“…However, it was known that the hydrogen transfer through the free surface of the liquid breeders by purge gas was smaller than that through the interface made by gas bubbling. The previous study [4,6] indicated that the hydrogen transfer ratio by the sweep gas was 10-100 times lower than that by the gas bubbling. This was because the free surface was covered by some oxides and/or nitrides, which suppressed the hydrogen transport.…”