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• Experimental and numerical analyses were conducted to investigate nonuniform void behavior at OFI. • Bubble characteristics were experimentally analyzed using image processing. • Various combinations of boiling models were performed to select adequate boiling models. • Experimental and numerical results showed that flow instability started with an increase in heat flux. • Dynamic motions of bubbles shifting left and right sides in the channel were observed. Numerical analyses were conducted to simulate the flow boiling phenomenon using different boiling models under natural circulation conditions. From the analyzed results, bubble behaviors were investigated at the onset of nucleate boiling (ONB) and the onset of flow instability (OFI). At the ONB, the predictions obtained for the bubble diameter, nucleation site density, and void fraction were compared with the experimental data. At the OFI, the bubble diameter, nucleation site density, void fraction, and bubble movement with respect to time were investigated. Among 4 cases with different combinations of boiling models, Case 1 was selected because the wall temperature, bubble diameter, nucleation site density, and void fraction were well predicted considering the agreement between the experimental and computational fluid dynamics results. When flow instability started as the heat flux increased, bubble behavior was nonuniform although heat flux was uniformly generated on the heated surface. [ABSTRACT FROM AUTHOR]

An appropriate method for selecting the axial power distribution that results in the most conservative thermal margins was developed in this study. Nuclear physics calculations provided axial power distributions with various shapes based on the core states and cycles. A total of 17 cycles were required to reach the equilibrium core from the initial core. The axial power distributions were sorted based on the power peaking and sum-to-peak (STP) values. In the present study, three different power peaking criteria were used to sort the axial power distributions: the upper 50%, upper 25%, and upper 10%. After the first sorting based on the power peaking, the axial power distributions were grouped based on the power peak location in the axial direction. In each group, the axial power distribution with the highest STP was chosen for thermal margin analyses. In addition, the axial power distributions in the hottest channel and fuel assembly from the initial to the equilibrium core were obtained. Since the axial power distributions in the hottest channel and fuel assembly were bounded by the axial power distribution with the upper 10% F Q , the thermal hydraulic analysis results gave the more conservative thermal margins using the selection method presented in this study. The Onset of Nucleate Boiling (ONB) temperature margin with the upper 10% F Q was 22.5 °C, but it was 25.0 °C for the hottest channel. Therefore, the axial power distribution used for thermal hydraulic and safety analyses should be selected using the appropriate method presented in this study. [ABSTRACT FROM AUTHOR]

Abstract: This paper provides the results of a cost–benefit analysis for BeO–UO2 nuclear fuel, material that contains a small volume fraction of Be in uranium oxide matrix. According to the cost–benefit analysis results, the optimized BeO content was about 4.8 wt% when the BeO and uranium oxide prices were assumed to be $317/kg and $64/kg, respectively. The benefit of increasing the thermal conductivity of BeO–UO2 fuel was greater than the burden of the expensive BeO material price if the BeO content was 4.8 wt%. In addition, the break-even point of the nuclear fuel cycle cost as a function of the burn-up was 4.4mills/kWh when compared economically between BeO–UO2 fuel and UO2 fuel. Therefore, BeO–UO2 fuel seems economically efficient when the burn-up is more than 60MWD/kg because the nuclear fuel cycle cost of BeO–UO2 fuel was lower than that of UO2 fuel. [Copyright &y& Elsevier]