Repositories actinides

The throwaway fuel cycle does not recover the energy values present ia the irradiated fuel. Instead, all of the long-Hved actinides are routed to the final waste repository along with the fission products. Whether or not this is a desirable alternative is determined largely by the scope of the evaluation study. For instance, when only the value of the recovered yellow cake and SWU equivalents are considered, the world market values for these commodities do not fully cover the cost of reprocessing (2). However, when costs attributable to the disposal of large quantities of actinides are considered, the classical fuel cycle has been the choice of virtually all countries except the United States. 

The primary issue is to prevent groundwater from becoming radioactively contaminated. Thus, the property of concern of the long-lived radioactive species is their solubility in water. The long-lived actinides such as plutonium are metallic and insoluble even if water were to penetrate into the repository. Certain fission-product isotopes such as iodine-129 and technicium-99 are soluble, however, and therefore represent the principal although very low level hazard. Studies of Yucca Mountain, Nevada, tentatively chosen as the site for the spent fuel and high level waste repository, are underway (44). 

Despite the problems of direct experimental evaluation of plutonium stability constants, they are needed in modeling of the behavior of plutonium in reprocessing systems in waste repositories and in geological and environmental media. Actinide analogs such as Am+3, Th+, NpOj and UOj2 can be used with caution for plutonium in the corresponding oxidation states and values for stability constants of these analogues are to be found also in reference 20. 

Francis AJ, Gillow JB, Dodge CJ, et al. 1998. Role of bacteria as biocolloids in the transport of actinides from a deep underground radioactive waste repository. Radiochim Acta 82 347-354. 

Because of the multivalent nature of the actinide ions, understanding the radiation-induced change of the valence-state of the actinide in solutions under self-irradiation or external irradiation is a challenge in radiation chemistry. Some of the ions are strong a-emitters. It is also important from a practical viewpoint that the solution chemistry of actinide ions is closely related to the storage and the repository of the wastes. Much work combined with experiment and simulation has been conducted and reviews were summarized [136,140-144]. 

In repository of HLW in salt mine, the understanding of radiolysis of concentrated NaCl up to 5 M is important. In addition, a-radiolysis by actinide ions dissolved is also taken into account. There are several reports mentioned above [186-192]. 

Th-oxyhydroxide species readily dissolve upon dilution below the solubility limit, it is not veiy likely that such actinide(IV) colloids play a role away from the source in the far field of a repository. In the near field of a repository, however, they may be predominant species controlling the solubility of tetravalent actinide species such as U(IV) and Pu(IV) and thus the source term. Unusual stability at high ionic strength has been also reported for amorphous SiOz colloids (Iler 1979 Healy 1994) which also cannot be explained solely by electrostatic repulsion. Formation of oligomeric or polymeric silicate species at the colloid-water interface are thought to exert additional steric stabilization by preventing close approach of those particles. 

The major repository of transuranic elements entering aquatic systems is the bed sediment (1-4). A significant portion is thought to arrive at the bed sediment surface as a result of association with, and subsequent settling of, suspended particulate matter. Concentrations of plutonium and americium in sediments relative to those in water reportedly range from 1 x IO" to 3 X 10 (32,33,34). Little information is currently available for other actinides of interest relative to nuclear fuel cycle wastes (Th, U, Cm and Np). 

The favored method for permanent storage of radioactive waste is deep geologic repositories. This option is the only option for unprocessed spent fuel assemblies and for most HLW. (An alternative, supplemental strategy discussed below is to remove some of the actinides in the HLW by chemical separations prior to geologic storage.)... 

Typical heat production in the moderator-fuel blanket is 750- 1500 MW. The excess heat is used to generate electricity that helps to pay for the operation of the facility. The transmuted material will have 20% of the original plutonium and minor actinides of the input material and will contain significant fission product activities. This transmuted material can be put into geologic storage, reducing the long-term hazard of the repository material. The overall feasibility of this accelerator transmutation of waste (ATW) has not been established yet. 

