Overview of Naval Reactors Program Development of the Supercritical Carbon Dioxide Brayton System John Ashcroft Kenneth Kimball Michael Corcoran Knolls Atomic Power Laboratory Schenectady NY

Abstract The Naval Reactors (NR) Program, and its two Prime Contractor Laboratories, Knolls Atomic Power Laboratory (KAPL) and Bettis Laboratory, are exploring the supercritical carbon dioxide (S-CO2) Brayton energy conversion system. This non-ideal gas Brayton system may offer improved efficiency and power density relative to ideal gas Brayton systems and may offer physical and operational simplicity relative to a saturated or superheated steam Rankine system. The NR Program work has included initial concept development and integration with S-CO2 flow and heat transfer measurements, instrumentation development, leak testing, and materials compatibility testing. System testing work has also been initiated, both in-house and through component vendor development. The design of a 100 kWe integrated system test (IST) is underway with planned operation in 2010. Initial scoping of a 1-3 MWe S-CO2 system test (SCST) is underway. Introduction The S-CO2 Brayton system was originally envisioned [1-3] as a means to provide high energy conversion system efficiency at moderate temperatures. Little practical development work followed the initial interest in the cycle. However, recent renewed interest in advanced reactors for Generation IV and the Global Nuclear Energy Partnership (GNEP) has spurred renewed interest in S-CO2 in the United States [4.5] and abroad [6-9] The S-CO2 Brayton system is similar in configuration to ideal gas loop closed Brayton systems. The simplest closed system consists of a heat source heat exchanger, a turbine and generator, a recuperating heat exchanger, a waste heat removal heat exchanger (a “precooler”), and a compressor. Figure 1 shows a slight modification, with two separate turbines for driving the compressor and the generator. A unique characteristic of the S-CO2 system compared to a Helium Brayton system is that the cold end of the loop operates at temperatures and pressures near the critical point of carbon dioxide, as shown in Figure 2. Cooling the CO2 to below (to the left of) the “pseudo-critical” line leads to a rapid increase in CO2 density and specific heat. This results in lower compressor work and high heat transfer coefficients in the precooler and the recuperator. This lower compressor work allows for higher efficiency than is possible with an ideal gas at the same turbine and compressor inlet temperatures.

Figure 1: Simple Recuperated Close Loop S-CO2 Brayton System with Separate Turbine Driven Compressor and Turbine Generator. The other unusual feature of the S-CO2 system is the compact size of the turbomachinery. The high density of the CO2 working fluid in the turbine and compressor allows for small, high speed components, with fewer stages, and the potential for integral electric generator arrangements, as shown in Figure 3. This compact and simpler turbomachinery may allow for lower component cost and a smaller energy conversion system footprint than is possible with steam or ideal gas Brayton systems.

Organization Argonne National Laboratory Barber Nichols Inc.

Figure 2: Temperature-Entropy Diagram for an Example S-CO2 Brayton Loop. Note that the cold end of the loop operates below (to the left of) the pseudocritical line for CO2. Here the fluid density is large, reducing the required compressor work relative to an ideal gas Brayton cycle.

Brayton Energy Chromalox, Hydro-Pac, Micro Motion, Micropump Concepts NREC Curtis-Wright EMD Dresser Rand ePower LLC Heatric Hoffman Process Inc. John Crane Inc. Massachusetts Institute of Technology NASA Glenn Research Center Sandia National Laboratory

Figure 3: Turbine-Compressor Generator Integral Arrangement. This concept is based on Turbomachinery and Generator Contract Development Contracts like those Described in Table 1.

NR Program Efforts The NR Program efforts and plans include preliminary system thermodynamics and sizing evaluations, fundamental S-CO2 flow and materials testing, component development, system control modeling, and system testing. The efforts include collaboration with DOE National Laboratories (Sandia and Argonne) and NASA, as well as with subcontracts with Universities and component vendors. Table 1 provides a list of NR Program collaborators and subcontractors.

