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Experimental Study on Critical Heat Flux Behavior in Single Fuel Pin with and Without Wire Spacer

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Experimental Study on Critical Heat Flux Behavior in Single Fuel Pin with and Without Wire Spacer EXPERIMENTAL STUDY ON CRITICAL HEAT FLUX BEHAVIOR IN SINGLE FUEL PIN WITH AND WITHOUT WIRE SPACER Dan Tri LE*, Noriaki IN ABA** and Minoru TAKAHASHI** *Department of Nuclear Engineering, Tokyo Institute of Technology Nl-18, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8550, Japan [email protected]. ac .j p **Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology Nl-18, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8550, Japan inaba@2phase .nr.titech. ac.j p [email protected] Abstract: Light water reactor could have fast neutron spectrum with high conversion ratio nearly equal unity by using tight lattice fuel assembly with wire spacer. Besides, the critical heat flux (CHF) data base for grid spacer has a great quantity of studies but it is not in case of wire type. In order to investigate coolability in tight lattice core in boiling water reactor (BWR), an experiment of critical heat flux (CHF) was conducted using single pin flow channels with a heater pin with and without wire. We determined the critical heat flux for this system by varying the inlet temperature from 333 to 373 K and mass fluxes from 200 to 700 kg/m1 2s and the pressure was atmospheric pressure. The result show the CHF data in two-phase flow condition base on inlet and outlet condition in both case of heater pin with and without wire. The CHF values were higher with wire than without wire due to the effect of wire and spiral flow. Keywords: Critical Heat Flux, Tight Lattice, Two-phase Flow, Wire Spacer. 1. INTRODUCTION Nowadays, the fourth generation reactors are the focus of most study in the nuclear engineering. The fuel used for the fourth generation reactors are Plutonium and natural Uranium instead of the U. Most fourth generation reactors are liquid metal-cooled fast reactor. It is known that the liquid metal-cooled fast reactors (LMFR) that have the breeding ratio higher than unity meet the requirement of sustainability. However, the LMFR poses some unique design problems compared to the light water reactors which have been commercialized and operated for long years. On the other hand, tight lattice core could have a high conversion ratio of nearly equal to unity even in light water reactors (LWR), particularly in boiling water reactors (BWR). The use of wire spacers is more suitable for the tight lattice core than the use of grid spacer. However, the tight lattice core has smaller coolant volume compared with normal core. From thermal hydraulic point of view, the coolability or heat removability is one of key issues for the feasibility of the tight lattice core with wire spacers. The most crucible feature of coolability is the CHF of the fuel rod in the BWRs. There have been numerous studies on CHF so far. Nevertheless, the studies on CHF of tight lattice core with wire spacer are still few. In case of thermal-hydraulics for nuclear reactor, there have been a lot of studies on the CHF data base for grid spacer, but it is not in case of wire type. Thus, the critical heat flux or burnout phenomenon in tight lattice core is one of the most important studies for such kind of reactor. The CHF experiment was performed by Cheng & Muller [1] who reported that the CHF in hexagonal rod bundle with wire spacer was higher than that with grid spacer in low quality condition. The study was performed by using Freon-12 because of low latent heat and low critical pressure. The results were transferred from Freon-12 to water condition by using the fluid-to-fluid scaling law of Courtaud, et al [2], In 2009, the study of Diller, et al [3] showed that the advantages of fuel assembly with wire spacer type over grid spacers are their significant reduction in pressure drop and increase in CHF. This study was performed by simulating the hexagonal fuel assembly of pressurized water reactor (PWR) core. Their studies contributed to the evaluation of wire spacer compared with grid spacer and for the upgrade or developing a new type of light water reactor. Besides, the fuel bundle with wire spacer has advantages not only in thermal-hydraulic design of fuel but also in nuclear design of fuel. In 2009, Olander [4] performed the study about nuclear fuels, and showed that by using the hydride fuel with wire spacer, the fuel assembly could have good fuel performance and could reduce the core size. By reviewing of some studies for wire spacer, it can be seen that wire spacer has some interesting advantage, particularly