Effects of the onset time of natural circulation on safety in an air ingress accident involving a HTGR

Abstract

An air ingress accident is a major safety issue pertaining to high-temperature gas-cooled reactors (HTGRs). When a helium depressurization accident occurs, air enters the core through broken pipes. The natural circulation (NC) phase of this accident leads to massive air ingress, causing extensive oxidation of the core graphite structure. The oxidation results in three main safety issues: an increase in the core temperature, degradation of the core graphite structure, and explosive CO accumulation in the upper plenum. As a result, the onset time of the natural circulation is one of the key parameters related to reactor safety. Using GAMMA (GAs Multi-component Mixture transient Analysis) we performed a one-dimensional (D) analysis of GT-MHR in the same manner used by the conventional safety tools. A 2D analysis was also done to determine the 2D effects, including the degree of stratified-flow induced-air ingress in the horizontal ducts. The 1D and 2D simulations showed that NC occurs at around 360 h and over 500 h, respectively (fin et al., 2011). In an effort to determine the effect of the onset time of NC on safety, we performed a sensitivity study of the NC onset time when it is intentionally controlled. Due to the limitation of the 1D simulation to represent multi-dimensional stratified flows in hot and cold pipes, the 1D and 2D results are quite different. In the 1D calculation, the bottom reflector temperature is very sensitive to the onset time of NC, whereas it is insensitive in the 2D simulation. According to the 2D analysis, however, a strong-back flow from the air in the cavity is observed in the lower part of the reactor core and at the entrance of the broken pipes. As a result, the bottom reflector temperature gradually decreases in the 2D analysis irrespective of the onset time of NC. Based on these calculation results, the possibility of CO detonation and damage to the graphite structure are examined. Our conclusions are that there are large safety margins of the maximum fuel temperature, CO detonation, and damage to the graphite structure and even larger margins considering the more realistic 2D simulation. (C) 2012 Elsevier B.V. All rights reserved.