Calculations to support JET neutron yield calibration: Contributions to the external neutron monitor responses

Highlights: ► We developed a simple but quick-running computational model of the JET tokamak. ► 5–10% of the neutrons hitting the fission chambers penetrate the tokamak wall, other come via ports. ► The highest contribution to a certain fission chambers is via the closest port and the second highest contribution is via the port closest to the neutron source. ► The torus hall wall significantly affects the external fission chambers response due to back scattering of neutrons. - Abstract: Neutron yield measurements are the basis for the determination of the absolute fusion reaction rate and the operational monitoring with respect to the neutron budget during any campaign for JET, the Joint European Torus. After the 2010 changes of the JET plasma-facing materials (Carbon wall to ITER-Like Wall transition), confirmation of the neutron yield calibration will be ensured by direct measurements using a calibrated 252Cf neutron source deployed inside the JET vacuum vessel. In order to thoroughly understand the transport of neutrons from the vacuum vessel to the fission chamber detectors mounted outside the vessel on the transformer limbs and thus to computationally support the JET neutron calibrations project, we developed a simple but quick-running computational model of the JET tokamak for performing Monte Carlo neutron transport calculations. From the modelling we find that a minority of the neutrons hitting the fission chambers penetrate the tokamak wall, whilst most come via the ports. The highest contribution to a fission chamber response comes via the port nearest to a point neutron source and the second highest contribution comes via the next nearest ports. If the port is blocked by a massive object, the fission chamber response is decreased by up to the contribution of that port. It was observed that the torus hall wall significantly affects the response of each external fission chamber due to back scattering of neutrons. The whole process of understanding and improving the knowledge of the neutron yield calibration for JET is of great interest for ITER, where the methods and procedures for calibrating the neutron yield monitors are still being developed, but the requirement is for 10% accuracy in the fusion yield determination, as it is in JET.