The safety assessment of reactor thermohydraulics under different accident scenarios needs accurate prediction of conjugate heat transfer in two-phase flow. Specifically, code prediction of flow regime, cladding temperature, and heat flux in prototypic (pressures, mass and heat fluxes) steady state and transient conditions is required for accurate assessment of system margin to CHF and possibility of core damage. There is a scarce of data available and applicable for validation of numerical codes in prediction of heat transfer at high pressures and temperatures. A test campaign aiming to generate the data for validation of STH codes modelling of two-phase flow has been carried out at the Royal Institute of Technology (KTH) in Stockholm. The results are relevant to LWRs (including SMRs), and cover two-phase flow steady state conditions including approaches to CHF. The campaign was performed on High Pressure Water Test (HWAT) facility. The facility can operate at prototypic conditions in terms of pressure, temperature, flow rate and heat flux. The setup consists of a thermohydraulic loop with 3.68 m long heated section and a condenser being the ultimate heat sink. The effective height of the main loop is nine meters. The heated section is a tube with 18.9 mm inner diameter, heated using direct current. The cases tested within the campaign cover two-phase flows at pressures reaching 12.3 MPa and thermal powers up to 1.62 MW/m². The paper provides a concise literature review on two-phase heat transfer at reactor prototypic conditions, describes the experimental setup, and methodology used to calibrate GOTHIC model. Model validation is carried out focusing on approaches to critical heat flux. Conclusion on code validity and outlook for further experimental work is provided.

An experimental setup has been designed and manufactured at the Royal Institute of Technology (KTH) in Stockholm to investigate the internal structure of boiling two-phase water flow under prototypical Light Water Reactor (LWR) core conditions, including those relevant to PWR, BWR and SMR designs. The setup is based on the High-pressure WAter Test (HWAT) loop, designed for 25 MPa pressure, 1 kg/s water mass flow rate and 1 MW thermal power. The facility has been updated with a new test section and advanced instrumentation systems to enable measurements in both forced convection and natural circulation, under steady-state and transient operations. This novel experimental setup allows for the first-time measurements of radial distributions of local two-phase flow parameters under high-pressure LWR core conditions. The resulting data is intended to enhance the fundamental understanding of boiling two-phase flow phenomena, contribute to the development of closure laws (including for polydispersed flow) and support the validation of computational codes (1-D and 3-D). The paper presents the loop design, the updated instrumentation with associated uncertainties, and data post-processing methods (including the derivation of dispersed phase length scales). Results from commissioning tests, such as heat balance tests and single-phase tests, are presented. Examples of high-pressure boiling two-phase flow measurements are presented and discussed. Fundamental behavior and associated key parameters, including radial distributions of void fraction, mixture velocity, interfacial length scales and polydispersed characteristics, are identified and quantified.

A benchmark activity on two-fluid simulations of high-pressure boiling upward flows in a pipe is performed by 12 participants using different MCFD (Multiphase Computational Fluid Dynamics) codes and closure relationships. More than 30 conditions from DEBORA experiment conducted by CEA are considered. Each case is characterised by the flow rate, inlet temperature, wall heat flux and outlet pressure. High-pressure Freon (R12) at 14 bar and 26 bar is boiled in a 19.2mm pipe heated over 3.5m. Flow rates range from 2000 kg m−2 s−1 to 5000 kg m−2 s−1 and exit quality x ranges from single-phase conditions to x=0.1 which leads to a peak void fraction of α=70%. In these high pressure conditions, bubbles remain small and there is no departure from the bubbly flow regime (François et al., 2011; Hösler, 1968). However, different kind of bubbly flows are observed: wall-peak, intermediate peak or core-peak, depending on the case considered. Measurements along the pipe radius near the end of the heated section are compared to code predictions. They include void fraction, bubble mean diameter, vapour velocity and liquid temperature. The benchmark covered two phases. In the first phase of the benchmark activities, experimental data were given to the participants, allowing to compare the simulation results and to develop, to select or to adjust the models in the CMFD codes. The second phase included blind cases where the participants could not compare to the measurements. In between the two phases, possible additional model adjustments or calibrations were performed. Overall, the benchmark involved very different closures and a wide range of models’ complexity was covered. Yet, it is extremely difficult to have a robust closure for all conditions considered, even knowing experimental measurements. The wall-to-core peak transition is not captured consistently by the models. The degree of subcooling and the void fraction level are also difficult to assess. We were not capable of showing superiority of some physical closures, even for part of the model. The interaction between mechanisms and their hierarchy are extremely difficult to understand. Although departure from nucleate boiling (DNB) was not considered in this benchmarking exercise, it is expected that DNB predictions at high-pressure conditions depend strongly on the near-wall flow, temperature, and void fraction distributions. Therefore, the suitability of the closures also limits the accuracy of DNB predictions. The benchmark also demonstrated that in order to progress further in models development and validation, it is compulsory to have new measurements that include simultaneously as many variables as possible (including liquid temperature, velocity, cross-correlations and wall temperature); also, a better knowledge of the local bubble sizes distributions is the key to discriminate performances of interfacial area modelling (IATE, MUSIG or iMUSIG models, considering for instance the possibility of two classes of bubbles having totally different behaviour regarding the lift force). Following this benchmark impulse, we hope that future activities will be engaged on high-pressure boiling water experiments with a continuation of models’ comparisons and development.

