[en] To introduce the printed circuit steam generator (PCSG), the design methods were developed, and the CFD analysis was performed. The design considerations were divided into three parts: thermal sizing, structural integrity evaluation, and specific design for water channels. The thermal sizing was performed by a 1D heat transfer analysis and the channel length was increased until it satisfied the target thermal power. The structural integrity was evaluated based on the ASME code and the calculated stresses were compared to the allowable stress. The flow instability, flow maldistribution, and fouling have negative effects on the performance. In order to prevent such phenomena, the mixing channel and orifice were designed. The CFD analysis on the final design of the PCSG was performed to evaluate the steam state at the outlet and flow distribution. Although a flow maldistribution (∼12 %) was observed, the steam state at the outlet was satisfied with the operating conditions.

[en] Many researchers have investigated the Onset of Nucleate Boiling (ONB) in a narrow rectangular channel. The power released from the edge of plate-type fuel is higher than the middle of plate-type fuel. Al-Yahia et al. investi-gated the effect of transverse power distribution on the subcoold boiling in a narrow channel. They found different thermal-hydraulic characteristic between non-uniform and uniform power distribution. Thus, non-uniform heating must be considered in research reactors which use the plate-type fuel. The objective of this experimental study is to find the effect of transversely non-uniform heating on the ONB. The effect of transversely non-uniform heating on the ONB in a narrow rectangular channel is studied. The experiments under non-uniform and uniform heating are performed, and the results are compared. The thermal power at ONB is decreased under non-uniform heating. Since the non-uniform heating causes the local high heat flux and wall temperature, ONB occurs at relatively low thermal power compared to uniform heating condition. The local heat flux and wall temperature at ONB under non-uniform heating are similar with the results of uniform heating condition. Since bubble generation is depends on the local condition, under the same system condition, the non-uniform heating has no effect on the local heat flux and wall temperature at ONB.

[en] Flow instability in a narrow rectangular channel (2.35 mm x 54.0 mm x 300 mm) is studied under uniform and non-uniform heating conditions since the power released from the nuclear fuel is not uniform in the axial and transverse directions. Transverse nonuniform heating may cause local boiling where local heat flux is relatively higher than other locations. This may occur boiling locally, which disturb or generate a different velocity profile compared with that under uniform heating. The velocity profile change is significant when the flow condition reaches the Onset of Flow Instability (OFI). In the present study, an experimental facility has been designed to study the effects of non-uniform heating on the velocity profiles. Experiments are carried out using two different ways to reach the OFI; decreases flow rate with constant power and increases power with constant flow rate. When the flow reached the OFI, the pressure drop changes show different trends. This is because the flow travels faster where there is a significant boiling than that where there is not. This study shows different boiling behaviours in a narrow rectangular channel under transversely uniform and non-uniform heating. (author)

[en] A printed circuit heat exchanger is expected to improve the safety of a sodium-cooled Fast reactor when it is used as the steam generator. To use such a Printed Circuit Steam Generator (PCSG), the Sodium-Water Reaction (SWR) should be evaluated first. Because the diameter and length of a sodium channel in the PCSG are 4 mm and 2 m, respectively, a numerical approach is useful to understand the SWR characteristics in a minichannel. CFD modeling is conducted using commercial code and the SWR in a single sodium channel is analyzed under operating conditions. H₂O at 16.8 MPa and 683 K is injected into the sodium channel through the cracks of different sizes (0.1-1.0 mm). Although the SWR is extreme in the early stage, most of the Na_(l) is pushed towards the inlet and outlet of the channel within 15.3-35.3 ms as H₂O inflowed. The peak temperature is around 1200-1600 K in the early stage of the SWR and the temperature reaches around 683 K. Moreover, the calculated depth of target wastage is around 0.62-3.34 μm. Therefore, the effect of the SWR is negligible in the PCSG core under operating conditions. (author)

