Safety Aspects of Tritium Storage in Metal Hydride Form

Air or nitrogen ingress accident scenarios into JET tritium storage containers, filled with uranium or intermetallic compound (IMC) hydrides, are discussed based on the experimentally determined kinetics of the reaction of these hydrides with air, O₂ and N₂. Reaction of uranium with air can occur at room temperature. For the initiation of the reactions of uranium with N₂ or of some intermetallic compounds with air, elevated temperatures are required. Temperature rises of the metal hydrides due to air ingress are estimated for various cases. Modern tritium storage containers are protected against air ingress by intermediate and secondary containments which can be either evacuated or filled with inert gas. Therefore, air ingress can only occur due to double failure: failure of secondary containment and process containment at the same time. At JET, the secondary containments are filled with N₂. However, even for N₂, temperature increases are expected during the ingress into uranium beds (U-beds) for particular scenarios. It is shown that the JET design would not fail in this event. The calculation also shows that the smallest temperature rises during air, O₂ or N₂ ingress are expected for a getter bed design with free space above the metal getter layer for the gas to flow from inlet to outlet tube, because the reaction with the metal powder is limited by the diffusion rate of the gas through the powder. Estimates with ZrCo as getter are also made.