Determination of dry density and water content of compacted bentonite blocks using a dual-source (133Ba and 137Cs) gamma-ray attenuation technique

The dry density and water content of bentonite buffers are critical parameters governing the long-term performance of high-level radioactive waste repositories, making accurate non-destructive testing essential for quality assurance. This study develops and validates a dual-source ( 133 Ba and 137 Cs) gamma-ray attenuation methodology for the simultaneous determination of these two properties in compacted bentonite blocks. Experiments were conducted on specimens prepared with a range of water contents (10–18%) and compaction pressures (10–35 MPa), and a novel mathematical framework, the 'Sensitivity Determinant (K)', was introduced to analyze the system's error propagation characteristics. The results demonstrated high accuracy, with the proposed method showing a strong linear correlation to conventional gravimetric measurements for both dry density (R 2 = 0.9863) and water content (R 2 = 0.9546). The maximum errors were found to be 0.0214 g·cm −3 for dry density and 1.69 percentage points for water content. A slight systematic discrepancy was identified, the origin of which was traced to an overestimation of the transmitted intensity of the low-energy (81.0 keV) 133 Ba gamma-rays, caused by their sensitivity to the material's physical state (loose powder vs. compacted block) and to surface desiccation effects. The proposed K-framework successfully provided a theoretical explanation for these observed error trends. This study validates the dual-source gamma-ray attenuation technique as a reliable and practical non-destructive tool for evaluating the key properties of compacted bentonite blocks, contributing to the advancement of quality control technology for the safety of geological repositories.