[en] The neutron-induced radioactivity of the torus vacuum vessel presents a significant exposure source to in-vessel workers. Assessment of the torus radiation field is of prime importance in accurately predicting the likely doses. Correlation of the ambient dose-rate, the time spent in-vessel, and the actual doses to individuals show an apparent inconsistency, with the effective dose being approximately half of the expected dose. This paper aims to show that simple comparison of the ambient dose-rate, and the effective dose-rate is not appropriate in this situation because of the unique isotropic irradiation geometry that applies to tokamak vessel structures. The relationship between these two dosimetric quantities is compared, based on data from published sources, and discussed in relation to observations of doses to JET in-vessel workers. The measured effective dose to ambient dose ratio, which is found to be in the range 0.5-0.6, agrees with calculated values. Future shutdowns at JET in 2001 and 2003 will continue to involve manned entries to install new components and perform upgrades. For continued demonstration of JET policy on ALARP, and for optimising manpower and equipment resources, it is important to have a firm basis for predicting the doses to vessel workers

[en] The international fusion program is progressively more focused on producing plasmas of reactor relevance, requiring major progress in burning plasma diagnostics i.e. neutron, alpha particle, He ash, Tritium retention and Isotopic Composition measurements. In the last years it has emerged very clearly at JET that these diagnostics also provide crucial information about critical physical phenomena. First of all, several burning plasma measurements can improve significantly the diagnostic capability of the ion fluid. Neutron spectroscopy provides clear and direct measurements of, e.g., plasma rotation and fuel ion supra-thermal components, beside the ion temperature. During the JET Trace Tritium Experimental (TTE) campaign, spatially resolved neutron measurements were essential in obtaining the isotopic composition, the transport of the hydrogen isotopes and in assessing the merits of various heating schemes and their current drive capability. Burning plasma diagnostics can also strongly contribute to the physics of energetic particles and their interaction with the main plasma. Gamma ray spectroscopy is now an established method to determine the spatial localisation and to visualise the trajectories of ? particles and fast deuterons. During TTE the slowing down of fusion born alphas was measured for the first time with this approach in various plasma configurations and the effect of the q profile on the energetic particle confinement was studied in detail. Completely new measuring techniques are also being developed at JET to determine the distribution function of the energetic particles, using high Z impurities, and to detect the He ash. These new approaches require new research in atomic physics, to derive the necessary cross sections. Significant efforts are also devoted to the refinement of the fuel composition and tritium retention measurements. JET burning plasma diagnostics can therefore provide essential data for the study of reactor relevant issues and for the design of such measurements in ITER. They can also promote interesting new research in other related fields, from atomic physics to detector technology, and are likely to create interesting spin-off. (Author)

[en] Optimisation of auxiliary plasma heating by means of Ion Cyclotron Radio-Frequency (ICRF) and Neutral Beam Injection (NBI) as envisaged for the future fusion reactors is one of the main priorities in present research at today's tokamaks. Therefore, investigation of the production of fast ions during heating and of the subsequent fast ion behaviour in magnetically confined plasmas, together with an evaluation of the resulting bulk ion heating efficiency, are of essential importance for fusion reactor development. Gamma-ray diagnostics, based on the measurement of the gamma-ray emission from nuclear reactions between fast ions and the main plasma impurities, is a valuable technique for studying the fast particle energy distributions. Gamma-ray spectrometry provides information on the energy distribution, and the measurement of emission profiles supplies information on the spatial distribution of the reaction sites. Since 1987, the γ-ray emission from JET plasmas has been systematically monitored and used successfully in the analysis of heating effects during ICRF and NBI heating in the JET tokamak. The classical character of the fast ion slowing down behaviour has been demonstrated and estimates have been obtained of the fast particle confinement time. The study of sawtooth crashes has demonstrated dramatic spatial redistribution of fast particles and other effects. In recent JET experiments to study the ITER-relevant ICRH scenarios (³He)D and (³He)⁴He, γ-ray measurements provided information on the fast ion population, with the effective temperature of the energetic tail ions being deduced with the help of a γ-ray spectrum simulation code, GAMMOD. In this paper, the main y-ray results are presented and the capabilities of gamma diagnostics are discussed in the light of the ITER-project programme

No abstract available

No abstract available

[en] We present the results of analysis of triton burn-up process using the data from diamond detector. Neutron monitor based on CVD diamond was installed in JET torus hall close to the plasma center. We measure the part of 14 MeV neutrons in scenarios where plasma current varies in a range of 1-3 MA. In this experiment diamond neutron monitor was also able to detect strong gamma bursts produced by runaway electrons arising during the disruptions. We can conclude that CVD diamond detector will contribute to the study of fast particles confinement and help predict the disruption events in future tokamaks.

