Scoping analysis of the influence of initial temperature on D/T-air ignition risk in a vacuum chamber

• Ignition/explosion of D/T-air mixture in Tokamaks when a LOVA (Loss Of Vacuum Accident) occurs is an event that must be assessed in fusion safety analysis. • 3D unsteady CFD simulations can give realistic flow patterns and predict the local and instantaneous reaction rates. • Detailed chemistry of hydrogen/oxygen is necessary to accurately predict whether the combustion starts or not. • A parametric study taking into consideration three initial temperatures of a D/T mixture in a spherical vessel subject to LOVA has been carried out. DEMO (Demonstration Fusion Power Reactor) is a new nuclear power plant aiming at demonstrating that electricity can be produced from fusion reactions. In such a facility, all potential safety issues must be assessed and addressed in order to minimize the risk of accidents. Among those, fires and explosions represent some of the main threats during plant operation. One of the reference accident events is the LOVA (Loss of Vacuum Accident) in the DEMO Tokamak due to the postulated breach of one of the lines Vacuum Vessel (VV) connected volumes. The accident transient sequence considers the air entering the VV, initially in vacuum condition, to mix with the deuterium-tritium therein contained and the possible flame ignition in case of heat source. When simulating such a complex phenomenology, several initial and boundary conditions, as well as input parameters representative for the considered safety case shall be defined. To this end, as a preliminary analysis, a LOVA in a simple spherical tank, containing a hot equimolar mixture of deuterium and tritium, was considered. The volume of this vessel was taken equal to 1 m 3 and an air ingress through a pipe whose axis is directed radially with respect to the tank was simulated numerically by means of open-source software OpenFOAM to simulate the air expansion in the vessel as well as the chemical reactions that may occur. Three initial temperatures of the D/T mixture were considered as conditions that may occur: 400 K, 900 K and 1300 K, based on the temperature range that is expected to exist on the plasma-facing components and in the vessel during operation and when decay heat is produced. On the basis of the observed fields of temperature, combustion intensity and water mass fraction, the risk of ignition was assessed for each considered case.