Rod Channel Research Articles

Incorporation of nanoscale zero-valent iron particles inside the channels of SBA-15 silica rods by a “two solvents” reduction technique

A new reduction method named a “two solvents” reduction technique was developed for incorporation of nanoscale zero-valent iron particles (NZVIs) inside the channels of SBA-15 silica rods under mild conditions. The resulting NZVIs/SBA-15 composites were compared with the ones prepared by the conventional liquid phase reduction method in structure, morphology and reactivity. All the samples were characterized by X-ray diffraction (XRD), N2 adsorption–desorption isotherms, transmission electron microscopy (TEM) and all-direct-reading plasma atomic emission spectrometry (ICP-AES). Results showed that abundant ultrasmall zero-valent iron particles were synthesized and well dispersed in the mesopores of SBA-15 silica rods by the new reduction technique, whereas larger iron particles were supported and aggregated on the surface of the silica rods by conventional reduction method. Batch experiment demonstrated that NZVIs incorporated inside the silica channels had higher reactivity for the removal of Cr(VI) in aqueous solution than those supported on the surface.

Thermal–hydraulics analyses for 1/6 prismatic VHTR core and fuel element with and without bypass flow

Presented are the results of thermal–hydraulics analyses for a Very High Temperature Reactor (VHTR) 1/6 core and hexagonal fuel element, with and without helium-coolant bypass flow in interstitial gaps and in the control rod channels. Owing to the complexity and massive size of the VHTR, a full core analysis requires extensive and parallelized computation capabilities and a long time (weeks to months) to complete. These demanding requirements are mostly due to the 3-D computational fluid dynamics (CFD) simulation of the helium flow in the 10-m long coolant channels in the reactor core. Results demonstrate the effectiveness of coupling a 1-D helium flow in the channels together with a recently developed and validated convective heat transfer correlation, to a 3-D heat conduction in the graphite and fuel compacts in the core fuel elements. This simplified thermal–hydraulics analysis methodology for VHTR markedly reduces computation time and memory requirements, while maintaining accurate results. The helium bypass flow in interstitial gaps between fuel elements in the VHTR core provides additional cooling of the edge regions, but increases the temperature near the center of the prismatic fuel elements. Results also show that helium “bleed” flow through the control rod channels minimally affects the temperature distribution within the fuel elements. The heat generation in the corner burnable poison rods in the VHTR core fuel elements affects the temperature only in the close vicinity of the rods.

Numerical Analysis of Flow and Heat transfer in Sub-channel of Supercritical Water Reactor

Investigation on the thermal-hydraulic behaviour in the High performance Light Water Reactor (HPLWR) fuel assembly has obtained a significant attention in GENERATION-IV. In this paper, CFD analysis is carried out to study the heat transfer behaviour of supercritical water in sub-channels of squared annular assembly. Effect of various parameters, e.g. thermal boundary conditions and pitch-to-diameter ratio on the thermal–hydraulic behaviour is investigated. Boundary conditions, i.e., constant heat flux at the outer surface of cladding are applied. The effect of pitch-to-diameter ratio (P/D) on the distributions of surface temperature and heat transfer coefficient (HTC) was analyzed .For typical (gap between two rod) channel analysis, it is found that HTC increases with p/d first and then decreases significantly when p/d is <1.2. If P/D is larger than 1.2 heat transfer deterioration (HTD) occurs as main stream enthalpy is quite small. One/eight channel of 40 pin fuel assembly show that the structure of the secondary flow mainly depends on the rod bundle configuration as well as the pitch-to-diameter ratio, whereas, the amplitude of the secondary flow is affected by the thermal boundary conditions, as well. Furthermore, the HTD under wire-wrapped spacer were evaluated.

