[en] The onset of the flow instability (OFI) and critical heat flux (CHF) in narrow rectangular channels are very crucial subjects for design and safety analysis concerning the systems such as high power research reactors (RRs) using plate-type fuels, since the phenomena could lead damage to fuels due to the initiation of local degradation of heat transfer. Therefore, experimental and analytical studies listed below have been performed to improve the understanding and predicting performance of both phenomena in a narrow rectangular channel heated on both sides, focusing on low pressure and low mass flux (LPLF) conditions, in consideration of transient or abnormal conditions of RRs.First, two-phase flow excursion instability (FI) especially for a downward flow was experimentally investigated. Three test channels with different gap sizes (2.5, 3.3 and 4.1 mm) for the same channel width (40 mm), the heated width (30 mm) and the length (350 mm) were adopted for the experiment. In addition, top plenum was opened to simulate a single channel of multi-channels in open-pool-type RRs. For several im-posed heat flux values (ranging0−1000kW/m²) and high inlet subcooling (43-75 K), subcooled flow boiling was investigated while reducing the fluid mass flux starting from a sufficient high-mass-flux condition (i.e. mass-flux-controlled system) up to the occurrence of the flow excursion. The minimum mass flux conditions in which a stable flow is sustainable were identified via 47 FI data points and were compared with relevant correlations. The results showed that FI was highly affected by gap size of channel, and therefore new empirical correlation reflecting the gap effect were suggested. The visualization of the flow boiling using a high speed camera was clearly demonstrating that the FI is triggered by the abrupt increase of pressure drop due to the coalescence of facing bubbles (for Pe < 14000) or wavy vapors (for Pe > 14000) on opposing heated surfaces, rather than onset of significant void (OSV), which has been generally conceptualized.Secondly, the characteristics of premature and stable CHF for downward flow were experimentally investigated using a narrow rectangular channel with gap of 2.35 mm. The flow boiling was developed as the wall heat flux was increased for the imposed mass flux of 500 and 1000kg/m²s, and inlet subcooling of 52 -74 K (i.e. heat-flux-controlled system). In addition, top plenum was closed to adopt inlet throttling, while lower plenum was connected to the open pool so as to maintain atmospheric pressure at the exit. The results showed that pressure drop fluctuation was inherently involved in the flow boiling, regardless of the amount of throttling, after a distinct initiating point called as the onset of pressure drop fluctuation (OPDF). It was also verified that coalescence of bubble (or vapor) layers on opposing heated surfaces triggered the OPDF as the expansion of the liquid-vapor interface became bounded by the other layer on the opposite side. The fluctuation was amplified as the exit quality was increased and the flow regime passed through the unstable slug or churn flow. For low inlet throttling conditions, premature CHF was induced at the maximum fluctuation during those flow regimes. On the other hand, stable CHF was obtained in the re-stabilized flow regime as the annular flow was developed only for large inlet throttling conditions with an additional pressure drop across the valve of more than 0.4 bar for the experimental configuration.Thirdly, a database including 62 OPDF data and 54 CHF data was constructed with additional experimental data for various system settings. 24 CHF data were stable CHFs, which were identified based on the characteristic of pressure drop fluctuations. Parametric trends analysis, flow regime analysis and correlation assessment were performed with the database. Based on the analysis, the upper and lower limits of CHF were newly proposed. Especially for stable CHF, it was verified that the onset of liquid entrainment in the annular flow triggered stable CHFs for downward flow in a narrow rectangular channel, instead of onset of slug or churn-annular flow transition, which was proposed in relevant literatures. In addition, it was certified that the limited systematic effects such as plenum or heater material effect was involved in the database.Fourthly, we conducted more detailed investigations focusing on the growth of bubble layers (BLs) not only for downward flow but also for upward flow. Furthermore, image processing algorithm was newly developed to automatically extract quantified bubble layer thickness (BLT) from vast number of images. It was verified that triggering mechanism is identical for both upward and downward flow from visualization, that is bubble blockage due to mergence of facing BLs on opposite side, but OFI for upward flow occurred at 25 % higher heat flux under same fluid conditions. Total 194 BLT data were obtained for various fluid conditions by using image processing algorithm. Parametric trends analysis showed that the maximum BLT increases with increasing heat flux, and decreasing mass flux and inlet subcooling. From the results, R factor which has been utilized as a key parameter in existing OFI and OSV correlations was adopted for development of new correlation. It was well represented that OFIs were enhanced for upward flow against downward flow due to delay of BL growth and the merging phenomena. Finally, new mechanistic model based on force balance approach was proposed for the prediction of the merging point and therefore OFI, since maximum BLTs were comparable to bubble departure diameter

