Behavior of Tritium in the Vacuum Vessel of JT-60U

Abstract

Disassembly of the JT-60U torus was started in 2010 after 18 years D2 operations. In future the vacuum vessel will be treated as non-radioactive ones after the clearance procedure under the Japanese regulation depending on the tritium (T) contamination level. Note that the vessel was manufactured from Inconel 625 steel. Therefore, it was very important to study the hydrogen isotope behavior in Inconel 625 from viewpoint of the clearance procedure. Inconel 625 specimen was exposed to the D2 (92.8 %) – T2 (7.2 %) gas mixture at 573 K for 5 hours. The tritium release from the specimen at 298 K was controlled for about 1 year. After that a part of tritium remaining in the specimen was released by heating up to 1073 K. Other part of tritium trapped in the specimen was measured by chemical etching method. Most of the chemical form of the released tritium was HTO. The contaminated specimen by tritium was released continuously the diffusible tritium under the ambient condition. In the tritium release experiment, the amount of desorbed tritium was about 99% during 1 year. It was considered that the tritium in Inconel 625 was released easily. From these results, the behavior of tritium in the vacuum vessel of the JT-60U torus will be evaluated and discussed