Nuclear Analysis Supporting the Design Optimization of the ITER Radial Gamma-Ray Spectrometer (RGRS)

The Radial Gamma-ray Spectrometer (RGRS), located in the port cell of Equatorial Port No 1, is a cornerstone diagnostic system within ITER’s Research Plan, needed for advancing the understanding of fast ion behavior, runaway electrons, and measuring fusion power accurately. The system comprises four high-resolution LaBr₃ scintillator spectrometers, with lines of sight traversing the Bioshield to monitor the plasma. This study presents detailed neutron and gamma-ray transport analyses that support the ongoing design optimization of the RGRS. Monte-Carlo simulations were conducted using ITER’s 40-deg Tokamak Model (C-Model) and a standalone model of the equatorial port cell behind the Bioshield plug, using secondary neutron and gamma-ray sources. The calculations were performed to assess and validate the design improvements of RGRS aimed at optimizing the diagnostic configuration for ITER DT plasma conditions. The work also introduces an optimized input methodology that enables faster neutronics analyses during iterative design refinements. Furthermore, the results provide a preliminary assessment of neutron and gamma-ray backgrounds inside the RGRS detectors for spectral regions relevant to diagnostic operation. This can further contribute to performance analyses for evaluating detector response.