Bentonite is a clay material being considered as one of the engineered barriers to isolate the spent nuclear fuel, buried from the geosphere (natural barrier). Many studies of bentonite have been performed on the chemical and geotechnical properties of bentonite (Marcos 1997), but data on its petrophysical (electrical) characteristics are scanty. The migration of hazardous elements from the nuclear waste repository to the geosphere may be significantly affected by the porosity of the bentonite. To measure eventual changes in the porosity of bentonite in situ, electrical resistivity is a useful petrophysical parameter. In our study the electrical behaviour of the bentonite MX-80 with different values of density and water content is being analyzed in laboratory conditions. The electrical resistivity is measured with a speetral lP (induced polarization) system built around a two-channel signal analyzer (HP35665), which operates in a frequency range from 0.0156 Hz to 10 kHz (Vanhala & Soininen, 1995). A constant voltage sine signal is fed through the sample and a reference resistor. Potential difference across the sample is an input to the analyzer through a differential amplifier. Changes in porosity may be inferred from resistivity data which are obtained at different density and moisture values.


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