Table of contents

Confinement and the removal of tritium are key subjects for the safety of ITER. The ITER buildings are confinement barriers of tritium. In a hot cell, tritium is often released as vapour and is in contact with the inner walls. The inner walls of the ITER tritium plant building will also be exposed to tritium in an accident. The tritium released in the buildings is removed by the atmosphere detritiation systems (ADS), where the tritium is oxidized by catalysts and is removed as water. A special gas of SF₆ is used in ITER and is expected to be released in an accident such as a fire. Although the SF₆ gas has potential as a catalyst poison, the performance of ADS with the existence of SF₆ has not been confirmed as yet. Tritiated water is produced in the regeneration process of ADS and is subsequently processed by the ITER water detritiation system (WDS). One of the key components of the WDS is an electrolysis cell. To overcome the issues in a global tritium confinement, a series of experimental studies have been carried out as an ITER R&D task: (1) tritium behaviour in concrete; (2) the effect of SF₆ on the performance of ADS and (3) tritium durability of the electrolysis cell of the ITER-WDS. (1) The tritiated water vapour penetrated up to 50 mm into the concrete from the surface in six months' exposure. The penetration rate of tritium in the concrete was thus appreciably first, the isotope exchange capacity of the cement paste plays an important role in tritium trapping and penetration into concrete materials when concrete is exposed to tritiated water vapour. It is required to evaluate the effect of coating on the penetration rate quantitatively from the actual tritium tests. (2) SF₆ gas decreased the detritiation factor of ADS. Since the effect of SF₆ depends closely on its concentration, the amount of SF₆ released into the tritium handling area in an accident should be reduced by some ideas of arrangement of components in the buildings. (3) It was expected that the electrolysis cell of the ITER-WDS could endure 3 years' operation under the ITER design conditions. Measuring the concentration of the fluorine ions could be a promising technique for monitoring the damage to the electrolysis cell.

A new solution for high-gain direct-drive fusion with the indirect drive beam irradiation of the Laser Mégajoule is presented here. This solution uses 2 rings per hemisphere at polar angles of 49° and 59.5°. The 2D calculations with the hydrodynamics code FCI2 lead to a thermonuclear energy of 32 MJ with less than 1 MJ incident laser energy.

A multi-scale model for particle diffusion in porous structures is used to study the effect of the internal structure of graphite on atomic hydrogen transport and inventory in graphite. The diffusion of trace amounts of atomic hydrogen are modelled as a trapping–detrapping mechanism within the porous network typical in graphites. Activation energies for the different traps are taken from experiments and from molecular dynamics simulations. Different diffusion mechanisms dominate at different graphite temperatures. It is seen that the trace diffusion coefficient of hydrogen scales as the square of the jump lengths of the dominant mechanism. Depending on the detrapping mechanism and on the internal structure of graphite, the jump lengths after each detrapping event can vary over a few orders of magnitude. This gives rise to the possibility of anomalous diffusion in graphite. The effect of closing the pores of graphite on atomic hydrogen diffusion is also presented.

In recent campaigns, Tore Supra has focused its efforts on the physics optimisation and operation of stationary scenarios with high input power (up to 8 MW), duration of more than 60 s and vanishing loop voltage (up to 80% of non-inductive current). For physics integration, Tore Supra has been equipped with a large number of new real time sensors. The control of the lower hybrid (LH) wave deposition profile width measured by the hard x-ray camera has been achieved with the parallel refractive index n_|| varying from 1.7 to 2.3 and the LH-wave power using different types of control techniques. In addition, the control of the current profile with the parallel refractive index of the LH-launched spectrum has also been combined with loop voltage control using the central solenoid voltage as actuator. The effect of electron cyclotron heating as an actuator has been examined for control of temperature bifurcation phenomena such as internal transport barrier. On the basis of the experimental power flux analyses, the seven infrared cameras monitoring the five antennas and the toroidal pumped limiter have been used for implementing new power load limit avoidance control schemes which have been successfully combined with the LH-deposition profile and loop voltage control.

