Flow visualization of liquid metals by neutron radiography

Fusion Engineering and Design 27 (1995) 607-613

ELSEVIER

Fusion Engineering and Design

Flow visualization of liquid metals by neutron radiography N. Takenaka a, T. Fujii a, A. Ono b, y . Motomura °, A. Turuno d a Department of Mechanical Engineering, Kobe University, I b Faculty o f Cross-Cultural Studies, Kobe University, I-1

1 Rokkodai, Nada, Kobe 657, Japan Rokkodai, Nada, Kobe 657, Japan ° Graduate School of Science and Technology, Kobe University, 1-1 Rokkodai, Nada, Kobe 657, Japan d Department of Reactor Technology, Japan Atomic Energy Research Institute, Tokai, Naka, Ibaraki 319-11, Japan

Abstract

Experimental techniques were developed to visualize liquid metal flows by neutron radiography. Lead-bismuth eutectic was used as a working fluid. Tracer and dye injection methods were employed for the visualization. A tracer made of gold-cadmium intermetallic compound and a dye made of l e a d - b i s m u t h - c a d m i u m alloy were developed. It was shown that both methods were suitable for flow visualization in a liquid metal. The use of this visualization technique to study thermal hydraulics in a liquid metal-cooled blanket for a fusion reactor is discussed.

1. Introduction

Thermal hydraulics of liquid metals is important in designing blankets for a magnetic confined fusion reactor. Since a liquid metal has high thermal and electrical conductivity, the flow characteristics are often different from those of an ordinary liquid such as water, especially in thermal convection and under a magnetic field. In general, the heat transfer of a liquid metal is suppressed by an applied magnetic field. However, it has been reported that heat transfer of a liquid metal was enhanced in natural convection in liquid alkali metals [1-3] and forced convection in mercury with a grid [4]. The details of the enhancement phenomena have not been clarified. It is impossible to simulate such flows by water to understand the enhancement mechanism. Flow visualization is a popular method for studying thermal hydraulics. Tracer and dye injection methods are often used for the visualization of a transparent fluid flow. Small particles suspended in the fluid visual-

ize the flow in the tracer method. In the dye injection method, a dye-like ink injected into the fluid follows the flow. However, the visualization of a liquid metal flow is difficult since a liquid metal is opaque to visible rays. Therefore, it is necessary to develop a new visualization technique to study thermal hydraulics of a liquid metalcooled blanket for a fusion reactor. Since most metals have small attenuation coefficients to neutron rays, neutron radiography is applicable to visualize a flow in a metallic container and a liquid metal flow. Various applications of neutron radiography can be found in Refs. [5-9]. Real-time neutron radiography was applied to flow visualization in a metallic container by Cimbala et al. [10] and to visualization of particle motion in a spouted bed by Ogino and Kamata [11], an organic fluorinated hydrocarbon liquid being used as a working fluid in an aluminium container since it is visible by neutron rays. Takenaka et al. [12] showed that cyclotron-based neutron radiography was applicable to the visualization of liquid metal

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flows. The thermal neutron flux of the system was about 106n cm -2 s -~ [13], which was sufficient to visualize the liquid metal flow qualitatively but not sufficient to obtain quantitative data like a flow vector field. The purpose of this paper is to demonstrate the visualization and measurement techniques for a liquid metal flow by high-flux thermal neutron radiography and to discuss the application of these techniques to studies of the thermal hydraulics in a liquid metal-cooled blanket. The real-time thermal neutron radiography system of the JRR-3M nuclear reactor at the Japan Atomic Energy Research Institute (JAERI) [14] was used for the visualization test. The thermal neutron flux of the present reactor system is over l0 s n cm 2s-1, which is one of the highest flux systems to be reported.

2. Principle of neutron radiography on the present application Radiography is a technique for visualizing the structure of an object by measuring the difference in the attenuation rates of radio rays to the materials constituting the object. Fig. 1 shows the mass attenuation coefficients, gin, of X-rays (line) and neutron rays (symbols) to the elements [9]. The mass attenuation coefficients of X-rays increases with increasing the atomic number of the irradiated material. On the other hand, those of neutron rays depend on the particular elements. They are large for small atomic number elements such as hydrogen and some special elements such as cadmium and are small for aluminium and stainlesssteel wall materials, heavy metals such as lead and bismuth and the alkali metals potassium and sodium. Lithium has a high mass attenuation coefficient. Details of the attenuation coefficients of alkali metals will be discussed later. Since the mass attenuation coefficients of some liquid metals and the wall materials are small and those of some elements are very large, neutron radiography is suitable for visualizing the behaviour of multiphase flows in a metallic wall and for visualization of a liquid metal flow.

