Conference Agenda

In this study, a modified point reactor kinetics model is developed to account for the advection of delayed neutron precursors (DNPs) in a molten salt fast reactor (MSFR). Accurately capturing the behavior of delayed neutrons is crucial for MSFR transient analysis, as they have a significant impact on reactor control and overall stability. The point kinetics parameters, including prompt neutron generation time and the effective delayed neutron fraction, are calculated using an extended Monte Carlo N-Particle (MCNP) code. This version of the code is specifically modified to incorporate the effects of fuel circulation, which is a unique characteristic of molten salt reactors compared to traditional solid-fuel reactors. The modified model is implemented into the FLUENT using a user-defined function (UDF) to perform transient analyses for the unprotected loss of flow (ULOF) scenario. The reactor’s response to a sudden reduction in fuel flow is studied, focusing on how the core average temperature and reactor power evolve over time. The absence of recirculation zones in this transient scenario has significant effects on the inlet and outlet temperatures of the reactor, which are critical for evaluating the reactor's safety characteristics. The velocity and temperature fields within the reactor core during the ULOF event are analyzed in detail. The model is benchmarked against two independent models from the Politecnico di Milano and the Technical University of Delft, showing a good agreement with referenced results. This comparison validates the accuracy and reliability of the modified point reactor kinetics model for MSFR transient analysis.

In recent years, there has been renewed interest in Molten Salter Reactors (MSRs) for their potential advantages compared to reactors that rely on solid fuel. In response to such interest, many methods and codes have been developed to capture the unique features of MSRs. Among them, the System Analysis Module (SAM) is a modern system analysis tool that provides fast-running, modest-fidelity, whole-plant transient analyses capabilities, essential for fast-turnaround design scoping and engineering analyses of advanced reactor concepts. For liquid-fuel MSR, the complex physics and chemistry involved in MSR operation, such as reactor kinetics, fluid flow, heat transfer, and salt composition dynamics, pose significant challenges for system-level modeling. Specific modeling capabilities including are needed for system-level transient simulation. This paper presents recent advancements in SAM capability enhancements for system-level modeling of MSRs, focusing on improved simulation fidelity, computational efficiency, and multi-physics integration. Key enhancements include the development of species transport, Delayed Neutron Precursor (DNP) drift, modified Point Kinetics Equations (PKE), decay heat modeling, key fission product behavior, salt corrosion, and thermal-hydraulic coupling, as well as the code robustness and performance enhancements for MSR applications. The code enhancement allows for better predictive accuracy in safety analysis, transient behavior, and operational optimization, thus supporting the design and licensing of next-generation MSRs. Results from case studies are presented to demonstrate the benefits of these enhancements in accurately capturing key reactor transient behaviors.

Liquid-fueled molten salt reactors (MSRs) represent the only reactor design utilizing liquid nuclear fuel, wherein the molten fuel salt generates heat continuously during circulation, exhibiting unique fluid dynamics characterized by an embedded internal heat source. The presence of this internal heat source significantly influences wall heat transfer characteristics; however, experimental studies on molten salt heat transfer with internal heat sources remain scarce, leaving existing modified heat transfer models for fuel salts unvalidated by direct experimental evidence. This paper proposes an innovative experimental approach combining microwave heating and hot air heating to simultaneously simulate internal heat generation within molten salt and controlled wall heat flux. A rigorous calculation methodology for wall heat transfer coefficients under these coupled conditions is also established. The findings provide valuable insights and a methodological framework for experimental investigations of internal heat source-coupled heat transfer phenomena in liquid-fueled molten salt reactor systems.

In this study, the influence of internal heat sources on turbulent wall cooling heat transfer in a pipe was analyzed, targeting the heat exchangers in molten salt reactors (MSRs). The non-homogeneous problem arising from internal heat sources was solved using the superposition principle. Numerical calculations were performed from the entrance region to the fully developed region to account for the cumulative effects of internal heat generation. The local and mean Nusselt numbers (Nu) were calculated for a range of Reynolds numbers (Re) from 5 to 10⁶, Prandtl numbers (Pr) from 1 to 10, and internal heat source parameters (Ω) from 1 to 10³. The results indicate that the presence of internal heat sources under wall cooling conditions enhances the heat transfer rate. This enhancement becomes more pronounced with increasing Ω and decreasing Re and Pr. However, due to the thin viscous sublayer in turbulent flow, the maximum enhancement rate remains below 12%. Therefore, a region where an internal heat source produces a meaningful enhancement rate (≥ 5%) was identified. A correction factor was developed to account for the enhancement effect within this range. This study provides fundamental insights into the effects of internal heat sources and offers a quantitative basis for the design and performance evaluation of MSR heat exchangers.

This paper describes sensitivity analyses that were performed using the existing MSRE model for the STH code SPECTRA. The work described in this paper concentrated on the influence of uncertainties in salt properties on the heat transfer in MSRE during normal operation. The results of the current study lead to the following conclusions:

• Thermal conductivity has the highest (positive) effect on the heat transfer coefficient.
• Viscosity and specific heat have similar effect but with different sign; viscosity decreases the heat transfer coefficient, while specific heat increases it.
• The effect of density is smaller than the effect of thermal conductivity.
• Extended sensitivity analysis included the sensitivity indices from the Plischke method. It confirmed the strongest effect of thermal conductivity, followed by viscosity and specific heat (both having similar effect).
• Very similar results were obtained using both deterministic and random sampling methods.

This paper describes sensitivity analyses that were performed using the existing MSRE model for the STH code SPECTRA. The work described in this paper concentrated on the influence of uncertainties in the delayed neutron precursors (DNP) on the results of the MSRE low power transients: pump start-up and coastdown. The results of the current study lead to the following conclusions:

• There is a positive effect of the delayed neutron precursor yield fraction on the reactivity effect in both pump coastdown and pump startup transients.
• The effect is strong for the middle groups and weak for the long-living group and the short-living groups. The long-living group travels through the complete loop several times before it decays. The short-living groups decay mainly in the core, independently of the flow rate, and therefore these groups are basically not affecting the pump transients.
• Very similar results were obtained using both deterministic and random sampling methods.