The design of heat exchangers for use in a supercritical carbon dioxide (sCO 2 ) Brayton cycle power converter must provide for acceptable performance for duty cycle events encompassing anticipated transients and postulated accidents. This paper presents the results of a comprehensive analysis of thermal transients for sCO 2 cycle heat exchangers, with emphasis on the sodium-to-CO 2 heat addition heat exchanger. A range of transients, from normal operation to severe accidents, were simulated with the coupled PDC and SAS4A/SASSYS-1 system level dynamic analysis computer codes. For each transient, the calculated change in the heat exchanger wall temperature is determined as a measure of the thermal loading.

Supercritical carbon dioxide Brayton cycle power converters can benefit advanced nuclear reactors, as well as small modular reactors, by reducing the plant cost and increasing plant electrical output. The sCO 2 cycles can also be designed for operation under direct dry air cooling. This paper presents the results of the coupled control analysis of a sCO 2 cycle for a 100 MWe sodium-cooled fast reactor. The plant control mechanisms were investigated and optimized for load following operation.

Supercritical carbon dioxide Brayton cycle power converters can benefit advanced nuclear reactors, as well as small modular reactors, by reducing the plant cost and increasing plant electrical output. The sCO 2 cycles can also be designed for operation under direct dry air cooling. The paper presents the results of the coupled control analysis of a sCO 2 cycle for a 100 MWe sodium-cooled fast reactor under changing ambient air temperatures. The optimum plant operation modes are identified.

The capability to utilize dry air cooling by which heat is directly rejected to the air atmosphere heat sink is one of the benefits of the supercritical carbon dioxide (sCO 2 ) energy conversion cycle. For the selection and analysis of the heat exchanger options for dry air cooling applications for the sCO 2 cycle, two leading forced air flow design approaches have been identified and analyzed for this application; an air cooler consisting of modular finned tube air coolers; and an air cooler consisting of modular compact diffusion-bonded heat exchangers. The commercially available modular finned tube air cooler is found to be more cost effective and is selected as the reference for dry air cooling.

Commercially available and cost effective finned tube air coolers are an enabling technology that makes practical dry air cooling for the supercritical carbon dioxide (sCO 2 ) Brayton cycle by which heat is directly rejected from CO 2 to the air atmospheric heat sink. With dry air cooling, sCO 2 Brayton cycle conditions need to be re-optimized to increase the main compressor inlet temperature and pressure (e.g., 35 °C and 8.2 MPa) relative to water cooling to limit the air cooler size to a practical value, and to increase the compressor outlet pressure (e.g., 25 MPa) to maintain a high efficiency. With reoptimization, the plant efficiency for the AFR-100 Sodium-Cooled Fast Reactor Nuclear Power Plant (NPP) is similar to that with once-through water cooling, while the NPP capital cost per unit output electrical power ($/kWe) is roughly estimated to be only 2 % greater. For the AFR-100 application, no unique benefit is identified for the sCO 2 Brayton cycle relative to the superheated steam cycle with respect to the capability to use dry air cooling.

Although a number of power conversion applications have been identified or have even been developed (e.g., waste heat recovery) for supercritical carbon dioxide (S-CO 2 ) cycles including fossil fuel combustors, concentrated solar power (i.e., solar power towers), and marine propulsion, the benefits of S-CO 2 Brayton cycle power conversion are especially prominent for applications to nuclear power reactors. In particular, the S-CO 2 Brayton cycle is well matched to the Sodium-Cooled Fast Reactor (SFR) nuclear power reactor system and offers significant benefits for SFRs. The recompression closed Brayton cycle is highly recuperated and wants to operate with an approximate optimal S-CO 2 temperature rise in the sodium-to-CO 2 heat exchangers of about 150 °C which is well matched to the sodium temperature rise through the core that is also about 150 °C. Use of the S-CO 2 Brayton cycle eliminates sodium-water reactions and can reduce the nuclear power plant cost per unit electrical power. A conceptual design of an optimized S-CO 2 Brayton cycle power converter and supporting systems has been developed for the Advanced Fast Reactor – 100 (AFR-100) 100 MWe-class (250 MWt) SFR Small Modular Reactor (SMR). The AFR-100 is under ongoing development at Argonne National Laboratory (ANL) to target emerging markets where a clean, secure, and stable source of electricity is required but a large-scale power plant cannot be accommodated. The S-CO 2 Brayton cycle components and cycle conditions were optimized to minimize the power plant cost per unit electrical power (i.e., $/kWe). For a core outlet temperature of 550 °C and turbine inlet temperature of 517 °C, a cycle efficiency of 42.3 % is calculated that exceeds that obtained with a traditional superheated steam cycle by one percentage point or more. A normal shutdown heat removal system incorporating a pressurized pumped S-CO 2 loop slightly above the critical pressure on each of the two intermediate sodium loops has been developed to remove heat from the reactor when the power converter is shut down. Three-dimensional layouts of S-CO 2 Brayton cycle power converter and shutdown heat removal components and piping have been determined and three-dimensional CAD drawings prepared. The S-CO 2 Brayton cycle power converter is found to have a small footprint reducing the space requirements for components and systems inside of both the turbine generator building and reactor building. The results continue to validate earlier notions about the benefits of S-CO 2 Brayton cycle power conversion for SFRs including higher efficiency, improved economics, elimination of sodium-water reactions, load following, and smaller footprint.

