Numerical study of air ingress transition to natural circulation in a high temperature helium loop. (January 2018)

• Record Type: Journal Article
• Authors: Franken, Daniel; Gould, Daniel; Jain, Prashant K.; Bindra, Hitesh
• Is Part Of: Annals of nuclear energy. Volume 111(2018)
• Page Start: 371
• Page End: 378
• Publication Date: 2018-01
• Subjects: HTGRs -- Air ingress -- Diffusion -- Natural circulation; Nuclear energy -- Periodicals; Nuclear engineering -- Periodicals; 621.4805
• DOI: 10.1016/j.anucene.2017.09.007
• Languages: English
• ISSNs: 0306-4549

Abstract

Highlights: Computational fluid dynamics (CFD) model is developed to study transition from diffusion to natural circulation air ingress. CFD model represents the high temperature 'h' shaped experimental loop. Model predictions of transition times strongly agree with experimental results. Natural circulation velocities predicted from the model match closely with the experimental data. Abstract: The generation-IV high temperature gas cooled reactors (HTGRs) are designed with many passive safety features, one of which is the ability to passively remove heat under a loss of coolant accident (LOCA). However, several common reactor designs do not prevent against a large break in the coolant system and may therefore experience a depressurized LOCA. This would lead to air entering into the reactor system via several potential modes of ingress: diffusion, gravity currents, and natural circulation. At the onset of a LOCA, the initial rate of air ingress is expected to be very slow because it is governed by molecular diffusion. However, after several hours, natural circulation would commence, thus, bringing the air into the reactor system at a much higher rate. As a consequence, air ingress would cause the high temperature graphite matrix to oxidize, leading to its thermal degradation and decreased passive heat (decay) removal capability. Therefore, it is essential to understand the transition of air ingress from molecular diffusion to natural circulation in an HTGR system. This paper presents results from a computational fluid dynamics (CFD) model to study the air ingress transition behavior. These results are validated against an h-shaped high temperature helium loop experiment (McIlroy et al., 2010). Details are provided to quantitatively predict the transition time from molecular diffusion to natural circulation.