Journal of Fusion Energy, Voi. 3, Nos. 5/6, 1983

11.5. Heat Removal

Akira Inoue, 1 Haruki Madarame, 2 and Tatsuzo Tone 3

II.5.1. OUTLINE OF HEAT-REMOVAL STUDIES IN JAPAN

In Japan, studies on heat-removal technology in fusion reactors are being made at laboratories of universities and the Japan Atomic Energy Research Institute (JAERI). In the former, fundamental studies mainly on fluid flow and heat transfer in commercial power plants are performed, whereas technology for the JT-60 device and for the currently planned next- generation tokamak, the Fusion Experimental Reac- tor (FER), is developed in the latter.

Studies at universities were started in 1974, sup- ported by a Grant-in-Aid for Scientific Research, MBS (Special Research Project on Nuclear Fusion). A two-phase flow blanket cooling scheme was pro- posed and has been investigated systematically. Vari- ous types of two-phase flow including liquid-metal single flow have been studied, based upon the in- dividual interest of researchers. Later, studies on gas cooling and water cooling were begun. The present research activities at universities and the JAERI are explained in the following.

II.5.2. ACTIVITIES AT UNIVERSITIES

At laboratories of ~miversities, fundamental studies related to fluid flow and heat transfer in the MCF as well as in the ICF blanket have been con- ducted.

A. Tohoku University (Department of Nuclear Engineering)

Liquid-metal mist cooling for a high-tempera- ture surface at a high beat flux in a fusion reactor has been investigated. The major objectives of this study are to obtain experimental data on sodium mist cool- ing and to prove the superiority of this heat-transfer performance applied for cooling a high-temperature surface under fusion-reactor conditions. It also must be proved that the liquid-metal mist flow has no eddy-current path in itself in magnetic fields which suppresses MHD resistances in the flow.

A sodium-argon mist flow sprayed from a noz- zle is jetted vertically to the heat-transfer target, which is made of SUS-304 or tantalum, and heated up to the desired initial temperature by an electric furnace. The heat transfer is measured by a quench method. Sodium mist drops and their velocities are measured by both the LDV and the sampling-probe method.

Heat-transfer data have been obtained for the temperature range of the heat-transfer surface from 300 to 1000~ Measurements of mist drop sizes and their velocities have been tried by the above-men- tioned methods.

For the purpose of proving the performance in magnetic fields, a magnet will be installed at the test section, which can give a magnetic field of maximum 2 T. Several module tests are also planned which simulate cooling systems for the first wall or hot surface of the diverter.

1Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology.

2 Department of Nuclear Engineering, The University of Tokyo. ?Japan Atomic Energy Research Institute.

425

B. The University of Tsukuba (Institute of Engineering Mechanics)

Studies on the narrow-gap cooling of tandem- mirror fusion-reactor components have just started at

0164-0313/83/1200-0425503.00/0 (~u1983 Plenum Pubii~hirig Corporatior~

426 Inoue et aL

the Institute of Engineering Mechanics, The Univer- sity of Tsukuba.

One aim of these studies is to make clear the critical heat flux of these tubes cooled by subcooled water inside them. In future, the research results will be connected with real applications picked up at the conceptual design works.

C. The University of Tokyo (Faculty of Engineering)

Characteristics of liquid-metal flow and heat transfer under strong magnetic fields have been studied at the Heat Transfer Laboratory of the CTR Blanket Engineering Research Facility, The Univer- sity of Tokyo.

Na-Ar two-phase flow heat transfer was in- vestigated with a sodium loop - 5 liters/rain in flow rate and with a magnetic field of -1 .5 T.

The MHD pressure drop in the two-phase flow is considerably smaller than that in Na single-phase flow with the same Na mass flow rate, whereas the heat-transfer coefficient is slightly smaller. The heat removal in a CTR blanket with two-phase flow was shown to be attractive by this experiment.

In parallel, the effect of magnetic field on natu- ral convection was studied using NaK as the working fluid. The heat-transfer rate is raised in a low mag- netic field but is diminished in a stronger field.

