Extensive research is in progress in the United States and abroad to develop the technology needed to provide safe, environmentally acceptable repository systems for the geological disposal of high level radioactive waste materials in several candidate host rock media.
This technology includes
the development of computer codes for determination of stresses and deformations (thermomechanical effects) resulting from emplacement of the hot waste canisters in the host rock,
in-situ testing to determine critical stress and deformation properties to aid in site selection and evaluation, and
generic field testing to determine the influence of repository construction (excavation effects) and to provide input for and validation of repository modeling.
Although satisfactory for many other civil and mining engineering applications, many of the instruments and measurement techniques currently available for use in the above-mentioned activities lack either the required accuracy (particularly for small deformations in hardrock) and/or reliability (high failure rates in the hot, corrosive, or wet hostile field environments for extended time periods). This conclusion is based on the findings cited and discussed later.
It is the objective of this paper to discuss accuracy and reliability and limitations of the state-of-the-art instruments and measuring techniques utilized to determine stress, displacement, and deformational characteristics in the above-mentioned environments. Also included in the discussion are comments relating to their sensitivity, stress and temperature ranges, advantages and disadvantages.
The accuracy requirements for instrumentation depend on the
function for which they are used,
the in situ environment (temperature, etc.)
the media in which they are placed (salt, granite, etc.),
the time frame of interests (exploration phase, construction phase, monitoring phase), and
volume of influence of the required measurement.
Thus, the accuracy required for measurements, for example, of displacement in salt differ from measurements in granite. The basic response of the rock media (elastic versus plastic, etc.) also will dictate the choice and thus the accuracy of the instrumentation. The required accuracy for fracture aperture measurements is high because permeability of fractured rock systems is related to the aperture of the fracture cubed. Estimates of required instrumentation accuracy were made by the In Situ Tests and Model Validation working group at a workshop for Thermomechanical Modeling for a Hardrock (Crystalline) Repository (Holzer and Ramspott, 1979). Instrumentation accuracy requirements for crystalline rock are estimated to be:
Temperature: 1 C for heat conduction and 0.1 C for convection
Stress: plus or minus 10 percent (into within 1 MPa)
Displacement: 1 micron for fracture permeability and 0.1 mm resolution for rock stability
Strain: 10−5 or better
Modulus of Deformation: approximately 5 percent
Coefficient of thermal expansion: approximately 5 percent.
These accuracy requirements need to be evaluated further for crystalline rock and for other rock types (salt, argillaceous rock, tuff).
Stress is determined either as a change of stress with time or as the complete or absolute stress at a given time.