• Energy Research

International audience

Plasma-facing materials for next generation fusion devices, like ITER and DEMO, will be submitted to intense fluxes of light elements, notably He and H isotopes (HI). Our study focuses on tritium (T) retention on a wide range of W samples: first, different types of W materials were investigated to distinguish the impact of the pristine original structure on the retention, from W-coated samples to ITER-grade pure W samples submitted to various annealing and manufacturing procedures, along with monocrystalline W for reference. Then, He and He-D irradiated W samples were studied to investigate the impact on He-damages such as nano-bubbles (exposures in LHD or PSI-2) on T retention.We exposed all the samples to tritium gas-loading using a gentle technique preventing any introduction of new damage in the material. Tritium desorption is measured by Liquid Scintillation counting (LSC) at ambient and high temperatures (800 °C). The remaining T inventory is then measured by sample full dissolution and LSC. Results on T inventory on He exposed samples highlighted that in all cases, tritium desorption as a gas (HT) increases significantly due to the formation of He damages. Up to 1.8 times more T can be trapped in the material through a competition of various mechanisms, but the major part of the inventory desorbs at room temperature, and so will most likely not take part to the long-term trapped inventory for safety and operational perspectives. Unfortunately, investigation of “as received” industrial W (used for the making of plasma-facing materials) highlighted a strong impact of the pre existing defects on T retention: up to 2.5 times more T is trapped in “as received W” compared to annealed and polish W, and desorbs only at 800 °C, meaning ideal W material studies may underestimate T inventory for tokamak relevant conditions. Keywords: Tungsten, Helium, Tritium inventory, Plasma-wall interactions

The control of tritium inventory and permeation is crucial for the safe operation of fusion reactors. To measure these phenomena, a novel tritium experiment is introduced and used to measure tritium permeation through Eurofer97 at room temperature, with several possibilities for the downstream conditions: air or water and overhead air. A pre-loading condition was shown to be required for this experiment. This direct comparison revealed that tritium is preferentially found in the water phase and secondarily in the air phase.

The paper presents complementary approaches based on experimental and numerical works to address the behavior of tokamak-relevant tungsten particles loaded with tritium. Sampling of particles inside the WEST tokamak have been realized thanks to an in situ particle collection system called Duster Box. This method allowed to identify various types of tungsten particles among them spherical shaped micro-particles between 5 µm and 30 µm in diameter. Based on these results a surrogate tungsten powder has been provided by means of spheroidization process and sieving method. Moreover, the powder tritium retention capacity was measured and specific activities of 90 MBq.g−1 and 280 MBq.g−1 were obtained for particles with 17 µm and 11.5 µm median diameters, respectively. Considering such tritium activities trapped in the particles, Monte-Carlo simulation were performed to estimate the electrostatic self-charging rates and the corresponding electrical charge carried by the radioactive tungsten dust. The results of these experiments provide robust data for the assessment of the dispersion of toxic/radioactive material in the environment that could follow a loss of containment.

Two populations of dust particles were found during the first phase of operation of WEST. The one that dominates by size and weight comes from the delamination of tungsten coatings covering graphite tiles and the emission of droplets of molten materials during off-normal events. Sizes vary from several microns to tens of microns. More generally, micron-sized dust particles due to the erosion of all materials present in the vacuum vessel were collected. In addition, nanocavities were found at the surface of tungsten dust sampled after He plasmas and were attributed to He trapping in the form of nanobubbles. Tungsten nanoparticles constitute the second unexpected dust population. They are dominant by their number and were essentially found at the surface of micron-sized particles. They may result either from the condensation of an oversaturated vapor above molten tungsten or come from ion-neutral clusters growing in plasma regions of low temperature until the appearance of solid particles.

Fusion energy has the potential to provide sustainable solutions to global energy needs for the next generations. However, despite decades of intense international efforts many scientific and technological breakthroughs need to be achieved before fusion become available and economically viable. In addition, and prior to industrial development of the fusion technology, it is worth addressing possible negative environmental and health impacts. For instance, the interactions between the plasma and refractory materials called plasma facing components (PFC) like tungsten, will generate tritiated dust. The aim of the study is to address the fate in water and biological media of W nanoparticles that might be released in case of Lost Of Vacuum Accident (LOVA). The dilution of particles in TRIS, LHC9 and pulmonary media did not strongly affect the average size of the particles while the dilution in Saline medium lead to substantial aggregation. The results proved that oxidative dissolution of W nanoparticles occurred in several aqueous/biological media (TRIS, LHC9 and Lung media) with increasing time. From the different dissolution rates as a function of the tested media, it seems that the oxidative dissolutions are rate limited by diffusion in the oxidized layer surrounding the metallic core of particles. The mechanisms of dissolution involved $\mathrm{W}^{4+}$ and $\mathrm{W}^{6+}$ corroded layers prior to $\mathrm{W}^{6+}$ dissolution. Knowledge provided by these dispersion–dissolution experiments helped to determine the environmental mobility and persistence as well as the bio-durability of these tungsten nanoparticles. As dissolution has potential to influence the toxicity of particles, it is a crucial parameter to consider in the risk assessment of particles.