The power output of fusion experiments and fusion reactor-like devices isA new, more detailed, calibration is being provided 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 providecorrections 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 modelsto 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. measured in terms of the neutron emission rates which relate directly to the fusionyield rate. 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. The purpose of this paper is to give an overview of the arrangements being developed to allow a new calibration to be made.
COBISS.SI-ID: 25476647
After the coated CFC wall to ITER-Like Wall (Beryllium/Tungsten/Carbon) transition in 201011, confirmation of the neutron yield calibration will be ensured by direct measurements using a calibrated 252Cf neutron source deployed by the in-vessel remote handling boom and Mascot manipulator inside the JET vacuum vessel. The paper describes preliminary calculations and the results of numerical study of the effect of source holder on neutron detector response. The source baton was designed in such a way, that it does not significantly affect the neutron spectrum, angular neutron flux distribution or activation detector response. All effects are approximately equal to or less than 1%. The largest disturbance to the neutron flux angular distribution and to the neutron spectrum arises from the source capsule. Hence one should obtain as much information as possible about the capsule and the 252Cf source material in order to avoid additional systematic errors.
COBISS.SI-ID: 26263847
After the coated CFC wall to ITER-Like Wall (Beryllium/Tungsten/Carbon) transition in 20102011, confirmation of the neutron yield calibration will be ensured by direct measurements using a calibrated 252Cf neutron source deployed by the in-vessel remote handling boom and Mascot manipulator inside the JET vacuum vessel. Neutronic calculations are required to calculate the effects of the JET remote handling (RH) system on the neutron monitors. We developed a simplified geometrical computational model of the JET remote handling system in MCNP. In parallel we developed a script that translates the RH movement data to transformations of individual geometrical parts of the RH model in MCNP. After that a benchmarking of the model was performed to verify and validate the accordance of the target positions of source and RH system with the ones from our model. In the last phase we placed the JET RH system in the simplified MCNP model of the JET tokamak and studied its effect on neutron monitor response for some example source positions and boom configurations. As the correction factors due to presence of the JET RH system can potentially be significant in cases when the boom is blocking a port close to the detector under investigation, we have chosen boom configurations so that this is avoided in the vast majority of the source locations. Examples are given.
COBISS.SI-ID: 26907943
Neutron yield measurements are the basis for the determination of theFrom the modelling we find that a minority of the neutrons hitting the fission chamberspenetrate 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. absolutefusion reaction rate and the operational monitoring with respect to the neutron budget during any campaign for JET, the Joint European Torus.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 andprocedures 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. After the 2010 changes of the JET plasma-facing materials (Carbon wall to ITER-Like Wall transition), confirmation of the neutron yield calibration willbe 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 performingMonte Carlo neutron transport calculations.
COBISS.SI-ID: 25476391
The power output of fusion experiments and fusion reactor-like devices is measured in terms of the neutron yields which relate directly to the fusion yield. In this paper we describe the methods used to make the in-situ calibration of JET in April 2013 and the analysis of its early results. The target accuracy of this calibration was 10%, just as in the earlier JET calibration and as required for ITER, where a precise neutron yield measurement is important, e.g. for tritium accountancy. We calibrated the two main systems which carry the JET calibration, ie the external Fission Chamber detectors and the Activation System. This was the first direct calibration of the Activation system in JET. We used the existing JET remote-handling system to deploy the 252Cf neutron source and developed the compatible tooling and systems necessary to ensure safe and efficient deployment in these cases. The scientific programme has sought to better predict and understand the limitations of the calibration, to optimise the measurements and other provisions, to provide corrections for perturbing factors (e.g. presence of the remote-handling boom and other non-standard torus conditions) and to ensure personnel safety and safe working conditions. Much of this work has been based on an extensive programme of Monte-Carlo calculations and examples are given. First analysis of the activation results is complete and we review the results and errors. The analysis of the fission chamber data is still under way and we will discuss the findings so far.
COBISS.SI-ID: 27277351