Experimental Evaluation of the Bypass Flow in the VHTR Core

In the core of Very High Temperature Gas-Cooled Reactor for the hydrogen production, there is the bypass flow that does not pass through coolant holes within the graphite blocks. The main bypass flow path is the gap channel between the core blocks. The amount of effective coolant and the temperature of the outlet coolant decrease as the bypass flow rate increases. Since the bypass flow affects the success of hydrogen production, it is important to evaluate the amount and the distribution of the bypass flow. In this study, the experimental facility to measure the amount of the bypass flow was designed and set up. The measuring experiments and CFD analysis to evaluate the bypass were carried out with several gap sizes and the combinations of the blocks. As the results, the bypass flow ratio increases as gap size, it is independent of the inlet mass flow rate but decreases as the number of fuel block in the unit cell increase. INTRODUCTION In past several years, the environmental and economic problems of fossil fuel energy have been on the rise. In order to resolve the problems of fossil fuel energy, the hydrogen energy has been promoted in many countries. Massive production of hydrogen using a nuclear energy is a practical way to feed fuel required for the hydrogen economy and the extensive studies have been carried out on the technological investigation for hydrogen production. By the way, the coolant temperature of Pressurized Water Reactor (PWR) is not high enough to produce the hydrogen. Therefore, Very High Temperature Reactor (VHTR) is being developed for the hydrogen production. VHTR is a gas cooled reactor which uses helium gas as the coolant. The heat from the high temperature helium coolant makes it possible to produce the hydrogen effectively. The prismatic modular reactor (PMR) is a type of VHTR. The reactor core of PMR mainly consists of an assembly of hexagonal graphite elements. Most of the coolant passes through the coolant channels within the fuel blocks of the core, but some fraction of the flow bypasses coolant channels. The core flow that does not pass through coolant channels within the fuel element is called the bypass flow [1]. The bypass flow increases with the bypass flow area due to the fast neutron-induced graphite element shrinkage and core barrel swell during the power operation. The bypass flow affects the temperature distribution, average temperature of the outlet coolant and properties of graphite element. The amount of effective core coolant flow and the temperature of the outlet coolant flow decrease as the bypass flow increases. Therefore, the amount of the bypass flow should be minimized to obtain high temperature of coolant flow enough to produce the hydrogen. In order to minimize the bypass flow, it is needed to evaluate the amount and distribution of the bypass flow exactly. In addition, although the bypass flow is expected to vary from 10 % to 25 % or more of the total core coolant flow [2] the experimental data supporting this expectation have not been reported to our knowledge. Thus, the object of this study is to evaluate the bypass flow by experimental measurement and CFD analysis. EXPERIMENTS In order to measure bypass flow rate, the unit-cell concept is introduced. The unit-cell is the equilateral triangular section that is formed by connecting the center of three hexagonal blocks in the core as shown in Fig. 1. Experimental facility was designed as shown in Fig. 2. The experimental facility consists of the wind tunnel, test section and blower. The wind tunnel and the blower are installed to make a uniformly distributed flow at the inlet of the test section. The total length and the width of the experimental facility are 7.75 m and 1.4 m, respectively. The length of the test section is 793 mm and the cross-section of the test section is an equilateral triangle of 426.7 mm on a side. Three blocks are installed into the test section. The types of the block are standard fuel block and reflector block. In this study, control rod block and reserve shutdown channel block are not considered. The blocks used in the experiment are one sixth part of the actual core block. Hence cross-sectional area of the unit-cell is one half of single core block. At the inlet of the experimental facility, flow measuring channels are installed to measure the inlet flow rate. The additional channels to measure flow rate are installed at the end of each block to gather the flow passing through the coolant channels within the block. The inner diameter and length of the additional channel are 0.12 m and 1.8 m, respectively. Since the gravitational effect is small and the properties of the helium and air are similar [3] the working fluid is the atmospheric air instead of the helium gas and horizontal direction of the flow is selected. The Bi-Directional Flow Tube (BDFT) (Ref. 4) is used to measure the flow rate as shown in Fig. 3. This mass flow-meter measures the pressure differences between the front and back side of the probe, and converts it to mean velocity of the channel. 1 SMiRT 19, Toronto, August 2007 Transactions, Paper # S03/5