Heat Transfer Experiment and Simulation of the Verification Facility for High Power Rotating Tritium Target System

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1 J Fusion Energ (2015) 34: DOI /s ORIGINAL RESEARCH Heat Transfer Experiment and Simulation of the Verification Facility for High Power Rotating Tritium Target System Gang Wang 1 Zhen Wang 1 Qianfeng Yu 1 Yong Song 1 Yican Wu 1 Wenlong Cheng 2 Published online: 20 June 2015 Springer Science+Business Media New York 2015 Abstract High intensity D T fusion neutron generator (HINEG) is important for research and development work of fusion reactors, of which the rotating target system is one of the key components. The design of the cooling technology principle verification facility both for tritium target systems of the and n/s HINEG was proposed. The facility employed the jet array cooling method. The first stage heat transfer experiment was carried out and the heat transfer processes was simulated by CFD method, which aimed at the investigations of the new cooling enhancement technology. The experimental results, which had a good agreement with the numerical simulation results, show that the maximum temperature of the target surface was about 89.4 C under 60 kw heating condition and the average equivalent convection heat transfer coefficient of the jet array cooling was about 57,000 W/(m 2 K). All the results show that the verification facility could achieve the cooling requirement for n/s neutron yield (60 kw) design objective and the jet array cooling enhancement technology was effective. Keywords Rotating target system Cooling enhancement Jet array cooling HINEG Neutron generator & Qianfeng Yu qianfeng.yu@fds.org.cn 1 2 Key Laboratory of Neutronics and Radiation Safety, Institute of Nuclear Energy Safety Technology, Chinese Academy of Sciences, Hefei , Anhui, China University of Science and Technology of China, Hefei , Anhui, China Introduction As fusion reactor is one of the most promising solution methods of the sustainable development for nuclear energy utilization, many research and development works for fusion reactor are in progress [1 5], of which the International Thermonuclear Experimental Reactor (ITER) Project is the world-wide biggest fusion reactor research project. The neutron energy of ITER is 14 MeV and the neutron generated by a deuterium tritium (D T) fusion neutron generator is just the same. 14 MeV neutrons can be used in the fusion basic experimental research, medical isotope imaging, neutron photography, neutron weapon and so on. The world-wide typical D T fusion neutron sources are shown in Table 1, in which the high intensity D T fusion neutron generator (HINEG) is now in construction and n/s is the objective of the first phase. The second and third phases for HINEG are designed to be and n/s. The tritium target system is one of the key components of a D T fusion neutron generator. A higher neutron yield will cause a bigger heat load on the target system. The way to enhance the heat transfer is one of the most important technical problems for tritium target system design. There are three kinds of normal cooling type used by tritium target systems, which are single jet, interlayer and groove water cooling methods. The advanced tritium targets of RTNS-II and SNEG-13 are world-wild typical, of which the heat loads are 60 and 20 kw respectively and the cooling types are the groove water cooling [6 8]. A rotating target system for *10 12 n/s neutron generator was proposed and developed by Institute of Nuclear Energy Safety Technology (INEST) for the first phase of HINEG. And the neutronics and thermal experiments of this target system have been carried out [9].

