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The supercritical carbon dioxide Brayton cycle (S-CO<SUB>2</SUB> cycle) has attracted much attention as an alternative to the Rankine cycle for sodium-cooled fast reactors (SFRs). The higher cycle efficiency of the S-CO<SUB>2</SUB> cycle results from the considerably decreased compressor work because the compressor behaves as a pump in the proximity of the CO<SUB>2</SUB> vapor-liquid critical point. In order to fully utilize this feature, the main compressor inlet condition should be controlled to be close to the critical point of CO<SUB>2</SUB>. This indicates that the critical point of CO<SUB>2</SUB> is a constraint on the minimum cycle condition for S-CO<SUB>2</SUB> cycles. Modifying the CO<SUB>2</SUB> critical point by mixing additive gases could be considered as a method of enhancing the performance and broadening the applicability of the S-CO<SUB>2</SUB> cycle. Due to the drastic fluctuations of the thermo-physical properties of fluids near the critical point, an in-house cycle analysis code using the NIST REFPROP database was implemented. Several gases were selected as potential additives considering their thermal stability and chemical interaction with sodium in the temperature range of interest and the availability of the mixture property database: xenon, krypton, hydrogen sulfide, and cyclohexane. The performances of the optimized CO<SUB>2</SUB>-containing binary mixture cycles with simple recuperated and recompression layouts were compared with the reference S-CO<SUB>2</SUB>, CO<SUB>2</SUB>-Ar, CO<SUB>2</SUB>-N<SUB>2</SUB>, and CO<SUB>2</SUB>-O<SUB>2</SUB> cycles. For the decreased critical temperatures, the CO<SUB>2</SUB>-Xe and CO<SUB>2</SUB>-Kr mixtures had an increase in the total cycle efficiency. At the increased critical temperatures, the performances of CO<SUB>2</SUB>-H<SUB>2</SUB>S and CO<SUB>2</SUB>-cyclohexane with the recompression layout were superior to the S-CO<SUB>2</SUB> cycle when the compressor inlet temperature was above the critical temperature of CO<SUB>2</SUB>.
A sodium-cooled fast reactor (SFR) is one of the strongest candidates for the next generation nuclear reactor. However, the conventional design of a SFR concept with an indirect Rankine cycle is subjected to a possible sodium-water reaction. To prevent any hazards from sodium-water reaction, a SFR with the Brayton cycle using Supercritical Carbon dioxide (S-CO<SUB>2</SUB>) as the working fluid can be an alternative approach to improve the current SFR design. However, the S-CO<SUB>2</SUB> Brayton cycle is more sensitive to the critical point of working fluids than other Brayton cycles. This is because compressor work is significantly decreased slightly above the critical point due to high density of CO<SUB>2</SUB> near the boundary between the supercritical state and the subcritical state. For this reason, the minimum temperature and pressure of cycle are just above the CO<SUB>2</SUB> critical point. In other words, the critical point acts as a limitation of the lowest operating condition of the cycle. In general, lowering the rejection temperature of a thermodynamic cycle can increase the efficiency. Therefore, changing the critical point of CO<SUB>2</SUB> can result in an improvement of the total cycle efficiency with the same cycle layout. A small amount of other gases can be added in order to change the critical point of CO<SUB>2</SUB>. The direction and range of the critical point variation of CO<SUB>2</SUB> depends on the mixed component and its amount. Several gases that show chemical stability with sodium within the interested range of cycle operating condition were chosen as candidates for the mixture; CO<SUB>2</SUB> was mixed with N<SUB>2</SUB>, O<SUB>2</SUB>, He, and Ar. To evaluate the effect of shifting the critical point and changes in the properties of the S-CO<SUB>2</SUB> Brayton cycle, a supercritical Brayton cycle analysis code with a properties program, which has the most accurate mixture models, was developed. The CO<SUB>2</SUB>-He binary mixture shows the highest cycle efficiency increase. Unlike the CO<SUB>2</SUB>-He binary mixture, the cycle efficiencies of CO<SUB>2</SUB>-Ar, CO<SUB>2</SUB>-N<SUB>2</SUB>, and CO<SUB>2</SUB>-O<SUB>2</SUB> binary mixtures decreased compared to the pure S-CO<SUB>2</SUB> cycle. It was found that the increment of critical pressure led to a decrease in cycle operating pressure ratio which resulted in a negative effect on total cycle efficiency. In addition, the effects from changed minimum operating condition and property variations of multi-component working fluid changed the recuperated heat in the cycle which was closely related to the cycle performances.
