A contribution is given to the large scale HAW-experiment in the Asse mine in Germany. G.S.F Muenchen and E.C.N Petten are testing in a one-to-one scale test facility the complete technical system of an underground repository, in order to improve the final concept for high level radioactive waste (HAW) disposal. The Delft laboratory of Mining Technology/Rock Mechanics has installed a measuring system consisting of 50 transmitters and 50 receivers combined with two ten-channel transientrecorders for the long term observation and recording of cataclastic effects in the rock salt around a heater borehole Bl. Possible macro- and microfracturing will be detected by means of so called acoustic cross-hole measurements. Fans of acoustic P-wave velocities are determined providing information about the structural changes of the rock salt in a pillar and in the wall of a gallery at the 800 m level. The floor of the gallery is also monitored and information is gathered about the behaviour of the rock salt around an installed heater. Some results will be given of the measurements of the underground experiment in the period 1986 to 1988. The observed and expected structural changes will be explained by means of recent field tests and model experiments in the true triaxial cell in Delft. Preliminary processing of the data is carried out with tomographic imaging techniques. One of the objectives of the large field test is the development of measuring techniques. This type of measurements will be of value in refining the model, describing the behaviour of the salt.
Main objective of the European Community's research programmes on "management and disposal of radioactive waste" is to find effective means for ensuring the safety of man and his environment against the potential hazards arising from such wastes. Ways are sought for isolating the Waste from the environment for thousands of years with no need for human intervention. Such conditions can be achieved by disposal in the deep underground in geological formations, which are very unlikely to be disturbed by natural causes or by human activity. One of the studied concepts is the disposal in rock salt in 300 to 600 m deep bore-holes, drilled from about 800 m depth. Many safety studies, design calculations and thermomechanical tests have already been carried out in order to develop this concept. The next step in the development of this technology is a pilot scale demonstration test, to be carried out by the Gesellschaft fuer Strahlen- und Umwelt Forschung mbH, Muenchen (G.S.F.) in cooperation with the Netherlands Energy Research Foundation (E.C.N.). Objectives of this large HAW test are a.o, the measurement of brine and gas release, the demonstration of the transport and handling techniques of high active waste, the validation of thermomechanical models and the further development of several measuring techniques related to safety aspects of a repository. (Rothfuchs, 1988). The developing of the acoustic cross-hole technique has to be seen as a method to get insight in the development of microfracturing, its intensity and direction.