2 J Fusion Energ (2015) 34: Table 1 World-wide typical D T neutron sources and their target systems Accelerator name RTNS-I RTNS-II FNS OKTAVIAN HINEG Status To be moved To be moved In operation To be moved In construction Location USA USA Japan Japan China Facility LLNL LLNL JAERI Osaka Univ. INEST High voltage (kv) Ion source type Duoplasmatron Duoplasmatron Duoplasmatron Duoplasmatron ECR Ion beam D? D? D? D? D? Ion current (ma) Target diameter (cm) (50) Target beam spot size (cm) (1 2) Rotating speed (rpm) Cooling medium Water Water Water Water Water Continuous neutron yield (n/s) To achieve the second and third objectives of HINEG, a high power rotating tritium target system should be developed. As the final objective of HINEG needs a solution of about 200 kw heat transfer, normal cooling types would not meet the requirement effectively. Some new cooling methods should be employed [10, 11]. In this paper, the design of the cooling technology principle verification facility both for tritium target systems of the and n/s neutron generators was proposed. The preliminary experiment was carried out and the heat transfer process was simulated by CFD method, which aimed at the investigations of the new cooling enhancement technology. Introduction of the Verification Facility For a rotating tritium target of D T fusion generator, on which the titanium tritide coating is plated, neutron and a particle are generated when D? beam bombards the titanium tritide coating. The power of D? beam will be 60 kw (400 kv, 150 ma) for n/s neutron yield or 200 kw (400 kv, 500 ma) for n/s neutron yield and the beam diameter is designed to be 1 3 cm, which will cause very high power densities which are much bigger than the one of n/s rotating target system. As almost 99 % of the beam power will turn to heat deposition on the target, the target would melt if it was fixed and not cooled. On the other hand, the tritium would release heavily when the titanium tritide coating temperature was higher than 200 C [9]. These are the key challenges of the high power tritium target system development. The key technology for cooling enhancement is chosen to be the jet array cooling method. This method employs an array of jets to cool the target. The single jet cooling can Fig. 1 Schematic of verification facility for cooling technology increase the turbulivity and decrease the boundary layer. And the cooling effect will be enhanced when a system is cooled by jet array than a single jet. The design of the cooling technology principle verification facility, which is developed by Institute of Nuclear Energy Safety Technology [12 16], is presented in Fig. 1 and the main parameters are shown in Table 2. Figure 2 is the exploded view of the verification facility. The facility is mainly composed by insulation layer, heat source, target surface, rubber seal ring, jet nozzles, chamber structure and coolant inlet pipes. In the first stage experiment, the heat source type was chosen to be fixed induction heating. The insulation layer is used to avoid the radiation heat transfer between the heat source and the atmosphere around the facility. There are 60 jet nozzles, including 12 inner and 48 outside nozzles. Preliminary Experimental Research A real verification facility without insulation layer is presented in Fig. 2. To investigate the actual cooling performance of the real verification facility, a series of

3 1254 J Fusion Energ (2015) 34: Table 2 Main parameters of cooling technology verification facility Parameters Design/value Heat source type Induction heating Heat source power 0 80 kw Cooling type Jet array cooling Coolant Water Number of jet coolant inlet 60 Total flow rate 30 L/min Target material CuCrCz Target diameter 500 mm Target thickness 2 mm Chamber structure material 304 L Fig. 2 A real cooling technology verification facility (without insulation layer) preliminary heat transfer experiments were carried out, which aimed at the source power objective of n/s neutron yield (60 kw). When the facility operates, the target surface will be heated by the fixed induction heat source. The water is forced by pump and goes through the coolant inlet pipes and jet nozzles, carries away a part of the heat on the target surface and cools the surface. Then the heated water will go down through the coolant outlet pipe and be cooled by a water chiller and forced to go into the coolant inlet pipes again, which leads a cooling circulation. The power of the heat source and water flow rate Table 3 Cooling experimental results of the verification facility Q (kw) T s ( C) h [W/(m 2 K)] , , , , , ,766.6 can both be regulated so that the facility can operate under different heating conditions. The heat source powers in the experiments were 10, 20, 30, 40, 50 and 60 kw. The inner and outside diameters of the heating annulus were 450 and 470 mm respectively, which was equivalent to the 2 cm diameter D? beam heating condition. The water flow rate was about 30 L/min. The equivalent convection heat transfer coefficient h of the jet array cooling is calculated as follows: Q h ¼ ð1þ AðT s T w Þ h: equivalent convection heat transfer coefficient, Q: heat power, A: heat transfer area, T s : maximum temperature of target surface, T w : temperature of water at jet nozzle. The experimental results are shown in Table 3. T w was set to be 16 C. The maximum temperature of the target surface increased linearly as the source power increased. And the tested maximum temperature of the target surface was about 89.4 C under 60 kw heating condition, which was much smaller than the limited temperature (200 C). The average value of equivalent convection heat transfer coefficient of the jet array cooling under all tested conditions was calculated to be about 57,000 W/(m 2 K). The test error of the heating source was below 5 % and temperature test error was smaller than 1 %. So the precision of test results were acceptable in this study. All the experimental results show that during the first stage heat transfer experiment with fixed heat source, the verification facility could achieve the cooling requirement for n/s neutron yield (60 kw) design objective and the jet array cooling method was effective. Heat Transfer Simulation Calculation Model To compare with the experimental results of the verification facility, the numerical heat transfer simulation was carried out by CFD method. A three dimensional calculation model was built, of which the mesh model is presented