Lanthanides are coextracted with actinides and then separated from actinides, which are forecasted to be sent to a repository. The lanthanide elements comprise a unique series of metals in the periodic table. These metals are distinctive in terms of size, valence orbitals, electrophilicity, and magnetic and electronic properties, such that some members of the series are currently the best metals for certain applications. Increased use of the lanthanides in the future is likely, because their unusual combination of physical properties can be exploited to accomplish new types of chemical transformations. These elements coextracted with actinides and then separated from the latter, could in the future be recovered and used (among the lanthanides, only 151Sm is a long-lived isotope (half-life 90 years)).4... 

Currently proposed licensing regulations for geologic nuclear waste repositories require a performance assessment involving long-term predictive capabilities. Previous work (J- 5) has shown the importance of solubility controls for modeling maximum actinide concentrations in repository groundwaters. However, until reliable data are available on the actinide solid phases that may be present or that may precipitate in the environment, the solubility of solid phases such as hydrous oxides that have fast precipitation kinetics can be used to initially set maximum solution concentration limits. 

Rai, Dhanpat Strickert, R. G. Swanson, J. L., "Actinide Solubilities in the Near-Field of a Nuclear Waste Repository," In Workshop on Near-Field Phenomena in Geologic Repositories, (August 31 - September 3, 1981, Seattle, Washington), Nuclear Energy Agency of OECD, Paris, France, 1981 pp. 13-20. 

Actinide Containment in Geologic Repositories." U.S. Department of Energy Report PNL-SA-9549, Pacific Northwest Laboratory, Richland, Washington, 1978. 

The actinide retardation data for the hydrothermally-altered repository component experiments (4 and 5) in Table VI are preliminary and their interpretation is hindered by the lack of attainment of a steady-state condition for actinide migration in the systems These data indicate that only a small fraction of the actinides are retained by the rock core Comparing these results to those of the unaltered fissure experiments, where Pu and Np were almost completely retarded by the rock core, one could conclude that altering the rock tends to lower its ability to retard actinide migration Another way to discuss these same data, and one that leads to a completely opposite conclusion, is in terms of the amount of activity retained by the rock core, or, the rate of actinide loss from the groundwater in terms of A (dpm/mL) of 2 Pu and 237Np ... 

These quantities are both higher for experiment 4 and 5 than for the three experiments with unaltered repository components The loss of Pu through the core for experiments 1 and 2, for example, was 0 4 and 0 1 dpm/mL, respectively in experiments 4 and 5 the loss of Pu through the core appears to be 20 to 40 times greater This exercise is intended to show that there is no legitimate way to compare the behavior of altered and unaltered basalt from these data Experiment 6, yet to begin, should clarify this situation. What is clear from comparing the data in Table VI is that actinide behavior in altered and unaltered repository situations will be quite different ... 

Auxiliary experiments are underway and are being planned to elucidate the mechanisms of actinide retardation in altered and unaltered repository conditions. 

The CTH actinide separation process was developed as a possible means to reduce the expected long term dose to man from a geologic repository containing solidified radioactive waste from the reprocessing of spent nuclear fuel The distribution data for the elements present in significant amounts in the high level liquid waste (HLLW) from a Purex plant, the general principles and the flowsheet have been described in detail elsewhere A... 

Introduction actinide solubilities in reference waters. In this section, the environmental chemistry of the actinides is examined in more detail by considering three different geochemical environments. Compositions of groundwater from these environments are described in Tables 5 and 6. These include (i) low-ionic-strength reducing waters from crystalline rocks at nuclear waste research sites in Sweden (ii) oxic water from the J-13 well at Yucca Mountain, Nevada, the site of a proposed repository for high-level nuclear waste in tuffaceous rocks and (iii) reference brines associated with the WIPP, a repository for TRU in... 

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