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Geochemistry and Environmental Mobility of Iodine-129

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Geochemistry and Environmental Mobility of Iodine-129 2005 International Nuclear Atlantic Conference - INAC 2005 Santos, SP, Brazil, August 28 to September 2, 2005 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 85-99141-01-5 GEOCHEMISTRY AND ENVIRONMENTAL MOBILITY OF IODINE-129 Eduardo Figueira da Silva Divisão de Rejeitos Radioativos (DIREJ / CNEN - RJ) Comissão Nacional de Energia Nuclear Rua General Severiano, 90 22260-001 Rio de Janeiro, RJ [email protected] ABSTRACT Iodine-129 is the longest lived of the iodine isotopes and is considered to be one of the largest potential contributors to the doses resulting from nuclear waste repositories. It is generated in significant quantities in nuclear reactors and its importance is due to its very long half-life (1.6 × 107 years) and relative mobility under environmental conditions. The present paper reviews the important aspects concerning the generation, emission and global circulation of 129I in the geosphere. Following, geochemical aspects relevant to the mobility of iodine in the environment are analyzed within the framework of the safety of nuclear waste repositories. Iodine species usually found in nature are identified, in addition to species expected to be found in the near-field of repositories. Previous studies concerning the precipitation and sorption behaviors of the major species of iodine are critically reviewed. Finally, some aspects of the microbial activity influences on the mobility of 129I are presented. Although a reasonable amount of work has been done on trying to understand the geochemistry of 129I over long time frames, there is still a large amount of uncertainty in characterizing its behavior under environmental conditions and in the near-field of engineered repositories. In order to be able to predict the long- term behavior of a nuclear waste repository with respect to 129I, it is first necessary to have a better understanding of the geochemistry and mobility of this radionuclide, especially with respect to sorption and microbial interactions. 1. INTRODUCTION Iodine-129 is the longest-lived of the volatile radionuclides produced in the nuclear fuel cycle [1, 2], with a half-life of approximately 1.6 × 107 years, formed as a product of fission of uranium or plutonium and accumulates in reactor fuel in proportion to the fission in that fuel. The decay of 129I requires a very long period (1.1 × 108 years) to reach 1% of the original amount. Together with 14C, it persists for a period often considered to perpetuity for mankind, and will eventually enter the environment unless geological processes can be exploited to ensure its isolation [2]. 129I emits soft beta particles, a weak gamma ray and an X-ray, representing an ingestion and inhalation hazard [2,3]. When people breathe of ingest iodine it first moves to the thyroid gland because Thyroxin, a growth and metabolism regulating hormone produced in the thyroid, is rich in iodine. Therefore, the thyroid is the organ at the higher risk from exposure to radioiodine, which is of especial concern for fetus and children [4,3]. If released from engineered or geological barriers, 129I will enter the global circulation of iodine, mixing with the much larger quantity of stable iodine and following it in its movement through the environment. This circulation involves the accessible iodine reservoirs, particularly the deep ocean and its sediments. The geochemical behavior of 129I over the long periods of time until its decay to an innocuous isotope will determine its circulation patterns, the environmental impact, and ultimately the dose to humans. 2. GENERAL PROPERTIES, OCCURRENCE AND RADIOLOGICAL IMPACT Iodine is an essential element for many organisms and is often added to dietary salt, because deficiency of I causes goitre. Natural sources are most important environmental sources for the stable isotope 127I, while nuclear bomb tests and accidents have provided most of the radioactive isotopes. Iodine is naturally associated with Li, Na, K, B, P, W, F, Br, Cl, SO4, CO3 (brines, evaporites) [5] and its environmental mobility is usually very high. Iodine-129, the longest lived of the iodine isotopes, decays by the route [2]: β- 16 × 106 IT (1.0 ns) 129I 129mXe 129Xe (stable) 150 keV 39.6 keV Only 7.52% of the 129Xe transitions are by gamma ray emmission. The remainder yield conversion electrons (mainly 5 to 10 keV) and xenon X-rays [2]. Iodine has a complex chemistry. The element is volatile and reacts to form many volatile organic compounds. It may undergo oxidation or reduction to yield very soluble salts. It is intimately involved with the life cycle, and its concentration in the environment is frequently the result of biochemical processes. In the human it is concentrated in the thyroid gland, and the concerns of radioiodine largely involve its effect on this gland [2]. Several natural processes contribute