Various Bypass Flow Paths and Bypass Flow Ratios in HTR-PM

In HTR-PM, gaps between graphite and carbon blocks are widely distributed in the reactor core. The cold helium flows may be bypassed and not completely heated, therefore, the accurate prediction of bypass flows is a key problem related to the reactor safety. The flow network method was employed to represent interconnected bypass flow paths and calculate the bypass flow rates. Bypass paths in irregular shapes were simulated by CFD to get local and friction resistances. Typical bypass flow paths, included flow through sealing ring to the pebble discharge tube, flow through small holes to the control rod channels and flow through vertical gaps to the hot plenum, were modeled to calculate corresponding bypass flow rates. Large gaps, where might induce high flow rates at two ends of the bypass paths, should receive more concerns.

Store‐operated channels regulate intracellular calcium in mammalian rods

Exposure to daylight closes cyclic nucleotide-gated (CNG) and voltage-operated Ca(2+) -permeable channels in mammalian rods. The consequent lowering of the cytosolic calcium concentration ([Ca(2+)](i)), if protracted, can contribute to light-induced damage and apoptosis in these cells. We here report that mouse rods are protected against prolonged lowering of [Ca(2+)](i) by store-operated Ca(2+) entry (SOCE). Ca(2+) stores were depleted in Ca(2+)-free saline supplemented with the endoplasmic reticulum (ER) sequestration blocker cyclopiazonic acid. Store depletion elicited [Ca(2+)](i) signals that exceeded baseline [Ca(2+)](i) by 5.9 ± 0.7-fold and were antagonized by an inhibitory cocktail containing 2-APB, SKF 96365 and Gd(3+). Cation influx through SOCE channels was sufficient to elicit a secondary activation of L-type voltage-operated Ca2+ entry. We also found that TRPC1, the type 1 canonical mammalian homologue of the Drosophila photoreceptor TRP channel, is predominantly expressed within the outer nuclear layer of the retina. Rod loss in Pde6b(rdl) (rd1), Chx10/Kip1(-/-rdl) and Elovl4(TG2) dystrophic models was associated with ∼70% reduction in Trpc1 mRNA content whereas Trpc1 mRNA levels in rodless cone-full Nrl(-/-) retinas were decreased by ∼50%. Genetic ablation of TRPC1 channels, however, had no effect on SOCE, the sensitivity of the rod phototransduction cascade or synaptic transmission at rod and cone synapses. Thus, we localized two new mechanisms, SOCE and TRPC1, to mammalian rods and characterized the contribution of SOCE to Ca(2+) homeostasis. By preventing the cytosolic [Ca(2+)](i) from dropping too low under sustained saturating light conditions, these signalling pathways may protect Ca(2+)-dependent mechanisms within the ER and the cytosol without affecting normal rod function.

Matrix metalloproteinase-9 and -2 enhance the ligand sensitivity of photoreceptor cyclic nucleotide-gated channels

Photoreceptor cyclic nucleotide-gated (CNG) channels are the principal ion channels responsible for transduction of the light-induced change in cGMP concentration into an electrical signal. The ligand sensitivity of photoreceptor CNG channels is subject to regulation by intracellular signaling effectors, including calcium-calmodulin, tyrosine kinases and phosphoinositides. Little is known, however, about regulation of channel activity by modification to extracellular regions of CNG channel subunits. Extracellular proteases MMP9 and -2 are present in the interphotoreceptor matrix adjacent to photoreceptor outer segments. Given that MMPs have been implicated in retinal dysfunction and degeneration, we hypothesized that MMP activity may alter the functional properties of photoreceptor CNG channels. For heterologously expressed rod and cone CNG channels, extracellular exposure to MMPs dramatically increased the apparent affinity for cGMP and the efficacy of cAMP. These changes to ligand sensitivity were not prevented by destabilization of the actin cytoskeleton or by disruption of integrin mediated cell adhesion, but could be attenuated by inhibition of MMP catalytic activity. MMP-mediated gating changes exhibited saturable kinetic properties consistent with enzymatic processing of the CNG channels. In addition, exposure to MMPs decreased the abundance of full-length expressed CNGA3 subunits, with a concomitant increase in putative degradation products. Similar gating effects and apparent proteolysis were observed also for native rod photoreceptor CNG channels. Furthermore, constitutive apparent proteolysis of retinal CNGA1 and retinal MMP9 levels were both elevated in aged mice compared with young mice. Together, these results provide evidence that MMP-mediated proteolysis can regulate the ligand sensitivity of CNG channels.