[en] The nano-scopic light collection is investigated for the Active Matrix Flat-Panel Imagers (AMFPIs). The simulations using two kinds of screens are shown for light collection of x-rays. Enhancement of the light collection is accomplished by the microlens system incorporated with x-ray detector. For digital radiographic and mammographic applications, indirect detection imagers use Gd₂O₂S:Tb or CsI:Tl scintillation screens to convert the x-ray into visible photons. The light collection efficiencies for Gd₂O₂S and CsI are obtained. In Gd₂O₂S, the 27 kVp and 82 μm are the highest light collection cases in both Lambertian and Isotropic geometries. In CsI, 20 keV and 150 μm case have the highest light collection efficiency. So, x-ray energy and scintillator thicknesses are considered as the optimized light collection. The optimum thickness and x-ray energy combination are used for the detector of this study. In this paper, it is concluded that the screens between 17 kVp and 25 kVp have higher light collections, which could be considered as the clinical purposes if it is necessary. This energy range is compared with other energy cases, which are examined in the study. - Highlights: → The light collection efficiency could be increased by the microlens optically focusing method. → The lens focuses on the lights of the nano-scale photon distributions. → This depends on the angular distribution of the photons. → The quantum quantity is obtained by the x-ray energy and screen thickness. → The image blur can be decreased by the photon detection numbers in the photodetector.

[en] To investigate the thermal stabilities of CdSe and Cd-free quantum dots (QDs) in QD–silicone polymer nanocomposites, we synthesized both CdSe and CuInS₂ (CIS) QDs capped with different surface ligands (oleic acid (OA) and 1-dodecanethiol (1-DDT)). A comparison of the quantum yields of dried QDs after exposure to different temperatures revealed that the CIS QDs and 1-DDT ligands exhibited higher thermal stabilities than the CdSe QDs and OA ligands, respectively. We also prepared QD–silicone nanocomposites containing different types of QDs by varying the curing temperature and time and observed that nanocomposites containing CdSe QDs exhibited discoloration at high temperatures, whereas those containing CIS QDs did not. Moreover, the highest quantum efficiency (QE) of the QD–silicone nanocomposites was observed in the case of CIS QDs capped with 1-DDT and the lowest was observed in the case of CdSe QDs capped with OA. Because the dispersion states of QDs were not substantially affected at high temperatures, we attributed the high QE of nanocomposites containing the CIS QDs capped with 1-DDT to the superior thermal stability of CIS QDs and their surface ligand (i.e., 1-DDT).

[en] Highlights: • One-dimensional multi-channel approach is applied to predict the minimum point of pressure drop under non-uniform heating. • The pressure drop-mass flow rate curves between uniform and non-uniform heating are different. • In the single-phase flow, the pressure drop curve is the same for the uniform and non-uniform heat flux cases. • In the experiment, the pressure drop is abruptly increased after the minimum pressure drop is reached. • The predicted pressure drop curve is not matched well with the experiment after the minimum pressure drop. • For a better prediction, the flow regime effect in narrow rectangular channels should be considered. - Abstract: It is necessary to accurately predict the minimum point of pressure drop to ensure the safety of nuclear reactors. However, the non-uniform heat flux distribution along the transverse direction is encountered when the plate-type nuclear fuels are used. This study shows the effect of a transversely non-uniform heat flux on the minimum point of the pressure drop. The pressure drop-flow rate curve under the non-uniform heat flux was obtained by the experiment, and the trend of curve was different with the one of uniform heat flux case. Under the non-uniform heat flux, even when the inlet mass flow rate decreased, the value of the pressure drop was constant for a while with the development of a two-phase flow. With further reduction of inlet mass flow rate, the pressure drop started to decrease until the minimum point of the pressure drop was reached. Moreover, the inlet mass flow rate at the minimum point of pressure drop is much lower than that in the uniform heat flux case. For a detail analysis, the numerical approach is proposed along with the application of multi-channel concept. A single narrow rectangular channel is divided along the transverse direction, and the heat flux is given non-uniformly to the divided channels. Although the pressure drop is separately calculated for each divided channel, the mass is transferred between the channels. In the calculation, the mass flow rate is non-uniformly distributed in the transverse direction. If the mass flow rate is uniformly distributed, the non-uniform heat flux causes an unbalanced pressure drop because of the non-uniform distribution of void fraction. As a result, at the edges where the void fraction is high, the mass flow rate is transferred to the middle of channel to balance the pressure drop in transverse direction. When the void fraction in the middle becomes significantly large, the minimum point of the pressure drop can be obtained.