University of Virginia University of Wisconsin

Role System Modeling Precooler and Recuperator Heat Transfer Turbomachinery Concepts Sandia Test System Hardware and Test Development IST Turbomachinery, Electrical Control, Support Systems Compact Heat Exchangers System Instrumentation and Other Support Equipment

Turbomachinery Concepts CFD Models for Compressors Generator Concept Development Turbomachinery Concepts Compressor Test Approach Generator Concept Development Compact Heat Exchangers IST Tube/Shell Heat Exchangers Evaluation of Dry Gas Seals System Development System Modeling Component Sizing Materials Testing Aerodynamic, Rotordynamic and Bearing Evaluations Dual Shaft Closed Loop Brayton Testing Closed Loop Brayton Testing S-CO2 System Testing Component and System Model Development Magnetic and CO2 Film Bearing Development Blowdown Testing Materials Testing

Table 1. NR Program S-CO2 Development Subcontractors and Collaborators with Their Roles. Concept Development NR Program thermodynamic evaluations have extended the range of power level and temperature for which the S-CO2 Brayton energy conversion system can be considered relative to the GNEP and Generation IV concept focus. The GNEP and Generation IV thermodynamic studies [11] of S-CO2 systems have linked the energy conversion system to a high temperature reactor. With a high temperature heat source (e.g., ~500 °C or 932 °F), a “recompression” cycle appears most promising.

Near term testing of the system can be done with a relatively modest temperature heat source and at a much lower power level. For systems with low turbine inlet temperatures, near 300 °C (572 °F), the added complexity and losses associated with the additional recuperator and compressor needed for the recompression cycle is not warranted. The value of the recompression cycle is also lessened if the component efficiencies are less than have been extrapolated in the literature. Isentropic efficiency of about 80% for the compressor and 85% for the turbine are predicted for near term tests [10, 11]. Note that many recent evaluations [23, 24] of S-CO2 assume much higher turbomachinery component efficiencies as well as high heat exchanger effectiveness. While these papers point out the long term potential of the S-CO2 technology, it is unlikely that near term tests will live up to these predictions. Figure 4 shows the predicted system efficiency over a range of temperatures for a simple system and a recompression system using the anticipated component performance capabilities for near term tests. The figure compares the predicted efficiency to that of a steam system and a helium Brayton system.

Fundamental S-CO2 Testing The behavior of CO2 near the critical point is of concern due to the rapid variation of properties such as density, specific heat, and speed of sound. Figure 5 shows this rapid variation in properties as a function of temperature at pressure of 1100 psia, just above the critical pressure of 1070 psia.

Figure 5. Variation in CO2 Properties Near the Critical Point. This data, taken from the NIST “REFPROP 8” [13], shows properties at 1100 psia, just above the 1070 psia critical pressure.

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Preliminary tests have demonstrated that despite the rapid variation in properties, the pressure drop in a flowing loop is predictable and follows typical in-tube friction factor correlations based on the local properties. Figure 6 shows a comparison of predicted local pressure drop versus the data collected from a flowing loop [12]. Rapid variation in density near the critical point also results in a strong natural circulation flow capability in CO2, which has been confirmed in testing.

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Figure 4: Comparison of the Simple and Recompression Cycle Efficiency versus Temperature. Note that the system efficiency does not improve for the recompression cycle relative to the simple cycle at low temperatures. S-CO2 Brayton systems will provide system efficiencies that equal or exceed those of the Helium cycle at temperatures beyond the applicability of steam.

Heat transfer rates at precooler and recuperator operating conditions have been studied by Argonne National Laboratory and KAERI [14]. Heat transfer rates in S-CO2 are expected to vary around the loop depending on flow rates and state point conditions. In addition to process induced differences in heat transfer, some research has shown enhanced heat transfer in vertical and horizontal tubes (greater than Nusselt correlations suggest) near critical and pseudo-critical points during cooling of S-CO2 and deteriorated heat transfer during heating of SCO2, while other researchers have not shown these changes [14]. Conflicting heat transfer rate results dictate that additional testing at specified state points and with specific geometries will be a necessary prerequisite for accurate prediction of component performance in S-CO2 systems.

screening results for Elastomers tested in supercritical CO2 [17].

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Fe-20Cr-24Ni-6Mo-0.5Si (AL-6XN)

Figure 7. Test Conditions and Results for Stainless Alloys Materials Tests in Supercritical CO2 at 825 °F, 2800 psig for 500 hours.

Figure 6. Comparison of the Local Pressure Drop Measured in Test to Standard Friction Factor Correlation. Despite the proximity to the critical point, pressure drop data agrees well with existing correlations that use local conditions.