Numerical simulation of large scale vortex structure and flow pulsation in rectangular channels

The quasi periodic large scale vortex structure and flow pulsation in rectangular channels are investigated theoretically. Narrow gap or slot is not the necessary condition for the quasi periodic large scale vortex structure. The quasi periodic large scale vortex structure could be observed in flat rectangular channels with the same widths if appropriate velocity profile is available. The vortex structure and flow pulsation in velocity interface could be strengthened by the adjacent velocity interfaces. The effect of channel size on the vortex structure is significant. The quasi periodic large scale vortex structure and flow pulsation vanishes with the channel length decreasing. This work is helpful for the recognition of quasi periodic large scale vortex structure and strong turbulent mixing in rod bundles and channels.

Engineered Tobacco mosaic virus mutants with distinct physical characteristics in planta and enhanced metallization properties

Tobacco mosaic virus mutants were engineered to alter either the stability or surface chemistry of the virion: within the coat protein, glutamic acid was exchanged for glutamine in a buried portion to enhance the inter-subunit binding stability (E50Q), or a hexahistidine tract was fused to the surface-exposed carboxy terminus of the coat protein (6xHis). Both mutant viruses were expected to possess specific metal ion affinities. They accumulated to high titers in plants, induced distinct phenotypes, and their physical properties during purification differed from each other and from wild type (wt) virus. Whereas 6xHis and wt virions contained RNA, the majority of E50Q protein assembled essentially without RNA into rods which frequently exceeded 2μm in length. Electroless deposition of nickel metallized the outer surface of 6xHis virions, but the central channel of E50Q rods, with significantly more nanowires of increased length in comparison to those formed in wtTMV.

ICONE19-43819 WATER EXPERIMENTS ON THERMAL STRIPING IN REACTOR VESSEL OF JAPAN SODIUM-COOLED FAST REACTOR : COUNTERMEASURES FOR CONTROL RODS AND RADIAL BLANKET ASSEMBLIES

Design study of an advanced loop-type sodium-cooled fast reactor, JSFR, has been carried out in a frame work of Fast Reactor Cycle Technology Development Project (FaCT) in Japan. As the temperature differences among the control rod channels, blanket assemblies and the core fuel assemblies are around 100℃, temperature fluctuation due to the fluid mixing at the core outlet may cause high cycle thermal fatigue at the bottom of Upper Internal Structure (UIS). Then, a water experiment was conducted using a 1/3 scale 60 degree sector model of the core and reactor upper plenum. Characteristics of temperature fluctuations near the cold fluid outlets were obtained and it was confirmed that several countermeasures can reduce temperature fluctuations at the bottom of UIS.

Study of Interfacial Friction Force for Bubble Flows in a 2×1 Rod Channel Simplifying BWR

Abstract Most of the recent subchannel analysis codes are based on a multifluid model, and an accurate evaluation of the constitutive equations in the model is essential. In order to get an accurate interfacial friction force in two-phase bubble flows, experimental data on drag coefficient and interfacial area concentration have been obtained for air-water flows in a 2×1 rod channel simplifying a boiling water nuclear reactor fuel rod bundle. In order to know the effects of liquid properties on the data, the temperature of the test water was changed from 18°C to 50°C. The data are compared with the existing correlations reported in literatures. As a result, the semitheoretical correlation of Hibiki and Ishii (2001, “Interfacial Area Concentration in Steady Fully-Developed Bubbly Flow,” Int. J. Heat Mass Transfer, 44, pp. 3443–3461) was found to give the best prediction against the present interfacial area concentration data. The correlation of Delhaye and Bricard (1994, “Interfacial Area in Bubbly Flow: Experimental Data and Correlations,” Nucl. Eng. Des., 151, pp. 65–77) also gave a reasonably good prediction if their correlation was modified by incorporating liquid property effects. As for the drag coefficient, no correlation exists, which can predict the present data well. Therefore, we developed a new correlation, including three dimensionless numbers, i.e., bubble capillary number, Morton number, and Eötvös number. The correlation predicted the data of Liu et al. (2008, “Drag Coefficient in One-Dimensional Two-Group Two-Fluid Model,” Int. J. Heat Fluid Flow, 29, pp. 1402–1410) as well as the present data well.

