Source term evaluation is an important element in the assessment of efficiency of a Severe Accident Management (SAM) strategy. The identification of phenomena and parameters that present major contributions to the uncertainty in the magnitude and timing of the releases and quantify the uncertainty is vital for comprehensive risk analysis. In this work source term evaluation was performed using MELCOR for two accident scenarios, large break LOCA and station blackout, that leads to containment failure due to ex-vessel phenomena such as formation of non-coolable debris bed and steam explosion. Cases without containment failure were also analyzed for the effect of melt debris release characteristics on fission product release in both accident scenarios. It was observed that initial vessel failure mode was due to failure of the penetrations. When the debris being ejected was limited to only molten mass, instead of solid and molten mass, it also led to creep-rupture of the vessel lowerhead wall. This mode of ejection also shows a remarkable increase in the cesium and iodine source term release to the environment. When the containment does not fail, it is observed that there is lesser accumulation of fission products inside the containment and the retention inside the pressure suppression pool is enhanced in the scenarios with only molten debris ejection.

The dissolution of austenitic steel in liquid lead-based alloys can induce a phase transformation characterized by a sharp dissolution front separating ferrite and austenite grains, a process commonly referred to as ferritization. Although widely reported, the mechanism driving this transformation remains under debate. This study re-examines ferritization as a discontinuous reaction via a migrating liquid film and proposes a thermodynamically consistent model for the initiation and propagation of the dissolution front. The proposed mechanism is supported by experiments at 500–550°C, literature evidence, and diffusion calculations. Under low oxygen conditions, Cr transport through liquid Pb channels is identified as the rate-limiting step, setting the theoretical corrosion rate in stagnant environments. High-speed erosion-corrosion tests show enhanced corrosion rates, driven by erosion-limited channel lengths that locally boost mass transport. In contrast, under moderate oxygen concentrations relevant for lead-cooled fast reactor (LFR) operation, the rate-limiting step shifts to metal transport across a nanometer-scale amorphous oxide layer at the reaction front. Other Ni-containing austenitic steels, including alumina-forming austenitic (AFA) alloys and Ni-based high-entropy alloys (HEAs) can also be susceptible to discontinuous reactions under direct contact with liquid Pb-based alloys, lacking the self-healing oxide protection as observed in alumina-forming ferritic steels. This limitation may present a concern for the long-term use of bare austenitic steel in liquid Pb environments.

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.

In engineering systems operating under high Schmidt (Sc) or Prandtl (Pr) number flow conditions, the demand for near-wall mesh refinement increases significantly, underscoring the need for cost-effective modeling approaches that avoid additional computational overhead. Existing models, which are predominantly designed for low-Sc flows, overlook temporal filtering effects, resulting in inaccuracies in theoretical description and mass transfer predictions. This paper addresses the impact of high Sc or Pr by refining the single-layer scalar diffusivity model. It introduces a switch between scalar filtering and eddy viscosity-dominated regions, leveraging two parameters: κ Sc, accounting for temporal filtering effects, and κ Re, addressing variations in Reynolds number. In addition, we adopted a complementary outer layer term to model the upwarding trend in low frictional Reynolds number condition. Using the two-layer model with unity Sc and/or Pr, a close agreement with the von-Kármán constant in the velocity boundary layer was observed. The modified model demonstrated strong agreement with scalar profiles across a broad range of Sc and friction Reynolds numbers (Reτ) in direct numerical simulation and large eddy simulation data, demonstrating its accuracy at low Reτ and predictive performance at high Reτ. The two-layer model improves the prediction of turbulent mass transfer, providing better alignment with high Sc engineering correlations than existing wall model approach. This study provides valuable insight for modeling the mass and heat transfer processes under high Sc or Pr conditions.

Steam injection through spargers into a Pressure Suppression Pool (PSP) is used to prevent containment overpressure during primary system depressurization in normal operation and accident conditions. Direct Contact Condensation (DCC) of steam induces mass, momentum, and heat sources and thermal stratification can develop in the pool if the momentum source is not sufficient to overcome the buoyancy created by the heat source. The thermal stratification reduces the heat storage capacity of the pool, increasing the pool surface temperature and thus containment pressure compared to mixed pool configuration. In this work we analyze the results of the large scale pool experiments in PANDA and PPOOLEX facilities to develop better understanding of the phenomena and regimes that govern the multi-scale system. Specifically, we compare the results of the tests performed with the steam injection through the sparger head (horizontal injection) and the load reduction ring (downwards injection). We demonstrate that the response of the pool in terms of stable position of the thermocline and velocity of the thermocline motion can be described with proper selection of the non-dimensional scaling parameters for both facilities and directions of the steam injection. We also summarize and interpret other observations from the tests that are important for understanding of the pool thermohydraulic phenomena and development of respective predictive capabilities.

