Analysis of hydrogen deflagration/detonation effects in vacuum pressure suppression tank

The ICE (Ingress of Coolant Event) scenario is considered one of the most relevant safety issues related to the ITER Vacuum Vessel (VV). In case of the postulated accident, a mixture of steam and non-condensable gases will be discharged from the VV to the Vapour Suppression Tanks (VSTs) through the relief lines and the sparger systems. After the condensation of the steam in the VST water pool, a mixture of non-condensable gases could accumulate in the free volume of the VST. In case of combination of ICE with an ingress of air in the VV the non-condensable gases may contain a mixture of H2 and O2 with the potential of leading to a hydrogen deflagration or detonation. These events have a relatively low probability but could have heavy consequences. As a primary mitigation measure, “controlled” combustion of gas mixture is foreseen by means of ignitors located in several positions inside the VST to prevent the formation of dangerous concentrations of H2-O2. In this paper, the effects and the consequences on the walls and internals of a VVPSS tank caused by “controlled” deflagration as well as of the worst-case hydrogen detonation are investigated by means of CFD and FEM analyses. To perform the CFD analyses, a simplified three-dimensional VST model has been built to simulate different cases of hydrogen deflagration. These calculations allowed the evaluation of the main physical parameters, namely the pressure, the temperature time histories, and the Heat Transfer Coefficients as function of the temperature, which were then used for the thermo-mechanical analyses (by FEM code) of the VST sparger system. Results presented in terms of temperature distribution vs. time in the case of deflagration and in terms of pressure transients on the wall of the VST in the case of detonation are relevant input data for the structural verification of the VVPSS tank.