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Description
ITER Divertor Neutron Flux Monitor (DNFM) is designed to measure the total neutron yield and the fusion power in a wide dynamic range (1014 to 3x1020 n/s). Each DNFM detector unit is comprised of 235U and 238U fission chambers with independent electrode systems coated with either 500, 50 or 5 mg of uranium, allowing it to cover the entire dynamic range of fusion power. This setup also provides sensitivity to different energy groups: 235U is sensitive to neutrons in a wide energy range, especially at low energies, while 238U is sensitive to neutrons above the threshold of ~ 1 MeV.
ITER in situ calibrations are planned prior to the Start of Research Operation (SRO) and the DT-1 operational phase. NG-24M sealed tube neutron generator will be mounted on a robotic arm deployed inside the vacuum vessel from the Equatorial Ports. The in situ calibration campaigns aim to determine the calibration coefficients of the DNFM fission chambers that convert the detector count rates into total neutron yield. The presence of a robotic arm in the vacuum vessel introduces a bias in the calibration measurements due to neutron scattering on its structure. One of the objectives of this work is to quantify this source of bias. Monte Carlo neutron transport simulations employing a detailed model of the ITER machine, material compositions, and neutron source characteristics are used to calculate the uranium fission rate in DNFM fission chambers.
In this work, the OpenMC code is used to assess the impact of the robotic arm on the detector count-rates. In four of the six cases considered, the presence of the robotic arm alters the fission rates by up to 13%, with the largest contribution originating from the redistribution of neutrons below 1 MeV. We further demonstrate that positioning the neutron generator closer to the detector and away from the plasma axis leads to a neutron spectrum profile that is significantly closer to that of the volumetric plasma source.
The assessment underlines the need for detailed geometric modelling of both the in situ calibration campaign and of the machine during operation. This work contributes to the determination of the DNFM calibration coefficients and the associated uncertainties. Neutron transport simulations performed with the OpenMC code enable this bias to be explicitly evaluated and incorporated into the calibration coefficients, thereby compensating for its influence. The obtained results allows us to formulate a set of proposed calibration source positions.
The work is supported with the task agreement between the Institution “Project Center ITER” and the ITER International Fusion Energy Organization “Neutron calibration equipment design, modelling and testing” (IO/25/TA/450000025) as of 12th of December 2025.
The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.
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