A Separate Effect Facility (SEF-POOL) was designed to measure the time-averaged momentum induced by steam injection into a subcooled water pool. Recent analysis of large-scale pool data has shown that the turbulence generated by the steam injection affects not only velocity field in the vicinity of the steam injection point but also integral pool behavior (thermal mixing and stratification). Unfortunately, the application of existing techniques for the velocity field measurements (such as Particle Image Velocimetry) is difficult due to presence of small gas bubbles and significant temperature gradients in the liquid. In this paper we introduce an experimental approach to quantification of the velocity field using Bubble based Particle Tracking Velocimetry (Bub-PTV) in which the streamwise velocity is inferred by stereoscopic tracking of air bubbles entrained by the flow. This paper presents the development of in-house code for bubble tracking and preliminary results obtained from the tests using water injection into a water pool. These water injection tests are intended to verify the setup of the experiment (e.g. air generating system, stereo cameras) and provide databases for code development and validation. The results are also compared with Computational Fluid Dynamics (CFD) simulations performed in ANSYS Fluent, and good agreement was achieved. The experimental measurements suggest that the proposed approach can provide a 3D velocity field measurement of the jet. Moreover, it indicates the potential of Bub-PTV as a reliable technique for measuring downstream axial velocity fields induced by steam injection.

The time-averaged Effective Momentum Source (EMS) induced by steam injection into a subcooled water pool was measured in a Separate Effect Facility (SEF-POOL) under a wide range of injection conditions. Post-test simulations of large-scale pool experiments conducted in PANDA and PPOOLEX facilities indicate that diffusion of the momentum is another important factor that determines the downstream momentum transport and dynamics of the stratified layer. Thereby, an experimental quantification approach was introduced to measure the streamwise velocity profiles induced by steam injection. It is achieved by using Bubble based Particle Tracking Velocimetry (Bub-PTV) where the velocity is inferred by stereoscopic tracking of the injected air bubbles. In the previous work, we validated the approach using tests with water injection. In this paper, we discuss Bub-PTV application to steam injection tests. The Bub-PTV code was further developed to improve the performance of bubble detection under the steam injection conditions. The momentum induced by steam injection diffuses much faster compared to the single-phase liquid jet injection. We demonstrate that, in the far field where steam has condensed completely, the jet can be simulated using a single-phase solver with the Effective Heat and Momentum sources (EHS/EMS) models along with an additional turbulence source term to account for the turbulence generated in the process of steam condensation. Good agreement can be achieved on the downstream velocity profiles if the added turbulence source is properly calibrated.

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.

This article presents the experimental results and the phenomenological analyses of the H2P4 tests performed in the PANDA facility, within the OECD/NEA HYMERES project. H2P4 series is characterized by two phases: (i) the steam injection in the water pool with development of thermally stratified layer and pressurization of the system, and (ii) water spray injection above the pool to depressurize the gas space. This article analyzes two tests that are characterized by different sparger submergence levels. The results indicate that the sparger submergence affects the pool's thermal behavior, and the pressurization history. A decrease in submergence height leads to a faster pressurization of the vessel. An analysis of the experimental data is carried out using mass and energy balances for the liquid and gas volumes. This analysis contributes to the understanding of the pool phenomena and suggests that mass and energy are lost through vaporization at the liquid–gas interface and a significant amount of steam is condensed at the walls.

Nordic Boiling Water Reactors employ filtered containment venting and ex-vessel debris coolability in the deep pool located under the reactor pressure vessel as cornerstones of their severe accident management strategy. This paper focuses on the uncertainty analysis of the source term in accident sequences that result in filtered containment venting to the environment using the MELCOR code. The impact of uncertain MELCOR modeling parameters and modeling options on the timing and magnitude of the source term released to the environment has been evaluated in accident sequences initiated by a large break LOCA and SBO. The performed simulations illustrate the effect of MELCOR modeling parameters and options on the code's predictions of severe accident progression, event timing, and the magnitude of the source term released to the environment in different accident scenarios. Furthermore, the results highlight the importance of various retention mechanisms that limit the release of fission products into the environment.