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The supercritical carbon dioxide (sCO21) heat removal system, which is based on multiple closed Brayton cycles with sCO2 as the working fluid, is an innovative, self-propelling and modular heat removal system for existing and future nuclear power plants. Previous studies analysed its design, layout, control and operation. In addition, this novel study considers different sudden failures during the accident progress, e.g. failure of single sCO2 cycles, control systems and valves. These abnormal conditions were investigated with the thermalhydraulic system code ATHLET for a generic Konvoi pressurized water reactor. In most cases, the failure of a single sCO2 cycle can be compensated. On the one hand, failure of the fans of the gas cooler leads to a pressure increase which may be mitigated by an inventory control system or cycle shutdown. On the other hand, unintended fan speed-up may cause compressor surge without adequate countermeasures. Furthermore, the system can operate under the cyclic blow-off from the steam generator safety valves when the relief valves are not available. Finally, the unintended closure of the valve which controls the steam flow through the compact heat exchanger triggers a fast cycle shutdown but a subsequent restart might be possible., Conference Proceedings of the European sCO2 Conference5th European sCO2 Conference for Energy Systems: March 14-16, 2023, Prague, Czech Republic, p. 11

The supercritical carbon dioxide (sCO21) heat removal system, which is based on a closed Brayton cycle with sCO2 as a working fluid, is an innovative heat removal system for existing and future nuclear power plants. This paper provides the design, layout and control of the system based on assumptions developed in the project sCO2-4-NPP. A self-propelling operational readiness state enables a fast start-up and consumes only 12% of the design thermal power input. The system is analysed over a wide range of ambient and steam-side conditions in ATHLET, using performance maps for the turbomachinery, which were designed recently. The performance analysis suggests that it is a good option to operate the system at the design compressor inlet temperature of 55 °C at any boundary condition. With decreasing thermal power input, the rotational speed of the turbomachinery must be decreased to keep the system self-propelling. Moreover, the turbomachinery design with a higher surge margin is preferred. By controlling the compressor inlet temperature via the air mass flow rate and turbine inlet temperature via the turbomachinery speed, the heat removal system is successfully operated in interaction with a pressurized water reactor.

Innovative heat removal systems are currently investigated for use in existing and future nuclear power plants. One of them is the supercritical carbon dioxide (sCO21) heat removal system, which is based on a closed Brayton cycle with sCO2 as a working fluid. This paper provides the design and layout of the sCO2 cycle based on assumptions developed in the project sCO2-4-NPP. The system is analysed over a wide range of ambient and steam-side conditions in ATHLET, using performance maps for the turbomachinery, which were designed recently. Bypasses are considered in the layout of the cycle to cope with special operation conditions, e.g. start-up. Different operational readiness states for the system are shown, which enable a fast start-up of the system. Air mass flow rate control is implemented to keep the compressor inlet temperature constant with controller parameters depending on the ambient temperature. The performance analysis of the system suggests that it is a good option to operate the system at the design compressor inlet temperature of 55 °C at any ambient or steam-side boundary condition. With decreasing thermal power input, the rotational speed of the turbomachinery must be decreased to keep the system self-propelling. Turbomachinery design with a higher surge margin is preferred and different operation strategies are feasible and need to be tested in interaction with the nuclear power plant. 1sCO2 is defined as carbon dioxide at supercritical conditions with p > 73.8 bar and T > 31 °C, Conference Proceedings of the European sCO2 Conference4th European sCO2 Conference for Energy Systems: March 23-24, 2021, Online Conference, p. 14

In the frame of EU-project sCO2-flex the design of a 25 MWe supercritical CO2 (sCO2) Brayton cycle will be designed. The system will be optimized to meet flexibility requirements, while reducing environmental impact and focusing on cost efficiency. In the context of a sCO2 Brayton cycle, the gas cooler is a key component to achieve a high overall efficiency. Close to the critical point, due to varying properties, heat transfer and pressure drop of carbon dioxide (CO2) are difficult to predict. In case of vertical flow, acceleration and buoyancy effects induced by strong density gradients can cause a significant deterioration of the heat transfer. In this publication, the cooling heat transfer coefficient (htc) is investigated in a 3 mm diameter tube with vertical flow orientation. Commonly used calculation methods of the heat transfer coefficient are presented. Although developed for heating of sCO2, the mixed convection criterion of Jackson and Hall [7] is used to evaluate the heat transfer deterioration. The effects of the CO2 mass flux of 141 − 354 kg/m2s and bulk fluid temperatures of 20 − 50 °C with a constant pressure of 80 bar on the heat transfer were examined. The transition between forced and mixed convection can be explained by the htc -values. The upwards flow shows a steady decrease in the htc with the reduction of the mass flux. However, the downwards flow shows significant effects of buoyancy. At low mass flux the distinct peak in the htc at the pseudocritical temperature (Tpc) disappears., Conference Proceedings of the European sCO2 Conference4th European sCO2 Conference for Energy Systems: March 23-24, 2021, Online Conference, p. 146

