In connection with the geotechnical study of a proposed underground nuclear plant, a 300 m deep test hole was drilled at Ontario Hydro's Darlington site approximately 65 km east of Toronto. The hydrofracturing method was used to determine the state of stress in the Paleozoic limestones (0–220 m depth) and the Precambrian granitic gneiss (220–300 m). The results of these measurements indicated a state of high horizontal stress along the depth of boring and a consistent orientation of principal stresses in each geologic unit. These results were confirmed by a borehole TV camera survey, and by overcoring tests at shallow depths in the same general area. The measured stresses were used in an assessment of the stability of large reactor caverns for housing the nuclear components of a proposed 4 × 850 MWe underground nuclear power plant in Precambrian hard rock. It was found that under ambient temperature conditions the state of stress alone did not critically affect cavern stability. Next, the occurrence of thermal shock in the cavern under a loss-of-coolant accident condition was also analyzed. To aid in the design analysis of the thermomechanical stability, a series of tests were carried out on the thermal properties of Precambrian rocks at elevated temperatures, including thermal conductivity, thermal diffusivity and coefficient of thermal expansion. The test results on thermal properties are presented in the paper, along with the results of the finite element analysis and other pertinent rock mechanics tests performed.

INTRODUCTION

Underground siting of nuclear power plants has been receiving increased interest in various parts of the world owing to some potentially important benefits over equivalent surface installations. Reactors housed in deep rock provide an additional safety margin in containing the release of radionuclides, in reducing the damage due to earthquake in high seismic areas, in protecting against sabbotage, aircraft crashing, hurricanes, etc., in supplying high-pressure gravity-fed cooling water to the fuel core in emergency situations, and in minimizing the amount of land use and environmental impact (Oberth and Lee, 1979). These and other potential advantages are still being weighed in the United States against the major drawback of higher construction costs. Norway and Sweden, however, built their first underground power facilities some 20 years ago (McHugh, 1964; ENEA Report, 1962). In Canada, Ontario Hydro has recently embarked on an extensive investigation of the technical and economic implications of building a large 4 × 850 MWe underground CANDU (Canadian Deuterium Uranium) power station in Ontario. The location of the proposed power plant will have to satisfy several siting requirements including the proximity to a major load center to cut transmission costs, and the availability of an adjacent large body of cooling water. A third requirement, that of suitable host rock formation to ensure water tightness and cavern stability was the subject of a preliminary geotechnical investigation which included a geological surface study at the Darlington site, the drilling and coring of an NQ borehole (UN-l), and a number of borehole tests such as permeability and stress measurements.

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