The CO<SUB>2</SUB> bubble volume fraction, eruption velocity, flash depth and mass emission of CO<SUB>2</SUB> were determined from multiple wellbore CO<SUB>2</SUB>-driven cold-water geysers (Crystal and Tenmile geysers, in Utah and Chimayo geyser in New Mexico). At shallow depths the bubble volume fraction ranges from 0 to 0.8, eruption velocities range from 2 to 20 m/s and flash depths are predominately shallow ranging from 5 to 40 m below the surface. Annual emission of CO<SUB>2</SUB> is estimated to be (4.77+/-1.92)x10<SUP>3</SUP>, (6.17+/-1.73)x10<SUP>1</SUP>, (6.54+/-0.57)x10<SUP>1</SUP>t/yr for Crystal, Tenmile and Chimayo geysers, respectively. These estimates are coherent with Burnside et al. (2013) showing that the rate of CO<SUB>2</SUB> leakage from wellbores is greater than fault-parallel or diffuse CO<SUB>2</SUB> leakage. The geyser plumbing geometry consists of a vertical wellbore which allows for the upward migration of CO<SUB>2</SUB>-rich fluids due to artesian conditions. The positive feedback system of a CO<SUB>2</SUB>-driven eruption occurs within the well. Active inflow of CO<SUB>2</SUB> into the regional aquifers through faulted bedrock allows geysering to persist for decades. Crystal geyser erupts for over 24 h at a time, highlighting the potential for a wellbore in a natural environment to reach relatively steady-state high velocity discharge. Mitigating high velocity CO<SUB>2</SUB>-driven discharge from wellbores will, however, be easier than mitigating diffuse leakage from faults or into groundwater systems.
One of the promising future nuclear energy systems, the Sodium-cooled Fast Reactor (SFR) has been actively developed internationally. Recently, to improve safety and economics of a SFR further, coupling supercritical CO<SUB>2</SUB> power cycle was suggested. However, there can be a chemical reaction between sodium and CO<SUB>2</SUB> at high temperature (more than 400<SUP>o</SUP>C) when the pressure boundary fails in a sodium-CO<SUB>2</SUB> heat exchanger. To ensure the performance of such a system, it is important to employ a cleaning agent to recover the system back to normal condition after the reaction. When sodium and CO<SUB>2</SUB> react, solid and gaseous reaction products such as sodium carbonate (Na<SUB>2</SUB>CO<SUB>3</SUB>) and carbon monoxide (CO) appear. Since most of solid reaction products are hard and can deteriorate system performance, quick removal of solid reaction products becomes very important for economic performance of the system. Thus, the authors propose the conceptual method to remove the byproducts with a chemical reaction at high temperature. The chemical reaction will take place between the reaction byproducts and a cleaning agent while the cleaning agent is inert with sodium. Thus, various sodium-based compounds were first investigated and three candidate substances satisfying several criteria were selected; sodium bromate (NaBrO<SUB>3</SUB>), sodium chlorate (NaClO<SUB>3</SUB>), and sodium tetrafluoroborate (NaBF<SUB>4</SUB>). The selected substances were thermally analyzed with the TG/DTA studies. Unfortunately, it was revealed that all candidate substances did not react with Na<SUB>2</SUB>CO<SUB>3</SUB> and decomposed before 600<SUP>o</SUP>C. However, since no study has been performed on the issue of cleaning byproducts of Na-CO<SUB>2</SUB> reaction so far, this study provides the basic guideline for the future study and suggests the future research direction and preferred characteristics of cleaning agent of sodium CO<SUB>2</SUB> reaction byproducts.
