Management of Commercially Generated Radioactive Waste - U.S. ...

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5.2.3.1 Excavation and Underground Development
The excavation of rooms and tunnels underground will induce a new stress state and dis-
placement field in the host environment. The nature of these stresses and displacement
fields depends on the cross-sectional geometry of the excavation, the layout of the tunnels
and rooms, and the extraction ratio (the ratio of the volume removed to the volume remain-
ing) (Koplick et al. 1979).
Fracturing around the perimeter of the tunnels and rooms and effect on in-situ stress
states and its implications for long-term containment are two potential impacts being con-
sidered in the excavation of a repository.
Vast experience has been gained in the excavation of various kinds of underground
facilities. Fracturing during drilling and blasting operations is limited by controlling
such parameters as the size and type of charge, the configuration of drill holes, and the
sequence of detonation. Controls of these types are used extensively in the excavation of
undergound facilities intended for storage purposes and for long-term operations (Svanholm
et al. 1977); examples are caverns for compressed air and natural gas storage. In-situ
tests are in progress to confirm their suitability for the excavation of mined geologic
repositories (Hustrulid 1979). It is believed that no further technological advances are
needed in this area (Guiffre et al. 1979).
5.2.3.2 Thermal Effects
The thermochemical impacts of principal interest in repository design are those that
would accelerate the degradation of the waste package and the migration of radionuclide away
from the package. The introduction of heat into the system will change the environment in
which the waste was placed. The design of a waste package capable of withstanding the heat-
altered environment is discussed in Section 5.1.2.
The introduction of heat into the natural system will induce stresses in the host rock
and surrounding media (IRG 1979 and NAS 1979). These stresses will be superimposed on the
existing stresses and must be considered in design to ensure structural stability of the
repository. The heat generated by the emplaced waste will cause the rock mass to expand,
thus inducing surface uplift. In the long term, as the heat generation rate decreases, the
surface will subside. Displacement of the overlying rock mass may cause fracturing in the
rock, thereby giving rise to perturbations in the hydrologic flow regime. In addition, heat
may modify the thermal and mechanical properties of the rock; for example, an increase in
temperature will enhance the ductility of a rock but reduce its ultimate strength.
5.2.3.3 Radiation Effects
The effects exerted in the host rock by irradiation have generally been considered to
be of secondary importance. To date, most of the laboratory and theoretical studies have
concentrated on the effects of radiation on salt. The information available on radiation
effects on salt and on other geologic formations of interest for waste disposal has been