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Measuring the void fraction between fuel rods contributes to optimizing the geometrical design of fuel bundles for boiling water reactors (BWR). In this article, a method for reconstructing...

In dry air with a relatively short gap length under non-uniform electric field, the channel heating breakdown mechanism becomes critical to the electrical insulation design of power apparatus. We have experimentally clarified that the input energy density into the discharge channel almost decides the occurrence of channel heating breakdown in dry air. This implies that the rise of the channel temperature due to Joule heating causes the rise of the channel conductivity and the current, leading to the breakdown. In this paper, we construct the discharge channel heating model by the resistance circuits in consideration of the temperature dependence of conductivity and the shape of discharge channel, and investigate the process leading to the channel heating breakdown in dry air. The electrode system in this model is needle-plane electrodes with the gap length of 15 mm based on our experiments. At first, we verified that the process of channel heating breakdown can be explained by the temperature dependence of conductivity by using the preliminary zero-dimensional model with uniform temperature distribution in the discharge channel. With the given applied voltage and initial temperature, the temporal evolution of current and conductivity was calculated. Calculation results revealed that the temporal evolution of conductivity and the time to breakdown were determined by not only the conductivity itself but also the temperature dependence of conductivity. Subsequently, we constructed the discharge channel heating model in consideration of the radial and axial temperature distribution. With the given initial temperature distribution, the calculated temporal evolution of current agreed with the measured current waveform just before breakdown. This result revealed that the temperature rise in the center column of discharge channel is the most important factor for the breakdown process. The calculated temporal evolution of the temperature distribution along the center axis revealed that the high temperature region would develop from the tip of the needle electrode toward the center of the gap. In addition, the calculated temporal evolution of the radial temperature distribution suggested that the temperature at the center column of the discharge channel would rise sharply just before breakdown. In conclusion, in the channel heating breakdown in dry air, we clarified the process of the current concentration on the center axis of the discharge channel, as well as the progress of high temperature region or high conductivity region in the axial direction from the tip of the needle electrode.

To study the dc insulation design of gas insulated switchgear (GIS), the insulation characteristics under lightning impulse (LI) voltage with a superimposed dc voltage (superimposed voltage) must be clarified. The paper experimentally examined the GIS breakdown characteristics under this superimposed voltage. The test models simulating an insulator creepage surface were used for which consideration of the influence of dc voltage among various other GIS insulating elements is particularly important. To be specific, a cylindrical model made of epoxy resin or fiber-reinforced plastic (FRP) as the material and a conical epoxy spacer model were tested. For the cylindrical model, a cap-shaped electrode was placed on the insulator and a small gap was established between the end of the electrode and the insulator. When the dc breakdown voltages were measured using these samples, they were higher for the applied voltage of positive polarity than that of negative polarity for all samples. The post-test observation of the electrification condition revealed greater electrification on the insulator surface for the applied voltage of positive polarity. The electrification charges are considered to have relaxed the electric field and increased the breakdown voltage. Subsequently, the breakdown test was conducted using a superimposed voltage, whereby a foregoing dc voltage was applied to samples for a certain period, whereupon a LI voltage was applied with the dc voltage continually applied. The breakdown voltage when the LI voltage and dc voltage had equivalent polarity was approximately same to the LI alone breakdown voltage. Conversely, when they were opposite in polarity, the breakdown voltage under the superimposed voltage obviously tended to decrease from the LI alone breakdown voltage. It is considered attributable to the fact that the insulator surface was electrified by the foregoing dc voltage and applying LI voltage opposite in polarity to this electrification intensified the electric field where the breakdown started to occur. Accordingly, it emerged that the GIS breakdown characteristics changed significantly depending on the polarity combinations of LI and dc voltages. The influence of these polarities must be taken into consideration when studying the GIS dc insulation design.

Now that gas insulated switchgear (GIS) for ac systems is becoming increasingly compact as specifications are rationalized, more consideration of their insulation characteristics for residual dc voltage is required. Furthermore, with dc power transmission technology drawing more and more global attention, clarifying the insulation characteristics of GIS for dc voltage is increasingly important. The insulating portions for which the influence of dc voltage must be taken into consideration are solid insulators, such as insulating spacers. Under dc voltage, since the electric field distribution in an insulator differs from that under ac or impulse voltage and is governed by the resistance characteristics, clarifying its characteristics is crucial to study the GIS dc insulation design. As a solid insulator, focusing on fiber-reinforced plastics (FRP) used for GIS, for example, insulating rods, as well as partially treated epoxy resin; this paper experimentally investigated the bulk and surface electric conductivity under dc voltage, using the electric field, temperature, and other factors as parameters. As a result, the bulk electric conductivity of FRP in an edgewise direction exceeded that in the penetrating direction by one digit. It emerged that the electric conductivity of an insulating material with orientation like FRP varied depending on its direction. It was also found that, despite the fact the bulk and surface conductivity depended on the electric field for both FRP and epoxy resin, the variation width was relatively narrow within the range of the actual GIS operating electric field. The bulk and surface electric conductivity were also temperature-dependent, which meant the variation width was relatively wide. Furthermore, the surface electric conductivity was measured in SF6 gas and in the air to investigate the influence of the ambient atmosphere, whereupon it emerged that the electric conductivity was higher in air due to the adherence of moisture. As mentioned above, the electric conductivity of an insulator varies due to various factors, such as the influence of the material orientation, electric field, temperature, and moisture. Consequently, the electric field distribution inside the insulator also changes, meaning these electric conductivity characteristics must be taken into consideration to study the GIS dc insulation characteristics.

