Conference Agenda

Small Modular Reactors (SMRs), particularly Light Water-SMRs, represent a viable option for near-term nuclear deployment in Europe, building upon established Light Water Reactor technology while incorporating evolutionary design modifications like passive safety systems. While these systems offer advantages such as independence from external power and component minimization, they face potential functional failures due to Natural Circulation issues. Therefore, passive system failures must be addressed in SMR design and safety reviews. Current guidance on passive safety system requirements and failure mode modeling methodologies needs consolidation. Short-term research priorities include reliability analysis of ThermalHydraulic phenomena driving system operation and related Uncertainty Analysis. Building on ENEA's MELCOR input-deck developed within the Horizon Euratom Safety Analysis of SMR with Passive Mitigation Strategies-Severe Accident (SASPAM-SA) project, this work applies the REPAS (Reliability Evaluation of Passive Safety Systems) methodology to the Emergency Heat Removal System (EHRS), a key passive safety feature for decay heat removal in advanced designs. REPAS will help analyze various system states, including low-probability scenarios, to understand EHRS behavior and its plant impact.

The flow characteristics in the upper plenum of molten salt (MSR) and high-temperature gas-cooled reactors (HTGR), during the pressurized conduction cooldown accident (PCC) scenario, are dominated by natural convection jets emitted from the top of the reactor core. Benchmark experiments are necessary to validate the Computational Fluid Dynamics (CFD) codes currently being used to further characterize this phenomenon. This study focuses on providing benchmark data for the upper plenum PCC scenario. The experimental facility is a scaled-down generic model of the upper plenum of MSRs and HTGRs. This study produces velocity field measurement data, via Particle Image Velocimetry (PIV). Data from optical fiber-distributed temperature sensors, thermocouples, and volumetric flow data are additionally used to calculate the conjugate heat transfer characteristics of the plenum and provide boundary conditions for the CFD models. The test matrix consists of isothermal and non-isothermal cases. In the non-isothermal case, fluid flow is driven by natural convection and buoyancy forces while the isothermal case is driven by pump-induced pressure gradients. This paper presents a detailed description of the experimental methods and analysis techniques utilized in this study and the results of multiple isothermal and non-isothermal test cases.

Deep pool reactors are reactors that operate at low pressures and are usually built near residential areas for heating and cooling, so the design of the reactor system requires high intrinsic safety characteristics. The design of non-energetic safety systems for deep pool reactors is characterized by multiple novel safety devices, and the transient operating characteristics of the devices are critical to the safety and stability of the reactors. In this paper, a set of experimental system that can reproduce the non-energetic waste heat export function of the pool reactor is set up, and the transient characteristics of the equipment under the loss-of-flow accident are experimentally investigated. The experimental results verify the on-time response of the relevant non-energetic equipment after the design accident. Extended working condition numerical studies were also carried out using a system analysis program.The experimental results demonstrate that the deep pool reactor can export the waste heat of the reactor through its own non-energetic safety system under a loss-of-flow accident.

Passive safety systems are an economically interesting alternative to conventional active systems, which are also more robust against many external influences, as they do not rely on an external, active drive. Accordingly, they continue to function even if large parts of the plant infrastructure are damaged or unavailable, as was the case in Fukushima Daiichi accident, for example. However, passive heat removal systems in particular pose major challenges for designers and the thermal-hydraulic calculation tools they use. One reason for this is the coupling or feedback between the heat flow that is introduced into the coolant and the mass flow of the coolant through the system, which results from the heat input. In addition, state of the art heat transfer models obviously cannot capture the heat transfer for passive systems precisely enough. As a result, attempts to recalculate experimentally determined heat transfer rates of passive heat removal systems with fluid dynamic codes have sometimes resulted in considerable deviations between experimental and numerical data. This paper presents two new heat transfer models developed specifically for passive systems. It is described how they can help to better calculate and predict the performance of related systems. The models were developed primarily on the basis of experimental data recorded at the COSMEA and NOKO test stands and published by the Helmholz Center Dresden Rossendorf. In addition, the model development was supported by CFD calculations executed to better understand the underlying mechanisms.

In the Small Innovative Helium-Xenon Cooled MObile Nuclear power System (SIMONS), the reactor core is horizontally oriented. To investigate the thermal-hydraulic effects of this configuration, a computational fluid dynamics (CFD) methodology is utilized. The flow and heat transfer dynamics of the core assembly under natural circulation conditions are examined, utilizing the passive gas circulation test loop at Shanghai Jiao Tong University as a reference model. The influence of heating power on mass flow rate, Reynolds number, pressure drop, and Nusselt number is examined, delineating the power range that enables flow self-compensation within the natural circulation system. Moreover, this investigation delved into the distribution patterns of mass flow rate, temperature and heat transfer across various channels under conditions of reduced natural circulation. The findings revealed that with a decrease in mass flow rate, there is a progressive increase in the proportion of flow rate and heat transfer within the lower-positioned channels, And the matching degree between the flow rate and heat exchange of each channel decreases. Furthermore, the study observed a pronounced thermal stratification in the upstream chambers at reduced flow rates, which can be attributed to the more pronounced heating exerted by the bottom of the assembly, coupled with the obstruction of flow in the upper chamber by the buoyant ascent of low-density Helium-Xenon mixture.