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Process of Air Ingress during a Depressurization Accident of GTHTR300 时间:2018-09-20 00:00 来源:未知作者:admin 点击: 次 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.
1. Introduction
One of the next-generation nuclear plants is a Very-High-Temperature Reactor (VHTR). The VHTR has strong interests of development worldwide. In addition to broad economical appeal resulting from unique high temperature capability, the reactor provides inherent and passive safety. The Japan Atomic Energy Agency (JAEA) has successfully built and operated the 30 MWt High Temperature engineering Test Reactor (HTTR) and is now pursuing design and commercial systems such as the 300 MWe Gas Turbine High Temperature Reactor 300 (GTHTR300). Also, in order to deploy a commercial Gas Turbine High Temperature Reactor 300 for Cogeneration (GTHTR300C) in around 2030, JAEA is now carrying out design study [1, 2].
When a double coaxial pipe connecting a reactor and a Gas Turbine Generator (GTG) module breaks, air is expected to enter the reactor core from the breach. The depressurization accident is one of the design-basis accidents of the GTHTR300. When the depressurization accident occurs, air is expected to enter the reactor core from breach and oxidize in-core graphite structures. Air ingress process in the VHTR is known to follow two sequential phases. Density of the gas mixture in the reactor gradually increases as air enters by the molecular diffusion and natural convection of the gas mixture in the first stage of the accident. Finally, the second stage of the accident starts after the natural circulation of air occurs suddenly throughout the entire reactor. The natural circulation of air suddenly occurs as once sufficient buoyancy is established [3]. On the other hand, under specific boundary condition, the onset time for producing natural circulation is short [4, 5].
A related study investigated the air ingress process and development of a passively safe technology to prevent air ingress. In order to clarify the safety characteristics of the GTHTR300C in a pipe rupture accident, a preliminary analysis of air ingress was performed [6]. The previous paper described the influence of local natural circulation in parallel channels on the air ingress process during a primary pipe rupture accident in the VHTR [7]. The duration of the first stage of the air ingress process was also discussed with analytical results of the reverse U-shaped passage with parallel channels.