4 J Fusion Energ (2015) 34: Fig. 3 Calculation mesh model of target surface of verification facility Fig. 4 Simulation and experimental results for verification facility in Fig. 3. The facility model was simplified and the only concern was the target surface. The target surface model was meshed by ANSYS ICEM code. As the target surface was not complex, considering the computational accuracy and economy, the final mesh quantity was chosen to be about 80,000. The mesh type was adaptive unstructured. The calculation model was heat conduction equation. The main boundary conditions of this heat transfer simulation were set as follows: The initial and coolant inlet temperature at the jet nozzles was set to be 16 C. The heat source was added to the heating annulus of the target surface, and the value was set to be 10, 20, 30, 40, 50 and 60 kw for six different conditions respectively. The condition upon the target surface was assumed to be adiabatic condition, corresponding to the insulation layer. The convection heat transfer coefficient of the target surface back was assumed to be 57,000 W/(m 2 K) according to the experimental results. The total coolant flow rate was 30 L/min and assigned to 60 jet nozzles averagely. Heat Transfer Analyses Corresponding to the experimental research, a series of heat transfer processes of the verification facility under different heat source powers were simulated by FLUENT code. And the simulation results are presented in Fig. 4. As shown in Fig. 4, the tested and simulation maximum temperatures on the target surface both increased linearly with the heat source power increasing. The numerical and experimental results had good agreement and the differences under the same heating source were all very small (below 8 %). That means the test result of equivalent convection heat transfer coefficient of the jet array Fig. 5 Simulation result of target surface temperature distribution under 60 kw condition cooling in the experiments was acceptable, which shows the correctness and rationality of the experimental results and cooling technology employed by the verification facility to a certain extent. The steady temperature distribution of the target surface of the verification facility under 60 kw heating condition is presented in Fig. 5, in which the maximum temperature was about 93.5 C and about 4.1 C bigger than the experimental result (5 % difference). Conclusion In this paper, a principle verification facility design for high power rotating tritium target system of the D T fusion neutron generator was proposed, which aimed at the cooling technology verification. This facility employed jet array cooling method as the key technology

5 1256 J Fusion Energ (2015) 34: for the high power heat transfer. The first stage experiments and a series of numerical simulations for the heat transfer processes of the facility under different heat source power conditions were carried out. The experimental results show that maximum temperature of the target surface was about 89.4 C under 60 kw heating condition and the average value of equivalent convection heat transfer coefficient of the jet array cooling was about 57,000 W/(m 2 K), which had a good agreement with the numerical simulation results. All the results show that the verification facility could achieve the cooling requirement for n/s neutron yield (60 kw) design objective and the jet array cooling method was effective. The second stage experiments will be carried out in next step, which will employ a rotating heat source and aim at the cooling technology verification for n/s neutron yield (200 kw). Acknowledgments The authors appreciate the efforts of the other members in FDS Team, as well as the support of the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA ) and National Magnetic Confinement Fusion Energy Development Program (Grant No. 2014GB112000). References 1. Y.C. Wu, FDS Team, Nucl. Fusion 47, 1533 (2007) 2. Y.C. Wu, J.P. Qian, J.N. Yu, J. Nucl. Mater , 1629 (2002) 3. L. Qiu, Y.C. Wu, B. Xiao, Q. Xu, Q.Y. Huang, B. Wu, Y. Chen, W. Xu, Y. Chen, X. Liu, Nucl. Fusion 40, 629 (2000) 4. Y.C. Wu, J.Q. Jiang, M.H. Wang, M. Jin, FDS Team, Nucl. Fusion 51, (2011) 5. Y.C. Wu, FDS Team, Fusion Eng. Des. 84, 1987 (2009) 6. C.M. Logan, D.W. Heikkinen, Nucl. Instrum. Methods 200, 105 (1982) 7. D.W. Ramey, H.L. Adair, IEEE Trans. Nucl. Sci. NS-30, 1575 (1983) 8. G. Voronin, M. Kovalchuk, M. Svinin et al., Proc. EPAC 94(3), 2678 (1994) 9. G. Song, G. Wang, Q.F. Yu, W. Wang, C. Chen, Y.C. Wu, J. Fusion Energ. 34, 183 (2015) 10. W.L. Cheng, B. Xie, F.Y. Han, H. Chen, Exp. Therm. Fluid Sci. 45, 198 (2013) 11. M. Pais, L. Chow, E. Mahefkey, J. Heat Transf. 114, 211 (1992) 12. Y.C. Wu, FDS Team, Fusion Eng. Des. 81, 2713 (2006) 13. Y.C. Wu, FDS Team, Fusion Eng. Des. 82, 1893 (2007) 14. Y.C. Wu, FDS Team, J. Nucl. Mater , 1410 (2007) 15. Y.C. Wu, FDS Team, J. Nucl. Mater , 122 (2009) 16. Y.C. Wu, FDS Team, Fusion Eng. Des. 83, 1683 (2008)