INTRODUCTION

ABSTRACT

Case histories and field observations indicate that the combination of thermally induced stresses, excavation and in-situ stresses may cause thermomechanical failure (or thermal spalling) of the rock mass around an underground opening under certain thermal loading conditions. This paper discusses the factors and mechanisms underlying the thermomechanical stability of caverns excavated in hard rock. In particular, the thermal transients and thermoelastic stresses in the rock mass around a conceptual reactor cavern in Precambrian granitic rock in a high horizontal stress field under hypothetical accident conditions are analyzed, with the temperature dependence of rock properties taken into consideration. Based on the results of the analysis, the stability of the rock cavern against thermomechanical failure is evaluated and protective measures to minimize the possibility of such failure are discussed. Typical results of thermal transient and thermal stress analyses for a conceptual nuclear waste repository in hard rock are also presented.

In recent years, a number of investigations have been carried out by Ontario Hydro staff on the feasibility and rock mechanics design of various proposed underground facilities, including underground nuclear power station, fuel oil storage, energy storage and nuclear waste disposal. Rock caverns used for the storage of fuel oil or disposal of nuclear waste will be subjected to temperatures well above ambient. Likewise, rock caverns housing nuclear power plants may also be subjected to temperature rises under accident conditions (i.e. loss of coolant or rupture of steam generators) . These changes in temperature will induce a thermal stress field in the rock mass around the cavern opening, in addition to in-situ and excavation stresses. For many of the Paleozoic and Precambrian rocks of Ontario, Canada, a state of high horizontal in-situ stress has been observed. Tunnels and caverns constructed in these rock types generally experience a concentration of horizontal compressive stresses in the roof and floor, as manifested by field observations. In some cases, the resultant thermoelastic stress (i.e. thermal stress plus in-situ and excavation stresses) may be high enough to cause stress-induced instability of the rock mass around the cavern. This type of thermally induced rock failure has been observed in Canada and elsewhere and is generally referred to in the literature as thermal spalling. The effects of thermoelastic stresses on rock mass stability are discussed in this paper, along with case studies of a conceptual underground nuclear reactor cavern and a conceptual nuclear waste repository in crystalline hard rock.

CASE HISTORIES OF THERMOMECHANICAL FAILURE

A review of case histories of thermal spalling in rock was undertaken by Tsui (1978), in connection with Ontario Hydro's Underground Nuclear Power Station Study (1979). The number of well-documented case histories is limited. It was known, however, that various degrees of thermal spalling had occurred in some oil storage caverns in Sweden and Finland (Morfeldt, 1974; Johansson and Lahtinen, 1976), as well as in borehole heater experiments for nuclear waste disposal studies. The most noteworthy case history of thermal spalling involved a diesel-exhaust passage in an underground electric generation.

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