[en] Dissipating heat flux is the one of major considerations in design of systems such as thermosyphon reboilers, nuclear power plants (NPPs), and research reactors, etc. Critical heat flux (CHF) is dissipating limit at which boiling heat transfer coefficient sharply decrease and catastrophic failure of the heated surface happen. Especially for downward cooling, counter-current vapor develop at low mass flux and minimum CHF can be induced, which is related to counter-current flow limitation (CCFL) or flooding. The counter-current flow limited CHF (CCFL CHF) have been reported in several papers. Mishima and Nishihara conducted CHF experiment with thin rectangular channels under low mass flux/atmospheric pressure for both downward and upward flow. Their CHF experiment data for downward flow showed mass flux region in which CHFs are very low. They reported that CHF in this region is related to counter-current upward vapor flow. They developed a CHF correlation based on well-known Wallis-type CCFL correlation

[en] Concern on the hydraulic design on DNB estimation under abnormal operating condition is slightly different. The abnormal operating condition may occur at postulating an isolated flow blockage in the hot channel which may enhance or deteriorate critical heat flux(CHF) at the unblocked condition. As clad temperatures increases during an accident, internal fuel rod pressure may cause clad swelling or ballooning which could lead to coolant blockage. Most of experiments on flow blockage is focused on the heat transfer and flow distribution under these accident condition. The objective of this study is to evaluate the local flow distribution prediction of MATRA code on flow blockage condition as a prerequisite study to conduct the DNB estimation under an isolated flow blockage condition. The PNL 7x7 experiment is selected as a validation data of subchannel code because this experiment provides the local flow distribution averaged on subchannel area. Sequential validation works of MATRA code on PNL 7x7 experiments were performed by same author groups.

[en] For realization of fusion power plant, various technical subjects should be overcome in the future. Among them, fusion-specific safety standards and licensing regulations should be developed at a proper time in order to avoid additional delay in construction of the plant. Although these needs are clearly expected from licensing of fusion DEMO (DEMOnstration) plant, there are no study on regulation concepts of fusion DEMO plant since technology and design of fusion power plant is being developed even now. It is a big dilemma to initiate this research in such situation to prepare for meeting regulatory demands expected in 2030s. For the early stage of this research, safety objectives, which are the ultimate goals for safety and the foundation of safety infrastructure, should be considered. Since these are very inclusive and implicative to be top tier of safety document system, it can be generally discussed and expected for fusion power regardless of phase of fusion technology development. In the present study, first, existing safety objectives for fissile nuclear power are reviewed. Then examples of safety goal for fusion facilities including ITER licensing are summarized to read the trend of safety goals for the fusion facilities. Finally, based on these reviews and reasonable considerations, safety objectives for fusion power plant in the future are proposed

[en] After Fukushima Daiichi nuclear accident, safety of nuclear power and public acceptance as well become the most crucial issue. Not only how to make the system safe but also how to establish society acceptance is big problem. Actually, this kind of problem is closely linked to how to regulate nuclear power plant. In the not too distant future, innovative nuclear reactors including sodium-cooled fast reactor (SFR), high temperature gas-cooled reactor (HTGR) and fusion reactor will be realized, which have more safety features than water-cooled fissile reactors. Even they have inherent safety features; however, buildup of such as innovative power plant is impossible without providing persuasive regulations based on reasonable regulatory framework. As we have seen from lessons of Fukushima accident, failure of reasonable regulation and occurrence of terrible accident could spoil realization of plan. For the case of fusion power plant, Korea has domestic plan to begin construction of fusion DEMO (DEMOnstration) plant at 2022. Although the needs of regulation for fusion power are clearly expected from licensing of fusion DEMO plant, there are no study on regulation concepts of fusion DEMO plant since technology and design of fusion power plant is being developed even now. Expedite study on development of regulatory framework for fusion power is positively necessary to prepare for meeting regulatory demands at a proper time. In the present study, first, domestic plan to realize innovative reactor plant including fusion power plant are organized. In addition to this, previous study on development of regulatory framework for innovate reactors are reviewed. Technology-neutral safety approach for establishment of regulatory frame has been come into spotlight and has being developed to utilize the methodology for deriving safety requirement regardless of reactor design. Technology-neutral safety approach for new reactor designs is also reviewed. Then, inherent safety features of fusion power to be expected are suggested based on the report of 'Safety and Environmental Assessments of Fusion Power: SEAFP', which is resulted from European fusion development agreement (EFDA). Finally, feasibility of technology-neutral safety approach for fusion power is explored

[en] The purpose of this study was to evaluate the effectiveness of thoracic paravertebral block (TPVB) for management of pain during and after percutaneous radiofrequency ablation (RFA) of hepatic tumor. All patients were divided into non-TPVB (4 patients, 4 sessions of RFA for 4 tumors) and TPVB group (5 patients, 7 sessions of RFAs for 7 tumors). Ultrasound (US)-guided TPVB was performed at T7 level. The 15 mL of 0.375% ropivacaine was injected into right paravertebral space before RFA. If patients complained pain and asked analgesics or experienced pain with verbal numerical rating scale (VNRS) of more than 4, fentanyl 25 µg (up to 100 µg), pethidine 25 mg, and midazolam 0.05 mg/kg (up to 5 mg) were sequentially given intravenously during RFA. Total intravenous morphine equivalence of analgesics before, during, and after RFA was 129.1 mg and 0.0 mg in non-TPVB and TPVB group, respectively. US-guided TPVB may be an effective and safe anesthetic method for decreasing or eliminating pain during and after RFA for hepatic tumor and helpful in decreasing the usage of opioids