Validation of the ANL Plant Dynamics Code with the experimental data from integral S-CO 2 cycle facilities has been continued. Several code modifications as well as modeling approaches and assumptions were introduced to improve both the code’s capabilities in modeling the experimental loops and the agreement of the code prediction with the experimental data. The lessons learned from the code improvement and modeling experience important for the validation of the codes with the experimental data from small-scale integral loops are presented.

The ANL Plant Dynamics Code (PDC) is the current state-of-the-art capability for one-dimensional system level transient analysis of supercritical carbon dioxide (S-CO 2 ) Brayton cycle power converters. Earlier validation of code models was carried out with data from testing of individual S-CO 2 components such as a small-scale compact diffusion-bonded heat exchanger and compressor tests. The steady-state part of the PDC has been compared with experimental data from the Sandia National Laboratories (SNL) small-scale S-CO 2 Brayton cycle demonstration. In this work, predictions of the PDC code are assessed through comparison with SNL S-CO 2 loop transient data. Code modifications were needed to properly simulate the actual experimental runs due to the unique features of the small-scale SNL loop. Overall, good agreement with the measured data is predicted by the PDC, although the code predictions could be improved in some cases. Future code improvements for comparisons with future SNL loop data are identified based upon the results.

The ANL Plant Dynamics Code (PDC) for the analysis of supercritical carbon dioxide (S-CO 2 ) Brayton cycle power converters has been under development at Argonne National Laboratory for several years. It is the leading state of the art capability for modeling S-CO 2 cycles at the system level. The SAS4A/SASSYS-1 Liquid Metal Reactor Code System is the leading capability for modeling a SFR or LFR at the system level. The SAS4A/SASSYS-1 code combines reactor dynamics with thermal hydraulics calculations. Coupled together, the two codes allow users to carry out for the first time very detailed transient calculations of an entire plant incorporating a SFR with a S-CO 2 cycle energy converter, including simulation of both design and beyond-design events. The developed coupling approach utilizes the restart capability of SAS4A/SASSYS-1 and allows the data transfer between the two codes at each time step. The coupling approach is demonstrated by carrying out S-CO 2 cycle control calculations in which the entire plant follows a user-defined electrical grid demand scenario. The automatic control mechanisms on the S-CO 2 cycle side allow following of the load changes in the entire 0–100% range. On the reactor side, an autonomous (i.e., no deliberate control other than startup and shutdown) operation is assumed where the reactor power is adjusted by the internal reactivity feedbacks to match the heat removal rate by the balance-of-plant.

The ANL Plant Dynamics Code (PDC) for the analysis of supercritical carbon dioxide (S-CO 2 ) Brayton cycle power converters has been under development at Argonne National Laboratory for several years. In previous years, limited validation of the PDC models on an individual basis was carried out using experimental data obtained from facilities directed at individual components. Recently, experimental data from the SNL/BNI small-scale S-CO 2 Brayton cycle demonstration that is being assembled in a staged fashion has been provided to ANL. The loop configuration with a single turbo-alternator-compressor (TAC) was modeled with the Plant Dynamics Code and the performance prediction of individual components and the entire loop obtained from the PDC was compared with the experimental data. Overall, reasonably good agreement is obtained for steady state conditions around the loop when a value is inferred for the heat loss downstream of the turbine such that a steady state loop energy balance can be established.

The Supercritical Carbon Dioxide (S-CO 2 ) Brayton Cycle is a promising advanced alternative to the Rankine saturated steam cycle and recuperated gas Brayton cycle for the energy converters of specific reactor concepts belonging to the U.S. Department of Energy Generation IV Nuclear Energy Systems Initiative. A new plant dynamics analysis computer code has been developed for simulation of the S-CO 2 Brayton cycle coupled to an autonomous, natural circulation Lead-Cooled Fast Reactor (LFR). The plant dynamics code was used to simulate the whole-plant response to accident conditions. The specific design features of the reactor concept influencing passive safety are discussed and accident scenarios are identified for analysis. Results of calculations of the whole-plant response to loss-of-heat sink, loss-of-load, and pipe break accidents are demonstrated. The passive safety performance of the reactor concept is confirmed by the results of the plant dynamics code calculations for the selected accident scenarios.