Wall Forum 2021

The ITER Test Blanket Module (TBM) is tasked with demonstrating efficient breeding and extraction of tritium inside the environment of a nuclear fusion reactor. In order to meet the strict requirements for tritium self-sufficiency necessary for future fusion power plants, and to address safety concerns and minimize radioactive waste, retained T inventory in and T losses out of the TBM must be accurately assessed. In this work, at the request of F4E, T losses due to retention and permeation in the ITER Water-Cooled Lithium Lead (WCLL) TMB are studied with the aid of the TESSIM-X diffusion-trapping code. Due to the presence of neutron-induced traps in the structural materials, T losses in the TBM are now expected to be a factor of 5 - 10 times larger than what was previously thought. [mehr]

The Creation-Relaxation Algorithm (CRA) [1] has attracted a lot of interest recently as it offers a parameter-free method for generating high-dose (>1 dpa) microstructures using atomistic lattice statics with a clear interpretation of the damage dose. We have shown that this method provides quantitative estimations of experimentally measurable properties [2], and having atomistic detail allows us to watch key processes like loop habit plane rotation and coalescence occur. But the method operates in the zero temperature limit, and the lack of true dynamics means the defect microstructures produced are typically too dense. By contrast, massively overlapping Molecular Dynamics (MD) cascades produce closer estimates to experimental quantities [3], but are so much more expensive that they have been limited to the low dose (<0.1 dpa) regime. Here we show how to link the two methods, using MD cascades to relax CRA simulations, and so achieve the high doses needed. We show how to find the void content of an atomistic simulation, and from this demonstrate we can accurately model deuterium retention measured in nuclear reaction analysis experiments [4]. [1] Phys. Rev. Mater. 4:023605 (2020) [2] Phys. Rev. Lett. 125:225503 (2020) [3] J. Nucl. Mater. 528:151843 (2020) [4] arXiv:2106.12938 (accepted Phys. Rev. Mater 2021) [mehr]

Ions are used to simulate, accelerated-irradiation damage in materials. Most studies focus on displacement damage using self-ions or low-energy protons. Energetic protons between 16 – 30 MeV have the ability to induce combined-displacement and transmutation damage, over macroscopic ranges of 300 – 500 µm in tungsten. However the samples are radioactive post irradiation and radiation protection measures must be followed.Additionally, electronic losses from ions are converted into heat, which form steady state heat loads on the sample. For 16 MeV protons with 10 µA current on a 10 mm diameter sample, the heat loads amounts to ~2 MW/m2. This is simultaneously inflicted on the sample alongside irradiation damage.Pilot irradiations on tungsten have been performed to doses of 0.006 dpa and further analysed with scanning electron microscopy and instrumented indentation. The hardness results are compared against self-ion irradiation, 3 MeV proton irradiation and show similar radiation hardening increase. Transmutation estimates using FISPACT-II were compared against gamma spectroscopy results.The irradiation ideology, methodology and post irradiation results will be detailed and explained in the presentation. [mehr]

In fusion reactors tungsten is exposed to high doses of neutron radiation, which causes an embrittlement of the material. The irradiation leads to microstructural damage and causes transmutation. The microstructural damage can be simulated by heavy ion irradiation. In this work, tungsten ion-radiation was used as an easy to control damage source. Fine grained drawn tungsten fibers with a diameter of 16 µm were used as sample material. Due to limited penetration depth of the ions, the fibers needed to be thinned to a diameter of 5 µm. Oriented towards previous investigations, irradiation experiments with several radiation doses were conducted on the thinned fibers. GIRAFFE, a newly developed device for tensile tests was qualified and used for the evaluation of the irradiated samples. After the tensile tests the diameter of the broken fibers were measured as an indication for their ductility. It was found, that radiation doses up to 10 dpa does not show a measurable influence on the ductility of tungsten fibers. [mehr]

Fuel retention in tungsten (W) as a plasma-facing material, especially of the radioactive hydrogen isotope (HI) tritium, presents severe concerns for operation cost and safety of future fusion devices. In tungsten with very low intrinsic H solubility, HI retention is dominated by trapping at irradiation-induced defects. In our previous work, strong lattice distortion was observed in W surfaces after deuterium (D) plasma exposure with kinetic ion energies significantly below the thresholds for production of stable Frenkel pairs, which caused formation of a D-supersaturated surface layer(D-SSL) containing ~10 at.% of retained D. We recently proposed and experimentally verified a physical model for the SSL production by HI plasmas at sub-threshold ion energy based on hydrogen atom-ion synergy effects. However, the connection between the observed defect microstructures and the unexpectedly high concentrations of retained HIs in the SSL has remained unestablished. In the present work, we exposed W samples to HI plasma and characterized them with transmission electron microscopy (TEM) to determine the defect microstructures in the HI-SSL. High quality TEM thin foil specimens were preparedby adopting a back-thinning electropolishing approach. In planar view, contrast images in kinematical two-beam bright-field conditions confirmed the formation of “black-spot” clusters and their raft structures after a 1×1024 m-2 exposure to D ions of215 eV at 300 K. The average defect size and number density measured 4-5 nm and 1022 m-3, respectively. Since the same SSL defect microstructure forms under H plasma with doubled ion energy as for D plasma exposure, both bulk and foil Wsamples were simultaneously exposed to a series of H ion fluences to track the evolution of the defect microstructure in the HSSL. In order to clarify the correlation between the microstructure and the concentration of retained H, hydrogen depthprofiles were acquired with 1H-15N nuclear reaction analysis on the bulk W samples. The analysis of the defect nature and cluster geometry in the HSSL under different H ion fluences is ongoing. The present work is expected to provide an indepth understanding of HI retention in W materials upon injection of energetic projectiles (ions, charge-exchange neutrals, neutrons) in future fusion devices. [mehr]