Effects of helium implantation with heavy ion irradiation on deuterium permeation in yttrium oxide coating

Abstract

In a fusion reactor, strict control of tritium is required from the viewpoints of sufficient fuel balance and radiological hazard. Fabrication of tritium-impermeable ceramic coatings is a promising solution, and ceramic coatings have been investigated for several decades. In recent years, several irradiation tests have been conducted for the ceramic coatings to understand irradiation effects on their characteristics and functions. However, there are few experimental reports on multi-irradiation tests of neutrons and alpha particles produced by nuclear reactions and radioactive decay of tritium. In this study, yttrium oxide coatings were fabricated by reactive and non-reactive radio frequency magnetron sputtering, and their microstructures and deuterium permeation behaviors were investigated after heavy ion irradiation and helium implantation tests simulating irradiation damage by neutrons and generated alpha particles. All irradiated samples showed the formation of an amorphous layer and a nanocrystalline layer or region. In particular, an iron-helium-simultaneous-irradiated sample contained voids including nanometer-size spheres, indicating the formation of helium bubbles. The irradiated samples showed grain growth and/or crystallization at a higher temperature than the unirradiated samples during deuterium permeation tests. That suggests that the voids and the helium bubbles prevented structural change at low temperatures. Moreover, the helium bubbles tended to be more stable than the voids. On the other hand, nickel-single and nickel-helium-sequential-irradiated coatings deposited by non-reactive sputtering showed few voids and helium bubbles, resulting in a different behavior of deuterium permeation.