A novel design of the three-step Langmuir probe (TSLP) array has been developed to investigate the zonal flow (ZF) physics in the HL-2A tokamak. Three TSLP arrays are applied to measure the three-dimensional (3D) features of ZFs. They are separated by 65 mm in the poloidal and 800 mm in the toroidal directions, respectively. The 3D properties of the geodesic acoustic mode (GAM) ZFs are presented. The poloidal and toroidal modes of the radial electric fields of the GAM perturbations are simultaneously determined in the HL-2A tokamak for the first time. The modes have narrow radial wave numbers (k_rρ_i = 0.03–0.07) and short radial scale lengths (2.4–4.2 cm). High coherence of both the GAM and the ambient turbulence separated by toroidal 22.5° along a magnetic field line is observed, which contrasts with the high coherence of the GAM and the low coherence of the ambient turbulence apart from the field line. The nonlinear three wave coupling between the turbulent fluctuations and the ZFs is a plausible mechanism for flow generation. The skewness and kurtosis spectra of the probability distribution function of the potential perturbations are contrasted with the corresponding bicoherence for the first time, which support the three wave coupling mechanism.

The use of high heat flux plasma facing components (PFCs) in steady state fusion devices requires high reliability. These components have to withstand heat fluxes in the range 10–20 MW m⁻² involving a number of severe engineering constraints. Feedback from the experience of various industrial manufacturings showed that the bonding of the refractory armour material onto the metallic heat sink causes generic difficulties strongly depending on material qualities and specific design. As the heat exhaust capability and lifetime of PFCs during plasma operation are directly linked to the manufacturing quality, a set of qualification activities such as active infrared thermography, lock-in and acoustic measurements were performed during the component development phases following a qualification route. This paper describes the major improvements stemming from better measurement accuracy and refined data processing and analyses recent developments aimed at investigating the capability to qualify the component in situ during its lifetime.

Low activation ferritic/martensitic steels are good candidates for the future fusion reactors, for, relative to austenitic steels, their lower damage accumulation and moderate swelling under irradiation by the 14 MeV neutrons produced by the fusion reaction. Irradiation of these steels, e.g. EUROFER97, is known to produce hardening, loss of ductility, shift in ductile to brittle transition temperature and a reduction of fracture toughness and creep resistance starting at the lowest doses. Helium, produced by transmutation by the 14 MeV neutrons, is known to impact mechanical properties, but its effect at the microstructure level is still unclear. The mechanisms underlying the degradation of mechanical properties are not well understood, despite numerous studies on the evolution of the microstructure under irradiation. This impedes our ability to predict materials' behaviour at higher doses for use in the future fusion reactors. Simulations of these effects are now essential. An overview is presented on molecular dynamics simulations of the primary state of damage in iron and of the mobility of a dislocation, vector of plasticity, in the presence of a defect.

The influence of the magnetic ergodization on edge turbulence and turbulence-induced transport has been investigated by Langmuir probes in TEXTOR under three different static DED configurations. Common features are observed. With DED, the edge equilibrium profiles are altered and the resultant positive E_r is in agreement with modelling. In the ergodic zone, the potential fluctuations are strongly reduced and the local turbulent flux changes direction from radially outwards to inwards. In the same zone, the turbulence properties are profoundly modified by energy redistribution in frequency spectra, suppression of large-scale structures and reduction of the radial and poloidal correlation lengths for all frequencies. Meanwhile, the fluctuation poloidal phase velocity changes sign from the electron to ion diamagnetic drift, consistent with the change of the E_r × B flow, whereas the slight radially outward propagation of fluctuations is hindered by the DED. In the laminar region, the turbulence correlation is found to react to the observed reduced flow shear. Before the DED the Reynolds stress displays a radial gradient at the plasma edge while during DED the profile is suppressed, suggesting a rearrangement by the DED on the flow momentum profile.

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