https://scholarworks.unist.ac.kr/handle/201301/68 Tue, 12 May 2026 05:57:05 GMT 2026-05-12T05:57:05Z https://scholarworks.unist.ac.kr/handle/201301/91664 Title: Numerical Analysis of Heat Pipe Performance Considering Helium Gap Effects Author(s): Celik, Faruk; Bang, In Cheol Abstract: Heat pipe-cooled microreactors are a promising innovation for nuclear power generation because of their reliable passive heat transfer capabilities. However, concerns related to creep failure and differential thermal expansion pose significant challenges, potentially leading to structural failure and radioactivity release. Additionally, startup challenges, such as ensuring consistent heat transfer during initial reactor operation and managing transient thermal conditions, also need to be addressed to achieve reliable performance. The helium gap can provide benefits during the startup process by enhancing thermal insulation, which helps in controlling the heatup rate and reducing thermal stresses; thereby, it helps to have more stable and consistent reactor startup. This protective gap also helps in minimizing the likelihood of radioactive release by maintaining the structural integrity of the reactor components. This study explores the impact of the helium gap, the space between the condenser wall and the sleeve tube, on the thermal performance of heat pipes and an axial flow heat exchanger. A numerical approach is employed to evaluate how variations in the helium gap affect thermal resistance, heat transfer efficiency, and overall system performance. Results show that the helium gap thickness significantly affects the heat pipe's performance. A thickness of 0.0065 cm or 0.015 cm improves startup performance, while 0.1 cm has a negative impact. Adding a helium gap may slightly reduce system power output by lowering the coolant temperature in the heat exchanger. However, using an optimal helium gap thickness appears to be a promising design strategy to enhance both the safety and performance of heat pipe-cooled microreactors. Tue, 31 Mar 2026 15:00:00 GMT https://scholarworks.unist.ac.kr/handle/201301/91664 2026-03-31T15:00:00Z https://scholarworks.unist.ac.kr/handle/201301/91660 Title: Experimental investigation of transient behavior in sodium heat pipe system for microreactor Author(s): Jin, Ik Jae; Bang, In Cheol Abstract: Heat pipe cooled microreactors rely on passive heat removal through multiple heat pipes, and the thermal response of individual heat pipes under transient conditions is important for reactor thermal management and safety evaluation. However, experimental investigation on the transient thermal behavior of sodium heat pipe systems under representative operational and accident-related conditions remains limited. In this study, experimental investigations were conducted on a sodium heat pipe system under vertical and horizontal orientations to examine transient thermal behavior during start-up, steady-state operation, shutdown, loss of cooling, and inclination angle change scenarios. Temperature responses along the heat pipe and heater surface were measured to evaluate orientation dependent thermal characteristics. The results showed orientation-dependent differences in temperature distribution and deactivation behavior. During steady-state operation, the vertical configuration exhibited a more uniform temperature distribution in the condenser section and lower temperatures compared with the horizontal configuration. Under shutdown and loss of cooling conditions, temperature fluctuations associated with geyser boiling behavior were observed in the vertical orientation. The results provide experimental database for the transient thermal response of sodium heat pipe systems and contribute to understanding heat removal characteristics relevant to the thermal management and safety evaluation of heat pipe cooled microreactors. Sun, 31 May 2026 15:00:00 GMT https://scholarworks.unist.ac.kr/handle/201301/91660 2026-05-31T15:00:00Z https://scholarworks.unist.ac.kr/handle/201301/91651 Title: Combined Effects of Alkaline Groundwater and Gamma Irradiation on the Durability of Geopolymer Waste Forms Author(s): Kwon, Seongye; Lee, Yulim; Oh, Yujin; Park, Jaeyeong Abstract: The long-term durability of metakaolin (MK)-based geopolymer waste forms was evaluated under simulated disposal environments. Specimens incorporating Cs, Sr, and Co were fabricated and subjected to ANSI/ANS 16.1 leaching tests in deionized water (DI), concrete-saturated groundwater (CGW), and CGW combined with gamma irradiation (CGW + gamma). Compressive strength, leachability index (LI), microstructure, and surface chemistry were analyzed to evaluate mechanical and chemical stability. All specimens initially satisfied the disposal acceptance criterion (>= 3.45 MPa). The compressive strength remained stable in DI but declined under CGW due to alkali-induced degradation. Under the CGW + gamma condition, severe strength loss occurred, and several specimens fractured completely. Leaching tests revealed substantial LI reduction for Cs, Sr, and Co. Computed tomography imaging showed increased porosity under CGW + gamma, while X-ray photoelectron spectroscopy analysis confirmed Cs and Sr surface enrichment with the reduction of Co3+ to Co2+. These results indicate that while MK-based geopolymers demonstrated strong resistance to individual degradation factors, they experienced significant structural deterioration under combined alkaline and radiation conditions, providing critical insights for the safe disposal of radioactive wastes. Wed, 31 Dec 2025 15:00:00 GMT https://scholarworks.unist.ac.kr/handle/201301/91651 2025-12-31T15:00:00Z https://scholarworks.unist.ac.kr/handle/201301/91595 Title: Evaluation of long-term thermal durability of neutron-shielding resin in dry storage casks Author(s): Hwang, Sia; Kang, Tae Uk; Kim, Min Ji; Kang, Woo Jun; Kim, Hee Reyoung Abstract: This study evaluated the long-term thermal durability of the neutron-shielding resin RNS-NR under long-term thermal exposure. RNS-NR was exposed to 170 degrees C in open-type and closed-type chambers, with weight loss monitored for 1500 h and material characterization conducted after 2000 h. TGA, XRD, and ICP-OES were used to evaluate thermal, structural, and compositional stability. After 1500 h, the weight-loss rate was 0.447 wt% in the closed-type chamber and 1.380 wt% in the open-type chamber, indicating greater volatile removal in the open configuration. TGA showed major decomposition at 262.54 degrees C, suggesting that the mass loss at 170 degrees C was dominated by moisture release rather than polymer degradation. XRD confirmed no detectable phase change of aluminum hydroxide after thermal exposure. ICP-OES analysis of the water-rich condensate collected from the closed-type chamber detected boron at 17.6 ppm and zinc at 188 ppm, while aluminum was not detected. These results indicated minor transfer of B and Zn into the condensate. Based on this observation, ICP-OES analysis of solid samples showed that the boron content decreased from 1.37 wt% to 0.60-0.30 wt% in the open-type chamber and to 0.48-0.31 wt% in the closed-type chamber, with retention dependent on sampling location. Tue, 30 Jun 2026 15:00:00 GMT https://scholarworks.unist.ac.kr/handle/201301/91595 2026-06-30T15:00:00Z