Nuclear Technology / Volume 205 / Number 6 / June 2019 / Pages 830-846
Technical Paper / dx.doi.org/10.1080/00295450.2018.1523639
During a removal of legacy materials from one hot cell at the Idaho National Laboratory in 2010, five metal capsules and some loose zeolite material were identified as krypton (Kr) immobilization test specimens produced in the late 1970s under the Airborne Waste Management Program (AWMP). This AWMP research and development effort examined the encapsulation of 85Kr within a collapsed zeolite structure for use as a potential waste form for long-term storage. The recovered capsules appeared to have been placed to the side and remained untouched after the AWMP was halted in the mid-1980s. These reclaimed capsules and loose material presented a unique opportunity to study a potential 85Kr waste form after three half-lives had elapsed. The first phase of this study included two parts: The first was to assess the physical condition of the capsule walls, and the second was to examine the Kr-containing material within the capsules. The first part of this study was previously reported and noted that substantial corrosion was observed throughout each capsule wall of the two previously breached capsules that were examined. One of these capsules had been hot isostatic pressed (HIPed) and one was not HIPed. The second part of the study examined the materials contained in the two previously breached capsules. There appears to be a relatively uniform distribution of Kr and rubidium throughout the pellets examined. The chemical composition of the pellets appears to be consistent with 5A molecular sieves. The material contained within the HIPed capsules showed ~1 at. % lead (Pb). The origin of the Pb is currently indeterminate. X-ray diffraction analysis shows a significant shift from the 5A structure, most likely due to the Kr encapsulation/sintering process that occurred when the samples were made. Calculations based on the energy dispersive spectrometry elemental analysis show a residual Kr level within the pellets that is within a factor of 2 of the reported Kr capacities for this type of processed material. This provides a clear indication that a significant fraction of the Kr initially encapsulated in the material remained within the waste form even following a significant breach of the capsule wall. As a result, it would appear that this Kr immobilization method, even in non-HIPed form, is very promising as a waste form for long-term storage. The successful analysis of these two breached capsules forms a solid basis for the future analysis of the remaining unbreached capsules, which offers the opportunity to provide an even more complete understanding of the long-term Kr retention performance of this promising waste form.