Rod Channel Research Articles

Numerical study on the growth characteristics of bubble in a petal-shaped fuel rod channel

Based on the VOF multiphase flow model and bubble growth model, a numerical analysis model of bubble growth of subcooled flow boiling in a petal-shaped fuel rod channel was established. The growth characteristics of a single bubble and the influence of thermal parameters were analyzed. Firstly, the growth rate, shape, and trajectory of the bubble were investigated under typical working conditions. Secondly, the influences of inlet subcooling, inlet velocity, and wall superheat on the bubble growth characteristics were clarified. Finally, the existing bubble growth rate prediction correlation was evaluated based on the results of numerical calculations. The results show that the bubble growth rate is fast at the early stage of bubble growth when the evaporation heat flow from the microlayer plays a major role. With the disappearance of microlayer evaporation, the bubble diameter increases slowly. The bubble diameter decreases continuously when the evaporation heat flow in the overheating layer is less than the condensation heat flow after the bubble lift-off. Compared to the rectangular channel, the bubble diameter in a petal-shaped fuel rod channel has a larger equivalent diameter and more obvious lateral movement. With the increase of inlet flow velocity and inlet subcooling, the total evaporation heat flow of the bubble decreases, while the condensation heat flow increases, which results in the decrease of bubble diameter. The influence of increasing wall superheat on bubble diameter is opposite to that of increasing inlet flow velocity and inlet subcooling. Based on the numerical results, a new prediction correlation for bubble growth rate in a petal-shaped fuel rod channel was developed, whose prediction error is within ±20 %. The related research results help to further reveal the subcooled boiling heat transfer mechanism in a petal-shaped fuel rod channel.

Core Optimization for Extending the Graphite Irradiation Lifespan in a Small Modular Thorium-Based Molten Salt Reactor

The lifespan of core graphite under neutron irradiation in a commercial molten salt reactor (MSR) has an important influence on its economy. Flattening the fast neutron flux (≥0.05 MeV) distribution in the core is the main method to extend the graphite irradiation lifespan. In this paper, the effects of the key parameters of MSRs on fast neutron flux distribution, including volume fraction (VF) of fuel salt, pitch of hexagonal fuel assembly, core zoning, and layout of control rod assemblies, were studied. The fast neutron flux distribution in a regular hexagon fuel assembly was first analyzed by varying VF and pitch. It was demonstrated that changing VF is more effective in reducing the fast neutron flux in both global and local graphite blocks. Flattening the fast neutron flux distribution of a commercial MSR core was then carried out by zoning the core into two regions under different VFs. Considering both the fast neutron flux distribution and burnup depth, an optimized core was obtained. The fast neutron flux distribution of the optimized core was further flattened by the rational arrangement of control rod channels. The calculation results show that the final optimized core could reduce the maximum fast neutron flux of the graphite blocks by about 30% and result in a more negative temperature reactivity coefficient, while slightly decreasing the burnup and maintaining a fully acceptable core temperature distribution.

Study on safety gap between fuel channels in NHR200-II

This paper presents a novel method for estimating the safety gap between deformed fuel channels. The irradiation causes deformation of the fuel channel, which narrows the control rod channel, directly affecting the safety of the reactor. Investigating the critical safety gap between fuel channels can be utilized for safety assessments in design. Specifically, we first define the safety gap and three typical dropping cases. Next, we analyze the forces on the control rod, including flow resistance and channel resistance. Then, we establish a nonlinear dynamic model and simulate three dropping cases. Finally, we explore the influence of the friction coefficient and the fuel rod bundle. We find that the critical safety gap for the NHR200-II reactor is -5.0∼-3.1 mm (standard case) and -4.2∼-2.5 mm (arbitrary case), respectively. Our study provides a novel solution to accurately evaluate the effect of deformed boxed channels on the control rod’s dropping.

Similar Binding Modes of cGMP Analogues Limit Selectivity in Modulating Retinal CNG Channels via the Cyclic Nucleotide-Binding Domain.

In treating retinitis pigmentosa, a genetic disorder causing progressive vision loss, selective inhibition of rod cyclic nucleotide-gated (CNG) channels holds promise. Blocking the increased Ca2+-influx in rod photoreceptors through CNG channels can potentially delay disease progression and improve the quality of life for patients. To find inhibitors for rod CNG channels, we investigated the impact of 16 cGMP analogues on both rod and cone CNG channels using the patch-clamp technique. Although modifications at the C8 position of the guanine ring did not change the ligand efficacy, modifications at the N1 and N2 positions rendered cGMP largely ineffective in activating retinal CNG channels. Notably, PET-cGMP displayed selective potential, favoring rod over cone, whereas Rp-cGMPS showed greater efficiency in activating cone over rod CNG channels. Ligand docking and molecular dynamics simulations on cyclic nucleotide-binding domains showed comparable binding energies and binding modes for cGMP and its analogues in both rod and cone CNG channels (CNGA1 vs CNGA3 subunits). Computational experiments on CNGB1a vs CNGB3 subunits showed similar binding modes albeit with fewer amino acid interactions with cGMP due to an inactivated conformation of their C-helix. In addition, no clear correlation could be observed between the computational scores and the CNG channel efficacy values, suggesting additional factors beyond binding strength determining ligand selectivity and potency. This study highlights the importance of looking beyond the cyclic nucleotide-binding domain and toward the gating mechanism when searching for selective modulators. Future efforts in developing selective modulators for CNG channels should prioritize targeting alternative channel domains.