to the 129I global inventory, such as cosmic rays interactions with xenon isotopes, which makes an equilibrium contribution of 250 kg, and spontaneous fission processes in natural 238U and 235U , resulting in an equilibrium inventory contribution of 113 kg [2]. However, anthropogenic 129I has been produced by nuclear programs in a significant amount. Nuclear explosions in the atmosphere have added about 170 kg of 129I, most of this amount deposited on the surface of the Earth and in the oceans. But it is the 129I that comes from reprocessing of spent nuclear fuel that is of greatest concern. It has been estimated [2] that the world nuclear program can potentially produce from reprocessing of spent nuclear fuel about 8 × 107 kg of 129I over the next thousand years, if the amount of uranium readily available in ores is utilized. Although the dilution factors of 129I/127I for the atmosphere and oceans are similar (2.2 × 10-12 and 4.4 × 10-12, respectively [2]), the amounts of stable iodine in these environmental compartments differ markedly. There are 108 kg of stable iodine in the atmosphere and 9.5 × 1013 kg in the oceans, and consequently the ocean iodine has more than 106 times the capacity for isotopic dilution than that in the atmosphere [2]. More recently, Kabata-Pendias and Pendias [6] claimed that the 129I/127I ratio has increased in recent times due to nuclear weapons tests, from values of the order of 10-12 to about 10-8. Iodine-129 is the anthropogenic nuclear fission product that may present the greatest potential hazard in the long-term exposure following the ingestion of food contaminated from accidental releases from facilities from the nuclear fuel cycle [7]. Nevertheless, when compared to other natural volatile radionuclides, such as 14C or radon, anthropogenic 129I is considered to contribute much less (i.e., about one millionth) to the potential collective dose committed than the background due to those natural radionuclides [2], if 129I is assumed to be uniformly dispersed globally. However, release of 129I might lead to important local concentrations if provisions for adequate dispersion are not taken. Dispersion of 129I into the environment should preferably be associated with a simultaneous effect of adequate dilution by stable iodine 127I. Dispersion media which already contain some significant amount of natural iodine, like the ocean, would therefore be advantageous in terms of dose [2]. Yet, recovery and concentration for storage or disposal offers advantages in reducing doses for the near term (about 104 years), and complies with international agreements prohibiting ocean INAC 2005, Santos, SP, Brazil. disposal of radioactive wastes. Most of the work done on 129I management has dealt with its recovery, concentration and immobilization in a suitable form for long term storage. 3. GENERATION, EMMISSION PATHWAYS AND CIRCULATION MODELS The radioactive isotopes from iodine belong to a larger family of fission products (Z = 30-66) of U and Pu. The majority of the raioisotopes of iodine is obtained by β--decay from Sn, Sb and Te. Two important facts should to be noticed [8]. First, the production of 129I in terms of activity is lower, when compared to other isotopes, generally with shorter life. The average iodine concentration is of 106 Ci/t, while the 129I concentration is only of 0.02 Ci/t. Second, the production of radioactive iodines is more important in fast reactors, where the fission yield is more important, than in the thermal reactors. The net production of 129I from 235U fission in a thermal reactor is about 1 µCi/MW.d, depending on the operation conditions [1]. It is estimated that the world production of the long-lived iodine, i.e. 129I, will reach 2 × 106 Ci by the year 2060. The amount of 129I present in the environment by then will be around 2 × 104 Ci, i.e., 120 tones of radioactive iodine [8]. The emission of 129I from reactors is very small when compared to the shorter lived isotopes [8]. On the other hand, emissions from reprocessing plants are much larger [8] and enter the environment as releases to the atmosphere and hydrosphere [9]. Although smaller, the emissions of 129I from reactors shall not be totally neglected, due to its long half-life. Two sources of emission of 129I from nuclear reactors are possible: the effluents stack and the liquid effluents. From reprocessing plants, the atmospheric emissions are originated from the effluents dissolution and degassing units, and can be retained with appropriate absorbing filters and storage. For the adequate long-term storage of 129I, it shall be maintained in sterile medium and not in contact with air [8,15]. A fraction of iodine is also lost by volatilization in the process of vitrification of wastes [10]. The location of fission and transmutation products in fuel is dependent on the chemical nature of these products and on the reactor operating conditions, which can control the ability of the fission products to migrate through the fuel.
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