High-Resolution Crystal Structures of Trimeric and Rod Phycocyanin

The phycobilisome light-harvesting antenna in cyanobacteria and red algae is assembled from two substructures: a central core composed of allophycocyanin surrounded by rods that always contain phycocyanin (PC). Unpigmented proteins called linkers are also found within the rods and core. We present here two new structures of PC from the thermophilic cyanobacterium Thermosynechococcus vulcanus. We have determined the structure of trimeric PC to 1.35 Å, the highest resolution reported to date for this protein. We also present a structure of PC isolated in its intact and functional rod form at 1.5 Å. Analysis of rod crystals showed that in addition to the α and β PC subunit, there were three linker proteins: the capping rod linker (LR8.7), the rod linker (LR), and only one of three rod–core linkers (LRC, CpcG4) with a stoichiometry of 12:12:1:1:1. This ratio indicates that the crystals contained rods composed of two hexamers. The crystallographic parameters of the rod crystals are nearly identical with that of the trimeric form, indicating that the linkers do not affect crystal packing and are completely embedded within the rod cavities. Absorption and fluorescence emission spectra were red-shifted, as expected for assembled rods, and this could be shown for the rod in solution as well as in crystal using confocal fluorescence microscopy. The crystal packing imparts superimposition of the three rod linkers, canceling out their electron density. However, analysis of B-factors and the conformations of residues facing the rod channel indicate the presence of linkers. Based on the experimental evidence presented here and a homology-based model of the LR protein, we suggest that the linkers do not in fact link between rod hexamers but stabilize the hexameric assembly and modify rod energy absorption and transfer capabilities.

Subunit-Specific Regulation of Photoreceptor CNG Channels by Phosphoinositides

Cyclic nucleotide gated (CNG) channels in retinal photoreceptor cells play a key role in vertebrate phototransduction. The ligand sensitivity of photoreceptor CNG channels is adjusted during adaptation and in response to paracrine signals, but the mechanisms involved in channel regulation are only partly understood. Heteromeric A3+B3 (cone) and A1+B1 (rod) channels are sensitive to regulation by PIP3 or PIP2, demonstrating a decrease in apparent affinity for cGMP. To determine what subunit types are necessary for PIP3 sensitivity, we generated heteromeric channels by co-expression of PIP3-insensitive A2ΔN (Brady et al., 2006) with B3 or B1 subunits.

Complementary conductance changes by Ikx and Ih contribute to membrane impedance stability during the rod light response

In addition to the HCN1 channels that mediate the h current, the kx current also performs signal filtering in rod photoreceptors. This current is known to be mediated by potassium channels and has similarities to the neuronal M current and EAG potassium channels. Although it is known that in filtering the light response of rods, Ih and Ikx undergo complementary conductance changes, the qualities and significance of these changes are not clear. Here we present an analysis demonstrating the filtering effect of HCN1 channels in salamander rods when Ikx is blocked, and a simulation of the rod light response demonstrating the magnitude and time course of the conductance changes by both currents. Finally, we propose that the purpose of opposing conductance changes by Ih and Ikx may be to optimize the propagation of signals in the rod network.

Low-Conductance HCN1 Ion Channels Augment the Frequency Response of Rod and Cone Photoreceptors

Hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels are expressed in several tissues throughout the body, including the heart, the CNS, and the retina. HCN channels are found in many neurons in the retina, but their most established role is in generating the hyperpolarization-activated current, I(h), in photoreceptors. This current makes the light response of rod and cone photoreceptors more transient, an effect similar to that of a high-pass filter. A unique property of HCN channels is their small single-channel current, which is below the thermal noise threshold of measuring electronics. We use nonstationary fluctuation analysis (NSFA) in the intact retina to estimate the conductance of single HCN channels, revealing a conductance of approximately 650 fS in both rod and cone photoreceptors. We also analyze the properties of HCN channels in salamander rods and cones, from the biophysical to the functional level, showing that HCN1 is the predominant isoform in both cells, and demonstrate how HCN1 channels speed up the light response of both rods and cones under distinct adaptational conditions. We show that in rods and cones, HCN channels increase the natural frequency response of single cells by modifying the photocurrent input, which is limited in its frequency response by the speed of a molecular signaling cascade. In doing so, HCN channels form the first of several systems in the retina that augment the speed of the visual response, allowing an animal to perceive visual stimuli that change more quickly than the underlying photocurrent.

An Additional Retinoid Binding Site in Rhodopsin

Recently we discovered that some dark-adapted salamander rods and cones generated an electrical response to the truncated retinal analogue, β-ionone. This finding was tempered by observations that exposure to β-ionone led to pigment bleaching, and that very high concentrations of β-ionone inhibited rod channels in patch experiments. Therefore we examined whether β-ionone could activate the visual pigments by attacking the chromophore-binding pocket or through an interaction at an alternate binding site. Microspectrophotometry showed an accumulation of β-ionone in green-sensitive rod outer segments that increased linearly with bath concentration indicating that β-ionone most likely partitioned into disk membranes. β-Ionone also went into blue-sensitive rod outer segments, however, uptake was higher than in green-sensitive rods at all bath concentrations tested, suggesting that β-ionone bound to at least one site on rhodopsin in addition to partitioning. X-ray diffraction of green-sensitive rod rhodopsin crystallized in the presence of millimolar concentrations of β-ionone revealed a binding site located near the extracellular/intradiskal side of rhodopsin. β-Ionone may have low efficacy and low binding affinity because rhodopsin with ligand bound retained the inactive state conformation. β-Ionone is not a native retinoid, so we also examined the effects of retinol. Dark-adapted green-sensitive rods exposed to retinol lost sensitivity to flashes due to direct rhodopsin activation. In addition, rods exhibited a relative increase in sensitivity to shorter wavelengths, consistent with the ability of retinol to act as an antenna chromophore. Similar effects were seen for a blue-sensitive rod. These results support the presence in opsins of a retinoid-binding site(s) in addition to the chromophore-binding pocket, and suggest that this alternate site(s) mediates a number of distinct ligand-specific effects.

The development and application of a novel camera system for visual inspection of graphite fuel channels at Magnox nuclear power stations

Remote visual inspection of channels in graphite bricks in Magnox reactor cores has become a vital part of the safety case for continued operation of these power stations. This paper gives a brief summary of the history of graphite inspection and the challenges faced inspecting the internal bore of a fuel (or control rod) channel 100 mm diameter and 8 m high in a hazardous and highly radioactive environment. The concept of a radical new design of inspection system, incorporating a conical mirror to provide a full 360-degree circumferential view, is introduced. The challenges of turning a concept into a viable working system in a short period of time whilst keeping key stakeholders on board are described. The system has gone through several stages of evolution following initial on-site operations and has now completed the detailed visual inspection of thousands of fuel channels in Magnox reactors across the UK. The results from this camera system have been important in supporting the continued safe operation of Magnox reactors.

Light responses in the mouse retina are prolonged upon targeted deletion of the HCN1 channel gene

Hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels contribute to pacemaker activity, and co-determine the integrative behaviour of neurons and shape their response to synaptic stimulation. Four channel isoforms, HCN1-4, have been described in mammals. Recent studies showed particularly strong expression of HCN1 channels in rods and cones of the rat retina, suggesting that HCN1 channels are involved in the shaping of light responses in both types of photoreceptors. Therefore, the loss of HCN1 channels should lead to pronounced changes in light-induced electrical responses under both scotopic and photopic conditions. This was tested using a mouse transgenic approach. We used immunohistochemistry and patch-clamp recording to study the distribution of HCN1 channels in the mouse retina. HCN1 channels were strongly expressed in rod and cone photoreceptors, as well as in some bipolar, amacrine and ganglion cell types. In electroretinograms (ERGs) from animals in which the HCN1 channel gene had been knocked out, the b-wave amplitudes were unaltered (scotopic conditions) or somewhat reduced (photopic conditions), whereas the duration of both scotopic and photopic ERG responses was strikingly prolonged. Our data suggest that in visual information processing, shortening and shaping of light responses by activation of HCN1 at the level of the photoreceptors is an important step in both scotopic and photopic pathways.