The Supercritical Carbon Dioxide (S-CO<SUB>2</SUB>) Brayton cycle has been gaining attention due to its compactness and high efficiency at moderate turbine inlet temperature. Previous S-CO<SUB>2</SUB> cycle research works in the field of nuclear engineering were focused on its application to the next generation reactor with higher turbine inlet temperature than the existing conventional water-cooled nuclear power plants. However, it was shown in authors' previous paper that the advantages of the S-CO<SUB>2</SUB> Brayton cycle can be also further applied to the water-cooled Small Modular Reactor (SMR) with a success, since SMR requires minimal overall footprint while retaining high performance. One of the major issues in the S-CO<SUB>2</SUB> Brayton cycle is the selection and design of appropriate turbomachinery for the designed cycle. Because most of the nuclear industry uses incompressible working fluids or ideal gases in the turbomachinery, a more detailed examination of the design of the turbomachinery is required for a power system that uses S-CO<SUB>2</SUB> as working fluid. This is because the S-CO<SUB>2</SUB> Brayton cycle high efficiency is the result of the non-ideal variation of properties near the CO<SUB>2</SUB> critical point. Thus, the major focus of this paper is to suggest the design of the turbomachinery necessary for the S-CO<SUB>2</SUB> Brayton cycle coupled to water cooled SMRs. For this reason, a S-CO<SUB>2</SUB> Brayton cycle turbomachinery design methodology was suggested and the suggested design methodology was first tested with the existing experimental data to verify its capability. After then, it was applied to the proposed reference system to demonstrate its capability and to provide fundamental information for the future design.
The supercritical carbon dioxide (S-CO<SUB>2</SUB>) Brayton cycle is being considered as a favorable candidate for the next generation nuclear reactors power conversion systems. Major benefits of the S-CO<SUB>2</SUB> Brayton cycle compared to other Brayton cycles are: (1) high thermal efficiency in relatively low turbine inlet temperature, (2) compactness of the turbomachineries and heat exchangers and (3) simpler cycle layout at an equivalent or superior thermal efficiency. However, these benefits can be still utilized even in the water-cooled reactor technologies under special circumstances. A small and medium size water-cooled nuclear reactor (SMR) has been gaining interest due to its wide range of application such as electricity generation, seawater desalination, district heating and propulsion. Another key advantage of a SMR is that it can be transported from one place to another mostly by maritime transport due to its small size, and sometimes even through a railway system. Therefore, the combination of a S-CO<SUB>2</SUB> Brayton cycle with a SMR can reinforce any advantages coming from its small size if the S-CO<SUB>2</SUB> Brayton cycle has much smaller size components, and simpler cycle layout compared to the currently considered steam Rankine cycle. In this paper, SMART (System-integrated Modular Advanced ReacTor), a 330MW<SUB>th</SUB> integral reactor developed by KAERI (Korea Atomic Energy Institute) for multipurpose utilization, is considered as a potential candidate for applying the S-CO<SUB>2</SUB> Brayton cycle and advantages and disadvantages of the proposed system will be discussed in detail. In consideration of SMART condition, the turbine inlet pressure and size of heat exchangers are analyzed by using in-house code developed by KAIST-Khalifa University joint research team. According to the cycle evaluation, the maximum cycle efficiency under 310<SUP>o</SUP>C is 30.05% at 22MPa of the compressor outlet pressure and 36% of flow split ratio (FSR) with 82m<SUP>3</SUP> of total heat exchanger volume while the upper bound of the total cycle efficiency is 37% with ideal components within 310<SUP>o</SUP>C. The total volume of turbomachinery which can afford 330MW<SUB>th</SUB> of SMR is less than 1.4m<SUP>3</SUP> without casing. All the obtained results are compared to the existing SMART system along with its implication to other existing or conceptual SMRs in terms of overall performance in detail.
