INFLUENCE OF INSTALLATION POSITION DEVIATION ON MEASUREMENT RESULTS OF AN ON-LINE RADIATION MONITORING INSTRUMENT IN HTR-PM: A MONTE CARLO

The high temperature gas-cooled pebble-bed modular reactor (HTR-PM), considered as the first pebble-bed modular type demonstration nuclear power plant in the world with the safety characteristics of the fourth-generation nuclear energy system, has been constructed in Shidao Bay, Shandong Province, China. It connected to the grid first time on December 20, 2021. The inherent safety of HTR-PM largely depends on the adoption of the tristructural-isotropic (TRISO) coated particle as the nuclear fuel, in which the SiC layer can retain nearly all the fission products. However, there are still small amounts of nuclides which can diffuse from the intact or defective TRISO coated particles at a high temperature. Along with the radionuclides produced from the fission reaction of uranium contamination in the matrix graphite and neutron activation reaction in the core, they dominate the radioactivity in the primary coolant. The radiation monitoring on the radioactive level of the primary coolant is essential to assess the fuel performance in the reactor core. An on-line gross γ monitoring instrument was designed to monitor the real-time variations of radioactivity in the primary circuit, which can provide a direct reflect of the fuel element performance in the core. It adopted NaI detector, which was aligned with the center of the primary pipe. While, in the actual installation environment, the center of the pipeline to be measured was deviated from the detector with a maximum distance of 37 mm. Obviously, this deviation will decrease the detection efficiency of the instrument. We adopted the MCNP (Monte Carlo N Particle Transport Code) to calculate the detection efficiency under normal condition, and implemented a sensitivity analysis of the influence of the pipe deviation on the detection efficiency of the instrument. Current quantitative calculations can provide valuable guidance for the on-line radiation monitoring instrument installation in the nuclear power plant.