SCO2 Power Cycle Symposium May 24-25, 2011 Boulder, Colorado

Investigation of Fundamental Phenomena Relevant to Coupling the Supercritical Carbon Dioxide Brayton Cycle to Sodium-Cooled Fast Reactors Sienicki, James J. Argonne National Laboratory 9700 South Cass Avenue, Argonne, Illinois 60439, USA [email protected] Claude B. Reed, David B. Chojnowski, Yoichi Momozaki, Craig D. Gerardi, Mitchell T. Farmer, and Dae H. Cho Argonne National Laboratory 9700 South Cass Avenue, Argonne, Illinois 60439, USA

Abstract The supercritical carbon dioxide (S-CO2) Brayton cycle is well suited for application to Sodium-Cooled Fast Reactors (SFRs) as an innovative and advanced power conversion technology. Utilization of compact diffusionbonded heat exchangers is envisioned for the sodium-to-CO2 heat exchangers. It is essential to have an understanding of phenomena specific to the design and use of such heat exchangers. To provide this understanding, a suite of small-scale science-based experiments are being designed and assembled at Argonne National Laboratory (ANL) to provide fundamental data on sodium plugging of sodium channels due to the precipitation upon cooling of dissolved oxygen contaminating the sodium, freezing and thawing of sodium in simple and prototypical sodium-to-CO2 heat exchanger sodium channels, draining and filling of sodium from heat exchanger sodium channels including the effects of heat exchanger orientation, effects of thermal shock upon sodium-to-CO2 heat exchangers, and sodium-CO2 interactions under prototypical conditions of release of CO2 into sodium

1. Introduction The supercritical carbon dioxide (S-CO2) Brayton cycle is an innovative and transformational technology that can be utilized to advance Sodium-Cooled Fast Reactor (SFR) technology. The S-CO2 Brayton cycle is well suited for application to SFRs. The cycle is highly recuperated and wants to operate with a CO2 temperature rise through the sodium-to-CO2 heat exchanger of about 150 °C. This is approximately equal to the temperature rise through the SFR core; for example, 155 °C for the Advanced Burner Test Reactor (ABTR) preconceptual design [1], providing an excellent match. The benefits of the S-CO2 cycle for SFR applications are elimination of sodium-water reactions, potential for lower capital cost than the traditional Rankine superheated steam cycle, higher efficiency than the Rankine superheated steam cycle for higher SFR core outlet temperatures (e.g., 45 % cycle efficiency at 550 °C) further reducing the plant cost per unit electrical power and increasing the plant net present value, remarkably small turbomachinery with the expectation of reduced component costs, and compact power converter size reducing the turbine generator building size and cost. Utilization of the S-CO2 Brayton cycle requires that suitably reliable and economical sodium-to-CO2 heat exchangers are designed and that sodium-to-CO2 interactions under prototypical conditions following postulated heat exchanger failure are understood. Development of the S-CO2 cycle at Argonne National Laboratory (ANL) has envisioned the use of compact diffusion-bonded sodium-to-CO2 heat exchangers such as the Hybrid Heat Exchanger (H2X) or Printed Circuit Heat ExchangerTM (PCHETM) technology of Heatric Division of Meggitt (UK) Ltd [2]. Compact diffusion-bonded heat exchangers are expected to have a low potential for failure (high reliability), long life, and significantly smaller volume relative to shell-and-tube heat exchangers. Failures in compact diffusionbonded heat exchangers, if they occur at all, are expected to be small cracks (microcracks) resulting in slow interaction of CO2 with sodium.

SCO2 Power Cycle Symposium May 24-25, 2011 Boulder, Colorado Investigations of fundamental phenomena relevant to sodium-to-CO2 heat exchangers are being carried out at Argonne National Laboratory (ANL). Five small-scale experiments addressing particular fundamental phenomena are described below.

2. Sodium Plugging Experiments The Small Sodium Test Loop (SSTL) Sodium Plugging Loop previously constructed and operated at ANL has provided preliminary data on the plugging of 2, 4, and 6 mm semicircular diameter stainless steel channels due to sodium oxide precipitation upon cooling of flowing sodium containing dissolved oxygen simulating postulated oxygen contamination of sodium. The results of early experiments are discussed in Reference [3]. Subsequently, sodium oxide was added to the sodium raising the dissolved oxygen concentration at high temperatures to an estimated maximum value of 740 parts per million (corresponding to 13.8 g of sodium oxide added). Figures 1 and 2 show the previous configuration of the SSTL and the results of plugging tests performed at different levels of dissolved oxide for plugging of the test section with 2 mm semicircular diameter sodium channels.

