Analysis of Simplified Hydrogen and Dust Explosion in the Vacuum Vessel Accident using MELCOR Code

The failure of the confinement barriers inside a penetration line between the VV and a port cell initiates the accident. Air ingress in VV results in formation of hydrogen/air explosive mixture and further explosion. Hydrogen explosion is assumed to trigger a dust explosion which leads to a large pressure peak creating a connection between VV and NBI Cell. Any other components related to the accident other than VV, suppression tank (ST), port cell, NBI Cell and Gallery are ignored in the analysis. Additional Free volume is assumed connected to the VV to simulate hydrogen/dust explosions. Figure 1 presents schematics of modeling of thermal-hydraulic analysis used in the accident analysis. For ITER design, three volumes of accident analysis report (AAR) presents analysis of selected postulated events important in ITER safety studies including hypothetical events. MELCOR 1.8.2 code was chosen as one of the several codes to perform ITER safety analysis because it models wide range of physical phenomena such as thermal-hydraulics, heat transfer, and other phenomena including aerosol physics. MELCOR can also predict structural temperatures using energy produced by radioactive heat or chemical reactions. Analysis has shown that hydrogen/dust explosion damaged the VV confinement barriers transporting dust from VV to the port cell, NBI cell and other penetrating lines. Unlike the accident analysis performed in the accident analysis report (AAR), the radioactive material was released into the environment shortly after the event. Simplified accident analysis was performed in attempt to perform fast safety analysis, however, multiple components and initial conditions not under consideration caused significant difference from the AAR analysis results