Deformation of rock salt in openings mined for the disposal of radioactive wastes

Abstract

Deformation of Rock Salt in Openings Mined for the Disposal of Radioactive Wastes With the storage of high-level radioactive waste in salt structures, unique mine stability problems will occur as a result of the elevated temperatures. To predict flow in rock salt, scale models of salt pillars and their surrounding rooms were fabricated from cores taken in the Carey salt mine, Lyons, Kansas. Tests were conducted at temperatures of 22.5°, 60°, 100°, and 200°C for axial loads of 2000, 4000, 6000, 8000, and 10000 psi at each temperature. These tests showed that marked increases occur in the rates of deformation of salt pillars at high loads and especially at elevated temperatures. For all combinations of axial loads and temperatures, it was observed that there is initially a high rate of deformation that diminishes with time. Creep rates were found to continue to decrease even after more than 3 years of testing. An empirical relationship between pillar deformation, stress, temperature, and time was developed from the tests and is expressed as $$\dot \varepsilon = 0.39 \cdot 10^{ - 37} T^{9.5} \sigma ^{3.0} t^{ - 0.70} ,$$ $$\varepsilon = 1.30 \cdot 10^{ - 37} T^{9.5\sigma3.0} t^{0.30} ,$$ where\(\dot \varepsilon\) = strain rate (in./in./hr),e = cumulative deformation (in./in.),T = absolute temperature (°K),σ = average pillar stress (psi), andt = time (hr). For comparative purposes, model pillar tests were conducted on samples of bedded salt, as well as dome salt from six different mines in the United States and from the anticlinal structure of the Asse II salt mine in Northeast Germany. In general, the deformational behavior for the various types of salt was similar at room temperature as well as at elevated temperature even though some variations in the rates of deformation were observed. From model tests it was also observed that greatly accelerated rates of deformation will occur in excavated cavities where thin shale beds occur in the pillars at the roof and floor interfaces. Since the shalcs serve as friction reducers, effective confining stresses in the roof and floor are not transmitted into the pillars; thus the pillars under these conditions are weaker than where shale partings are not present at the tops and bottoms of the pillars.