Flow-accelerated corrosion and erosion (FACE) phenomena can be crucial for performance of structural elements in heavy liquid metal (HLM) cooled reactor systems. Existing experimental observations indicate that turbulent flow characteristic can affect FACE, but there is no quantitative data that can be used for model development and validation. Main recirculation pump impellers, which operate at high relative velocities and rotational flow conditions can be especially vulnerable to FACE. For comparison, the core internals operate at lower velocities and in axial flow conditions, but at higher temperatures and neutron fluence. Hence, systematic experimental data is needed to improve our knowledge on FACE phenomena. The Separate Effect Test Facility for Flow-Accelerated Corrosion and Erosion (SEFACE) is designed to obtain such experimental data including high relative velocities (up 20 ms−1) and high temperatures (400 to 550 °C) of liquid lead. This article focuses on the hydrodynamic design of SEFACE. The aim of the design is to achieve well defined flow conditions for experiments and ensure safe operation of the facility. First, we examine three design concepts (i.e., forced convection loop, rotating cylinder, and rotating disk) and motivate the choice of the rotating disk approach for SEFACE. Second, we discuss different design options, i.e., a confined rotor–stator test chamber and the unconfined rotating disk configuration. We used Reynolds-Averaged Navier Stokes (RANS) calculations to identify and solve the issues stemming from the high rotational speed. These include, for instance, lead free surface deformation, radial pressure buildup, and axial bending forces due to asymmetric test chamber. The CFD-derived torque and power predictions in rotor–stator and rotating disk systems are verified with selected empirical turbulent friction factor correlations or/and DNS calculations. We demonstrate that the developed hydrodynamic design of SEFACE solves identified issues and enables obtaining experimental data under well-defined flow conditions. The findings are deemed to also be applicable to the design of rotating disk-type FACE installations for other liquid mediums.

Long-term material compatibility in heavy liquid metal (HLM) remains a challenge for the successful deployment of HLM-based technologies. Flow-accelerated corrosion and erosion (FACE) phenomena can lead to continual material deterioration, which needs to be considered throughout the reactor design stage. Nonetheless, known experimental data are inadequate to cover all the prototypical flow regimes during LFR's operation. Modelling of the FAC/FACE phenomena remains mostly in lumped parameter/subchannel scales, where the FAC model is coupled to the bulk flow of the pipe or subchannel. These methodologies might produce a sufficient prediction for the core internals; however, this might not be suitable for the pump impeller due to comparatively greater relative velocity and the occurrence of transient flow patterns near the rotating impeller. To establish an understanding of the connection between turbulence and FACE, the liquid lead-based Separate Effect Flow Accelerated Corrosion and Erosion (SEFACE) facility is currently under design at KTH in the framework of the Sustainable Nuclear Energy Research In Sweden (SUNRISE) project. SEFACE attempts to investigate FACE phenomena in the liquid lead and produce quantifiable validation data for model development. The paper divides itself into two parts. Part I refers to the study of operational conditions in SEFACE via Reynolds Averaged Navier Stokes (RANS) simulation, while Part II deals with the recent attempt on modelling time-dependent flow shear on rotating disks based on large eddy simulation (LES). The paper begins with a brief review of prior studies on flow-accelerated corrosion. Following that, the SEFACE facility's design concept is laid out considering several physical and operational constraints. A periodic wedge of the SEFACE test chamber is chosen to examine the facility's time-averaged behaviour. The k-ω shear stress transport (SST) model was employed for the simulations. The torque prediction on the rotating disk system is verified with the empirical frictional factor prediction. The latest hydrodynamic design enables SEFACE to be spun at 1200 revolutions per minute (corresponding to a maximum velocity of 21 m/s) without causing free surface deformation or excessive pressure. SEFACE permits the collecting of experimental data under the effect of various relative velocities in a single experiment round. The second part of the paper focuses on a recent attempt to determine the wall shear stress distribution on a rotating disk using wall-modelled large eddy simulation (WMLES S-Omega). The obtained amplitude and frequency of wall shear stress fluctuations will aid model development in future.

The EU Horizon 2020 project McSAFER was launched in 2020 to advance the safety research of small modular reactors (SMR) via thermal-hydraulic experiments with related code validation and numerical coupled multi-physics and multi-scale simulations. This paper provides an overview of the thermal-hydraulic experiments and code validation within the project and presents the status of these activities highlighting some of the results already obtained. Experiments are performed at three European laboratories with test facilities dedicated for the investigation of SMR-relevant phenomena. Fundamental heat transfer experiments with boiling up to critical heat flux under forced convection for rod configurations and conditions representative of SMRs are performed with the COSMOS-H facility. Performance of the helical coil steam generator, core cross flow phenomena and the general behavior of an SMR operating with natural circulation are investigated with the MOTEL facility. Heat transfer phenomena are investigated in conditions relevant for SMRs also in forced to natural circulation transients with the HWAT facility. Selected thermal hydraulic system, subchannel and computational fluid dynamics (CFD) codes are validated with the experimental data.

