Research on detection scheme of pebbles in fuel handling system pipelines in pebble-bed HTGR based on γ-ray measurement

In the pebble-bed high temperature gas-cooled reactor (HTGR), the fresh fuel pebbles and the graphite pebbles are loaded before reaching the stable stage. After several circulations, the spent fuel needs to be dispensed into the corresponding containers. During the circulations, we need to count the passing of pebbles, distinguish and count the discharged spent pebbles to meet the requirements by nuclear safeguarding. Therefore, along the pipelines, detectors with these certain functions are required. In this paper, by using the characteristics of pebbles' different γ radiation intensities, we design the schemes to use scintillator detectors to carry out the detection. Based on the above principle, the detection process is simulated and evaluated. Through the MCNP simulation, we estimate the relevant signals of four different pebbles -- fresh graphite pebbles and fresh fuel pebbles, irradiated graphite pebbles and spent fuel pebbles -- when they pass through the detector, to demonstrate the feasibility of the detection scheme to identify and count these pebbles.