Spent nuclear fuel radionuclides

Disposal of spent nuclear fuel and other radioactive wastes in the subsurface and assessment of the hazards associated with the potential release of these contaminants into the environment require knowledge of radionuclide geochemistry. Plutonium (Pu), for example, exhibits complex environmental chemistry understanding the mechanism of Pu oxidation and subsequent reduction, particularly by Mn-bearing minerals, is of major importance for predicting the fate of Pu in the subsurface. 

Previous review articles on spent nuclear fuel and UO2 corrosion have dealt with the details of uraninite alteration (Finch Ewing 1992), spent fuel corrosion in oxidizing and reducing environments (Johnson Shoesmith 1988 Wronkiewicz Buck 1999 Shoesmith 2000), and the evolution of spent fuel microstructure (Poinssot et al. 2002). In this Chapter, the behaviour of three important radionuclides (237Np, 99Tc, and 239Pu) is examined with examples of studies from spent fuel corrosion tests to illustrate the potential geochemical behaviour of these radionuclides. 

Galkin, B.Ya., Shishkin, D.N. 2001. Extraction withdrawal of cesium and strontium radionuclides from the solution of spent nuclear fuel. In Back-End of the Fuel Cycle From Research to Solutions. GLOBAL 2001, September 9-13, Paris, France. 

The reasons why solvent extraction has become the reference technique for the reprocessing of spent nuclear fuels at industrial scale (and will probably also be chosen in the future for the recovery of long-lived radionuclides) are the following (32, 33) ... 

The disposition of the radioactive waste resulting from spent nuclear-fuel reprocessing is one of the major problems of nuclear technology. Many of these wastes are high-level (HLW) and present a potential environmental hazard. Because of the expense associated with long-term storage, it is desirable to minimize the volume of radioactive waste and store it as material with maximal chemical stability to avoid the dissipation of radionuclides in the environment. 

Liquid wastes. Historically, the most important radioactive wastes have been liquid wastes that arise from chemical reprocessing of spent nuclear fuel for defense production purposes, i.e., for the purpose of extracting plutonium for use in nuclear weapons. These wastes contain varying concentrations of many radionuclides, primarily fission products and long-lived, alpha-emitting transuranium isotopes. 

After one year of intermediate storage of spent nuclear fuel and reprocessing, the initial activity of the HLW solutions is of the order of lO " Bq/1. The activity due to the presence of °Sr and Cs is about 10 Bq/1, and after lOy the activity of the HLW solution decreases approximately with the half-life of these radionuclides... 

Curtis, D. B., J, Fabryka-Martin, P. Dixon, R. Aguilar, and J. Cramer. 1994. Radionuclide release rates from natural analogues of spent nuclear fuel. Froc. 5th annual inti. conf. Am. Nuci. Soc. 4, pp. 2228-36. 

The Source Term Working Group was established to prepare a detailed inventory of and release rates for the radionuclides dumped at each Kara Sea disposal site. To this end, inventories were calculated for the spent nuclear fuel (SNF) and activated components at time of disposal and projected forward in time protective barriers, if any, were evaluated for their potential effect on radionuclide release and a number of model scenarios were developed to predict the potential release of the radionuclide inventory into the Kara Sea. 

Spent nuclear fuel can either be reprocessed (see O Sect. 52.3.4) or put directly into a deep underground repository like in, e.g., Sweden or Finland. In both cases, the material stored in the repository contains long-lived radionuclides like Np and Am as well as varying amounts of plutonium. Therefore, the storage facility has to fulfill strict safety requirements for a very long time, typically several hundred thousand years for unreprocessed fuel. 

Ion-exchange processes are also of utmost importance in the Swedish concept for final storage of nuclear fuel. According to this method, the spent nuclear fuel will be placed in copper canisters that are disposed into bentonite-filled shafts at about 500 m depth in the granite bedrock close to the Forsmark nuclear power plant. In this concept there are four barriers that will prevent radionuclides from leaking into the groundwater (1) the fuel itself is very insoluble in water, (2) the copper canister is chemically stable, (3) bentonite clay (sodium montmorillonite) is... 

One of the major issues related to the expanded use of nuclear power is the fate of plutonium and actinides. Although there are a number of fission product radionuclides of high activity ( Cs and °Sr) and long half-life ( Tc, 200,000 years I, 1.6 x 10 years) in spent nuclear fuel, actinides account for most of the radiotoxicity of nuclear waste because, after several hundred years, the radiotoxicity is dominated by Pu (half-life = 24,100 years) and Np (half-life = 2,000,000 years). Thus, a major part of the long-term risk is directly related to the fate of these two actinides in the geosphere. 

A report that summarizes the state-of-the-art analytical methods for assay of SNF was published by the Expert Group on Assay Data of Spent Nuclear Fuel (EGADSNF 2011). Their stated objective was to view the techniques that serve for destructive post-irradiation examination (PIE) for analysis of the isotopic composition and concentrations in spent nuclear fuel sample. First, the sampling procedures and sample dissolution methods are considered, followed by techniqnes for separating the radionuclides and the measurement procedures. However, it should be emphasized that... 

The nuclear fuel industry uses large quantities of F2 in the production of UFg for fuel enrichment processes and this is the major use of F2. Reprocessing of spent nuclear fuels involves both recovery of uranium and separation of from fission products. Short-lived radionuclides decay during a period of fuel storage (pond storage). After this, uranium is converted in the soluble salt [U02][N03]2, and then into UFs (see Box 7.3). 

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