Design of a robust and compact gamma sensor: Assessment of uranium detection in underground COx mudstone for nuclear waste management

摘要

In the context of deep geological radioactive waste disposal in France, various experiments have been designed, including in situ hydrogeological tests, chemical measurements and diffusion experiments. In this paper, we focus on in situ Diffusion of Radionuclides (DRN) tests, typically for actinide diffusion. These tests involved the construction of a powerful and qualified instrument that fulfills requirements for in situ radionuclides measurements. This implied a very specific design of four channels 1x1 inch cylinder YSO gamma-ray sensors, satisfying severe geometric and operational constraints while displaying high sensitivity and robust, real time on-line performance. The calibration of such sensors was assessed regarding their ability to quantify the minimum quantity of natural uranium that could be detected with and without the presence of Callovian-Oxfordian mudstone (COx) screens. Its design, calibration,and detection sensitivity are described in the present paper. The sensor sensitivity at 12 and 6 counts/second/g(U) without and with COx screening effect were determined for energy peak counting windows (50-105 keV) respectively. This was equivalent to a limit of detection (LOD) of uranium at 42.5 mg and 126 mg without and with a 7 cm thickness of COx sample. These sensors provided the first trial for in situ actinide diffusion tests in the context of nuclear waste management.