Benchmark experiments on neutron streaming through JET Torus Hall penetrations

Abstract

Neutronics experiments are performed at JET for validating in a real fusion environment the neutronics codes and nuclear data applied in ITER nuclear analyses. In particular, the neutron fluence through the penetrations of the JET torus hall is measured and compared with calculations to assess the capability of state-of-art numerical tools to correctly predict the radiation streaming in the ITER biological shield penetrations up to large distances from the neutron source, in large and complex geometries. Neutron streaming experiments started in 2012 when several hundreds of very sensitive thermoluminescence detectors (TLD), enriched to different levels in 6 LiF/ 7 LiF, were used to measure the neutron and gamma dose separately. Lessons learnt from this first experiment led to significant improvements in the experimental arrangements to reduce the effects due to directional neutron source and self-shielding of TLDs. Here we report the results of measurements performed during the 2013-2014 JET campaign. Data from new positions, at further locations in the South West labyrinth and down to the Torus Hall basement through the air duct chimney, were obtained up to about 60 m distance from the plasma neutron source. In order to avoid interference between TLDs due to self-shielding effects, only TLDs containing natural Lithium and 99.97% 7 Li were used. All TLDs were located in the centre of large Polyethylene (PE) moderators, with nat Li and 7 Li crystals evenly arranged within two PE containers, one in horizontal and the other in vertical orientation, to investigate the shadowing effect in the directional neutron field. All TLDs were calibrated in the quantities of air kerma and neutron fluence. This improved experimental arrangement led to reduced statistical spread in the experimental data. The MCNP Monte Carlo N-Particle code was used to calculate the air kerma due to neutrons and the neutron fluence at detector positions, using a JET model validated up to the magnetic limbs. JET biological shield and penetrations, the PE moderators and TLDs were modeled in detail. Different tallying methods were used in the calculations, which are routinely used in ITER nuclear analyses: the mesh tally and the track length estimator with multiple steps calculations using the Surface Source Write /Read capability available in MCNP. In both cases, the calculated neutron fluence (C) was compared to the measured fluence (E) and hence C/E comparisons have been obtained and are discussed.

Neutron streaming studies along JET shielding penetrations

“…For this reason, all associated parameters (such as the neutron self-shielding effect as well as the "shadowing" effect caused by the interference between individual TLDs and foils within the moderator) have been studied and taken into account in the calculations. A detailed description of the method has been presented [1,7]. As it can be seen from Table 1, a good agreement was observed between TLDs and activation foil measurements for most of the positions studied.…”

“…Moreover, the vacuum vessel has ports through which neutrons may escape. These complications were approximated with a zone composed of iron, plastic and copper which has been adjusted in thickness until the JET external fission chamber measurements were consistent with the fluxes in the MCNP model [1]. This approximation was necessary since the complexity in the torus hall is too great for more detailed modeling on a realistic timescale.…”

“…9). More details on the experimental procedure have been discussed elsewhere [1]. The comparison of the simulations against the experimental measurements is presented in Table 1.…”

“…Among others, benchmark experiments are being carried out at JET aiming to validate in a real fusion environment the neutronic codes and nuclear data applied in ITER nuclear analyses. In particular, measurements and calculations of the neutron fluence through the penetrations of the JET shielding aim to assess the capability of numerical tools to accurately predict neutron transport along the long paths and the complex geometries characterizing the ITER biological shield [1].…”

Neutronics experiments and analyses in preparation of DT operations at JET

“…The results from previous streaming experiments carried-out during 2012-2014 DD campaigns were recently published [3], showing a satisfying agreement over six orders of magnitudes between the calculations with MCNP5 Monte Carlo code [4] and the measurements performed with thermoluminescent detectors (TLDs).…”