Materials Testing Materials experience from the British CO2 cooled reactors has been used as a starting point for initial corrosion and materials performance screening at SCO2 pressures and temperatures [16]. While the pressures in an S-CO2 system are greater, the same range of materials can be considered. Initial NR program screening tests have shown predictable oxide formation for nickel alloys and stainless steel alloys at temperatures of 440 °C (825 °F) and 2800 psi, with more significant oxide formation seen in ferritic steels [17]. Figure 7 shows test results for stainless steel samples in pure supercritical CO2 at 2800 psi and 825 °F. Material compatibility studies have also been performed at the University of Wisconsin and MIT [18, 19]. While supercritical CO2 has a low solubility for metals and oxides, it is readily absorbed by many organic materials that might be used in system components such as valves and seals. Figure 8 shows initial

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Rapid depressurization testing of the high pressure SCO2 system have been performed at KAPL and through a subcontract with the University of Wisconsin. The depressurization results in rapid chilling and crystallization of the CO2 gas [15]. The mass flow rate and the temperature of the gas measured in these tests are consistent with computational fluid dynamics models. Accurate prediction of these trends is important for personnel and equipment safety associated with a leak, as well as for characterization of high pressure seals within the turbine and compressor.

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Figure 8. Weight Gains for Elastomers Tested in Supercritical CO2. Specimens were subject to 3 days of testing at 130 °F and 1800 psig. Component Development The NR program has investigated design concepts for turbomachinery and heat exchangers for test systems between 100 kWe and 12 MWe. For the turbomachinery in this power range, single stage radial flow turbines and compressors, arranged in a two shaft configuration have been considered. Figure 9 shows the optimal shaft speed for applications in this range. Note that optimal shaft speed is higher than typical steam systems synchronous turbomachinery. Multi-stage radial flow compressors and multi-stage axial flow turbines may allow for improved aerodynamic efficiency and lower shaft speeds, at the expense of rotordynamic design complexity. There are two challenges associated with the turbomachinery. The first is in the aerodynamic design of the compressor for operation near the critical point. The supercritical CO2 properties vary

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appreciably based on the inlet conditions, and the density change with pressure is quite different from that of an ideal gas. Therefore, “rules-of-thumb” and correlations associated with surge avoidance may not be applicable in this system. Test data is needed for a variety of compressor configurations. Until such a database is developed, a wide margin to the critical point (higher inlet pressure) and a wide margin to predicted surge are recommended. The second challenge is generator operation in the CO2 environment, which results from the integral turbomachinery arrangement (Figure 3). In order to avoid excessive windage losses (~10% with a pressure of ~800 psia [27]), the generator region must be maintained at low pressure. This is done by utilizing shaft seals to create a barrier between the turbine and compressor regions and the generator region. The shaft seals decrease the leakage, but a CO2 pump-down system is needed to draw this leakage gas out of the generator and return it to the main loop. With the pressure in the generator cavity reduced to less than ~200 psia, the windage losses are predicted to be acceptably low (less than 5% [27]). The design of the heat exchangers needs to balance performance, cost, and operations and maintenance. While recent literature points to the increased use of high pressure capable compact heat exchangers such as the printed circuit heat exchanger (PCHE) [20], the potentially high thermal stresses and fatigue in this monolithic design must be accounted for. Other heat exchanger types might also be considered. For the intermediate heat exchanger (IHX) and the precooler, tube-in-shell heat exchangers might better balance mechanical design, cost and performance requirements. Table 2 compares heat exchanger design features.

Option Shell/Tube IHX

PCHE Recuperator Plate/Fin Recuperator Shell/Tube Recuperator Shell/Tube Precooler Precooler with Enhanced Heat Transfer surfaces on Gas-side Compact Precooler

Evaluation High gas side T leads to high average through-wall T, leading to compact size. U-tube arrangement best for thermal stress. High pressure capability, but impact of thermal stress and fatigue needs to be evaluated. Better thermal stress performance if pressure loading is acceptable. Impractically large. Low T between CO2 and cooling water leads to very large heat exchanger. Likely best tradeoff between size and maintainability. Meaningful improvements over standard shell/tube heat exchanger required.

Smallest design, but maintainability with process cooling fluid could be an issue.

