fuel cycle, and guaranteed safety. Russia is prepared to participate actively in these efforts. SPECIAL CHARACTERISTICS OF SPENT FUEL THAT DETERMINE THE CONDITIONS FOR ITS STORAGE An analysis of underground spent fuel repositories that are currently operat- ing or under construction shows that an assessment of the risk of container seal failure involves consideration of a wide range of natural catastrophic phenomena, particularly those associated with hydrogeological processes. The geochemical aspects have received intense study in Russia. It is well known that nuclear fuel is based on the fuel element, a long, nar- row tube made of corrosion-resistant zirconium alloy (or other metals) and filled with uranium dioxide (UO2) tablets with a 235U isotope content higher than that found in natural mixtures of uranium isotopes. The tablets in fuel elements are manufactured by pressing and have a density 94 to 95 percent of the theoretical density of uraninite. The size of the fuel grains does not exceed a few microns. During the irradiation process, numerous cracks form in the fuel tablets and the space between the grains expands, and this leads to an increase in the area of its interaction with underground water in the event that the seals on the spent fuel containers are broken. Uranium dioxide is the conserving matrix for all elements formed in the nuclear reaction process. A number of elements such as Pu, Am, Cm, Np, Th, the rare earth elements, Nb, and Zr occur in the structural lattice of irradiated uranium dioxide, the stability of which prevents the leakage of these elements into under- ground water. Other elements such as Tc, Se, I, Cs, Sn, Sr, and products of their decay exist in the form of an unstructured mixture. They enrich the intergrain seams and microcracks in the uranium dioxide matrix. In the event of violation of the integrity of the fuel element coating, the gas-forming radionuclides ( 85Kr, 3H, and 14C) will primarily be released into the environment, and in the event of contact with underground water, nonstructured easily soluble radionuclides will also be released. Here the cracks will promote the migration of radionuclides from the spent fuel. It is therefore obvious that safe underground storage of spent 

96 SPENT NUCLEAR FUEL STORAGE FACILITIES nuclear fuel requires favorable conditions ensuring a high level of stability of the uranium dioxide and lack of interaction with underground water. The results of natural observations are undoubtedly key in ensuring the geochemical conditions for high stability of uranium dioxide in underground spent fuel storage facilities. These observations make it possible not only to characterize the behavior of actinides in a wide range of physical-chemical condi- tions but also to obtain information regarding slow-developing processes. Many researchers view deposits of uranium and thorium as natural analogs of spent fuel repositories. It has been established that the bulk of uranium in uranium ore deposits is concentrated in oxides. According to Russian experience, ore deposits are localized within the bounds of highly permeable zones in conditions of direct contact with underground water. Nevertheless, even in areas with increased water permeability, numerous cases have been noted in which deposits of uranium ore that have lain deep in the earth for hundreds of millions of years are found in practically ideal states of preservation. The reducing near-neutral properties of underground water are the main condition determining the high level of stability of uranium dioxide. The fact that the equilibrium concentration of uranium in underground water in reducing hydrogeochemical conditions is very low and totals less than 10 –8 mol per liter is of fundamental significance. Therefore, uranium ore deposits influenced by such water maintain high levels of stability. Rich major ore deposits in Canada and Russia are often cited in the literature as natural analogs for spent fuel repositories. Despite long contacts between the ores in these deposits and underground water, the uraninite displays a high level of preservation, which is due to the reducing properties of underground water. The concentration of uranium in the water from the ore deposits containing up to 40 percent uranium is practically no different from background concentration levels, totaling 10–8 mol per liter. However, a question arises: Will uranium dioxide irradiated in fuel elements behave in a geological environment similar to natural uraninite, given that the intensity of alpha-radiation on the surface of spent fuel is two to three orders of magnitude higher than that of uranium ores? To resolve this question, let us turn to the results of a study of the ore deposits of the Franceville uranium mining region in Gabon (West Africa). The best-known deposit in this region is Oklo, in the ores of which the existence of the natural nuclear reactor phenomenon was first established. Evidence of this is seen in the depletion of the isotope 235U in certain ore deposits as well as the presence of radioisotopes or end products of their decay formed as a result of the nuclear reaction. The deposits of the Franceville region were formed about 2 billion years ago in sandstones contain- ing an organic substance at a depth of 3,000 to 3,500 m. Calculations show that the content of 235U in the uranium at that time totaled 3.25 percent by mass, which corresponds to the level in the nuclear fuel of modern power reactors. In the presence of water, which played the role of a neutron moderator, processes 

