Process of Air Ingress during a Depressurization Accident of GTHTR300

Abstract

A depressurization accident is the design-basis accidents of a gas turbine high temperature reactor, GTHTR300, which is JAEA’s design and one of the Very-High-Temperature Reactors (VHTR). When a primary pipe rupture accident occurs, air is expected to enter the reactor core from the breach and oxidize in-core graphite structures. Therefore, it is important to know a mixing process of different kinds of gases in the stable and unstable density stratified fluid layer. In order to predict or analyze the air ingress phenomena during the depressurization accident, we have conducted an experiment to obtain the mixing process of two component gases and the characteristics of natural circulation. The experimental apparatus consists of a storage tank and a reverse U-shaped vertical rectangular passage. One side wall of the high temperature side vertical passage is heated and the other side wall is cooled. The other experimental apparatus consists of a cylindrical double coaxial vessel and a horizontal double coaxial pipe. The outside of the double coaxial vessel is cooled and the inside is heated. The results obtained in this study are as follows. When the primary pipe is connected at the bottom of the reactor pressure vessel, onset time of natural circulation of air is affected by not only molecular diffusion but also localized natural convection. When the wall temperature difference is large, onset time of natural circulation of air is strongly affected by natural convection rather than molecular diffusion. When the primary pipe is connected at the side of the reactor pressure vessel, air will enter the bottom space in the reactor pressure vessel by counter-current flow at the coaxial double pipe break part immediately. Afterward, air will enter the reactor core by localized natural convection and molecular diffusion.

Numerical Study of the Air Ingress Accident of the HTR-PM and Possible Mitigation Measures

“…A multidimensional code named GAMMA was developed by Korea Atom Energy Research Institute (KAERI) to predict the air ingress accident of hightemperature gas-cooled reactor [8] and the advanced air ingress-related models developed by INL was implemented into the GAMMA code. Shiga discussed the process of air ingress during a depressurization accident of GTHTR300 [9]. e results show that the onset time of natural circulation depends on the location of the horizontal coaxial gas duct and the competition between natural convection and molecular diffusion.…”