in thermal-hydraulic design. Reviewing of some studies related to CHF for wire spacer, it can be seen that the studies on CHF for the fuel assembly with wire spacer are still not complete. Therefore, the main objectives of this study are to investigate the characteristics of CHF in single fuel pin, in particular the effect of wire spacer on CHF. 2. EXPERIMENTAL APPARATUS AND PROCEDURE 2.1. Experimental Apparatus The test loop used in this study is shown in Fig.l. The test loop consists of the storage tank, the water circulation pump, the orifice flow meter, the preheater, the test section and the condenser. The water flows from the water tank through the circulation pump and the pre-heater and enters the test section. Steam after going through the test section is condensed prior to flow back to the water tank. There is some consideration about the position of the orifice flow meter in the circulation loop after a preliminary test. The orifice flow meter was at first located in the downstream of the pre-heater. In the case of inlet temperature near the saturation temperature, a small amount of steam bubbles flowed out of the pre-heater into the flow meter. Therefore, the location of the orifice flow meter was moved to the upstream of the pre-heater so that steam bubbles did not affect the measurement of water flow rate. Fig.l.Experiment apparatus 2.2. Test section The test section with the oriented vertically is shown in Fig.2, and the cross section of the flow channel is shown in Figs.3a and 3b. The main part of the test section are the heater pin, the copper electrode, the glass tube, the thermocouple and the wire spacer in case of experiment for heater pin with wire spacer. The heater pin was made of a thin stainless steel tube with an outer diameter, D, of 8 mm and a length of 420 mm. It was connected to the copper electrode at both ends by silver soldering. The electrodes were inserted into the stainless steel tube by 10 mm for the soldering and connection. Therefore, the length of the heater pin, L, that was used for experiment and data analysis was 400 mm. On the other hand, the length to diameter ratio, of around 50 was large enough to minimize the effect of heated length on CHF [3]. The wire spacer was made of a Teflon tube in which a stainless steel wire was inserted so that the spacer was electrically insulated to the heater pin (Fig.3c). The axial pitch of wire, is the axial distance over which the wire completely wraps around the heated rod (Fig.4a).The axial pitch of the wire spacer, H, was set with two different values 100 and 200 mm. For this two different values of the axial pitch, the value of HID less than 50 was the upper limit of the wire correlation for both CHF and pressure drop [3]. Gap size, S, is the distance from the outer diameter of heater pin to the inner diameter of the glass tube as show in Fig.4 (b). For the rod diameter of 8 mm, three different values of gap size were chosen: 1.1, 1.5 and 2.0 mm. By changing the rod diameter, wire diameter and glass inner diameter, we could change the size of the gap. The heater pin was directly Joule-heated by a direct current electrically, which provided an uniform heat flux on the heater pin surface. The maximum power and current of the power in this experiment were 15kW and 500A, respectively. Cy electrode Outlet hermocouples Wire wrai T1 T2 Glass tub L=400 mm Cu electrode Fig.2. CHF test section. Thermocouples were setup to detect the CHF occurrence through the rapidly increase of surface temperature. The thermocouples were mounted of the heater pin surface at the axial locations of 10 mm and 20 mm upstream from the downstream end of the heated length, being marked as T1 and T2, respectively (Fig.5b). The reach to the CHF was detected by a rapid and sudden increase of the surface temperature which was measured by the thermocouples. Figure 5a shows the technique about the setup of the thermocouples. Figure 5a is detail each of T1 or T2 in Fig. 5b. Since the heater pin was the stainless steel tube with direct heating by the current, the three point junction technique (Fig.5a) for compensation of voltage induced by the current between two points junctions were used to setup the thermocouples. C and A mean the wires of Type K thermocouples: Chromel and Alumel, respectively. The voltage induced by the current between C and A was eliminated by adjusting the resistance of the variable resistor under low power input condition where temperature did not increase appreciably. The glass tube used in this study is Pyrex glass and crystal glass. On the outside surface of the glass tube, we made the position indications to recognize the location of CHF.
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