Hydrogen explosion risk for EU-DEMO reactor considering tungsten dust reaction with steam

Research activities in the Safety and Environment work package of EUROfusion, are carried out to evaluate structure system and components failure that could compromise the integrity of the confinement barrier and cause the release of contaminants for the European DEMOnstration (EU-DEMO) reactor. In this framework, deterministic safety analyses are performed to study the consequences of a postulated initiating event and determine whether such a failure event is bound within the safety objectives established for the plant. This paper studies the hydrogen explosion risk for the reference EU-DEMO reactor and evaluates the capability of new engineering safety system, such as passive auto-catalytic recombiner and exhaust detritiation system venting lines integrated with the vacuum vessel pressure suppression system, to minimize explosion risk by reducing hydrogen inventory in the atmosphere mixture. The present study was carried out using the MELCOR 1.8.6 system code modified for fusion applications. The reference postulated initiating event selected for this study combines a small loss of vacuum accident and a small break loss of coolant accident. This scenario is characterized by two concurrent events with possibly explosive oxygen–hydrogen mixture formation. The ingress of air from the loss of vacuum breach provides a significant amount of oxygen, while the release of steam, because of the loss of coolant accident, results in an increase of hydrogen inventory by means of tungsten oxidation reactions. Such tungsten-steam reactions may occur either on plasma-facing hot surfaces or with tungsten dust, which recent studies highlighted as possibly playing a predominant role in hydrogen production. An external script has been developed to evaluate tungsten dust oxidation since a detailed model of such phenomenon is not currently present in MELCOR 1.8.6. First results highlighted that oxygen–hydrogen mixture reaches explosive limits inside the tanks of the vacuum vessel pressure suppression system. For this reason a new model of the vacuum vessel pressure suppression system with passive autocatalytic recombiners is studied to understand if the hydrogen explosion risk is mitigated.