Suitability of flat bedded salt formations in Germany as the site for a repository for heat-producing radioactive waste

The search for a site of a final repository for highly radioactive waste in Germany was renewed when the Site Selection Act (StandAG) came into force in 2013. In Germany the development of concepts for a final repository and safety analyses for a repository in a salt dome was prioritized for many decades, whereas repository concepts in clay and crystalline rock were first considered only two decades ago. The aim of a comparative site selection procedure is to find a site before 2031, which provides the best possible safety for the enclosure of highly radioactive waste over a time period of 1 million years. The fundamental safety-related consideration is the enclosure of toxic waste in a so-called containment effective geological area (ewG). The main aspect of a long-term proof of safety is logically the systematic proof of safe long-term enclosure of the deposited waste. The approach developed within the framework of appropriate research projects (e.g. Eickemeier et al., 2013) is essentially based on the proof of geotechnical integrity of the ewG as the fundamental geological barrier as well as the geotechnical barriers. Due to their unique characteristics, including imperviousness and plastic deformability, salt rocks have been used for decades in Germany and worldwide in mining and especially for energy storage. Whereas halite in salt domes (type steep​​​​​​​ inclined salt) is distributed particularly in northwest Germany, flatly deposited salt rock (type flat bedded salt​​​​​​​) dominates in middle Germany and salt pillows (type salt pillows) in parts of northeast Germany. Both types of “bedded salt”​​​​​​​ widely reflect in their lateral extension sedimentation-related deposition conditions, apart from diagenetically related alterations. Beginning with the presentation of the host rock-specific boundary conditions of the various rock salts, this article focuses on the appropriate procedures for the proof of integrity of the geological barrier rock salt, based on the available experiences, corresponding reference studies and analogous examples. In the results it is shown that repository concepts in bedded salt formations and especially in the constellation of salt pillows provide substantial safety-related advantages due to a site-specific multibarrier system with alternate deposition of salt and saliferous clay as well as an intact overlying rock covering.

Ground Subsidence Prediction Model and Parameter Analysis for Underground Gas Storage in Horizontal Salt Caverns

Due to a great demand of natural gas or oil storage in these years, horizontal caverns were proposed to fully use bedded salt formations of China. Under the same geological and operating conditions, the horizontal cavern would shrink more than traditional pear-shaped cavern, which might bring larger ground subsidence and affect the safety of storage facilities. A new prediction model was proposed in this paper for the time-dependent ground subsidence above horizontal caverns. The proposed model considered the impurity of bedded salt formations and simplified the horizontal cavern to an ideal cylinder. The shape of the subsidence trough was determined by the probabilistic integration method, and corresponding calculation formulas for the tilt, curvature, horizontal displacement, and horizontal strain were derived. Based on the assumption that the subsidence volume at the ground was proportional to the reduced volume of horizontal cavern, a formula for the reduced volume over time was established. FLAC3D was introduced to simulate the ground subsidence, and the results show that the proposed prediction model agreed well with the simulation results. Finally, the proposed prediction model was used to analyze the impacts of different stratigraphic parameters and design parameters. The results mainly show that, as the draw angle increases, the subsidence trough becomes deeper and narrower; as the depth of the cavern increases, the maximum subsidence first increases and then decreases, and the subsidence trough gradually becomes round; with the increase of the purity, the subsidence gradually decreases; with the increase of the creep properties and the stress exponential constant, the maximum subsidence first increases rapidly and then slowly approaches the limit; increasing the brine extraction velocity can shorten the cavern construction period and then reduce excessive ground subsidence; the subsidence decreases nonlinearly with the increase of internal pressure; with the increase of the cross section diameter and length of the horizontal cavern, the subsidence presents a significant nonlinear increase. In addition, unlike the traditional pear-shaped cavern, under the same conditions, the ground subsidence above the horizontal cavern according to this newly proposed model is much larger, and the ground subsidence contour line is no longer a standard circle. The findings of this study can help for better understanding of the prediction of ground subsidence above salt caverns and also provide a reference for the design and construction. However, the proposed prediction method is ideal and theoretical and should be further improved by engineering practice in the future.

Performance Analysis of Conceptual Waste Package Designs in Salt Repositories

ABSTRACTA performance analysis of commercial high-level waste and spent fuel conceptual package designs in reference repositories in three salt formations was conducted with the WAPPA waste package code. Expected conditions for temperature, stress, brine composition, radiation level, and brine flow rate were used as boundary conditions to compute expected corrosion of a thick-walled overpack of 1025 wrought steel. In all salt formations corrosion by low Mg salt-dissolution brines typical of intrusion scenarios was too slow to cause the package to fail for thousands of years after burial. In high Mg brines judged typical of thermally migrating brines in bedded salt formations, corrosion rates which would otherwise have caused the packages to fail within a few hundred years were limited by brine availability. All of the brine reaching the package was consumed by reaction with the iron in the overpack, thus preventing further corrosion. Uniform brine distribution over the package surface was an important factor in predicting long package lifetimes for the high Mg brines.