Release Behavior of Hydrogen Isotopes from JT-60U Graphite Tiles

Abstract

Release behavior of hydrogen isotopes from the graphite tiles used in JT-60U was observed using the thermal desorption method where temperature was stepwise elevated to 300, 600 and 1000 °C. When first wall tile was left under helium atmosphere at 600 °C for 8 hours, about 40 % of total amount of hydrogen and deuterium retained in the tile was released, although only a small amount of hydrogen isotopes was released at 300 °C, which is the base temperature of inner wall of JT-60U. This indicates that a higher temperature of inner wall causes hydrogen retention to reduce considerably. When the graphite tiles were exposed to hydrogen at 1000 °C, the release of deuterium and tritium was enhanced. It is considered that the deuterium and tritium left in the graphite tile was released by the isotope exchange reaction. In order to remove almost all deuterium or tritium from the graphite tile without combustion of graphite, isotope exchange method at high temperature is effective. It was found that the amount of hydrogen retained in the graphite tile was much larger than that of deuterium. This indicates that a large amount of deuterium trapped in the tiles during deuterium discharge experiments was replaced with hydrogen during hydrogen discharge experiments. Additionally, depth profiles of hydrogen isotope are discussed from the obtained release curves.