A paper on the impact of poor borehole sealing on repository performance written by Stefan Finsterle, Cal Cooper, Richard A. Muller, John Grimsich and John Apps, has been published in the peer-reviewed journal Energies. The paper is available online and for download.
A deep horizontal borehole repository offers strong isolation of nuclear waste. The safety afforded by waste isolation at depth relies largely on the natural barrier provided by the horizontal section of the borehole. A potential for vulnerability may be with the vertical section of the borehole that needs to be drilled to build and access the repository. It is important to measure and ensure that the vertical access hole does not provide a direct path through which radionuclides escape from the repository to the land surface. While the borehole will be backfilled and plugged after waste emplacement, it is difficult to assure that the engineered sealing barrier will remain effective over the very long time period for which the waste must be safely isolated.
To investigate the importance of borehole sealing on repository safety, we calculated the radiological exposure dose assuming that the backfill material is of poor quality or has lost its ability to inhibit water flow and radionuclide transport. Our computer simulations indicate that the release of radionuclides through the poorly sealed access hole is small, even if an earthquake destroyed the waste canisters and pushed water along the borehole and into faults. The estimated maximum dose from the release of radionuclides during these adverse events does not increase significantly compared to the nominal scenario and is two to three orders of magnitude lower than a 10 mrem dose standard.
Given that the long-term effectiveness of borehole sealing is difficult to assess or predict, it is reassuring that a deep horizontal borehole repository does not need to rely on the long-term integrity of its seals and backfill material.