Measurement of radioactive contamination and confirmation of the decontamination effects are important for the recovery from the nuclear accident at the Fukushima Dai-ichi Nuclear Power Plant. We have developed a gamma camera which can visualize the gamma-ray intensity distribution in real time. Experiments were conducted to investigate its performance. An energy resolution of 2.3 % and spatial resolution of 0.68 m at a distance of 5 m were confirmed. In addition, field tests were conducted in the Fukushima Dai-ichi Nuclear Power Plant. The gamma-ray intensity distribution was successfully visualized within as short a time as 10 seconds in an environment with an air dose rate of 1 - 10 mSv/h. Prominent gamma-ray radiation was found from the penetration holes, which connect the inside and outside of the primary containment vessel. From this result, it was found that shielding the penetration holes will improve the work environment during decontamination and cleanup activities. These results indicate the gamma camera will contribute to decontamination work and radiation exposure reduction for workers.

A new scanning technique for reducing artifacts due to the partial volume effect is presented which maintains the spatial resolution of the conventional method. These artifacts are stripes and they have a ring shape between the source and the rotation center. Experimental and simulation results are in good agreement. The scanning technique has two data acquisitions per one cross-section, “+” quarter-sensor offset and “−” quarter-sensor offset. The partial volume effect is reduced by mathematical averaging the corresponding data of two data sets and doing image reconstruction with the average data.

Receptor activator of nuclear factor-kappaB ligand (RANKL), which is expressed on the cell membrane of osteoblasts/stromal cells, stimulates osteoclastogenesis. We investigated the regulatory effects of interleukin-1beta (IL-1beta) and prostaglandin E2 (PGE2) on expression of RANKL in human periodontal ligament (HPDL) cells and the mechanisms involved in the PGE2 effect.The HPDL cells were treated with IL-1beta, alone or in combination with indomethacin (INDO) or NS398, a cyclooxygenase-2 (COX-2) inhibitor. The HPDL cells were also pretreated with H89, a protein kinase A (PKA) inhibitor or GF109203X, a protein kinase C (PKC) inhibitor and subsequently treated with PGE2, PGE receptor (EP)2 agonist, EP4 agonist, forskolin, dibutyryl cAMP (db-cAMP), or 3-(isobutyl)-1-methylxantine (IBMX). After each treatment, expression of EP2, EP4, or RANKL mRNA was analyzed by reverse transcription-polymerase chain reaction and Southern hybridization. Expression of RANKL protein was detected by Western blotting, and cAMP accumulation was determined using a cAMP enzyme immunoassay kit.IL-1beta stimulated the expression of RANKL at messenger RNA (mRNA) and protein levels in HPDL cells. Endogenous PGE2 partially mediated the IL-1beta-induced RANKL mRNA expression. Exogenously added PGE2 also stimulated RANKL expression at mRNA and protein levels in the cells. The PGE2-stimulated RANKL expression was mediated by EP2/4 and cAMP-dependent PKA, while PKC was possibly involved in the PGE2 action.Human periodontal ligament cells activated with inflammatory factors such as IL-1beta and PGE2 may directly stimulate osteoclastogenesis through RANKL, which is stimulated to express by these factors.

Two radar systems, with respective centre frequencies of 800 MHz and 1.1 GHz, are developed to examine concrete structures. The former radar is for the non-destructive inspection of relatively thick concrete structures such as tunnel walls. The latter is to detect rebars and/or probe behind the rebars for reinforced concrete structures. To improve the inspection ability, transmitting pulse amplitude was controlled by changing the feed signal to an impulse generator for higher power at greater depths. As reflected data are acquired by sequentially synchronising to the transmitting pulses, the radar could probe a surface region using a lower transmitting amplitude, and it could probe the deep region at a higher amplitude while reducing attenuation of the radio wave propagation. Antenna sensitivity was improved by using FD-TD (Finite Difference-Time Domain) analysis to match antenna impedance to electric properties of concrete Structures. The systems were successfully applied to probing artificial defects and metal wires or pipes. The 800 MHz radar demonstrated high sensitivity which was more than 10 times that of earlier equipment. The 1.1 GHz radar could locate the position of rebars and detect metal wires and pipes behind rebars as three-dimensional colour images.

We have been developing a void fraction distribution measurement technique using the three-dimensional (3D) time-averaged X-ray CT (computed tomography) system to understand two-phase flow behavior inside a fuel assembly for BWR (boiling water reactor) thermal hydraulic conditions of 7.2 MPa and 288 °C. Unlike CT images of a normal standstill object, we can obtain 3D CT images that are reconstructed from time-averaged X-ray projection data of the intermittent two-phase flow. We measured the 3D void fraction distribution in a vertical square (5 × 5) rod array that simulated a BWR fuel assembly in the air-water test. From the 3D time-averaged CT images, we confirmed that the void fraction at the center part of the channel box was higher than that near the channel box wall, and the local void fraction at the central region of a subchannel was higher than that at the gap region of the subchannel. A comparison of the volume-averaged void fractions evaluated by the developed X-ray CT system with those evaluated by a differential pressure transducer in a void fraction range from 0.05 to 0.40 showed satisfactory agreement within a difference of 0.03.Copyright © 2013 by ASME

Forward-biased Schottky CdTe detectors (In/CdTe/Pt) have been investigated with an electron linear accelerator as an X-ray source, in order to suppress the polarization effect. The forward-biased detectors which have considerably large leakage currents show no gradual decrease in signals with time due to the polarization, while the reverse-biased detectors show a gradual decrease. Moreover, the outputs in the forward-biased state are much larger than those in the reverse-biased state.