This study proposes preliminary guidelines for the design and operation of a supercritical carbon-dioxide (sCO21) heat removal system for nuclear power plants. Based on a thermodynamic optimization the design point is calculated incorporating an existing small-scale compressor map. The behavior of the cycle is tested under varying boundary conditions on the steam side of the compact heat exchanger. The simulations are carried out with the thermal-hydraulic system code ATHLET, which has been extended for the simulation of sCO2 power cycles. The extensions include the thermodynamic properties, heat transfer and pressure drop correlations as well as performance map based turbomachinery models, which take the real gas behavior of sCO2 into account. During the decay heat transient, compressor surge occurs in some of the simulated cases. In order to avoid compressor surge and to follow the decay heat curve, the compressor speed is reduced together with the steam temperature. This enables to operate one single system down to a thermal load of less than 50 % even under the design restriction caused by the application of the existing compressor performance map. 1 sCO2 is defined as carbon dioxide at supercritical conditions with p > 73.8 bar and T > 31 °C, Conference Proceedings of the European sCO2 Conference4th European sCO2 Conference for Energy Systems: March 23-24, 2021, Online Conference, p. 400

In the frame of the sCO2-QA (qualification of analysis tools for the evaluation of a residual heat-driven, self-sufficient system for decay heat removal) project, a residual heat-driven self-sufficient sCO2-operated decay heat removal system based on a Brayton cycle is simulated with the German thermal-hydraulics system code ATHLET (analysis of thermal-hydraulics of leaks and transients). The heat removal system consists of a compact heat exchanger in the containment, a turbo-compressor system located in the reactor building and a gas cooler in the outdoor area. The validation of ATHLET and other numerical codes as well as understanding the heat transfer characteristics of sCO2 near the critical point requires experimental data. At IKE (Institute of Nuclear Technology and Energy Systems), the SCARLETT (supercritical carbon dioxide loop at IKE Stuttgart) test facility is available for various experiments with sCO2. This publication includes an experimental investigation of the thermal stratification in heated horizontal sCO2 pipe flows. For this investigation, eight test series with overall 48 experiments were carried out in two pipes with inner diameters of 4 mm and 8 mm. The experiments were carried out at a pressure of approximately 7.75 MPa. The target values of mass flux were set at 400 kg/m2s and 800 kg/m2s and those of heat flux at 50, 90 and 130 kW/m2, resulting in a heat to mass flux ratio of 62.5-225 J/kg. The inlet Reynolds numbers are between 16000 and 120000. The measured parameters are the flow rate, the pressure, the inlet and outlet fluid temperature as well as the outer surface temperature along the test pipe in three different radial angles. The results show the influence of the pipe diameter, Reynolds number, mass and heat flux on the temperature stratification. Also, inflow lengths were determined for a fully developed temperature stratification. This data set can be used for the validation of computer codes., Conference Proceedings of the European sCO2 Conference4th European sCO2 Conference for Energy Systems: March 23-24, 2021, Online Conference, p. 38