Relating to the liquid wall in inertial confine- ment fusion reactors, some other experiments have been made. Characteristics of gas-cooled reactors have also been studied, especially the heat transfer from a suddenly heated wall.

D. Tokyo Institute of Technology

At the Tokyo Institute of Technology, two kinds of studies of both gas cooling and gas-liquid metal two-phase flow cooling have been investigated in two different laboratories.

(1) At the Department of Mechanical Engineer- ing for Production, gas-cooling heat transfer coupled with radiation has been studied in a duct with non- uniform heat flux.

The first wall and blanket in a magnetically confined fusion reactor are subject to a high heat flux which is apt to be spatially nonuniform. The objec-

tire of this research is to investigate theoretically and experimentally the effects of secondary flow and thermal radiation due to nonuniform heat flux on the heat transfer of a gas flowing duct.

One vertical wall of a horizontal rectangular duct is electrically heated in the experiment. The effects of thermal radiation and secondary flow due nonuniformity of the heat flux on the gas-cooling performance in the duct was examined. Theoretical analysis of heat transfer in a duct including the above-mentioned effects was performed to obtain a heat-transfer rate and wall temperature profile along the flow direction. The variation of cooling perfor- mance due to the aspect ratio of the rectangular cross section of duct was examined.

(2) At the Research Laboratory for Nuclear Re- actors, MHD effects on pressure drop, structures of interfacial wave and turbulent flow, and heat transfer have been investigated in an air-mercury stratified flow with a wavy interface under a vertical magnetic field. A lower MHD pressure drop than that in a single-phase mercury flow with the same mass flow rate was obtained, and deterioration of the heat- transfer rate due to laminarization of mercury flow was much reduced by a supply of turbulent energy through the interfacial wave from high-velocity gas flow. This study aims at obtaining fundamental data for gas-liquid metal (lithium, sodium, or potassium), two-phase annular mist cooling of blanket in a MCF reactor proposed in the laboratory. Advanced study has just been started by a newly constructed helium-lithium two-phase flow heat-transfer loop using 75 kg lithium with a transverse magnetic field of up to 1.5 T (Fig. 1).

A simulation experiment for transient dynamics and heat transfer of a wetted-wall ICF reactor is ready to start, using high-intensity electron beams with maximum 6 KW and 30 KeV.

E. Kyoto University (Department of Nuclear Engineering)

MHD effects on heat transfer of both natural convection and pool boiling in mercury and potas- sium, as well as flow characteristics and heat transfer in both mercury-argon two-phase flow and NaK- nitrogen two-phase annular dispersed flow, have been actively investigated using several kinds of experi- mental devices and loops.

Heat-Removal Studies 427

Fig. 1. Helium-lithium two-phase flow heat-transfer loop at the Research Laboratory for Nuclear Reactors, the Tokyo Institute of Technology.

F. Osaka University (Department of Nuclear Engineering)

Several kinds of research subjects related to ap- plications of liquid-metal technology to heat removal and energy conversion for MCF and ICF reactors have been actively studied.

The subjects are (i) MHD pressure drop of NaK, N a K - N 2, and Li liquid-metal flow; (fi) MHD effects on heat transfer of flowing Li; (iii)boiling of K (potassium) under a magnetic field; (iv) electromag- netic implosion of NaK annular flow for liner implo- sion fusion; and (v) free surface flow dynamics for an ICF reactor blanket.

The main facilities and devices to study these subjects are (i) a NaK Blowdown Experimental Facil- ity with a NaK inventory of 230 liters, (ii) a dc

magnet (2 T; pole faces; 50• cm2), (iii) a NaK free surface annular-flow experimental device (NaK, 50 liters) (iv) a Lithium circulation loop for MHD experiments (Li, 20 liters), (v) an ion-beam accelera- tor (max, 40 keV-5 A), and (vi) a variable-frequency motor generator.

