A three-dimensional multi-volume analysis of combustible mixture generation due to in-vessel LOCA of ITER using the GASFLOW code

Three-dimensional distribution calculations are performed for the ITER-FEAT vacuum vessel (VV), the connected pressure suppression pool (SP) and drain tank (DT). A first-wall coolant leak without plasma shutdown is simulated. The steam, hydrogen, and air sources for this sequence are taken from best-estimate MELCOR calculations. A new extended version of GASFLOW is used to model the ITER-FEAT specific phenomena in adequate detail. During the accident sequence, hydrogen initially appears only in the VV due to the steam/beryllium reaction. After opening of the valves, steam and hydrogen flow from the VV through the connecting lines to the SP and the DT. Because of the ongoing steam condensation occurring in the SP, the pressure there remains permanently at a lower level compared to the other components, resulting in a continuous flow of steam and noncondensable gases into this volume. After 10500 s of steam flow, also air starts entering the VV, and an accumulation of N2 and O2 takes place in the SP cover gas. Combustible and explosive H2-O2-N2 mixtures exist after 13600 s, and at 21000 s a stoichiometric H2/O2 ratio has formed, involving 13 kg of hydrogen