3. Experimental apparatus and procedure A schematic diagram of the thermal neutron radiography system at JAERI is shown in Fig. 2. Thermal neutron rays from the nuclear reactor were collimated by a duct made of polyethylene to obtain a parallel neutron beam, then the test object was irradiated with

the beam. The radiographic image of the object was converted into a visible image by a scintillation converter and was recorded by a high-sensitivity video camera. The speed of the video camera was 30 framess -1. The irradiation room is 900mm wide, 1400 mm long and 1850 mm high. Details of the present neutron radiographic system were reported by Matsubayashi and Turuno [14]. Lead-bismuth eutectic (45 wt.% Pb-55 wt.% Bi) was used as a liquid metal. The attenuation coefficients of both lead and bismuth are small, as shown in Fig. 1. The melting point of the eutectic is 125 °C and its specific gravity is about 10.6 near the melting point. It is easy to handle and was proposed for use as a liquid target of a high-energy accelerator by Takeda [15]. A tracer and a dye using cadmium metal, which has a high attenuation coefficient as shown in Fig. 1, were developed for the eutectic. The tracer must be well wetted by the fluid, small enough for the scale of the flow and larger than the spatial resolution of the imaging system. Its density should be close to that of the fluid. Since the specific gravity of cadmium metal is 8.65, which is smaller than that of the eutectic, an intermetallic compound of gold and cadmium, AuCd3, was employed to make the tracer. The specific gravity of the compound is about 10.7 and it is easily wetted by P b - B i eutectic using soldering paste. The dye must be soluble in the fluid and its density should be close to that of the fluid. Since the cadmium is soluble in P b - B i eutectic, a eutectic containing 10 wt.% cadmium was used as the black dye. Two methods for injection of the dye, injection of the liquid dye and of the solid dye, were tested. A braze container 180mm wide, 200mm high and 10 mm thick, for the forced convection experiment is shown in Fig. 3. It was designed as a two-dimensional model of a liquid target for a spallation neutron source [ 15]. The shape of the window of the target, the bottom of the container, was designed to avoid dead points of flow due to vortices at the corners. Nitrogen gas bubbles were injected through the needles between the spacers. Circulation was induced by means of the hydrostatic head. The tracer method was applied to the visualization of forced convection. Cubic AuCd3 particles of about 2 mm were tested as the tracer. The particles were coated with P b - B i using soldering paste and inserted into the eutectic in the test section before irradiation. Fig. 4 shows a test section for natural convection and solidification. The eutectic in a stainless-steel container 50 mm wide, 150 mm high and 19 mm thick was heated by electrical heaters on one side and was cooled by oil

N. Takenaka et al. / Fusion Engineering and Design 27 (1995) 607-613

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N. Takenaka et al. / Fusion Engineering and Design 27 (1995) 607-613

4. Visualized and image-processed results

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(~) Flow Meter Fig. 3. Test section for forced circulation flow.

Fig. 5 ( a ) - ( d ) show original and processed images to obtain a vector field of forced circulation flow by the tracer method. Fig. 5(a) shows one frame of the original image. Tracers suspended in the fluid can be seen as small dots which are moving in the fluid. Fig. 5(b) is an integrated image of 256 frames where some non-moving tracers can be seen. By subtracting the integrated image from the original image, the image of moving tracers can be obtained (Fig. 5(c)). The image in Fig. 5(c) shows noise due to fluctuations of neutron flux. Since the dots due to the noise were much smaller than the dots of the tracers,, spatial filters could be used to reduce the noise. Fig. 5(d) was obtained with a 3 × 3 image element spatial filter. A vector field can be obtained by pattern matching between two tracer images as shown in Fig. 5(d). Two consecutive frames 1/30 s apart were used and space correlation functions between the two frames were calculated at each point. The flow vector can be obtained at the peak of the correlation function. Fig. 6 shows an example of a vector field. It can be seen that no vortex is found near the bottom of the container but two vortices can be clearly seen between the spacers. An example of visualization of solidification is shown in Fig. 7. The molten eutectic was cooled at left-hand wall a n d the solid dye was injected into the liquid phase. The solid dye melted and diffused in the liquid phase, but it did not penetrate into the solid phase. The liquid phase was darkened by the dye and the solid phase was not. Therefore, the solid/liquid interface was clearly visualized.

5. Discussion of the application of neutron radiography to the thermal hydraulics in liquid metal-cooled blanket

C) Liquid Metal (Pb-Bi)

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on the other side. Natural convection was induced and solidification occurred when the temperature of the oil was low enough. The dye was inserted during the irradiation.