From retinal circuitry to eye diseases—In memory of Henk Spekreijse

This article summarizes our recent works on stratum-by-stratum structure-function rules for synaptic contacts between retinal bipolar cells and third-order retinal neurons in the inner plexiform layer. These rules were derived from large-scale voltage clamp recordings of various types of bipolar cells in the tiger salamander retina, and they appear applicable to bipolar cells in the mouse and other mammalian species. This review also gives a brief account of how we used pathway-specific knockout mouse models to dissect rod and cone signaling channels in the mammalian retina. Furthermore, studies on cellular and genetic mechanisms underlying several neurodegenerative retinal disorders are described.

Specifics of RBMK core cooling in beyond design basis accidents

The most dangerous beyond design basis accidents for RBMK reactors, leading to the worst consequences, are related to the loss of long-term heat removal from the core. Due to a specific design of RBMK, there are a few possibilities for heat removal from reactor core by non-regular means: removal of heat from graphite stack by reactor gas circuit, removal of heat from reactor core using control rods cooling circuit, depressurisation of reactor cooling system, supply of water into cooling system from low pressure water sources, etc. This paper presents the analysis of such heat removal by employing RELAP5, RELAP5-3D and RELAP/SCDAPSIM codes. The analysis was performed for Ignalina nuclear power plant with RBMK-1500 reactor. The analysis of result shows that the restoration of water supply into control rod channels enables to remove 10–30 MW of the generated heat from the reactor core. This amount of removed heat is comparable with reactor decay heat in long-term period and allows to slowdown the core heat-up process. However, the injection of water to reactor cooling system is considered as main strategy, which should be considered in RBMK-1500 accident management procedure.

Functional Activity of Photoreceptor Cyclic Nucleotide-Gated Channels Is Dependent on the Integrity of Cholesterol- and Sphingolipid-Enriched Membrane Domains

Rod and cone photoreceptor cyclic nucleotide-gated (CNG) channels play pivotal roles in phototransduction. This work investigates the functional significance of photoreceptor CNG channel association with membrane microdomains enriched in raft lipids, cholesterol and sphingolipids. The primary subunits of cone and rod CNG channels, CNGA3 and CNGA1, respectively, were heterologously expressed in HEK 293 cells, and channel activity was determined by ratiometric measurement of [Ca (2+)] i in response to cyclic guanosine monophosphate (cGMP) stimulation. CNGA3 was found to be largely insoluble following Triton X-100 extraction and cofractionationed with biochemically isolated membrane domains enriched in caveolin-1. Cofractionation of both natively expressed CNGA3 and CNGB1 (the modulatory subunit of the rod CNG channel) with the low buoyant density, caveolin-1-enriched membranes was also confirmed in mouse retinas. The functional significance of this association was established by the observed negative effects of depletion of raft lipids on the channel activity. Treatment with the cholesterol depleting agent, methyl-beta-cyclodextrin (MCD), significantly inhibited CNGA3 and CNGA1 activation in response to cGMP stimulation. MCD treatment lowered cellular cholesterol levels by approximately 45% without altering fatty acid composition, suggesting that the inhibition of channel activity by MCD treatment is not due to perturbation of other membrane lipids. Treatment with the sphingolipid biosynthesis inhibitor myriocin resulted in impaired activation and cytosolic redistribution of CNGA3, suggesting that the integrity of the membrane domains is critical for the channel cellular processing and plasma membrane localization. This study demonstrates the association of photoreceptor CNG channels with membrane domains enriched in raft lipids and indicates, for the first time, that raft lipids modulate the plasma membrane localization and functional activity of photoreceptor CNG channels.