Background: The heaviest T-z = 0 doubly-magic nucleus, Sn-100, and the neighboring nuclei offer unique opportunities to investigate the properties of nuclear interaction. For instance, the structure of light-Sn nuclei has been shown to be affected by the delicate balance between nuclear-interaction components, such as pairing and quadrupole correlations. From Cd to Te, many common features and phenomena have been observed experimentally along the isotopic chains, leading to theoretical studies devoted to a more general and comprehensive study of the region. In this context, having only two proton holes in the Z = 50 shell, the Cd isotopes are expected to present properties similar to those found in the Sn isotopic chain. Purpose: The aim of this work was to measure lifetimes of excited states in neutron-deficient nuclei in the vicinity of Sn-100. Methods: The neutron-deficient nuclei in the N approximate to Z approximate to 50 region were populated using a multinucleon transfer reaction with a Cd-106 beam and a Mo-92 target. The beamlike products were identified by the VAMOS++ spectrometer, while the gamma rays were detected using the AGATA array. Lifetimes of excited states were determined using the recoil distance Doppler-shift method, employing the Cologne differential plunger. Results: Lifetimes of low-lying states were measured in the even-mass Cd-102-(108) isotopes. In particular, multiple states with excitation energy up to MeV, belonging to various bands, were populated in approximate to 3 Cd-106 via inelastic scattering. The transition strengths corresponding to the measured lifetimes were compared with those resulting from state-of-the-art beyond-mean-field calculations using the symmetry-conserving configuration-mixing approach. Conclusions: Despite the similarities in the electromagnetic properties of the low-lying states, there is a fundamental structural difference between the ground-state bands in the Z = 48 and Z = 50 isotopes. The comparison between experimental and theoretical results revealed a rotational character of the Cd nuclei, which have prolate-deformed ground states with beta(2) approximate to 0.2. At this deformation Z = 48 becomes a closed-shell configuration, which is favored with respect to the spherical one.

In the present study, a multi-domain coupled computational fluid dynamics (CFD) approach is developed for the modeling of dryout and post-dryout heat transfer. For the fluid part, the thin film and gas core are modeled by the liquid film model and two-fluid model, respectively. For the solid part, the heating process is modeled by either using a constant heat source or solving the Joule heating problem. The fluid-solid conjugate heat transfer is calculated by using carefully designed coupling schemes which can automatically determine the operation mode for pre- and post-dryout regions. Unlike standalone simulations where only the inner wall temperature is predicted, coupled simulations are able to predict the outer wall temperature, allowing a direct comparison with experiments. Simulations were carried out for a wide range of flow conditions and validated against the corresponding steady state experiments. By newly introducing a film rewetting model, the current CFD code is capable of simulating the transient behavior of dryout. With the rewetting model, the coupled code successfully predicted the dryout hysteresis.

Lifetimes of negative-parity states have been determined in the neutron deficient semi-magic (N = 50) nucleus Rh-95. The fusion-evaporation reaction Ni-58(Ca-40, 3p) was used to populate high-spin states in Rh-95 at the Grand Accelerateur National d'Ions Lourds (GANIL) accelerator facility. The results were obtained using the Doppler Shift Attenuation Method (DSAM) based on the Doppler broadened line shapes produced during the slowing down process of the residual nuclei in a thick 6 mg/cm(2) metallic target. B(M1) and B(E2) reduced transition strengths are compared with predictions from large-scale shell-model calculations. state-of-the-art theory. Remarkably, the structural features up to moderate angular momentum of nuclei immediately below the N = Z = 50 shell closures can be described with high accuracy in a very simple way by shell-model calculations including only the g(9/2) and p(1/2) subshells. Of special interest is the neutron-proton pair coupling scheme which is expected to appear in the heaviest N=Z nuclei [1,2] and the seniority structure of the N = 50 isotones [3-7]. However, multiple core-excited states have been observed in the semi-magic nuclei of the Sn-100 region [8-10]. The theoretical study of those states is a challenging task, which requires a significantly larger model space for their interpretation. Transition probabilities between nuclear states provide important constraints for theoretical modelling of the structure of the nuclei of interest. Our previous lifetime study of the semimagic (N = 50) nucleus Ru-94 [ 11,12] provided information on the electromagnetic decay properties of neutron-core excited states. We now address lifetime measurements in its closest, more neutron deficient, isotone Rh-95 using the same DSAM technique. The experimental results have been interpreted within the framework of large-scale shell-model (LSSM) calculations.

One new pair of positive-parity chiral doublet bands have been identified in the odd-A nucleus Nd-135 which together with the previously reported negative-parity chiral doublet bands constitute a third case of multiple chiral doublet (M chi D) bands in the A approximate to 130 mass region. The properties of the M chi D bands are well reproduced by constrained covariant density functional theory and particle rotor model calculations. The newly observed M chi D bands in Nd-135 represents an important milestone in supporting the existence of M chi D in nuclei.