[en] Highlights: • Intergranular structure at near-surface on TiZrN was changed by laser carburization. • Carbon diffused through grain boundaries, following the Harrison Type-B behavior. • Diffused carbon bonded each other as the amorphous carbon in grain boundaries. • The pre-trapped amorphous carbon in grain boundaries interrupted hydrogen permeation. Hydrogen impermeability of carbon-doped TiZrN coatings was investigated in terms of carbon diffusion behavior and bonding states. The FFT pattern of a ring pattern and the inverse FFT with edge dislocations indicated formation of an amorphous phase and change in intergranular structure due to carbon diffusion through the grain boundaries of TiZrN. The diffused carbon followed Harrison Type-B behavior, because of localization at near-surface and increased compressive residual stress. The formation of amorphous carbon suggested the trapping at boundaries. The impermeability of the carbon-doped coatings was enhanced by 80 %, showing the contribution of amorphous carbon as a repellent.

[en] We present optical follow-up observation results of three binary black hole merger (BBH) events, GW190408_181802, GW190412, and GW190503_185404, which were detected by the Advanced LIGO and Virgo gravitational wave (GW) detectors. Electromagnetic (EM) counterparts are generally not expected for BBH merger events. However, some theoretical models suggest that EM counterparts of BBH can possibly arise in special environments, prompting motivation to search for EM counterparts for such events. We observed high-credibility regions of the sky for the three BBH merger events with telescopes of the Gravitational-wave EM Counterpart Korean Observatory (GECKO), including the KMTNet. Our observation started as soon as 100 minutes after the GW event alerts and covered 29–63 deg² for each event with a depth of ∼22.5 mag in the R band within hours of observation. No plausible EM counterparts were found for these events, but based on there being no detection of the GW190503_185404 event, for which we covered the 69% credibility region, we place the BBH merger EM counterpart signal to be M _g > − 18.0 AB mag within about one day of the GW event. The comparison of our detection limits with light curves of several kilonova models suggests that a kilonova event could have been identified within hours of the GW alert with GECKO observations if the compact merger happened at <400 Mpc and the localization accuracy was on the order of 50 deg². Our result shows great promise for the GECKO facilities to find EM counterparts within a few hours from GW detection in future GW observation runs.

[en] Highlights: • The hybrid system maximizes latent heat recovery and fuel conversion to the power. • Injecting the boiler flue gas into the gasifier improves oxy-coal gasification. • Reusing the boiler flue gas as a diluent in a gas turbine enhances its utilization. • Using additional heat exchangers provides the flexibility of thermal integration. • Extensive heat recovery and hybridization increase the net efficiency of the system. This study proposes the 2nd generation hybrid pressurized oxy-coal combustion power cycle that utilizes both fluidized-bed combustion coupled with the Rankine cycle and gasification connected to the Brayton cycle. The fluidized-bed boiler and gasifier are thermally connected by a flue gas stream flowing from the former to the latter. This improves thermal integration carbon conversion in the gasifier. The Rankine cycle is thermally connected to the Brayton cycle through the reheating process of intermediate pressure steam. This hybrid cycle enables the system to recover thermal energy from the pressurized flue gas with effective thermal integration in a way that it balances heat flow rates between hot and cold streams and increases the degree of freedom for heat flow control. The proposed system also reuses cold boiler exhaust gas as a diluent of the Brayton cycle to control its gas turbine inlet temperature, which raises the gas turbine flow rate and hence the gross power. The results indicate more than 9%-point and 1%-point increases in the gross and net efficiency, respectively, as compared to a single Rankine cycle system. The proposed hybrid system exhibits the net efficiency of 34.39% (HHV) higher than 33.04% (HHV) of the single oxy-PFBC cycle.