The air ingress accident is one of the postulated design basis accidents, a guillotine-type break of the main pipes connecting to the reactor vessel, which is still unclear if the present HTGRs can maintain a passive safe function for this type of break. In order to analyze the air ingress accident, we developed a multi-dimensional multi-component mixture analysis code (GAMMA) and investigated chemical reaction and thermo-fluid behaviors related to the accident. GAMMA includes the models to address the important physical phenomena: multi-component molecular diffusion, bulk and surface chemical reactions, and heat transport by conduction, convection and thermal radiation. Since the period of the transient is very long, about one month, the efficient analysis tool is necessary in order to get a solution numerically stable and computationally fast. Therefore, for fast code run, we adopt the Implicit Continuous Eulerian (ICE) technique which reduces a 10N ×10N matrix to an N×N pressure matrix. A concerned complex system can be configured by the linkage of a 1-D calculation module and a 2/3-D calculation module.
In order to verify and validate the GAMMA code, we assessed various experiments and benchmark problems on the chemical reaction and heat removal behaviors: molecular diffusion tests, graphite oxidation tests, air-ingress tests, pebble-bed heat removal tests, and reactor cavity cooling system performance tests. The calculation results of the GAMMA code are in a high level of agreement with the experimental data as well as those of the other analysis codes used for the conventional HTGRs. From the air ingress analyses for the reference gas cooled reactors, PBMR and GT-MHR, significant rise in fuel temperature is observed for an assumed large air volume but the peak fuel temperature is predicted below the fuel failure criterion (1600℃). Sensitivity analysis on the air volumes in a vault and the onset timings of natural convection shows that the major parameters affecting on the severity of air ingress are the air concentration in a vault, the natural convection flow rate, and the bottom reflector temperature. In particular, if the air supply into the core is limited by isolating the reactor cavity immediately following the break, the consequence of air ingress would be mitigated enough to maintain the fuel and internal structure integrity during the accident.
