In recent years interest in gas escape phenomenon from unlined mined rock caverns has increased considerably. This phenomenon is most critical to the feasibility of Compressed Air systems, closed surge chambers, and underground storage of hydrocarbons. It is of further interest in the long term sealing of radioactive waste repositories and in the potential siting of underground nuclear plants. All previous studies used extremely simplified analytical models and/or laboratory tests and attempted to define a "critical gradient" at which "bubble escape" would cease to occur. The predicted critical gradients ranged from 0.4 to 1.0. These ideas were sometimes translated into practice in the form of elaborate tunnel schemes with radiating pressurized boreholes in order to maintain sufficient gradients both during cavern construction and during use. This paper describes detailed laboratory experiments which were conducted to study two-phase countercurrent flow through simulated rock fractures. A modified Hele-Shaw parallel plate model was built which allowed variation of fracture aperture and orientation with respect to the pressurized cavity. Also, different entrance geometries were simulated to delineate their importance as far as bubble or slug initiation is concerned. An array of sensitive pressure transducers were used to obtain any small variation in the pressure field during bubble propagation. The deformation and volume of the bubbles were recorded via a high speed camera. The data is analysed in light of existing two-phase flow theories developed in the chemical and nuclear engineering fields. Engineering implications are discussed in detail. INTRODUCTION Gas escape from unlined rock caverns involves two distinct problems: the fundamental initiation and motion of gas "bubbles" through rock fractures, and, the calculation of overall gas losses from storage caverns. Solutions to the latter problem, at present, are best handled using an equivalent porous medium approach as suggested by Barton (1972) and Berg and Noren (1969). However, this provides no insight into the fundamental mechanism and parameters controlling the initiation of gas escape. It is this problem that the present research addresses. Interest in two-phase flow in the geotechnical area was originally restricted to the petroleum engineering field where drillstem and two-phase porous medium problems have received considerable attention. A phase is simply one of the states of matter and can be either a gas, a liquid, or a solid. The term two-component is sometimes used to describe flows in which two phases are not of the same chemical substance. Since the mathematics describing two-phase or two component flows are identical, it does not matter which definitions are chosen. Increasing use of underground space has aroused significant interest in developing a better understanding of single-phase and to a lesser extent of two-phase fracture flow. Recently Willet (1979a, 1979b) has given an excellent review of the history and present status of the economic use of the underground. Two-phase fracture flow may be a very important, if not critical, parameter in each of the following areas: nuclear power plants sited underground, crude oil and petroleum product storage.

This paper describes how a rather conventional rock cavern will be used for intermediate storage of spent nuclear fuel from the Swedish nuclear power stations. The plant and the spent fuel handling is generally described. The design requirements for the rock cavern postulate very high safety demands. An extensive reinforcement will therefore be carried out in order to guarantee the stability of the rock cavern. The cost of the rock cavern will be reasonably low, in spite of the extended costs of reinforcement, compared to the advantages which are obtained by this type of sub-surface storage. INTRODUCTION In order to increase present spent fuel storage capacity available at the Swedish nuclear power stations, the Swedish Nuclear Fuel Supply Company (SKBF) is planning to erect an intermediate spent fuel storage facility (CLAB). Oskarshamnsverkets Kraftgrupp Aktiebolag is assisting SKBF in realising the CLAB project, and the Swedish State Power Board (SSPB) is designing the building constructions. The other main consultants engaged in the project is Asea-Atom (Sweden) and Societe Generale pour les Techniques Nouvelles (France). CLAB will be located close to the Oskarshamn nuclear power station. All permits required to the facility were obtained in 1979. CLAB was then the first licensed spent fuel storage facility of its kind in the western world. The fuel elements will be stored in a rock cavern, which provides very good protection against external impacts such as acts of war and sabotage. The bedrock will also protect the environment by isolating the fuel in the unlikely event of any internal impact taking place. The advantages of this sub-surface storage are obtained at a reasonably low cost compared to the cost of a storage building on the surface designed to resist the same impacts. The rock cavern will be constructed using well-known techniques. This paper will describe how a rather conventional rock cavern can be used in combination with techniques for nuclear fuel handling. The description of the design and construction work is based on knowledge of the existing rock mass on the site. THE CLAB FACILITY - GENERAL DESCRIPTION AND LAYOUT The CLAB design is based on a storage capacity of 3000 metric tonnes of uranium, with provisions to permit an expansion of this capacity to 9000 tons. In terms of function, the CLAB facility can be divided into three main parts: fuel reception, storage and auxiliaries. Spent fuel from Ringhals, Barseback and Forsmark nuclear power stations will arrive at the harbour of the Oskarshamn power station on a ship specially designed for this purpose. The fuel elements will be transported in containers called casks. These casks are designed to provide adequate protection against damage during transport. The casks will be transported on a special vehicle from the ship or from Oskarshamn power station to the CLAB facility. The vehicle will bring the cask to the fuel reception building at CLAB. In this building, the casks will be cooled down and then placed in an unloading pool. The fuel elements will be unloaded under water, which provides protection against radiation, and then placed in special canisters.

The present paper presents methods for injection of brine solution into the subsurface investigated under the Compressed Air Energy Storage Project of the Department of Energy in the United States of America. The principal objectives in this investigation were two fold: Prevention of any possible pollution of ground water by the injected brine solution. Avoidance of over pressurizing the solution-deposit zone by the injection operation which will result in: induced undesirable fractures in both upper and lower confining formations, and inefficient and uneconomical injection operation. INTRODUCTION The continental United States has many rock salt deposits, as well as salt lakes and natural brines. Salt is found in 4 major basins as well as several minor basins (Fig. 1). Taken together, this means that over one half of the states contain salt deposits (Lefond, 1969). The injection of brine into geologic formations can cause contamination of existing portable ground water. The contamination is due to either extensive lateral spreading or vertical upward of downward movement of the brine. The direction and magnitude of brine movement is a direct function of the total geologic environment surrounding the injection point. The total geologic environment embraces the rock types, fractures, rock strengths and permeabilities, in situ state of stress, ground water regime(s), and geochemistry. During brine injection containment is ideally obtained by ensuring that the suitable injection sequence, which is highly permeable, is over/underlain by impermeable rocks. These rocks restrict both the vertical upward and downward movement of the brine. The lateral extent is dependent on the uniformity, isotropic aspects, of the injection formation. The operating fluid pressures active within the migrating brine, must be known to ensure that they do not cause either, a) tensile fractures or b) extend pre-existing fractures within the over/underlying confining rocks. The promotion of fractures markedly increases the permeability of the low permeability confining rocks, and consequently contamination of previously isolated ground water by the brine. CRITERIA FOR INJECTION FORMATION The injection formation should be: an extensive sedimentary formation: unfractured sandstone, limestone, dolomite, and unconsolidated sands are the general lithologic types used for injection; at least several hundred feet thick; hundreds of square miles in extent in order to minimize injection pressures and to provide a buffer zone against migration of brine to discharge areas; an area of relatively simple geologic structure without complex folds and faults (synclinal sedimentary basins are favorable; high formation pressures are unfavorable); of uniform hydraulic properties with high enough permeability so that excessive injection pressures can be avoided (according to Warner in 1977, a porosity of at least 10% to 20% and permeability greater than 100 millidarcys, or 0.2 ft/day, is required for large-scale brine injection); of low or negligible lateral movement of formation fluids, to a discharge point (grouting may be necessary to lower the permeability so that lateral confinement is maintained); a zone that is below the level of fresh-water circulation with the surface