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.
COBISS.SI-ID: 25476391
The power output of fusion experiments and fusion reactor-like devices is measured in terms of the neutron emission rates which relate directly to the fusion yield rate. The largest fusion power produced in magnetically confined experiments so far was at JET in 1997, when a peak value of 16 MW was achieved. Determination of such parameters requires a set of absolutely calibrated neutron detectors. At JET, the Fission Chamber neutron detectors were originally calibrated some 20 years ago by performing a set of in-situ calibrations using neutron sources and the absolute calibration has been maintained since then by cross calibrations against activation system measurements. After this elapsed time and a succession of changes to the internal and external JET structures, the JET Neutron yield calibration needs re-measurement. A new, more detailed, calibration is being provided in practical terms by means of an engineering programme of development of the robotic tools which will allow safe and accurate deployment of a strong 252Cf source for the measurements. It is led by a scientific programme which seeks to better understand the limitations of the calibration, to optimise the measurements and other provisions, to provide corrections for perturbing factors and to ensure personnel safety and safe working conditions. Much of this work is based on an extensive programme of Monte-Carlo calculations. These include the updating of previous JET models to provide continuity of comparison with previous understanding, the provision of fast models for side effect estimation and the development of a new more detailed JET model which will allow comparisons with the older more homogeneous model while coping with the demands of the new calibration.
COBISS.SI-ID: 25476647