Investigation of the subsurface conditions at a nuclear power plant site is important at all stages of the site evaluation process. The purpose of this investigation is to provide information or basic data for decisions on the nature and suitability of the subsurface materials. At each stage of the site evaluation, the investigation program should provide the data necessary for an appropriate characterization of the subsurface.
Related with power plants are spent fuel storage facilities (interim and permanent). Some of these facilities lie underground (geological disposal). The principle is to carefully place the radioactive waste in excavated tunnels in geological formations such as salt, hard rock, or clay. The detailed study of these formations is of the outmost importance for their long-term use.
A geophysical survey followed by a geotechnical campaign is often the most cost-effective and rapid means of obtaining subsurface information, especially over large study areas as those usually involved in nuclear power plants (several hectares). Geophysics can be used to select borehole locations and can provide reliable information about the nature and variability of the subsurface between existing boreholes. An isolated geologic structure such as a karstic feature might not be detected by a routine drilling program. An effective geophysical survey, however, could detect the presence of the karst and map its depth and lateral extent.
Other advantages of geophysical methods are related to site accessibility, portability, noninvasiveness, and operator safety. Geophysical equipment can often be deployed in heavily forested areas, on steeply dipping slopes, in marshy terrain or rock, and in other areas that might not be easily accessible to drill rigs or cone penetration test (CPT) rigs.
The typical information that can be derived is stratigraphical and structural:
- Descriptions of the extent and nature of subsurface materials.
- Characterizations of soil and rock (in terms of properties).
- Information on groundwater (the groundwater regime, locations and characteristics of the hydrological units, physical chemistry of the water).
It is very common to carry out seismic refraction and reflection surveys to provide continuous lateral and depth information for the evaluation of subsurface conditions. Interpretation of the survey results provides stratigraphic and structural geological information, information on the location of the groundwater table and an estimate of wave velocities at the site. Borings at a later stage provide vertical stratigraphic confirmation for the survey.
Shear-wave velocity models are also required (cross-hole. Down-hole, MASW) to calculate the dynamic elastic moduli of the subsurface materials.