The sCO2 sCO2-4-NPP European project aims to develop an innovative technology based on supercritical CO2 (sCO2) for heat removal to improve the safety of current and future nuclear power plants. The heat removal from the reactor core will be achieved with multiple highly compact self self-prope llant, self self-launching, and self self-sustaining cooling system modules, powered by a sCO2 Brayton cycle. Heat exchangers are one of the key components required for advanced Brayton cycles using supercritical CO2. To this end, two compact and highly effective brazed plates and fins heat exchangers are designed: a main heat recovery heat exchanger (CHX), wh which allows the heat transfer directly from the steam generator to the sCO2 sCO2-4-NPP cycle, a heat sink exchanger (DUHS), which evacuate the remaining heat to the atmosphere. An important work has been achieved in the frame of this project to conceive the pre liminary design of these components, in close collaboration between Fives Cryo, a French brazed plates and fins heat exchangers manufacturer, the Institut für Kernenergetik und Energiesysteme (IKE) of University of Stuttgart and KSG/GFS Institute, a simula tor center, both in Germany. In fact, several constraints needed to be taken into account. For instance, for the DUHS, the low fans power available for the cycle and the necessary air flow for effective heat exchange implies considering almost inexisting pressure drops on the air side. To this end, very specific design ideas has been adressed to meet the desired thermal duty. Also, this project benefits from the recent results achieved among the european project sCO2-flex, related to the mechanical resistance of heat exchanger components, the assembly process and their thermal and hydraulic performances, along with Fives Cryo expertise and background. A second challenge of the sCO2-4-NPP project is to qualify the designed plates and fins heat exchangers, at cycle operating conditions, in order to meet with pressure vessels codes and regulations according to nuclear requirements. This paper presents the work achieved on the design of DUHS heat exchanger components and a preliminary part of the qualification of this equipment according to nuclear power plants regulations., Conference Proceedings of the European sCO2 Conference4th European sCO2 Conference for Energy Systems: March 23-24, 2021, Online Conference, p. 274

In the frame of EU-project sCO2-flex the design of a 25 MWe supercritical CO2 (sCO2) Brayton cycle will be developed. The system will be optimized to meet flexibility requirements, while reducing environmental impact and focusing on cost efficiency. In the context of a sCO2 Brayton cycle, the gas cooler is a key component to achieve a high overall efficiency. Close to the critical point, due to varying properties, heat transfer and pressure drop of carbon dioxide (CO2) are difficult to predict. By performing experiments with the “SCARLETT” facility of the University of Stuttgart, expertise will be gained using CO2 as working fluid in the pseudocritical region. The results of the fundamental experiments will be used for validation and improvement of correlations to develop heat exchangers working with high efficiency. The heat transfer and pressure drop of carbon dioxide near the critical point cooled in a 2 mm diameter was investigated. The outer wall surface temperature is determined by soldering calibrated T-type thermocouples into the copper tube wall. Thermocouples are evenly distributed along the 1.2 m cooled length. The effects of the CO2 mass flux of 400-1400 kg/m²s, inlet pressures of 7.7-8.5 MPa, bulk fluid temperatures of 10-85 °C and the flow orientation (upward, downward and horizontal) on the heat transfer and pressure drop were examined., Conference Proceedings of the European sCO2 Conference3rd European Conference on Supercritical CO2 (sCO2) Power Systems 2019: 19th-20th September 2019, p. 255

Conference Proceedings of the European sCO2 Conference3rd European Conference on Supercritical CO2 (sCO2) Power Systems 2019: 19th-20th September 2019, p. 39

Conference Proceedings of the European sCO2 Conference3rd European Conference on Supercritical CO2 (sCO2) Power Systems 2019: 19th-20th September 2019, p. 30

The supercritical CO2 heat removal system is a very innovative reactor heat removal concept as it improves the safety of both currently operating and future BWRs and PWRs through a self-propellant, self-sustaining and self-launching, highly compact cooling system powered by an integrated Brayton-cycle using supercritical carbon dioxide (sCO2) as its working fluid. Since this system is powered by the decay heat itself, it provides new ways to deal with beyond design accidents. The turbine of a Brayton-cycle provides more energy than necessary to drive the compressor, which means that the sCO2-HeRo system provides electricity in addition. Therefore, this system can be an excellent backup cooling system for the reactor core in case of a Fukushima-like scenario, with a combined station blackout (SBO), loss of ultimate heat sink (LUHS) and loss of emergency cooling. In addition, this system might also be used as a heat removal system for the reactor in hot stand-by condition, removing the decay heat by keeping the reactor pressure vessel at operation temperature and pressure. The system is developed within a EU funded project called "Supercritical CO2 Heat Removal System, sCO2-HeRo". The objective of this project is to build a small-size demonstrator and install it at the PWR glass model at GfS in Essen, Germany. By means of this down-scaled demonstration unit, important operational data will be gained to demonstrate the feasibility of this heat removal system. OA platinum

