Improving the Scientific Basis for Managing DOE's Excess Nuclear Materials and Spent Nuclear Fuel (2003) / Chapter Skim
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5 Cesium-137 and Strontium-90 Capsules
5 Cesium-137 and Strontium-90 Capsules
Pages 52-59
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From page 52... ...
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~.
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From page 53... ...
: 20.8 inches Cesium inner and outer capsule: 31 6L SS Strontium inner capsule: Hastelloy C-276 Strontium outer capsule: 316L SS Cesium capsules,3.5 m3 Strontium capsules, 1.5 m3 15 Metric tons of capsules and contents Cesium,47 MCi Strontium, 20 MCi Total including daughter isotopes, 131 Mci 355 kilowatts (1.2 million BTU/hour) Sources: aROO, 2002; bDNFSB, 1 996;'NRC, 1 997b; ~DOE, 1 996b.
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From page 54... ...
Various mechanisms for capsule failure have been proposed including poor welds and phase changes in the material as a function of temperature (DNFSB, 19961. 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." DOE's current plans (DOE, 2002g)
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From page 55... ...
These options include · continued storage in the pools at the WESF facility, passive storage in air at a new facility, overpacking and disposal of the capsules at a geologic repos tory, and jj_ · vitrification into a glass or calcination into an oxide followed by disposal at a geologic repository. Storage 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.
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From page 56... ...
Topics for research include: the physical and chemical changes that will occur in the CsCI and SrF2 in a confined environment (e.g., in sealed capsules) and intense radiation field, the physical and chemical changes caused by short-lived isotopes decaying to other elements with different valencies, the moisture and other impurity concentration thresholds to keep gas generation to acceptable levels, and the phase changes for these materials as a function of the impurity levels typical in these capsules.
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From page 57... ...
Also, chloride and fluoride attack of the capsule material increases at higher temperatures. Opportunities for research on capsule integrity include: · developing more quantitative capsule surveillance methods than the "clunk" test to determine capsule material and weld integrity or bulging; · understanding the fundamental reason for the capsule failures to date, specifically, the mechanical and metallurgical aspects, and applying the knowledge gained to assess the remaining capsules; understanding capsule content phase changes and/or chemical reactions to enable predictions about their behavior if the capsules are moved from storage in pools to passive air storage; understanding the gas-generation potential of the capsule contents over time, especially as the elemental composition changes dramatically in a matter of decades; and · determining whether the capsule contents have a unique enough radioisotope signature to be able to identify them in the event of theft.
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From page 58... ...
Although the Hanford capsules appear to be theft resistant, immobilization research would also be applicable for stabilizing cesium and strontium from unused industrial and medical sources worldwide (IAEA 2003~. The chemical and physical form of the radioactive material would have a large effect on its dispersibility.
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From page 59... ...
Therefore, research opportunities exist to study the stability of the waste type chosen given these large radiation fields. Opportunities include research to: assess whether or not the current capsule form is viable for either long-term storage or direct disposal in a geologic repository, assess the merits of ceramic versus vitrified waste forms, including processes to produce them, · develop methods for uniformly dispersing radionuclides in glass or ceramic and methods for verification · understand how to ensure glass or ceramic stability for time periods of hundreds of years under the intense radiation fields without corroding or leaching, and understand the physical and chemical changes that will occur for these very rapidly decaying compounds in a confined environment (sealed capsules, vitreous or crystalline matrices)
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