The main results obtained so far are as follows: (i) the MHD pressure drop in a circular tube under a uniform magnetic field agrees with the prediction based on a uniform-velocity thick-wall model, (ii) heat transfer is supressed with an increasing magnetic flux density as a whole but presents a singular in- crease in a weak magnetic region, (iii) violent boiling inherent to liquid metal is suppressed by applying a magnetic field and the results are encouraging for a boiling two-phase flow cooling concept, and (iv) the implosion dynamics of a NaK liner present an appre-

428 lnoue et al.

ciably good agreement between experiment and the- ory.

G. Kyushu University (Department of Nuclear Engineering)

Thermal analysis of a gas-cooled blanket system is performed. The purpose of this study is to improve the insufficient heat removal capability of helium gas, which is considered to be the main obstacle to the concept of a gas-cooled blanket system. The main attention of the research is focused on a gas-solid suspension medium as a candidate for blanket coolant. In spite of inherent difficulties of the han- dling technique, a gaseous suspension is an excellent heat-transfer medium, and excellence is prominent under the condition of high temperature and high heat flux because suspended solid particles play the role of emitting or absorbing thermal radiation. Be- sides that, it could serve for breeding material as well if a suitable compound of the lithium is used as the suspended solid phase. Among various heat-transfer configurations examined so far, impinging jet of the suspension deserves special attention because it can be applied to the cooling of the first wall. The heat. transfer coefficient of the impinging jet was found to be higher by at least one order of magnitude than those encountered in usual gaseous heat-transfer ap- plications. Moreover, the heat-transfer problem of suspension flowing within helically coiled circular tubes is now being analyzed for the purpose of apply- ing it to the cooling of the tritium breeding region.

II.5.3. ACTIVITIES AT THE JAERI

At the Japan Atomic Energy Research Institute (JAERI), experimental activities for heat removal technology development are found in the JT-60 pro- gram and fusion reactor development program.

Since JT-60 operates with a long pulse (5 - 10 s), heat removal technology issues of its high-heat flux components may be almost equivalent to those of a fusion reactor. In particular, heat removal from beam dumps is one of the serious problems in the design of high-power high-energy neutral beam injectors (NBI). Experimental studies were made on burnout heat flux in highly subcooled forced-convection boiling of water

for the design of beam dumps for JT-60 NBI. (1) These dumps are composed of many circular tubes with two longitudinal fins. The tube was irradiated with nonuniformly distributed hydrogen ion beams of 120 to 200 kW for as long as 10 s. The burnout and film-boiling data were obtained at local heat fluxes of 8 to 15 M W / m 2. These values were as high as 2.5 times larger than those for the circumferen- tially uniform heat-flux case with the same parame- ters. From the experimental results, the beam dumps have been designed to receive energetic beam fluxes of as high as 5 M W / m 2, with a margin of a factor of 2 for burnout.

In the area of fusion-reactor technologies, a ther- mal-cycle test of a divertor plate is now in progress. (2) The wall is made of a composite solid of copper and tungsten. One of the issues of the composite solid is whether such a combination can endure a reasonable number of thermal cycles or not. In this test, an argon plasma jet is used as a heat source and a brazed test piece 5 cm in diameter is placed in a plasma flow conduit. The front face of the tungsten disk is heated by the argon plasma jet, and the back face of the copper disk is cooled by a water jet. An average heat flux of 2 M W / m 2 was applied for 1 rain, with a cool-down period of 10 min. The esti- mated plasma temperature was 104~ About 100 thermal cycles have been imposed on the test piece as of October 1983. There are no observable cracks in the tungsten, copper, and brazed portions. Several test pieces made by various brazing and bonding methods are to be tested in a series of experiments.

Surface heating tests simulating heat loads on the first wall of a fusion reactor started in late 1983 with electron beam sources of Kawasaki Heavy In- dustries, Ltd. The experimental test program involves tests of steady-state and disruption heat fluxes on the first wall of the Fusion Experimental Reactor (FER) design.

REFERENCES

1. H. Horiike et al., Burnout experiment in subcooled forced-con- vection boiling of water for beam dumps of a high power neutral beam injector. Nucl. Technol. Fus. 2:637 (1982).

2. T. Tone et al., Japanese contributions to the Japan-U.S. Workshop on Blanket Design/Technology, JAERI-M 83-005 (1983).