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Figure 1. Small Sodium Test Loop Sodium Plugging Loop. Figure 2. Effects of Dissolved Oxygen upon Plugging of 2 mm Semicircular Diameter Sodium Channels.

For the tests of Figure 2, the temperature of the stainless steel test section at the plug location decreases autonomously as the plug forms and the sodium flowrate decreases. The SSTL is currently being upgraded to provide improved sodium plugging data under well-controlled conditions at the plug location such that the temperature at the plug location remains unvarying during the plug formation process for interpretation of the plugging phenomena and for validation of future plugging models. The facility is being modified to provide for air cooling and heating of the existing test section with three 6 mm semicircular sodium channels over five heating zones to decrease the temperature of flowing sodium over the first three heater zones and increase the sodium temperature over the last two heater zones. Figure 3 shows the test section inside of an air duct through which the cooling air flows from right to left; the air is precooled with a chiller prior to entering the duct. The sodium also flows from right to left through the test section. The air cooling capability can readily cool the sodium temperature by about 155 °C as it flows through the first three heater zones. Ceramic band (radiant) heaters provide for shaping of the temperature profile. The sodium is heated as it flows through the last two heater zones by cast bronze platen (surface) heaters to assure that plug formation takes place within the body of the test section. The SSTL upgrading also includes replacement of the cold trap with a larger and more effective design as well as incorporation of a sodium plugging meter for determination of the dissolved oxygen concentration.

SCO2 Power Cycle Symposium May 24-25, 2011 Boulder, Colorado

Figure 3. Test Section with Three 6 mm Semicircular Diameter Sodium Channels, Air Cooling Duct, and Heaters.

3. Sodium Freezing and Thawing Experiments Inadvertent freezing or thawing of sodium is known to have caused failure of specific sodium components. A new small-scale sodium testing facility is being designed and fabricated to investigate fundamental phenomena involved in sodium freezing and melting in stainless steel sodium channels. The objective is to understand the potential for damage resulting from the freezing or melting of sodium inadvertently trapped inside of stainless steel channels such that the potential for damage can be shown to be nonexistent or can be designed out of the sodium-to-CO2 heat exchanger configuration. An innovative small experiment apparatus (Figure 4) for sodium freezing and thawing testing using small-scale test sections has been designed and is being assembled for both fundamental studies of freezing and melting in simple configurations as well as prototypical heat exchanger sodium channel configurations. Test sections of different length can be installed and removed using quick change VCR fittings on the lower tubing and incorporation of a coil into the upper tubing. Prior to freezing or melting testing, sodium is circulated by a small electromagnetic (EM) pump to achieve wetting of the stainless steel at elevated temperatures. In circulation mode, the EM pump raises the sodium height enabling sodium to spill over into the sodium supply vessel. Freezing and melting experiments are conducted with the EM pump turned off establishing sodium free surface levels well above the test section. Freezing tests can be conducted with the test section temperature lowered such that freezing occurs with liquid sodium communicating with the free surfaces or with the temperature lowered in the tubing outside of the test section forming solid sodium plugs such that freezing sodium is trapped between the plugs. The facility will include a vacuum pump to degas the stainless steel and sodium for investigation of the fundamental effects of dissolved gases upon adherence of the sodium to the stainless steel walls.

Figure 4. Sodium Freezing and Thawing Experiment Apparatus withoutTest Section Installed.

SCO2 Power Cycle Symposium May 24-25, 2011 Boulder, Colorado 4. Sodium Draining and Filling Experiments The design of a compact diffusion-bonded heat exchanger must assure that the sodium channels are large enough and do not incorporate obstacles that could prevent sodium from draining from the wetted stainless steel channels in a timely manner in the event that the intermediate sodium circuit is deliberately drained to minimize sodium spillage following detection of an intermediate sodium circuit leak. Bridging of heat exchanger channels by trapped sodium could result in excessive stresses and strains in the stainless steel structure due to subsequent sodium freezing and thawing, or oxidation of the sodium to form high melting temperature solid sodium oxide plugs. Small-scale sodium drain and fill experiments incorporating stainless steel test sections with prototypical sodium channels are being designed to experimentally verify that the heat exchanger sodium channel configuration and orientation provides for efficient draining and subsequent refill of sodium. In the facility illustrated in Figure 5, the test section is contained inside of a stainless steel vessel having a trunnion support such that the test section orientation can be varied from vertical to horizontal. A sodium loop provides for wetting of the stainless steel prior to the tests.

