Disposal of strontium and cesium capsules in geologic media: an analysis of technical feasibility

The storage configuration evaluated was one in which a single capsule would be contained within and positioned at the center of a 24-in. dia cylindrical canister which in turn would be emplaced in a slightly larger borehole located in the repository (tunnel) floor. The net effect of this arrangement would be that the capsule would radiate its heat to the canister/rock boundary which, due to its radial distance (12 in.), would experience an acceptably low heat flux into the rock and, hence, an acceptably low temperature response. Results of calculations indicate that for CsCl capsules there is no technical limitation related to rock temperatures, capsule wall temperatures, corrosion effects (over an assumed 25-year retrievability period), or capsule core temperatures which would suggest that they could not be disposed of in basalt or bedded salt. For the geologic disposal of SrF2 capsules, rock or salt temperatures, although higher than for disposal of CsCl capsules, would not be excessive; hence, there is no technical limitation related to the relatively high specific power of SrF2 capsules or its effect on the geologic media which would prevent disposal of SrF2 capsules in this manner. Temperatures within the capsule itself may create two marginal situations. First, the outer, stainless steel capsule wall may be ''sensitized'' such that it would be vulnerable to stress corrosion cracking if ever returned to a water basin. However, the objective of assuring ''easy retrieval'' would not be seriously compromised by this condition of sensitization. Secondly, the corrosion life of the Hastelloy C-276 inner capsule wall, although predicted to be acceptable in this application by the results of early corrosion studies, may remain a point of uncertainty, pending the completion of a SrF2/containment compatibility test program currently underway at Pacific Northwest Laboratory