Conference Agenda

The safety performance of SFR designs is commonly assessed through Unprotected Loss of Flow (ULOF) transients where active core cooling systems are lost. The importance of ULOF transient relies in its potential to progress into the coolant boiling phase and eventually into partial/even total core destruction. It requires the detailed consideration of the particular effects of various specific design characteristics (e.g., upper sodium plenum, absorber layers, discharge tubes, etc.) during the progression of the transient under consideration.

This work presents the results of SAS-SFR simulation for a 10 s halving time ULOF transient including transient power, reactivity effects and fuel thermal-mechanical and coolant thermal-hydraulic conditions. The SAS-SFR model used provides a precise description of the accident progression in all SA-channels, thus results of the first failing SA-channel are presented in detail to give a deeper insight of the physical phenomena taking place during the various accidental phases.

SAS-SFR calculations predict boiling onset in all SA-channels followed by clad motion in 30 out of 34 channels and fuel pin break-up in 20 out of 34 channels by the end of the calculation. Clad relocation does not block the coolant channel completely, thus after fuel break-up, mobile fuel is relocated outside the core and a strong negative reactivity shuts down the reactor without damaging the hexcan. Therefore, SA hexcan integrity is assured although pin failure cannot be avoided in the SFR low void core design analyzed. However unless cooling conditions are improved reestablishing the coolant flow, cladding integrity will be at risk.

Predicting tritium transport and permeation is a critical challenge in Fluoride Salt-cooled High-Temperature Reactors (FHRs). The coolant, Flibe (2LiF-BeF₂), generates significant tritium via neutron transmutation, which can permeate into the secondary system through heat exchangers. Accurately estimating the multi-dimensional tritium permeation rate is essential, particularly given the increasingly complex geometries of heat exchangers, necessitating a robust numerical tool. To address such technical needs, we have developed scalarMultiRegionFoam, a novel three-dimensional OpenFOAM solver. Built upon chtMultiRegionFoam, the solver incorporates scalar transport equation for the fluid domain and diffusion equation for the solid domain. Additionally, we have implemented a new boundary condition, scalarTransportCoupledMixed, to accurately model the fluid-solid interface. We validated the developed solver against three cases. First, we verified the governing equation within a single domain by comparing it against an analytical solution for a transient problem. Next, we assessed the boundary condition for the fluid-solid interface using a steady-state analytic solution. Finally, we validated the complete model against transient experimental data. The verification and validation results have demonstrated the reliability and accuracy of our solver, establishing it as a powerful tool for simulating 3D tritium transport in FHRs.

Reliability and stability are crucial for nuclear reactor operations, especially in a small-scale system such as micro-reactors, where instability can lead to power fluctuations resulting in localized overheating. Heat pipes often encounter instability during transient phases such as startup and load-following operations, which can induce geyser boiling. This phenomenon occurs when subcooled liquid is expelled to the condenser section, resulting in significant temperature oscillations. Understanding the instability mechanism in heat pipes is necessary to optimize heat pipe micro-reactor operation. This study presents a transient thermal performance of a closed water thermosyphon – a wickless heat pipe, serving as a preliminary step towards modeling a heat pipe for micro-reactor applications. The simulations are conducted using OpenFOAM, a Computational Fluid Dynamics (CFD) code with a Volume of Fluid (VoF) solvers to capture the two-phase flow and phase changes in a thermosyphon. This research evaluates various initial filling ratios, working fluid temperatures, and heating powers in a 2D domain to identify optimal conditions for mitigating thermal instabilities during the startup phase. The OpenFOAM model was validated by comparing it with existing literature that utilized ANSYS Fluent, and it gave us similar results. The CFD modelling and the results of this study will contribute to a better understanding of the thermosyphon’s behavior in transient conditions, serving as the preliminary study for future investigations into more complex heat pipe micro-reactor modelling. In addition to that, this research uses OpenFOAM, which is uncommon in existing literature, which will address a gap in existing literature.

In recent years, significant research and development efforts have focused on various aspects of supercritical water-cooled reactors (SCWRs), with a particular emphasis on thermal-hydraulic analysis. Computational fluid dynamics (CFD) modeling has been extensively used to predict the thermal-hydraulic behavior within SCWR fuel assemblies. This modeling is crucial for validating heat transfer characteristics near critical and pseudocritical points, especially when operating at pressures below the critical threshold. One key focus of this research is simulating trans-critical transients, where the system pressure drops from supercritical to subcritical levels, to understand the effects on the fuel assembly.

To ensure the accuracy and reliability of these CFD models, experimental data from simpler geometries such as single tube and small rod bundles are often used for validation. The available experimental setups provide valuable insights into the behavior of heat transfer under supercritical and subcritical conditions, enabling better predictions and optimizations for more complex fuel assembly designs. By leveraging both CFD modeling and experimental validation, researchers aim to enhance the understanding of SCWR thermal-hydraulic performance and improve the safety as well as efficiency of these advanced reactor systems. This ongoing research is critical for advancing the development of SCWRs, which hold promise for efficient and sustainable nuclear power generation in the future.

SAFARI-1 is a Materials Test Reactor (MTR) situated at Pelindaba in South Africa. SAFARI-1 is a tank-in-pool type reactor with plate-type fuel assemblies licensed to operate at 20 MW with two primary pumps. The reactor typically contains 26 fuel elements and 6 follower-type control elements. This paper will discuss the modelling and analysis approach for the SAFARI-1 reactor for normal and abnormal operation and compare results obtained against operating technical specifications (OTS) for the reactor. The operational safety of the reactor will be verified for a range of operating conditions including single failure and design bas accidents. Peak clad temperature is one of the critical parameters determining the viability of a planned cycle for the SAFARI-1 reactor. The limiting conditions of operation specify the limits for the fuel clad temperature and convection heat transfer coefficient. The maximum expected clad temperature can be calculated by performing an analysis of the neutronic and thermal-hydraulic behaviour of the proposed cycle. The transients analysed lead to an increase in fuel clad temperatures and in particular, clad temperature at the hot spot. In this analysis, onset of nucleate boiling (ONB) is used as an indicator for more in-depth analysis. The departure from nucleate boiling ratio (DNBR) is also examined for compliance.