[en] To compare the occurrence of transient severe motion (TSM) between gadoxetate disodium- and gadopentetate dimeglumine-enhanced MRI and between gadoxetate disodium-enhanced MRI scans obtained with and without the application of a modified breath-holding technique. We reviewed 80 patients who underwent two magnetic resonance examinations (gadoxetate disodium-enhanced MRI and gadopentetate dimeglumine-enhanced MRI) with the application of a modified breath-holding technique (dual group). This group was compared with 100 patients who underwent gadoxetate disodium-enhanced MRI without the application of the modified breath-holding technique (single group). Patient risk factors and motion scores (1 [none] to 5 [non-diagnostic]) for each dynamic-phase imaging were analysed. In the dual group, mean motion scores did not differ significantly between gadoxetate disodium- and gadopentetate dimeglumine-enhanced MRI (p=0.096-0.807) in any phase. However, in all phases except the late dynamic phase, mean motion scores of the dual group were significantly lower than those in the single group. TSM incidence did not differ significantly between gadoxetate disodium- and gadopentetate dimeglumine-enhanced MRI in the dual group (3.8% vs. 1.3%, p=0.620). With proper application of the modified breath-holding technique, TSM occurrence with gadoxetate disodium-enhanced MRI was comparable to that associated with gadopentetate dimeglumine-enhanced MRI. (orig.)

[en] Recently, data which prove that Wnt pathway activation may be an early event in multistep carcinogenesis in the stomach have been accumulating. We examined the effect of flavanone against β-catenin/Tcf signaling in AGS gastric cancer cells. Reporter gene assay showed that flavanone inhibited β-catenin/Tcf signaling efficiently. In addition, the inhibition of β-catenin/Tcf signaling by flavanone in HEK293 cells transiently transfected with constitutively mutant β-catenin gene, whose product is not phosphorylated by GSK3β, indicates that its inhibitory mechanism was related to β-catenin itself or downstream components. To investigate the precise inhibitory mechanism, we performed immunofluorescence, Western blot, and EMSA. As a result, our data revealed that there is no change of β-catenin distribution and of nuclear β-catenin levels through flavanone. In addition, the binding of Tcf complexes to DNA is not influenced by flavanone. The β-catenin/Tcf transcriptional target gene cyclinD1 was downregulated by flavanone. These data suggest that flavanone inhibits the transcription of β-catenin/Tcf responsive genes, by modulating Tcf activity without disrupting β-catenin/Tcf complex formation

[en] Functional activation of β-catenin/Tcf signaling plays an important role in early events in carcinogenesis. We examined the effect of naringenin against β-catenin/Tcf signaling in gastric cancer cells. Reporter gene assay showed that naringenin inhibited β-catenin/Tcf signaling efficiently. In addition, the inhibition of β-catenin/Tcf signaling by naringenin in HEK293 cells transiently transfected with constitutively mutant β-catenin gene, whose product is not phosphorylated by GSK3β, indicates that its inhibitory mechanism was related to β-catenin itself or downstream components. To investigate the precise inhibitory mechanism, we performed immunofluorescence, Western blot, and EMSA. As a result, our data revealed that the β-catenin distribution and the levels of nuclear β-catenin and Tcf-4 proteins were unchanged after naringenin treatment. Moreover, the binding activities of Tcf complexes to consensus DNA were not affected by naringenin. Taken together, these data suggest that naringenin inhibits β-catenin/Tcf signaling in gastric cancer with unknown mechanisms

[en] Disruption of circadian rhythm is a major cause of breast cancer in humans. Cryptochrome (CRY), a circadian transcription factor, is a risk factor for initiation of breast cancer, and it is differentially expressed between normal and breast cancer tissues. Here, we evaluated the anti-proliferative and pro-apoptotic activity of KS15, a recently discovered small-molecule inhibitor of CRY, in human breast cancer cells. First, we investigated whether KS15 treatment could promote E-box-mediated transcription by inhibiting the activity of CRY in MCF-7 human breast cancer cells. Protein and mRNA levels of regulators of cell cycle and apoptosis, as well as core clock genes, were differentially modulated in response to KS15. Next, we investigated whether KS15 could inhibit proliferation and increase sensitivity to anti-tumor drugs in MCF-7 cells. We found that KS15 decreased the speed of cell growth and increased the chemosensitivity of MCF-7 cells to doxorubicin and tamoxifen, but had no effect on MCF-10A cells. These findings suggested that pharmacological inhibition of CRY by KS15 exerts an anti-proliferative effect and increases sensitivity to anti-tumor drugs in a specific type of breast cancer. - Highlights: • Cryptochrome inhibitor (KS15) has anti-tumor activity to human breast cancer cells. • KS15 induces differential changes in cell cycle regulators and pro-apoptotic genes. • KS15 inhibits MCF-7 cell growth and enhances susceptibility to anti-tumor drugs.