The physical properties of liquid metals at temperature T(K) are given in Table 1 where Zt is the total macroscopic cross-section (the attenuation coefficient), P the density, I ~ the mass attenuation coefficient, P r the Prandtl number and ~o the electrical conductivity. Nuclear properties were taken from Ref. [8], the other properties of the liquid alkali metals and lead-bismuth from Ref. [16] and those of lithium-lead from Refs. [17] and [18], collected by Moriyama et al. [19]. Lithium and lithium-lead eutectic are proposed for use as a working fluid in the liquid metal-cooled blanket. Natural lithium has a high attenuation coefficient for thermal neutrons and is difficult to use as the working

N. Takenaka et al. / Fusion Engineering and Design 27 (1995) 607 613

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fluid for t h e v i s u a l i z a t i o n b y n e u t r o n r a d i o g r a p h y . T h e t h e r m a l a n d electrical p r o p e r t i e s o f t h e o t h e r alkali metals, s o d i u m , p o t a s s i u m a n d t h e i r eutectic ( N a K ) , are n o t so m u c h different f r o m t h o s e o f lithium, as s h o w n in T a b l e 1. T h e m a c r o s c o p i c cross-sections o f

these alkali m e t a l s are smaller t h a n t h a t o f l e a d - b i s m u t h . T h e r e f o r e , the t h e r m a l h y d r a u l i c s o f l i t h i u m c a n be e s t i m a t e d f r o m the v i s u a l i z a t i o n results w i t h s o d i u m , p o t a s s i u m or N a K , w h i c h c a n be o b t a i n e d easily by neutron radiography.

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N. Takenaka et al. / Fusion Engineering and Design 27 (1995) 607-613

cient for visualization. F o r the visualization with enriched 7Li, liquids containing 6Li are ideal dyes.

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L i t h i u m - l e a d eutectic also has a rather high attenuation coefficient. The thermal and electrical properties are close to those of l e a d - b i s m u t h eutectic. Therefore, the thermal hydraulics of LiPb can be studied by using PbBi. The attenuation coefficient of 6Li for thermal neutrons is high but that of 7Li is very low. The visualization of lithium and l i t h i u m - l e a d is possible with enriched 7Li. Comparing the macroscopic cross-sections of enriched 7Li with that of l e a d - b i s m u t h , 99% 7Li is visible and 99.9% 7Li and 99% 7 L i - P b eutectic are suffi-

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Fig. 7. Solid-liquid interface visualized by the dye injection method.

Table 1 Physical properties of liquid metals Metal

Li

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N. Takenaka et al. / Fusion Engineering and Design 27 (1995) 607-613

6. Conclusion It has been shown that neutron radiography is applicable to flow visualization in a liquid metal, which is difficult using other methods. L e a d - b i s m u t h eutectic was used as a liquid metal. The flow vector field can be obtained by the tracer method. The dye injection method can be applied to the visualization of solid/liquid interfaces. The application of neutron radiography to study the thermal hydraulics in a liquid metal-cooled blanket has been discussed. Flows in potassium, sodium and lithium and lithium-lead eutectic with the use of enriched 7Li can be visualized by neutron radiography.

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[8] J.P. Barton (ed.), Neutron Radiography, Kluwer, Dordrecht, 4th edn., 1994. [9] J.C. Domanus (ed.), Practical Neutron Radiography, Kluwer, Dordrecht, 1992. [10] J. Cimbala, D. Sathianathan, S.A. Cosgrove and S.H. Levine, Neutron radiography as a flow visualization tool, in S. Fujine, K. Kanda, G. Matsumoto and J.P. Barton (eds.), Neutron Radiography, Kluwer, Dordrecht, 3rd edn, 1989, pp. 497-504. [11] F. Ogino and M. Kamata, Application of neutron radiography to the study of liquid-solid two-phase flow, in J.P. Barton (ed.), Neutron Radiography, Kluwer, Dordrecht, 4th edn., 1994, pp. 339-346. [12] N. Takenaka, T. Fujii, A. Ono, K. Sonoda, S. Tazawa and T. Nakanii, Visualization of streak lines in liquid metal by neutron radiography, in J.P. Barton (ed.), Neutron Radiography, Kluwer, Dordrecht, 4th edn., 1994, pp. 355-362. [13] S. Tazawa, M. Yano and T. Nakanii, Cyclotron-based Neutron Radiography in Neutron Radiography, Reidel, Dordrecht, 1986, pp. 231-238. [14] M. Matsubayashi and A. Turuno, JRR-3M neutron radiography facility, in J.P. Barton (ed.), Neutron Radiography, Kluwer, Dordrecht, 4th edn., 1994, pp. 415-422. [ 15] Y. Takeda, Thermofluid behaviour of the lead bismuth eutectic target for the spallation neutron source at SIN, Nucl. Instrum. Methods Phys. Res A237 (1985). [16] JSME Data Book: Heat Transfer, 3rd and 4th edns., JSME. [17] V. Coen and T. Sample, Pb-17Li: a full characterized liquid breeder, in Fusion Technology 1990, Proc. 19th Syrup. on Fusion Technology, London, UK, Elsevier, Amsterdam, 1990, pp. 248-252. [ 18] B. Schulz, Thermophysical properties in the system LiPb, KFK 4144, 1986. [191 H. Moriyama,S. Tanaka, D.K. Sze, J. Reimann and A. Terlain, Tritium recovery from liquid metals, in Proc. 3rd Int. Symp. on Fusion Nuclear Technology, 1994, G1-3.