CREATING REGIONAL INTERNATIONAL STORAGE FACILITIES 97 similar to those in nuclear reactors occurred in certain ore deposits in Oklo over the course of 500 million years. A study of such ore deposits located at various depths from the current surface made it possible to assess how uranium dioxide and nuclear reaction products contained in it would behave in both reducing and acidic hydrogeo- chemical conditions. Thus, in an ore deposit at a depth of 250 meters in reducing conditions, uraninite is not affected by processes of secondary change. A detailed study of element mixtures and their isotopic components in such ores has shown that all elements occurring in the crystalline lattice structure of uraninite are maintained in it right up until their complete decay. The fact that Am, Pu, and Np behaved in just such a manner is shown by the end products of their decay, 209Bi and 207Pb, which, despite the substantial difference in their geochemical properties from those of uranium, were largely maintained in the composition of uraninite and only partially transferred in direct proximity to ore deposits. Such nonstructural elements as Rb, Cs, Sr, Mo, Cd, Xe, and I were almost completely transferred out of the ore deposits, while Ru and Sn were partially removed. On the whole, the results of geochemical studies of natural reactor zones show that in reducing conditions uraninite is characterized by very high stability and not only reliably maintains actinides in its structure but also very powerfully limits the outward movement of elements not included in the uraninite structure. Thus, the results of studies of the ore deposits of Oklo support the conclusion that this geological environment is capable of ensuring the safe long-term storage of spent nuclear fuel. ON SELECTING A SITE FOR AN INTERNATIONAL GEOLOGICAL SPENT FUEL STORAGE FACILITY IN RUSSIA For the majority of countries, selecting a site for the construction of a spent fuel repository is an exceptionally complex problem. In the 1970s, IAEA and a number of countries developed rules for decision making on repository locations. These decisions are to be made taking geological, economic, legal, and socioeco- nomic factors into account. In the various countries that use nuclear power, each of these factors has widely varying significance. It is especially difficult to resolve the problem of selecting an underground spent fuel repository site in countries with high population density and unfavorable geological conditions. Russia is among those countries with a large land area, low population density, and an enormous diversity of geological conditions. For these reasons it is possible to   Pourcelot, L., and F. Gauthier-Lafaye. 1998. Weathering conditions and behavior of fission prod- ucts in Oklo reactors. Proceedings of the Symposium on Scientific Basis for Nuclear Waste Manage- ment XXI 506:1071-1072. 

98 SPENT NUCLEAR FUEL STORAGE FACILITIES select spent fuel repository and storage facility sites in Russia with practically ideal geological conditions. Intensive studies have been made of the geological-geochemical conditions of uranium migration in the Transbaikal (Krasnokamensk) region characterized by long stability and chemical destruction of uranium ores. The major uranium deposits here are located in the caldera of an ancient volcano. During the 30 years that the enterprise has operated here, unique research has been conducted regarding conditions of migration and transformation of uraninite at depths rang- ing from the surface to 1,500 m underground. The primary uranium ore mineral here—pitchblende (mineral version close to uraninite)—is a natural analog of synthetic uranium dioxide, making up 96 percent of the volume of spent nuclear fuel. A study of U-Pb isotopic systems of unchanged pitchblende preserved in several parts of ore deposits has shown that these systems have remained completely closed over the course of the entire period (about 135 million years) since the deposit was formed. The majority of primary uranium ores have been subject in varying degrees to the effects of hydrothermal solutions, during the course of which in reducing conditions the pitchblende was replaced by a U-Si gel compound, which was also redeposited in the ore deposit veins. In one ore deposit subjected to such hydrothermal changes, an original methodology was used to conduct a quantitative calculation of the uranium balance. This study showed a practically complete absence of uranium transfer outside the bounds of the ore body. The uranium freed up by the substitution of uraninite remained in practically the same spot in a newly formed U-Si gel as a result of the action of the effective geochemical barrier, preventing further uranium migration. In addition to the assessment of the long-term safety of underground spent fuel storage, the results of this research study provide the basis for believing that the uranium dioxide in spent fuel, provided that it is preserved as a mineral phase, ensures the reliable long-term isolation of uranium and the transuranic and fission radionuclides “imprinted” in the uranium dioxide matrix. Under the influence of hydrothermal solutions, which could cause mineral transformations of the uranium dioxide matrix, the uranium will be effectively immobilized by the newly formed U-Si gel. The large volume of data obtained as a result of lengthy systematic studies of the conditions for the placement of an underground storage facility in this region, the presence of highly qualified personnel, and the consent for the most part of the local population argue in favor of Krasnokamensk as an important site for the possible placement of an international spent fuel storage facility.