Figure 5. Schematic Illustration of Sodium Draining and Filling Tests.

5. Thermal Shock Experiments Thermal shock resulting from anticipated transients and postulated accidents for a SFR plant incorporating a S-CO2 Brayton cycle is an important phenomenon for compact diffusion-bonded heat exchangers. A small-scale thermal shock testing facility is being designed for investigation of thermal shock-induced stresses and strains and the potential for heat exchanger failure by subjecting small-scale stainless steel heat exchangers incorporating

SCO2 Power Cycle Symposium May 24-25, 2011 Boulder, Colorado prototypical core configurations to cooldown and heatup transients and providing data for testing and validation of analysis approaches.

6. Sodium-CO2 Interaction Experiments While the S-CO2 cycle eliminates sodium-water reactions, there is a need to understand sodium-CO2 interactions under prototypical conditions following postulated failure of a compact diffusion-bonded sodium-to-CO2 heat exchanger. A small-scale sodium-CO2 interaction experiment facility is being assembled and tests will be conducted to provide fundamental data on the interactions between a vertical column of sodium and CO2 released through stainless steel micro leak configurations and fundamental data on the postulated self-plugging of stainless steel micro leak configurations under realistic conditions of sodium-to-CO2 heat exchanger failure. The small-scale experiment facility is the first anywhere focused upon the failure mode expected for compact diffusion-bonded sodium-to-CO2 heat exchangers in which the CO2 release rate is very slow due to anticipated microcrack formation. The sodium components are shown in Figure 6. Carbon dioxide will be injected into a vertical column of sodium through the side of the test vessel near its bottom. The sodium column height will be varied to investigate how the reactions and product species depend upon the sodium column height or interaction time. Product gases, if any, will be determined with a mass spectrometer on an exhaust line from the test vessel.

Figure 6. Sodium Components of Sodium-CO2 Interaction Facility (Left) and Test Vessel (Right).

SCO2 Power Cycle Symposium May 24-25, 2011 Boulder, Colorado 7. Summary The S-CO2 cycle is well suited for application to SFRs as typical SFR core temperature rises are approximately equal to the 150 °C CO2 temperature rise in the sodium-to-CO2 heat exchanger at which the heavily recuperated cycle wants to operate. A suite of small-scale science-based experiments are being designed and assembled at ANL to provide the fundamental data required for the design and utilization of compact diffusion-bonded heat exchangers for sodium-to-CO2 heat exchange.

Acknowledgements Argonne National Laboratory’s work was supported by the U. S. Department of Energy Advanced Reactor Concepts Program under Prime Contract No. DE-AC02-06CH11357 between the U S. Department of Energy and UChicago Argonne, LLC. The authors are grateful to Rick Kendall and Matt Hutmaker of DOE as well as Robert N. Hill (ANL/NE), the National Technical Director.

References 1.

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J. J. Sienicki, A. Moisseytsev, D. H. Cho, Y. Momozaki, D. J. Kilsdonk, R. C. Haglund, C. B. Reed, and M. T. Farmer, “Supercritical Carbon Dioxide Brayton Cycle Energy Conversion for Sodium-Cooled Fast Reactors/Advanced Burner Reactors,” Paper 181359, Global 2007: Advanced Nuclear Fuel Cycles and Systems, Boise, Idaho, September 9-13, 2007. D. Southall and S. J. Dewson, “Innovative Compact Heat Exchangers,” Paper 10300, Proceedings of ICAPP ‘10, San Diego, CA, June 13-17, 2010. Y. Momozaki, D. H. Cho, J. J. Sienicki, and A. Moisseytsev, “Experimental Investigations on Sodium Plugging in Narrow Flow Channels,” Nuclear Technology, Vol. 171, pp. 153-160, 2010.