Technical Report NTB 88-40

The disposal of radioactive waste is to take place by means of a long-term and safe emplacement in deep geological formations. While the Federal Republic of Germany foresees disposal in salt formations, other countries favour sedimentary rock, crystalline or volcanic formations. The assent given by the German Federal Government to the State of Lower Saxony to investigate other geological formations as well as rock salt on their suitability to host repositories also led to a cooperative engagement with Switzerland.

The Swiss concept for the disposal of radioactive wastes envisages emplacement in solid rock formations of great compactness. The underground rock laboratory Grimsel (GTS) is being operated by Nagra (National Cooperative for the Storage of Radioactive Wastes) in order to test and to further develop methods to determine the suitability of a repository site as well as to investigate the qualification of granite formations as a host rock for the emplacement of high-level radioactive wastes. This underground laboratory is used by Nagra in cooperation with external partners to study the mechanical, geological and hydraulic properties of the rock by means of a number of in situ tests. One of the experiments carried out by the Institut für Tieflagerung (IfT) of the Gesellschaft für Strahlen- und Umweltforschung (GSF) was the "heater test" (HT). A heater test simulates the power output of heat producing wastes at the time of their emplacement. Tests of this kind were carried out repeatedly at different sites (e. g. Stripa, Asse, Climax). The heater test at the GTS was adapted to the specific requirements of the Swiss concept for final disposal which foresees emplacement in poorly fractured rock with low permeability. The wastes to be emplaced are cooled down for approximately 30 to 40 years in an interim storage facility until their power output at emplacement is too low to raise the temperature of the assumedly present surrounding water above boiling point.

Taking these boundary conditions into consideration, the following objectives were defined for the heater test in the GTS:

1. objective:Test and development of methods for exploration, proof of qualification and monitoring of a repository for heat producing wastes in the crystalline host rock.

2. objective:Investigations of the thermal, mechanical and hydraulic reactions of a crystalline host rock to an artificial input of heat.

3. objective:Development and application of a suitable computer analysis for site specific modelling of the thermomechanical processes in the near field of a heat source in the crystalline rock.

Planning, performance and evaluation of the heater test took place under strict observance of the objectives to be attained. In the preparatory phase of the geomechanical and geophysical instrumentation the applicability of the systems employed elsewhere to that date was reviewed and adapted to the specific requirements of this test. During the operational phase of the systems over a test period of three years it was possible to prove their general efficiency. Possible shortfalls were counteracted by increased servicing efforts.

The measured thermal, mechanical and hydraulic reactions to the artificial power input are to be documented and discussed in detail in this report. The thermoinduced mechanical reactions remained locally confined and did not reach an order of magnitude which could prove detrimental to the suitability of the formation as regards hosting of a repository. Due to the pronounced strain restriction effect of the surrounding rock the deformations were only very slight.

The rock stresses, resulting from strain restriction due to heating, remained below approximately 10 MPa. This means that in a repository at 1200 m depth and at a maximum rock temperature of 100°C a sufficiently great span between the resulting total stress and the bearing capacity of the rock is to be expected. An increase of seismicity was not registered during the test phases. However, it cannot be ruled out that seismic energy was liberated in the crystal texture as a consequence of heating and cooling processes.

Water discharge and pressure measurements were taken in the course of rock hydraulics investigations. Both changes of discharge rates and pressure gradients were restricted locally and by time. The ventilation test in the GTS, which also investigated the change of the hydraulic conditions in the surroundings of a heated cavity, confirmed this result.

In the heater test a simple rotation-symmetrical FEM-model was used for the preparation and test-accompanying modelling of the thermomechanical conditions. The results initially served for the layout of instrumentation and for outlining of the test procedure. Later on – when compared to the measurements – they served to give an estimate of the efficiency of the selected model with a view to the allocated task. The selected simple model proved to be adequate to describe the spread of temperature and the development of deformations and stresses in the rock due to the artificial heat input.

After termination of the heater test investigations the following may be said with regard to the three above mentioned objectives:
 

1. objective:Suitable measurement techniques are available for exploration and monitoring of the geomechanics of a repository site for heat producing wastes. A heater test with the emphasis on rock stresses and hydraulics should not be dispensed with when developing the repository. 

2. objective:The reactions of the crystalline host rock to the power input remain locally confined. Thermoinduced hydraulic effects are only of short duration. The mechanical reactions alone as a consequence of heat input from the waste canisters do not represent a hazard to the safety of the repository. They should, however, be added to the site specific primary state in each case.

3. objective:The relevant thermomechanical reactions can be modelled with a relatively small effort. Indispensable, however, is the realistic description of rock parameters and boundary conditions, for which some specific laboratory and in situ tests are required. 

This report was compiled within the scope of a Research and Development Project, subsidized by the Bundesministerium für Forschung und Technologie, entitled: "Untersuchungsprogramm zur Erkundung ausgewählter Eigenschaften von kristallinen Felsformationen für die Endlagerung radioaktiver Abfälle (Förderkennzeichen KWA-5315-4)".