Conference Proceedings of the European sCO2 Conference3rd European Conference on Supercritical CO2 (sCO2) Power Systems 2019: 19th-20th September 2019, p. 125

The High Performance Light Water Reactor is a nuclear reactor concept of the 4th generation which is cooled and moderated with supercritical water. The concept has been worked out by a consortium of European partners, co-funded by the European Commission. It features a once through steam cycle, a pressure vessel type reactor, and a compact containment with pressure suppression pool. The conceptual design enables to assess its feasibility, its safety features and its economic potential.

The aim of this work is to investigate important aspects of ex-vessel molten corium cooling by bottom flooding through a porous concrete core catcher. These are the transport of cooling water through the porous concrete to the melt and the fragmenta-tion and porosity formation in the melt due to the interaction with cooling water/steam. Firstly, this work investigates the hydraulic parameters of the porous concrete core catcher. Cooling water flow simulations are performed by means of the code COCOMO3D for the passive distribution of the cooling water into the melt layer from below with sufficient flow rates over the large reactor cavity. These investigations show that the permeability values of two different layers of the porous concrete and their relation to one another have a significant effect on the rising superficial velocity of the cooling water and on the required pressure head for the coolant supply. A methodology is developed in this work in order to optimize the core catcher for hydraulic properties, which can be applied to various boundary conditions. Based on this methodology, for the inlet configuration that provides the cooling water around to the core catcher over the whole perimeter, various concrete pairings can be chosen to provide sufficient cooling water into the molten corium uniformly, with a feasible pressure head. Due to the restrictions on the design for back fitting, in these cases the water supply can be provided to the core catcher only from a very small inlet connection. Thus, very high velocities of cooling water are expected around the inlet region, and the linear friction laws are not adequate anymore. To gather the needed data for improving the modelling a dedicated experiment set-up is built within the framework of this work, and the relation between pressure and superficial velocity of water for porous concrete samples from CometPC core catcher, which are provided by KIT, is measured. The falling head method is used which enables the measurements for a wide range of pressure values. The non-linear friction law with the values for permeability and passability obtained from the measurements is then implemented into COCOMO3D. The simula-tions are performed for restricted water inlet case with the quadratic friction law for porous concretes. These simulations show that back fitting of the porous concrete core catcher device with limited water inlet configuration can raise many challenges. Increasing the area of the water inlet and providing the water uniformly from the perimeter of the porous core catcher device is the more feasible approach for the reactor application. Finally, the fragmentation and porosity formation phenomenon caused by bottom flooding is modelled in this work. The initial molten corium-steam interaction is assumed as the decisive phenomenon having a lasting effect on the fragmentation. Therefore, the fragmentation is modelled as a void fraction modeling of the two-phase flow of molten corium and steam in thermal equilibrium. In order to simulate this two-phase flow a new model for the interfacial friction force for molten corium-steam two-phase flow has been necessary. A new Bubbly-Channel two-phase flow interfacial friction force model is developed in this work in order to model the porosity formation during corium cooling by bottom flooding. With a stand-alone simulation program, this new model is validated against the COMET and CometPC experiments with varying boundary conditions and material properties. The results show very good agreement with the final porosity range achieved in the experiments. The new interfacial friction model and the material properties of liquid corium are implemented into COCOMO3D code. The COCOMO3D simulations of twophase flow of molten corium and steam are performed for CometPC plus experiment data. The flow pattern of steam and melt during these simulations shows good replication of the post-test morphology of solidified porous thermite from the experiments as well as the dispersion and ejection that happened during the experiment. Two further cases are simulated for the reactor case, which show that the partial ablation of the sacrificial concrete between core catcher and molten corium has an effect on the coolant distribution in the compact melt layer. These investigations show that being able to simulate molten corium as a moving liquid, due to the new model, provides more realistic modelling of coolant ingression into the compact melt layer via bottom flooding and the space that coolant actually occupies, hence the fragmentation phenomenon. The two-phase flow modelling of molten corium and steam presented in this work can be extended further in the future to three-phase flow of molten corium-steam-water with evaporation phenomenon in order to model the entire cooling and solidification process by bottom cooling.