RF Attenuation Measurements in Compacted Bentonite Buffer Materials Using Software-Defined Radios

Compacted bentonite buffers are crucial components in engineered barrier systems (EBS) for high-level radioactive waste (HLW) disposal. However, their physical properties significantly attenuate radio frequency (RF) signals, complicating wireless sensor implementation. This paper experimentally investigates the RF attenuation characteristics of various compacted bentonite buffers using software-defined radios (SDRs). Both powder and granule-type bentonite blocks were fabricated and evaluated under different compaction pressures and water contents. For measurements, the SDR transmits frequency-offset single-tone signals with pulse shaping, while changing the transmit power and carrier frequency. Experimental results indicate that granule-type bentonite exhibits greater RF signal attenuation than powder-type bentonite due to its higher dry density. Furthermore, even under identical dry density conditions, granule samples are more sensitive to water content variations than powder samples. Additionally, engineering-scale tests show that each additional 25 cm of buffer thickness increases signal attenuation by about 12 dB at a dry density 1.6 g/cm3. These findings provide valuable insights for designing wireless monitoring sensors in HLW disposal systems, highlighting the impact of bentonite type, density, moisture content, and thickness on RF signal propagation.