Conference Agenda

Natural circulation flow is widely adopted in Small Modular Reactors (SMR) to simplify system design and achieve passive heat removal. The characteristics of low-velocity natural circulation flow are affected by both forced and natural convection. Since the amount of natural circulation flow is dependent on pressure losses, such as the wall friction, understanding this mixed convection flow is essential for the system design and safety evaluation of SMR. However, most of the previous studies were conducted using air as a working fluid or under low-temperature water conditions. Therefore, this experimental study investigated the wall friction factor in high-temperature water flows. The wall friction factor was measured at a vertical annular channel under natural circulation flow conditions. Experiments were performed under Re of 690-5,020, and Gr of 10⁵-5.5×10⁷ at the heated channel. The gaps of the concentric annular channels were 2.9, 5, and 7 mm, respectively. Evaluation of the existing model showed that the forced convection wall friction model underpredicts the present experimental data under low-velocity and larger gap conditions. Under these conditions, secondary flow within the channel prevented development of flow. Accordingly, this caused continuous changes in the velocity profile, increased viscous dissipation, and greater pressure loss. To predict accurately the increased wall friction factor in mixed convection flow, a new model was developed based on the present experimental data. The model accounts for secondary flow induced by buoyancy and radial temperature gradients within the channel. The developed model demonstrated good prediction under low-velocity mixed convection flow conditions.

In this study, we proposed scaling criteria for wall boundary conditions of thin plates with volumetric heat generation based on analyses of free convection-conduction conjugate heat transfer. Unlike uniform wall temperature(UWT) or uniform heat flux(UHF) boundaries, nuclear fuels typically involve volumetric heat generation. To examine the effect of conjugate heat transfer, the parameter for scaling criteria of wall thermal boundary conditions was analytically derived using the perturbation method. To quantify this parameter, the governing equations were numerically solved using the Runge-Kutta method for free convective flow and the finite volume method for solid conduction. The results showed that when the axial conduction-to-convection ratio—defined as half the plate thickness divided by the product of the modified Biot number and plate length—is greater than 0.5, the solution converges to the UWT solution. Conversely, when this ratio is less than 0.01, the solution aligns more closely with the UHF solution. For plates with the same volumetric heat generation, the peak temperature is highest under UHF condition and lowest under UWT condition. Therefore, the scaling criteria for the wall boundary condition proposed in this study can make a significant contribution to the thermal design of nuclear fuels. Furthermore, the scaling criteria were validated against experimental data for both air-cooled and water-cooled free convective plates with volumetric heat generation. According to the scaling criteria proposed in this study, the air-cooled test data were closer to the UWT condition, whereas the water-cooled test data were closer to the UHF condition.

Natural convection in an inclined channel with downward facing heater were investigate to validate the performace of PECS(passive ex-vessel corium retaining and cooling system), the Korean core catcher developed for the exporting APR1000 reactor. PECS has inclined channels under the structure , and the channels are heated from the top surface when the molten corium drops on the structure by a severe accident. The VPEX(Variable PECS experimental facility)was designed and built to examine the phenomena in the PECS channel experimentally. The VPEX has the heating block made of the carbon-steel with stainless steel coating, which is the same as the PECS structure. And the heat flux distribution over the channel were given by CFD calculation considering the corium behavior on the PECS. The tests were performed for various parameters such as the heat flux, the inlet subcooling, the channel shape, and the pressure. The results shows that the PECS has sufficient cooling capability even with the 175% of the expected heat flux. Also, the behavior of natural convection in the PECS channel were calculated using 1-D code, NCir, and the results were compared with the tests. The calculation results vary by the two-phase friction model and the void fraction model, however, corresponds to the experiments well for certain models.

This study investigates heat transfer enhancement in two-phase natural circulation loops, focusing on the role of superficial velocity in Small Modular Reactors (SMRs). Natural circulation, driven by buoyancy forces, is a key passive cooling mechanism in SMRs, where compact designs necessitate efficient heat removal. Using the MARS-KS code, this research simulates flow behavior, void fraction, mass flow rate, and heat transfer coefficients under two-phase conditions, specifically analyzing the NuScale SMR design, reduced to 1/10th of the original size for experimental feasibility. The study examines the impact of the minimum and maximum superficial velocity ranges, representing variations in the cooling channel gap. The results demonstrate that optimizing superficial velocity enhances thermal efficiency by improving convective boiling and phase change dynamics, while maintaining system stability. Higher velocities lead to better heat transfer performance in the evaporator, riser, and condenser, with the increased flow velocity fostering more efficient heat dissipation. These findings indicate that controlling the cooling channel gap can optimize flow velocity and, consequently, heat transfer. This research provides a foundation for future experimental studies on cooling channel height control, which will further investigate the influence of gap adjustments on heat transfer. The results contribute to the development of more efficient and reliable passive cooling strategies in SMRs, advancing reactor safety, performance, and sustainability. By optimizing natural circulation and refining reactor designs, this study supports the ongoing efforts to enhance the safety, efficiency, and long-term stability of next-generation nuclear energy systems.

External reactor vessel cooling (ERVC) plays a vital role in preventing failure. Nucleate boiling on the curved lower head of the RPV is a key heat removal mechanism. While current models focus on coolability limits, early cooling through nucleate boiling is equally important. Nucleate boiling models rely on accurate heat flux partitioning, with single-phase heat transfer being a key contributor. However, the correlations used in system analysis codes are not suited to the curved geometry of the RPV's lower head. Understanding single-phase heat transfer on downward-facing curved surfaces is essential for developing accurate nucleate boiling models. This study addresses this gap by developing heat transfer correlations for single-phase flow on curved, downward-facing surfaces under constant heat flux. Experimental measurements and CFD simulations were used to develop two correlations: one for forced convection (Ri < 0.1) and another for mixed convection (0.1 < Ri < 10), within the ranges 1,000 < Re < 13,000, 2.56 < Pr < 4.36, and 0.001 < Ri < 10. The study highlights that in forced convection, curvature affects boundary layer development and heat transfer. For mixed convection, buoyancy effects are captured through the Buoyancy number (Bo), with correlations showing how buoyancy transitions from impairment to enhancement of heat transfer. These findings are vital for improving system codes used in nuclear safety analysis, particularly for predicting heat transfer during ERVC and ensuring the integrity of the RPV in severe accident conditions.