The storage and transport of water-borne heat directly in rock caverns and tunnels is today a relatively untested technique which is judged to offer economic and technical advantages when applied on a large scale. The storage of hot water however, causes a change in the chemical composition of the storage water and gives weathering products on the exposed minerals of the cavern wall. These exolution and weathering processes are very complex and it is at present necessary to determine the ion exolution rate for each rock type at a certain specific water quality.
A research project was initiated in the early 1970's by the Swedish Rock Mechanics Research Foundation (Befo) in order to solve these problems of large scale hot water (l500C) storages. This project was divided into various specific subjects dealing with the rock mechanic due to heating, the behaviour of concrete structures in hot water, the chemistry of water-rock interaction and the corrosion - precipitation in heat exchanger systems. The present results of water-rock interaction when storing hot water in crystalline rocks show changes in the natural chemical equilibrium between the rock and ground water which previously prevailed, chiefly as a consequense of increasing temperature and pressure. For most rock forming minerals, except the carbonate group, there is an increase of solubility due to increasing pressure and temperature. This causes a change in the chemical composition of the storage water and gives a precipitation on the exposed minerals of the cavern wall. This precipitate, however, does not prevent further ion exolution from the hidden mineral phases. It has also been proved that there is an equilibrium between the newly formed weathering material and the water phase. The ion content of the storage water reaches after a period of time a stable level. It is evident that the exolution processes are complex and even if it is possible to get data for a single mineral system, it is impossible to combine the results from tests of single mineral systems forming a certain rock type and thereby get a good estimation of ion exolution. Studies have been carried out on the mineral composition of the granites used in the weathering tests without getting any correlation between the solid phase and the water phase composition. Therefore, it is now necessary to determine the ion exolution rate for each rock type at a certain specific water quality.
PREINVESTIGATIONS AND LABORATORY METHODS.
The laboratory tests in this investigation have been run at 1500C and 0.6 mPa in small teflon lined autoclaves with a total volume of 200 cm3 and a water content of 75 cm3. (Fig. 1.) The water in the autoclave entirely covers the rock slices. All the reaction water is replaced after the first week and after that every fourth week as long as the test goes on. The first replacement is due to the initial rapid reactions. This procedure leads to an increasing ion exolution process for the succeeding four week periods.
