A Modest Proposal: A Robust, Cost-Effective Design for High-Level Waste Packages
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"waste package". A recent paper [5] has resurrected this approach by advancing several new concepts for EBS designs. Given the fact that EBS design is an activity that combines inputs from both materials studies and performance assessment, both of which are central to the Materials
Research Society's Symposium on the Scientific Basis for Radioactive Waste Management, it is
appropriate and timely to further expand on this new theme [5] of revised waste-package design. CONCERNS ABOUT WASTE-PACKAGE PERFORMANCE There are a range of postulated EBS/ waste-package designs throughout the world, involving a variety of waste forms and engineered barrier materials. Common components to such designs are (1) a nuclear waste form (most commonly spent nuclear fuel or borosilicate glass containing reprocessed HLW), (2) encapsulated in a container/ overpack, and (3) surrounded by a buffer/ backfill designed to restrict advective flow of groundwater past the waste form. The barrier materials, dimensions, and geometry of designs vary significantly, based on factors such as type of waste, expected geological conditions, and safety goals. In this paper, focus is placed on one of the most successful designs (Figure 1, right-hand side), that incorporates HLW-borosilicate glass partially filling a stainless-steel pour canister, encased in a thick (25-30 cm) mild-steel overpack, and surrounded by approximately a meter-thick buffer of compacted bentonite clay. Note that all of overpack thickness, except a 5-cm corrosion allowance, is a conservative mechanical-strength allowance to prevent crushing of the interior void of the HLW canister. Safety assessments of this design have shown it to be extremely robust in assuring compliance with international safety criteria [1-3]. 589 Mat. Res. Soc. Symp. Proc. Vol. 506 01998 Materials Research Society
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Concerns, however, have been raised regarding the performance and emplacement feasibility of such designs. Many of these concerns, which have proven difficult to fully resolve, are also relevant to waste-package designs for spent fuel [6-8]. A partial list of such concerns includes: • problems with the planned emplacement of intact clay buffer into locally wet boreholes, as recent engineering-scale tests at Asp6 and FEBEX/Grimsel have shown, • disruption of the clay layer by H 2 gas formation from long-term, anaerobic corrosion of the massive steel overpack,
"*erosion of buffer by water flowing in surrounding fractured rock, "*sinking of the dense overpack through the clay buffer, and "*expansion of iron corrosion products that may damage or deform the buffer, "•ability to emplace the barriers and to assure emplaced barriers meet design specifications. If the buffer is disturbed or disrupted, extremely adverse impacts on overall repository safety may arise. This includes loss of colloid filtration, transition from slow diffusive-release to potentially rapid advective-release of radionuclides, and significant increases of the uncertainties i
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