Table 2. Heat Exchanger Design Features and Options. Control System Development and Modeling The NR Program has developed system models and control algorithms to evaluate plant control during plant operations such as start-up and shut down, changes in power levels, and transients associated with plausible system failures. These models use the Nuclear Regulatory Commission‟s TRACE code, with modifications made for the SCO2 system [21]. Figure 10 shows an example model run for a startup and power up of the two-shaft 100 kWe IST configuration. In this example, the compressor shaft is first motored to provide flow in the loop. The heaters are then engaged to warm the loop to an intermediate temperature. Next the turbinegenerator shaft is started-up and quickly begins to produce power. The system is heated to full temperature and the shafts are brought to normal speed and power output.

Power Level (MWe) Figure 9. Approximate S-CO2 turbomachinery shaft speed and turbine wheel diameter as a function of output power level. The Symbols represent power level cases studied by BNI.

The models predict stable operation of the S-CO2 system using a number of different control schemes, including shaft speed control via load regulation, and thermal power control via throttle and recirculation valves. The system has also been shown to respond well to sudden changes in load [22]. These results agree with the modeling results produced by Argonne, MIT, and Sandia for large scale and test scale S-CO2 systems [23, 24, 26].

System Testing Plans The NR program is developing a 100 kWe system test and is planning 1-3 MWe system test. The 100 kWe test (the “IST”) is currently under construction, with all major components on order. Figure 11 shows the reference heat balance for the IST, Figure 12 shows an arrangement configuration. A modest turbine inlet temperature (~300 °C or 570 °F) is used in the test to alleviate materials challenges and facilitate heating with a liquid heat source (an organic heat transfer fluid).

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Figure 11. IST Heat Balance. The heat balance shows reduced component efficiencies reflective to the scale of the system. It also shows the impact of generator losses, windage, and leakage. Cooling Water Temperature Control HX

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Figure 10. Start-up Transient for the IST. (a) Heater Exit Temperature, (b) Compressor Inlet Pressure, (c) Turbine Shaft Power, and (d) Compressor Shaft Speed. Current Models can track system and component conditions throughout the range of the modeled operation.

Precooler

Figure 12. IST Component Layout.

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The test loop features a two-shaft arrangement with a constant speed turbine-generator and a variable speed turbine driven compressor. This arrangement was selected because it allows for the widest range of control options. The purpose of the test is to understand the integrated system features and to demonstrate system controllability. Initial testing of the IST at Bettis is planned for March 2010. Complete test hardware descriptions and test plans have been developed [25]. The test will followon and extend similar small scale system testing under development at Sandia National Laboratory and Barber Nichols Inc. [26]. The 1-3 MWe system test (the “SCST”) is planned to follow the IST. Initial scoping of this test series is underway. As with the IST, the SCST will study the integrated system performance features and the system controllability. Acknowledgements While the authors of this paper have assembled and described the work of the Naval Reactors Program on the S-CO2 system, the work has been done by a much larger number of devoted engineers and scientists working at Knolls Atomic Power Laboratory, Bettis Atomic Power Laboratory, and at our subcontractors. This overview paper would not be possible without the work of this team. References [1] Feher, E.G., “The Supercritical Thermodynamic Power Cycle”, Intersociety Energy Conversion Engineering Conference (IECEC), 1967. [2] Angelino, G., “Carbon Dioxide Condensation Cycles for Power Production”, Journal of Engineering for Power, 1968. [3] Hoffman J.R. and Feher, E.G. “150 kWe Supercritical Closed Cycle System”, Transactions of the ASME, Pg 70-80, 1971. [4] Wright, S.A. et al, “Supercritical CO2 Brayton Cycle Compression and Control Near the Critical Point”, ICAPP‟ 08, Anaheim CA, June 2008. [5] Pickard, P. “DOE Program on S-CO2 Cycle Development” MIT CANES Symposium on Supercritical CO2 Power Cycle, 2007. [6] Kato, Y. et al “Supercritical CO2 Gas Turbine Cycle Systems”, ICAPP 2007, Nice, France, May 2007. [7] Simon, N., Latge, C., Gicquel, L. “Investigation of Sodium – Carbon Dioxide Interactions with Calorimetric Studies”, ICAPP 2007, Nice France, May 2007. [8] Cha, J.E., et al, “Development of Supercritical CO2 Brayton Cycle for the KALIMER”, NUTHOS7, Seoul, Korea, October 2008.

