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• New experimental sCO 2 dataset with 196 experiments and 8950 data points. • Thermal inflow lengths were quantified with up to 480 d i for fully developed flow. • Experiments categorised in normal, enhanced and deteriorated heat transfer regimes. • Heat transfer deterioration in the new dataset is caused by buoyancy effects. • Two heat transfer correlations predict the experimental data with 30 % accuracy. Supercritical CO 2 shows great potential as a working fluid in power plant cycles due to its moderate critical pressure of 7.38 MPa and critical temperature of 30.98 °C, closely matching typical heat sink temperatures. Understanding the heat transfer characteristics of sCO 2 is crucial for designing cycle components. This publication presents a systematic analysis of sCO 2 heat transfer in two heated vertical pipes with 4 and 8 mm inner diameters, with both upward and downward flow at pressures of approximately 7.75, 8.00, and 9.50 MPa, and flow inlet temperatures between 5 and 40 °C, based on 196 experiments. The study investigates a range of mass fluxes from 400 to 2000 kg/m2s and heat fluxes from 10 to 195 kW/m2, resulting in a heat to mass flux ratio of 6 to 275 J /kg. The findings reveal enhanced, normal, and deteriorated heat transfer within the experimental dataset. Buoyancy effects are identified as the main cause of deteriorated heat transfer in the investigated parameter range, with flow acceleration showing no significant influence on heat transfer. A new proposed dimensional criterion categorises 36 out of 119 experiments with upward flow in the deteriorated heat transfer regime, accompanied by temperature peaks rising to 47 K. Thermal inflow lengths range from 0 to 480 inner pipe diameters, with some experiments not achieving a thermally fully developed flow over the entire pipe length. Based on a comparison with approximately 8950 experimental data points, two Nusselt correlations are found, capable of reproducing the experimental results with a mean absolute deviation of around 30 %. This publication provides valuable data for validating numerical models and developing correlations to predict the heat transfer of supercritical fluids. [ABSTRACT FROM AUTHOR]

Nuclear research reactors (RR) are essential facilities in countries implementing nuclear power plants and are used for experiments necessary for commercial reactor development, training and education programs, and many other applications not related to nuclear energy production (e.g., isotope production, neutron sources, materials science). Europe has a broad and very diverse landscape of RRs, many of which have been in operation for 30-60 years, are well maintained and regularly modernized. However, financial pressures caused by a combination of declining interest and the lack of a sound financial model have led to the closure of many of them (e.g. OSIRIS in Saclay, JEEP II research reactor at IFE Kjeller and BER2 in Berlin). These negative trends called for coordinated European action to assess the impact of the declining number of RRs. The Towards Optimized Use of Research Reactors (TOURR) project was a response to this challenge. Its main objective was to assess the status of the EU RR fleet and to develop a strategy for the refurbishment and construction of new RR in Europe. The assessment was based on analysed data obtained through extensive questionnaires sent to all operating European RR. The analysis revealed gaps in terms of lack of long-term funding, lack of manpower and lack of communication between RRs and their customers. It also showed threats of further European RR closures. Regarding the long-term EU RR strategy, the main recommendations of the TOURR project are to build (at least) two RRs, a medium-flux multipurpose reactor and a flexible zero-power facility. Both reactor cores could be part of a single facility built at the European level and accessible to all EU Member States. • Current status of the European Research Reactor fleet analysed. • Main gap in EU RR utilization is the lack of manpower and lack of financing. • RR research opportunities in new fields identified (e.g. processing of chemicals) • At least one new multipurpose medium-flux reactor is needed in EU. • In addition, results indicate the need for flexible zero-power reactor. [ABSTRACT FROM AUTHOR]

The supercritical carbon dioxide (sCO 2) heat removal system, which is based on multiple closed Brayton cycles with sCO 2 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 study considers different sudden failures during the accident progress, e.g. failure of single sCO 2 cycles, control systems and valves. These abnormal conditions were investigated with the thermal-hydraulic system code ATHLET for a generic Konvoi pressurized water reactor. In most cases, the failure of a single sCO 2 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 can 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. • Transient simulation of a supercritical carbon dioxide heat removal system. • Successful operation in interaction with a pressurized water reactor. • Analysis of different sudden failures, e.g. of single cycles, control systems and valves. • Some failures are mitigated by the applied robust design of the heat removal system. • For other failures, fast interventions or further design or control improvements are proposed. [ABSTRACT FROM AUTHOR]

This publication provides a dataset of 54 experiments, carried out with supercritical CO 2 (sCO 2) in two heated horizontal pipes with inner diameters of 4 and 8 mm at a pressure of approximately 7.75 MPa. The mass fluxes were set to 400 and 800 kg/m2s and the heat flux was varied from 30 to 130 kW/m2, resulting in a heat to mass flux ratio of 38–225 J/kg. The influences of the inner pipe diameter, the Reynolds number and the heat to mass flux ratio on the thermal stratification were studied. Temperature differences up to 90 K between the bottom and the top of the pipe have been detected. By varying the bulk fluid inlet temperature, the thermal inflow lengths have been quantified for a fully developed temperature stratification with up to 180 inner pipe diameters. The Petukhov and Richardson buoyancy criteria show that the experimental results are strongly influenced by buoyancy. For the Jackson buoyancy criterion, a new threshold of higher than 160 is proposed. The validation of four empirical Nusselt correlations shows that the Bishop correlation has the lowest total average deviation of 52%, especially for the bottom site of both pipes with an average deviation of 29%. • 54 Experiments with heated horizontal sCO 2 flows in pipes were carried out. • Thermal stratifications were investigated with temperature differences up to 90 K. • Thermal inflow lengths were quantified with up to 180 d i for fully developed flows. • Three different criteria for buoyancy and one for flow acceleration were tested. • Four different heat transfer correlations were validated against experimental data. [ABSTRACT FROM AUTHOR]

In this publication, the cooling heat transfer coefficient (h t c) 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 sCO 2 , the mixed convection criterion is used to evaluate the heat transfer deterioration. The effects of the CO 2 mass flux of 141 − 354 k g / m ² s and bulk fluid temperatures of 20 − 50 ° C with a constant pressure of 80 b a r on the heat transfer were examined. The upwards flow shows a steady decrease in the h t c 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 h t c at the pseudocritical temperature (T p c) disappears. The deteriorated heat transfer in the downwards flow showed significant lower wall temperatures compared to the upwards flow. The separate evaluation of the liquid-like and gas-like region indicates that the deterioration may vary based on the fluid properties. The results were regressed within +-15% in the forced convection region and +-40% in the mixed convection region. The comparison with the literature data of vertical cooling in 6 mm showed different behaviour which indicates an influence of the tube diameter. • Heat transfer of sCO2 depends strongly on the flow direction. • The flow direction influences the heat transfer much stronger at low mass flux. • Heat transfer deterioration leads to a strong decrease in the wall temperature. [ABSTRACT FROM AUTHOR]

The supercritical carbon dioxide (sCO 2 1) heat removal system, which is based on a closed Brayton cycle with sCO 2 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. • Design, layout and control of a supercritical carbon dioxide heat removal system. • Self-propelling operational readiness enables fast start-up. • Compressor inlet temperature may be kept constant at any boundary condition. • New turbomachinery design with a higher surge margin is preferred. • Successful operation in interaction with a pressurized water reactor. [ABSTRACT FROM AUTHOR]