Rock mechanics for nuclear waste management

There are presently over 430 nuclear power plants in operation throughout the world. They produce nuclear fuel waste containing long-lived radioactive elements that need to be isolated from man and the environment for thousands of years. In most countries, emplacement deep underground in stable rock masses (geological disposal) is being proposed as a method for the safe disposal of nuclear fuel waste.

The general issues related to the underground disposal of nuclear waste were described in the 1989 Report of the International Association of Engineering Geology Commission. Although many of these issues are not usually addressed in the civil or mining engineering communities, the issues of excavation stability during operation and excavation methodology, at depths of 300 to 1000 m, the target depth for a disposal vault, are addressed daily by the mining industry. Of most concern is the issue of long-term stability of the closed disposal vault, and the potential for the creation of new fracture surfaces in the rock mass surrounding the vault, which could lead to the migration of radionuclides from the disposal vault to the surface environment in the distant future.

As a result, the international nuclear community has invested considerable time and effort in unravelling the fundamental mechanisms of rock fracture. In Canada much of the research has been carried out at AECL's Underground Research Laboratory (URL) located in Southeastern Manitoba. A major five-year rock mechanics experiment at the URL was the Mine-by Experiment, completed in 1995. Many of the findings from that experiment are now published. One of the more interesting findings refutes the common notion that cohesion and friction are mobilized at the same displacements. This notion has significant ramifications for estimating strength around underground openings.

The deep geological disposal concept will require kilometres of tunnel and shafts at depth of 300 to 1000 m. At these depths the excavations will produce microseismic activity adjacent to the openings. Usually microseismic activity is associated with deep-level mining and hence there is a depth of experience that the nuclear industry can draw on. For example, practical experience indicates that damaging seismic events (rockbursts) are associated with deep mines where extraction ratios are greater than 0.6. For the general repository arrangements being considered by the nuclear industry, extraction ratios will be less than 0.3. Hence, at these low extraction ratios the microseismic event will be local to the openings and unlikely to cause rockbursts.

Canada was the first country to construct an underground facility dedicated to geotechnical research. Now, after 15 years of research, other countries have recognized the importance of (in situ) research. Today Sweden, Finland, Switzerland, Germany and United States have dedicated underground research facilities, with United Kingdom and Japan planning such facilities.

GRC is currently involved with the ZEDEX Project at the ÄSPÖ Hard Rock Laboratory in Sweden. This project, specifically designed to quantify the damage around a 5 m diameter drill-and-blast drift and a 5 m diameter TBM drift, is being carried out in conjunction with Prof. Paul Young at Keele University in the UK.