As the usage of ion exchange resins increases the inventory of spent ion exchange resins increases in nuclear power plants. This study is to find an environmental-friendly process to treat theses spent resins. The test samples were prepared by diluting the slurry made by wet ball milling the spent cationic exchange resins for 24h. The spent cationic exchange resins were separated from mixed ion exchange resins by a fluidized bed gravimetric separator. The decomposition of the samples was investigated with super-critical water oxidation (SCWO) equipment. A statistical test method - the central composite design as a statistical design of experiments - was adopted to find the optimum condition to decompose the spent exchange resins. The optimum condition was 60% of excess oxygen, 22.5min of residence time, 0.615wt% of NaOH, 358 of reaction temperature, and 3600psi of reaction pressure, which is a sub-critical condition. The liquid product of the decomposition has the characteristics of 80-185ppm of COD (Chemical Oxygen Demand), 4.0-6.0 of pH, and <1.0ppm of corrosive components (Ni, Fe, Cr, and Mo). The exhaust gas from the SCWO equipment contained NOx of 0ppm, SOx of 3ppm (environment exhaust standard in Korea: NOx 200ppm, SOx 300ppm). Co-substituted mock samples were prepared to simulate spent cationic exchange resins from nuclear power plants which can contain radioactive Co isotopes. The conditions to obtain organic compound destruction ratio which conforms the effluent stand for the mock samples were found. The treated water filtered with 0.2-filter contained less than 1ppm of Co. Thus Co recovery rate of more 99% was achieved.
The anti-solvent approach has been demonstrated as one potential industrial method to produce pharmaceutical co-crystal powders with high purity. In this study, we combined the anti-solvent method with cooling to maximize the yield of the solution-based co-crystallization between indomethacin (IMC) and saccharin (SAC). The cooling start time was the key process parameter; other parameters were fixed based on results of preliminary work. Highly pure IMC-SAC co-crystal powders were produced via the combined method, regardless of the cooling start time, and the yield was substantially enhanced. However, some material properties, such as crystallinity and particle size, were affected by the cooling start time; i.e., whether cooling was started before nucleation (pre-nucleation cooling) or after nucleation (post-nucleation cooling). When pre-nucleation cooling was applied, a greater degree of supersaturation led to nucleation of α-IMC and IMC-SAC together. The metastable α-IMC eventually transitioned to stable IMC-SAC co-crystal particles, followed by crystal growth. When post-nucleation cooling was applied, the transient α-IMC was not detected during the entire process.
An alternative approach to monitoring the pyrochemical process (pyroprocessing) for spent nuclear fuel treatment is proposed and examined. This approach relies on modeling and the real-time analysis of process readings. Using an electrorefiner model, named ERAD, cathode potential and cell current were identified as useful process readings. To provide a real-time analysis of these two process readings, an inverse model was developed based on fundamental electrochemical relations. The model was applied to the following operating modes: pure uranium deposition, co-deposition of uranium and plutonium, and co-deposition of uranium and zirconium. Using the cell current and cathode potential, the model predicted which species were depositing and their rates. The deposition rates predicted by the inverse model compared favorably to those calculated by ERAD.
The MARS-LMR code has been developed by the Korea Atomic Energy Research Institute (KAERI) to analyze transients in a pool-type sodium-cooled fast reactor (SFR). Currently, KAERI is developing a prototype Gen-IV SFR (PGSFR) with metallic fuel. The decay heat exchangers (DHXs) and the intermediate heat exchangers (IHXs) were designed as a sodium-sodium counter-flow tube bundle type for decay heat removal system (DHRS) and intermediate heat transport system (IHTS), respectively. The IHX and DHX are important components for a heat removal function under normal and accident conditions, respectively. Therefore, sodium heat transfer models for the DHX and IHX heat exchangers were added in MARS-LMR. In order to validate the newly added heat transfer model, experimental data were obtained from the JOYO and STELLA-1 facilities were analyzed. JOYO has two different types of IHXs: type-A (co-axial circular arrangement) and type-B (triangular arrangement). For the code validation, 38 and 39 data points for type A and type B were selected, respectively. A DHX performance test was conducted in STELLA-1, which is the test facility for heat exchangers and primary pump in the PGSFR. The DHX test in STELLA-1 provided eight data points for a code validation. Ten nodes are used in the heat transfer region is used, based on the verification test for the heat transfer models. RMS errors for JOYO IHX type A and type B of 19.1% and 4.3% are obtained, respectively. It was reported that a bypass flow of 20% was allowed in type A test. When the bypass flow model was added to MARS-LMR, the RMS error for type A reduced to 12.6%. The RMS error for the STELLA-1 DHX was estimated to be 3.5%. Through the comparative studies with various alternative heat transfer models in the tube and shell-side, the base heat transfer model in MARS-LMR was found to give good prediction for the STELLA-1 and JOYO experiment results.