Abbreviations

bcc Body-centered cubic CLAM China low activation martensitic steel CVD Chemical vapor deposition fcc Face-centered cubic HFR High flux reactor HIP Hot isostatically pressed ITER International Thermonuclear Experimental Reactor

PCA Prime candidate alloy

PRF Permeation reduction factor

RAFM Reduced activation ferritic/martensitic steel

4.16.1 Introduction

As fusion energy research progresses over the next several decades, and ignition and energy production

are attempted, the fuel for fusion reactors will be a combination of deuterium and tritium. From a safety point of view, these are not the ideal materials. The reaction of deuterium with tritium produces a-particles and 14.1 MeV neutrons. These neutrons are used not only to breed the tritium fuel, but also interact with other materials, making some of them radioactive. Although the decay of tritium produces only a low — energy p-radiation, it is difficult to contain tritium. Additionally, being an isotope of hydrogen, tritium can become part of the hydrocarbons that compose our bodies.

From the tritium point of view, the fusion facility can be divided into three components: the inner vessel area where the plasma is formed, the blanket where tritium production occurs, and the tritium exhaust and reprocessing system. There is the potential for tritium release in all the three sections of the facility. The tritium cycle for a fusion reactor begins in the blanket region. It is here that the tritium is produced by the interaction of neutrons with lithium. Specifically, the reaction is given symbolically as 6Li(n, a)3H. A neutron that has been thermalized, or lowered in energy by interaction with surrounding materials, is absorbed by 6Li to produce both an a-particle (helium nucleus) and a triton. Elemental lithium contains ^7.5% 6Li. As a breeder material in a fusion plant, lithium is enriched in the 6Li isotope to various degrees, depend­ing on the particular blanket design. The 7Li isotope also produces a small amount of tritium via the 7Li (n, a)3H + n reaction. The cross-section for this endo­thermic reaction is much smaller than that for the 6Li reaction. Upon release from the lithium breeder, the tritium is separated from other elements and other hydrogen isotopes. It is then injected as a gas or frozen pellet into the torus, where it becomes part of the plasma. A fraction of the tritium fuses with deuterium as part of the fusion process, or it is swept out of the chamber by the pumping system. If tritium is removed from the torus by the pumping system and sent to the reprocessing system, it is again filtered to separate other elements and other isotopes of hydrogen. All through the different steps, there is the potential for permeation of the tritium through the materials containing it and for its release to the environment. The probability of this occurring depends on the location in the tritium cycle. This chapter describes hydrogen permeability through two categories of materials that will be used in fusion reactors: candidate plasma-facing and structural materials.

The plasma-facing materials in future fusion devices will be heated by high-energy neutrons, by direct interaction of the plasma particles, and by electromagnetic energy released from the plasma. These plasma-facing materials must be cooled. It is primarily through the cooling tubes passing through the plasma-facing materials that tritium losses can occur in the primary vacuum vessel. The three mate­rials typically used for plasma-facing applications are carbon, tungsten, and beryllium. In this report, we describe the behavior of these materials as plasma­facing materials and how tritium can be lost to the cooling system.

The term ‘structural material’ is used here to describe materials that serve as the vacuum boundary in the main chamber, as the containment boundary for the blanket region, and as the piping for cooling and vacuum lines. These materials can be ferritic and austenitic steels, vanadium alloys, and zirconium alloys, as well as aluminum alloys in some locations, or potentially ceramics. We give a complete list of the different types of structural materials and review their tritium permeation characteristics.

Materials with a low permeability for tritium are being considered as barriers to prevent the loss of tritium from fusion plants. There are a few metals with relatively small values of permeability, but as a whole, metals themselves are not good barriers to the transport of tritium. Ceramics, on the other hand, are typically very good barriers if they are not porous. In most cases, the low permeation is due to extremely low solubility of hydrogen isotopes in ceramic mate­rials. Bulk ceramics, such as silicon carbide, may one day be used as tritium permeation barriers, but most of the current barrier development is for coatings of oxides, nitrides, or carbides of the metals themselves. We show in this review that many such oxides and nitrides may exhibit extremely good permeation behavior in the laboratory, but their performance as a barrier is significantly compromised when used in a radiation environment. We review the permeation parameters of materials being considered for barriers.

This report begins with a review of the processes that control the uptake and transport of hydrogen isotopes through materials. The parameters used to define these processes include diffusivity, solu­bility, permeability, trapping characteristics, and recombination-rate coefficients. We examine the transport of hydrogen isotopes in plasma-facing mate­rials, discuss the conditions that exist in the main torus, and look at the ways in which tritium can be lost there. Next, we consider the tritium transport properties of structural materials, followed by the transport properties in barrier materials, including oxides, nitrides, and carbides of structural metals, as well as low-permeation metals. The application of tritium barriers is discussed in some detail: both the theoretical performance of barriers and their observed performance in radiation environments, as well as an example of tritium permeation in the blanket of a fusion reactor. We conclude by summar­izing the tritium permeation properties of all the materials, providing the necessary parameters to help designers of fusion reactors to predict tritium losses during operation.