[en] In nuclear fusion reactors, DD and DT neutrons produced during operation induce the activation of the device components, thus the resulting radioactive nuclides induce high dose levels even when the machine is not operating. The problem of the activation is one of the key issues in Nuclear Fusion Technology for safe operation and maintenance and it is as more critical as the nuclear performances of the fusion machines increase. Hence in the past years many efforts have been made to develop reliable methods to predict the induced activation and the resulting shutdown dose rate. Suitable systems of codes, data and interfaces to predict the shutdown dose rate distributions in full 3D geometry have been recently developed mainly by FZK with the Rigorous 2-Step (R2S) scheme and by ENEA with the Direct 1-Step (D1S) method. These techniques are both based on the combined use of MCNP Monte Carlo code and FISPACT inventory code, but exploit different approaches. The R2S follows a classical approach with Monte Carlo transport calculations for neutrons and decay photons in two sequential steps, whereas in the D1S method neutrons and decay gammas are transported in a single run. Previous benchmarks performed at the Frascati Neutron Generator (FNG, Italy) and at the Fusion Neutron Source (FNS, Japan) facilities showed the effectiveness of both methods to predict dose rate, but the comparison with pre-existing, not-oriented for this purpose, dose rate measurements at JET resulted less satisfactory, therefore a dedicated benchmark experiment has been proposed. The experiment was conducted during the 2005-2007 campaign of JET in order to validate the computational methodologies in a reactor-like configuration. Dose rate levels calculated using D1S and R2S methods were compared with experimental data collected before, during off-operational periods and at the end of 2005-2007 JET campaign in two irradiation positions: close to the vessel with high sensitivity TLDs GR- 200A (natural LiF) detectors and one external position with an active detector of Geiger- Mueller type. In this work the results of the JET benchmark experiment are presented; the impact of the nuclear activation data, coming from different evaluations, the outcome of geometrical and/or materials uncertainties and the reliability of both methods in a real fusion reactors framework are discussed as well. (orig.)

[en] Neutrons produced by D-D and D-T plasmas induce the activation of tokamak materials and of components. The development of reliable methods to assess dose rates is a key issue for maintenance and operating nuclear machines, in normal and off-normal conditions. In the frame of the EFDA Fusion Technology work programme, a computational tool based upon MCNP Monte Carlo code has been developed to predict the dose rate after shutdown: it is called Direct One Step Method (D1S). The D1S is an innovative approach in which the decay gammas are coupled to the neutrons as in the prompt case and they are transported in one single step in the same run. Benchmarking of this new tool with experimental data taken in a complex geometry like that of a tokamak is a fundamental step to test the reliability of the D1S method. A dedicated benchmark experiment was proposed for the 2005-2006 experimental campaign of JET. Two irradiation positions have been selected for the benchmark: one inner position inside the vessel, not far from the plasma, called the 2 upper irradiation end (IE2), where neutron fluence is relatively high. The second position is just outside a vertical port in an external position (EX). Here the neutron flux is lower and the dose rate to be measured is not very far from the residual background. Passive detectors are used for in-vessel measurements: the high sensitivity Thermo Luminescent Dosimeters (TLDs) GR-200A (natural LiF), which ensure measurements down to environmental dose level. An active detector of Geiger-Muller (GM) type is used for out of vessel dose rate measurement. Before their use the detectors were calibrated in a secondary gamma-ray standard (Cs-137 and Co-60) facility in term of air-kerma. The background measurement was carried-out in the period July -September 2005 in the outside position EX using the GM tube and in September 2005 inside the vacuum vessel using TLD detectors located in the 2 Upper irradiation end IE2. In the present work, experimental background data and dose rate, the latter collected during some operation free days in the early phase of the 2006 JET campaign, are compared with the same quantities calculated using the D1S approach. The impact of key parameters (geometrical model, materials impurities, different sets of cross sections) to the calculated dose rates is discussed as well. (author)

[en] In 2011-2012, an experimental campaign with a significant yield of fusion neutrons was carried out on the JET tokamak. During this campaign the facility was equipped with two diamond detectors based on natural and artificial CVD diamond. These detectors were designed and manufactured in State Research Center of Russian Federation TRINITI. The detectors measure the flux of fast neutrons with energies above 0.2 MeV. They have been installed in the torus hall and the distance from the center of plasma was about 3 m. For some of the JET pulses in this experiment, the neutron flux density corresponded to the operational conditions in collimator channels of ITER Vertical Neutron Camera. The main objective of diamond monitors was the measurement of total fast neutron flux at the detector location and the estimation of the JET total neutron yield. The detectors operate as threshold counters. Additionally a spectrometric measurement channel has been configured that allowed us to distinguish various energy components of the neutron spectrum. In this paper we describe the neutron signal measuring and calibration procedure of the diamond detector. Fluxes of DD and DT neutrons at the detector location were measured. It is shown that the signals of total neutron yield measured by the diamond detector correlate with signals measured by the main JET neutron diagnostic based on fission chambers with high accuracy. This experiment can be considered as a successful test of diamond detectors in ITER-like conditions

[en] A detailed description of the 14 MeV neutron emission in plasmas heated by neutral beam injection has been carried out by coupling Monte Carlo calculations of the neutron emission spectrum with TRANSP modelling of the beam ion energy distributions. The model is used to study tritium beam injection experiments of the JET trace tritium campaign for internal transport barrier (ITB) and H-mode discharges. For ITB discharges, the measured neutron emission spectrum is well described by modelling using as input the beam ion distribution calculated with TRANSP. For H mode discharges the neutron spectrum can be reproduced only if high energy tritons are lost from the plasma, suggesting the possible role of low frequency tearing modes on the beam ions. The presented results are of relevance for tritium beam transport studies in trace tritium experiments and, more generally, for deuterium and tritium transport studies in high power experiments using neutron emission spectroscopy. (paper)