Inspired by JET: David Harber's sculpture at Cricket St Thomas. (Image: Julian Siret Source: EFDA)
Tritium, which is used in fusion reactors, is a radioactive isotope of hydrogen, and as such is shares the same chemical properties. Like hydrogen, it can easily permeate a large number of materials. In a colloquium delivered at JET (Joint European Torus) last week, Assistant Professor Takumi Chikada from the University of Tokyo outlined progress in research of ceramic coating that could contain tritium and protect tritium-carrying pipework.
“Without solving this problem it will be impossible to operate a fusion reactor,” he stated.
Because of its very small size, tritium tends to permeate through materials readily—an undesirable characteristic in a tritium processing plant, where tritium would be exposed to a large surface area as it passes through cooling, ducting and processing pipework.
Assistant Professor Chikada’s results showed that a layer of erbium oxide only tens of microns thick on a steel surface could reduce permeation of tritium by 100 000 times. Erbium oxide was originally chosen as an insulation coating because it has a high thermodynamic stability and is resistant to liquid lithium-lead—a proposed blanket material for fusion plants, which is corrosive to many materials.
Other materials are also being studied as tritium barriers, for example in the Karlsruhe Institute of Technology (KIT) which investigates aluminum oxide. It is unclear which coating will turn out to be the most suitable—erbium oxide and aluminum oxide each have advantages, but experiments to date have been with small samples, which can suffer from flaking if the wrong manufacturing process is used. Also irregularities in the crystal structure of the oxide can provide paths for tritium to permeate through.
“The development path is stony for barriers in the lab,” says Dr Wolfgang Krauss from KIT. “We must guarantee the quality (no defects, smooth surface etc) and qualify the barriers under irradiation. Also industrial relevance must always be considered.”
However as manufacturing techniques improve and explore multiple coatings of varied materials, it is likely that a good solution will be discovered, so we can be confident that the tritium will go where we want it to go—back into the tokamak!
Chikada, T., Suzuki, A., Kobayashi, T., Maier, H., Terai, T., & Muroga, T. (2011). Microstructure change and deuterium permeation behavior of erbium oxide coating Journal of Nuclear Materials, 417 (1-3), 1241-1244 DOI: 10.1016/j.jnucmat.2010.12.283
