The design and fabrication of beta sensor system for in situ diffusion tests in mudstone in France

Long term in situ diffusion experiments in the Callovian-Oxfordian mudstone (France) are designed in the context of nuclear waste management. β-emitters HTO and ³⁶Cl radiotracers are representative of neutral and anionic species in these experiments. Studies have been carried out to design an in situ beta monitoring system in order to quantify tracer migration in the rock pore water over time. The development, design and calibration of β-emitter radiation sensors were performed. An optimised geometry was calculated for the detection chamber of ³⁶Cl in solution (Ø×L=30×2mm²) via Monte-Carlo N-Particle transport simulation tools. A better SNR (Signal to Noise Ratio) was obtained with YSO cylindrical crystal (Ø×L=6.5×0.5mm²). A sensitivity of 0.21cpm/Bq/mL was measured using ³⁶Cl standards. The ³⁶Cl detection limits were 18Bq/mL and 14.2Bq/mL after 1h and 24h of accumulation time respectively. Such beta sensors, placed 350mm from the radiotracer injection borehole, could successfully discriminate ³⁶Cl anisotropic diffusion in various directions within 4 years. It will be tested in situ and can be adapted to other in situ experience.