[9] Fouillet, C. “AREVA Evaluations of S-CO2 Cycle for Sodium Cooled Reactor”, MIT CANES Symposium on Supercritical CO2 Power Cycle, 2007. [10] Fuller, B., “Turbo-Machinery Considerations Using Super-Critical Carbon Dioxide Working Fluid for a Closed Brayton Cycle”, MIT CANES Symposium on Supercritical CO2 Power Cycle, 2007. [11] Corcoran, M., Hoang, H.T., and Wuthrich, J.W., “Thermodynamic Study of a Supercritical CO2 Brayton Cycle”, Supercritical CO2 Power Cycle Symposium, Troy, NY, April 2009. [12] Milone, D.V., “Computational Model and Experimental Data on the Natural Circulation of Supercritical Carbon Dioxide”, Supercritical CO2 Power Cycle Symposium, Troy, NY, April 2009. [13] Lemmon, E.W., Huber, M.L., McLinden, M.O. “NIST Standard Reference Database 23: Reference Fluid Thermodynamic and Transport Properties-REPROP, Version 8” NIST, Gaithersburg, MD [14] Lomperski, S., Cho, D, Song, H., Tokuhiro, A. “Testing of a Compact Heat Exchanger for Use as the Cooler in a Supercritical CO2 Brayton Cycle” ICAPP ‟06, Reno NV, June 2006. [15] Anderson, M., Mignot, G., and Corradini, M., “Experimental Investigation of Critical Flow of Supercritical Carbon Dioxide” Supercritical CO2 Power Cycle Symposium, Troy, NY, April 2009,. [16] Shropshire, D., "Lessons Learned form GEN I Carbon Dioxide Cooled Reactors", Proceedings of the 12th International Conference on Nuclear Engineering , Arlington VA, Paper #ICONE1249380, April 25-29, 2004. [17] Tunison, H., “Materials Evaluation in S-CO2” Supercritical CO2 Power Cycle Symposium, Troy, NY, April 2009. [18] Ballinger, R. “Materials Testing in S-CO2” MIT CANES Symposium on Supercritical CO2 Power Cycle, 2007. [19] Cao, G., Anderson, M., Sridharan, K., Tan, L., and Allen, T.R., “Corrosion of Candidate Alloys in Supercritical Carbon-Dioxide”, Supercritical CO2 Power Cycle Symposium, Troy, NY, April 2009. [20] Li, X., Kininmont, D., Le Pierres, R., Dewson, S.J., “Alloy 617 for High temperature Diffusion Bonded Compact Heat Exchangers”, ICAPP‟ 08, Anaheim CA, June 2008. [21] Siebert, B., Hexemer, M., Dolatowski, J., and Wahl, G., “TRACE Modifications in Support of S-CO2 Transient Modeling”, Supercritical CO2 Power Cycle Symposium, Troy, NY, April 2009. [22] Hexemer, M. J., Hoang, H.T., Rahner, K. D., Siebert, B., and Wahl, G. D., “Integrated Systems Test (IST) S-CO2 Brayton Loop Transient Model Description and Initial Results”, Supercritical CO2 Power Cycle Symposium, Troy, NY, April 2009.

[23] Moisseytesev, A., and Sienicki, J.J. “Controllability of Supercritical Carbon Dioxide Brayton Cycle near the Critical Point”, ICAPP‟08, Anaheim, CA, June 2008. [24] Carstens, N.A., et al, “Dynamic Modeling of the S-CO2 Recompression Cycle” ICAPP „08, Anaheim CA, June 2008. [25] Howard, D. B., “The Integrated System Test Design and Status”, Supercritical CO2 Power Cycle Symposium, Troy, NY, April 2009.

[26] Wright, S.A, Fuller, R. Pickard, P.S., Vernon, M.E., “Initial Status and Test Results from a Supercritical CO2 Brayton Cycle Test Loop”, ICAPP „08, Anaheim, CA, June 2008. [27] Calfo, R.M., Farr, C.T., and Poole, G.E., “SCO2 Electrical Generator Efficiency Trade Study”, Supercritical CO2 Power Cycle Symposium, Troy, NY, April 2009

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