Tritium/hydrogen barrier development

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.

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4.16 – Tritium Barriers and Tritium Diffusion in Fusion Reactors

Most of the hydrogen-induced crack propagation mechanisms occuring in metals and alloys involve various interactions of hydrogen with deformation processes in materials. Modeling the kinetics of hydrogen entry with respect to its local concentration to assess a crack propagation rate first requires a detailed characterization of these interactions. The purpose of this paper is to review some data illustrating the influence of deformation on the hydrogen behavior in iron and nickel base alloys. Experimental data on hydrogen, deuterium and tritium diffusion and trapping show: (a) a strain dependence of the hydrogen ingress in b.c.c. alloys through natural or passive films; (b) a strain induced hydrogen trapping; (c) an accelerated transport of hydrogen in pre-strained samples or during dynamic straining; (d) a hydrogen enhanced localized plasticity; and (e) a deformation enhanced embrittlement. These effects are shown to depend on various parameters including the hydrogen activity, the material chemistry and microstructure, the strain rate and the temperature.

Influence of deformation on the hydrogen behavior in iron and nickel base alloys: a review of experimental data

In power plants, several major components such as steam generator tubes, boilers, steam/water pipe lines, water box of condensers and the other auxiliary components like bolts, nuts, screws fasteners and supporting assemblies are commonly fabricated from plain carbon steels, as well as low and high alloy steels. These components often fail catastrophically due to hydrogen embrittlement. A brief overview of our current understanding of the phenomenon of such hydrogen damage in steels is presented in this paper. Case histories of failures of steel components due to hydrogen embrittlement, which are reported in literature, are briefly discussed. A phenomenological assessment of overall process of hydrogen embrittlement and classification of the various damage modes are summarized. Influence of several physical and metallurgical variables on the susceptibility of steels to hydrogen embrittlement, mechanisms of hydrogen embrittlement and current approaches to combat this problem are also presented.

Hydrogen embrittlement in power plant steels

Hydrogen permeation of X80 steel with superficial stress in the presence of sulfate-reducing bacteria

This paper presents the first results on the effect of nitrogen implantation on hydrogen permeation in steels. Nitrogen can modify superficially the steel's chemistry and/or microstructure depending on the fluence and thereby affect the processes of hydrogen diffusion and trapping. The implantations were performed on low carbon steel specimens with different nominal doses (1% to 10% and 33% nitrogen in a superficial layer of approximately 100 to 120 nm). The corresponding microstructures were characterized and permeation tests were conducted at room temperature in a double electrolytic cell. The nitrogen implanted layers on iron affects the electrochemical behaviour of the surface and the permeation in the material. This effect depends on the nitrogen concentration in the layer and on the corresponding microstructure. A continuous Fe2N layer acts as an efficient barrier to hydrogen entry and permeation when the layer is located on the entry face of the permeation membrane. This effect is stronger when the implanted layer is on the downstream face of the membrane. The low permeability values are mainly attributed to a lower hydrogen solubility in the implanted layer, whereas hydrogen trapping on defects and nitride precipitates delay hydrogen penetration.

J. C. Pivin

Papers

Influence of nitrogen ion implantation on hydrogen permeation in an extra mild steel