Tritium is a radioactive form of hydrogen. Because it is radioactive, its release to the environment must be minimized. Most of the materials used in fusion reactors are metals that have a relatively high permeability for tritium. The fusion community has been working on barrier materials to minimize tritium release by permeation through structural materials. Unfortunately, most barrier materials work very well during laboratory experiments, but fail to meet requirements when placed in radiation environments.

This chapter presents tritium permeation characteristics of various materials used in fusion reactors, including plasma-facing, structural, and barrier materials. The parameters necessary for tritium release calculations for various regions of a fusion reactor are given.

/pdf/4-16-tritium-barriers-and-tritium-diffusion-in-fusion-3bie1pdem1.pdf

4.16 – Tritium Barriers and Tritium Diffusion in Fusion Reactors

Preparation technique and alloying effect of aluminide coatings as tritium permeation barriers: A review

An effective permeation reduction of gaseous hydrogen isotopes into a metal wall by introducing a barrier is essential in two main fields: prevention of hydrogen-embrittlement in steels and control of the tritium inventory in future nuclear fusion reactors. By far the most important advances and relevant studies originate from the Nuclear Fusion Community where tritium retention is an important issue, influencing safety issues. Stable permeation barriers are searched among materials with the lowest bulk hydrogen solubility and diffusivity. Besides a few specific pure metals, like beryllium and tungsten, dense oxides, nitrides and carbides have been mostly investigated. Coating techniques for preparation of well adhered and perfect barriers are evidently of the same importance as the material selection itself. Most attractive are the techniques where an ad-layer is formed simply by oxidation. Other methods require specific gas environments with strong electric and magnetic fields, which may represent a limit for the ad-layers uniform coverage over large and uneven areas. Evaluation of the achieved barrier performances is another challenging task. Several new methods, which can trace hydrogen isotopes in bulk at very low concentrations, often miss in the determination of their mobility. Also, they do not reveal the role of barrier defects. The classical gas permeation rate method through coated membranes is still the most reliable option to determine the actual Hydrogen Permeation Barrier (HPB) efficiency. At elevated temperature, the hydrogen permeation rate is recorded at the downstream side of a coated membrane exposed to a substantially higher hydrogen upstream pressure. By using modern vacuum instrumentation techniques, even the most effective barriers can be well characterised.

Hydrogen permeation barriers: Basic requirements, materials selection, deposition methods, and quality evaluation

The ternary compound TiAlN coating has been known to be superior to binary compound TiN in protecting tools, which may be damaged by high thermal load. In the present study, TiAlN coatings are deposited on stainless (AISI 316L) and carbon steel substrates by using DC sputtering technique. The structure of TiAlN coatings with different Al contents is studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results reveal that the structure evolves from the cubic B1 type of TiN to the hexagonal phase of AlN when the Al content increases from 0 to 70 at.%. The oxidation behavior of deposited TiAlN coatings with different Al concentrations are investigated at oxidation temperatures ranging from 300 to 800 °C using energy-dispersive X-ray (EDX) spectroscopy and X-ray diffraction (XRD) analysis. It is concluded that the oxidation resistance is enhanced at first and then decreases with increasing Al content, the addition of 40 at.% Al leading to the best anti-oxidation effect. Coatings of same composition exhibit the best barrier performance to hydrogen permeation.

Microstructure, oxidation and H2-permeation resistance of TiAlN films deposited by DC magnetron sputtering technique

Deuterium permeation of Al2O3/Cr2O3 composite film on 316L stainless steel

The efficiency of TiN-TiC bilayer coatings, deposited by ion-beam-assisted deposition on martensitic steel, as a hydrogen permeation barrier was investigated by a gas phase method; the hydrogen permeability in the TiN-TiC bilayers is very low, at least 104 times lower than in the steel substrate in the temperature interval 470–570 K. Possible physical mechanisms, responsible for the reduced permeability of the ceramic bilayers, are discussed. In particular, from our experimental results, it can be concluded that chemisorption and/or hydrogen jumping from surface sites to the first subsurface atomic layer represents the hydrogen permeation limiting process.

Analysis of the hydrogen permeation properties of TiN-TiC bilayers deposited on martensitic stainless steel

We have observed peculiar hydrogen transient fluxes through stainless steel membranes which cannot be explained in terms of the linear permeation laws. In particular, before attaining the steady-state regime, the hydrogen flux may show a maximum, depending on the membrane temperature. Moreover, by repeating the measurement without changing the relevant parameters, the steady-state permeation rate may strongly differ from the first measurement. To clarify whether the observed anomalous behaviour is due to surface or bulk effects we have modified the membrane surface layers by implantation of a reactive (N + ) or a non-reactive species (Ar + ). The subsequent H-permeation experiments show that the implanted surface, when facing the high-pressure side, does not permit the flux peaks (independently of the implanted species). Moreover, permeation processes are reproducible in the sense that there is no difference between the steady rates of the first and second measurements, except at high temperature.

Peculiar aspects related to the hydrogen permeation flux through a martensitic steel membrane before and after ion implantation

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Analysis of the hydrogen permeation properties of TiN-TiC bilayers deposited on martensitic stainless steel

Peculiar aspects related to the hydrogen permeation flux through a martensitic steel membrane before and after ion implantation