Given their approximately 30-year half-lives, the isotopes could decay significantly during storage thus reducing their hazard and difficulty of eventual disposal. Intermediate or long-term storage on site has the advantages of allowing monitoring and surveillance, providing physical protection, saving the material as a potential resource, and maintaining the material in disposal-ready condition while avoiding interstate transportation issues. The report states that the capsules “may have experienced chloride-induced stress corrosion cracking near the outer capsule welds due to lack of water chemistry requirements and control.”ĭOE’s current plans (DOE, 2002g) include continuing to provide safe and compliant storage as well as surveillance and maintenance Various mechanisms for capsule failure have been proposed including poor welds and phase changes in the material as a function of temperature (DNFSB, 1996). However, if the inner capsule is swollen, it will not move freely and will be a candidate for overpacking. The need for overpacking is typically determined by simply shaking the capsule and listening for the inner capsule to “clunk.” If the inner capsule moves freely, the capsule is deemed to be in good condition. Most capsules are considered in good condition, but 23 have had to be overpacked, i.e., sealed in a larger stainless steel container (ROO, 2002). Nevertheless, there are gaps in the technical basis for predicting incipient failures and understanding failure mechanisms. However, unlike most other elements, cesium can biomagnify (concentrate) as it moves to higher trophic levels (Whicker and Shultz, 1982).Ĭapsule integrity is essential for interim storage. Cesium and strontium have limited mobility in the environment due to adsorption on clays and other aluminosilicates. Cs-135, which is present along with the Cs-137, has a half-life of about 2 million years, and it will become the dominant source of radioactivity in the cesium capsules in about 600 years. However, the present radioactivity is so great that it will take over 800 years for the Sr-90 to decay to the level currently allowed for disposal as low-level waste, 0.04 Ci/m 3. 1 Compared to other nuclear materials in DOE’s inventory, Cs-137 and Sr-90 have relatively short half-lives, 30 years and 29 years, respectively. Dose rates range from 8,600 to 18,000 rems/hour for the Cs-137 capsules and from 20 to 420 rems/hour for the Sr-90 capsules. The main challenges for managing and dispositioning the capsules are due to the intense radiation and the relatively large amount of heat that they produce. Each strontium capsule contains approximately 33,000 Ci of Sr-90 and produces approximately 260 W of heat (see Table 5.1). Each cesium capsule contains on average approximately 35,000 Ci of Cs-137 plus an unspecified amount of Cs-135 estimated to be 0.7 Ci and produces approximately 190 W of heat. The chemical composition has been described as being relatively uniform (NRC, 1997b). In the capsules, cesium is in the form of cesium chloride (CsCl) and strontium as strontium fluoride (SrF 2). DOE plans to move the capsules to passive air storage however, their final disposition has not been determined (DOE, 2002g). Currently the capsules are stored at the Waste Encapsulation and Storage Facility (WESF) under 13 feet of water to cool the capsules and provide radiation shielding. Environmental Protection Agency, and the Department of Energy (DOE), dispositioning the capsules is among eight specifically identified priorities for accelerating Hanford Site cleanup (Tri Party, 2002). According to a recent letter of intent among the state of Washington Department of Ecology, the U.S. They have been described as the nation’s most lethal single source of radiation other than inside an operating reactor (Long, 2002). The almost 2,000 capsules contain a total of 67 million curies of radioactivity, which amount to about 37 percent of the total radioactivity at the Hanford site (ROO, 2002). The expected applications for the Hanford capsules did not materialize and ceased entirely in 1988 after one capsule being used in the commercial sector was found to be leaking (USNRC, 1989). The cesium and strontium were concentrated and sealed in stainless steel (SS) capsules (see Figure 5.1) for potential uses, for example, thermoelectric generators and sterilizers. In the early 1970s operators at the Hanford, Washington, site removed a large fraction of the Cs-137 and Sr-90 from the site’s high-level tank waste in order to reduce the requirements for cooling the tanks.
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