Post-Closure Performance Assessment for Deep Borehole Disposal of Cs/Sr Capsules

Abstract

Post-closure performance assessment (PA) calculations suggest that deep borehole disposal of cesium (Cs)/strontium (Sr) capsules, a U.S. Department of Energy (DOE) waste form (WF), is safe, resulting in no releases to the biosphere over 10,000,000 years when the waste is placed in a 3–5 km deep waste disposal zone. The same is true when a hypothetical breach of a stuck waste package (WP) is assumed to occur at much shallower depths penetrated by through-going fractures. Cs and Sr retardation in the host rock is a key control over movement. Calculated borehole performance would be even stronger if credit was taken for the presence of the WP.

Status of Deep Borehole Disposal of High-Level Radioactive Waste in Germany

“…A detailed safety analysis and assessment of a general concept for DBD in Germany has not been performed. Studies on DBD in the U.S. considered a radionuclide transport around the borehole [39,40]. The radionuclide transport from that depth would exhibit an extremely low release of radionuclides to the biosphere.…”

The State of the Science and Technology in Deep Borehole Disposal of Nuclear Waste

“…A more traditional post-closure performance assessment study is undertaken for deep borehole disposal of Cs/Sr capsules in the third paper on this topic [7]. The study considers waste emplacement of 0.21-m-diameter waste packages in crystalline rock at a depth of about 5000 m. The waste emplacement zone is 534 m deep, with a 2000-m-deep seal zone.…”

“…Waste types, including waste types suitable for deep borehole disposal and the circumstances under which a national waste management programme might wish to consider deep borehole disposal [1], and the status of deep boreholes as a potential option for disposal of high-level waste in Germany [2]; • Engineered barrier performances, including corrosion performances of engineered barrier systems [3] and potential cementing systems suitable for deep borehole disposal [4]; • Performance assessments, including the thermal evolution around heat-generating waste for a wide range of material properties and disposal configurations [5], geochemical analyses of deep brines focussed on fluid-rock interactions [6] and post-closure performance assessment calculations for the deep borehole disposal of Cs/Sr capsules [7] and • Safety cases, including deep horizontal drill-holes in sedimentary, metamorphic or igneous rocks [8] and safety case aspects for deep vertical boreholes [9].…”

Who Might Be Interested in a Deep Borehole Disposal Facility for Their Radioactive Waste?

“…In the USA, this approach has been suggested [16] for disposal of separated 137 Cs and 90 Sr, packaged from the 1960s to the 1980s in about 2000 small (c. 80 mm diameter x 550 mm length) capsules with a total volume of about 6 m 3 . This small volume contained about 4 × 10 18 Bq of radioactivity in 2006 [17], which will decline by more than half by 2050, owing to the short half-lives of the principal radionuclides in the capsules (although there is a small amount of much longer lived 135 Cs present as well). Safety assessments have been carried out to support the DBD disposal concept for these materials [18].…”

“…This small volume contained about 4 × 10 18 Bq of radioactivity in 2006 [17], which will decline by more than half by 2050, owing to the short half-lives of the principal radionuclides in the capsules (although there is a small amount of much longer lived 135 Cs present as well). Safety assessments have been carried out to support the DBD disposal concept for these materials [18]. The very high activity of these wastes, combined with their small volume, appears to make them ideal candidates for DBD, but their short half-lives challenges this perspective: after around 1000 years of decay these wastes would be classifiable as low-level waste.…”