The development and assessment of the coupled thermo-mechanical code, FRAPTRAN/CUPID

This paper introduces the coupling methodology of the thermo-mechanical code, FRAPTRAN/CUPID, and discusses the verification and validation calculations of the coupled FRAPTRAN/CUPID. In the verification calculation, which utilized 3 × 3 fuel rod channels, the coupled calculation produced consistent results compared to the standalone CUPID calculation. The validation calculations, based on two international and domestic LBLOCA tests (OECD-Halden IFA-650.5 and ICARUS-RT-20-02), demonstrate that the coupled FRAPTRAN/CUPID code effectively simulates the thermal–hydraulic behaviors of nuclear fuel rods during LBLOCA. It also provides valuable thermo-mechanical information regarding the deformation and burst of the nuclear fuel rods during LBLOCA. Finally, the paper presents an OPR1000 SLB calculation as a reactor application, utilizing the FRAPTRAN/MASTER/CUPID/MARS framework.

ИССЛЕДОВАНИЕ ОСНОВНЫХ ПАРАМЕТРОВ ГИДРОМЕХАНИЧЕСКОГО ДЕМПФЕРА ТРАНСПОРТНЫХ СРЕДСТВ, ИСПОЛЬЗУЕМЫХ В АГРОПРОМЫШЛЕННОМ КОМПЛЕКСЕ

The article presents materials related to the development of a promising design of a hydraulic damper in-stalled in the suspension of vehicles used in the agro-industrial complex. A special feature of the hydraulic damper design is the transition from vertical channels to horizontal channels in the piston and stem of the vi-bration damper. In this case, the vertical sections of the rod channels are mated with the horizontal sections of the piston channels. The presence of such calibrated holes in the piston and rod, as well as due to the ab-sence of bypass valves, can significantly reduce sudden pressure changes in the vibration damper during high-speed movements of both the rod and the piston. This significantly increases the smoothness of the ride, and, consequently, the comfort and safety of the vehicle and, thus, improves the working conditions of the personnel operating it. Analytical studies have been carried out, as well as hydrodynamic and strength calcu-lations of such determining parameters as the pressure of the working fluid and its flow rate. Recommenda-tions are given on the choice of rod and piston material for a promising design of a hydraulic damper for ve-hicles. A method of engineering calculations for determining the parameters of the throttling system is pro-posed, which allows to establish the criteria for the operability of the developed technical solution with a view to its possible application in industrial conditions.

Characteristics of bubble behavior in petal-shaped fuel rod bundle channels under different flow conditions

The petal-shaped fuel element is a novel type of fuel element suitable for small-scale reactors. The motion of bubbles in the rod bundle channel has a significant impact on the surrounding flow and temperature fields, thereby influencing the two-phase flow and heat transfer characteristics. To investigate the dynamics of bubbles in the rod channel of the petal-shaped fuel rod, we simulated the rising motion of bubbles with different inlet flow rates using the volume of fluid method (VOF). We obtained the trajectory, rising speed, and shape changes of the bubbles with varying initial sizes, and analyzed the flow field around the bubbles. Our results indicate that the transverse displacement of bubbles in the radius range of 0.5 mm–1.25 mm increases gradually with their initial size, and the degree of deformation and maximum rising velocity of the bubbles also increase. However, bubbles with a radius of 1.5 mm experience less transverse displacement due to their larger size and contact with multiple fuel rods during the ascent process. Furthermore, the transverse displacement distance of the bubble increases and then decreases with the increase of fluid flow rate in the channel, while the aspect ratio of the bubble remains unaffected by the inlet flow rate. The study of bubble behavior sheds light on the mechanism of bubble motion in the channel of petal-shaped rod bundles, and provides a foundation for the study of two-phase flow and heat transfer in the channel.

Experimental study on coupled quenching behavior in a dual-cooled annular rod channel

In this paper, a bottom reflooding experiment was performed in a single annular rod channel to investigate the coupled quench characteristics of the dual-cooled annular fuel. An indirect-heated annular rod with the axial uniform power shape was used. The asymmetric quench front propagation between the external and internal sides of the annular rod was observed. The internal cladding was quenched earlier than the external cladding, owing to heat accumulation in cylindrical shell and the blockage of the vapor flow by the grid spacers in external channel. A unique “secondary quenching” phenomenon was observed at the internal side of annular rod due to the radial heat conduction. The effects of reflooding velocity, inlet subcooling, initial cladding temperature and heating power on quenching behavior were also studied. The present study can contribute to the understanding of the coupled quenching characteristics of dual-cooled annular fuel and obtains valid data for the development of reflooding models and the validation of thermal hydraulics codes of annular fuel.

Modal analysis of lead-bismuth eutectic flow in a single wire-wrapped rod channel

The thermal-hydraulics as well as flow-induced vibration of wire-wrapped rod bundles calls for accurate and efficient liquid metal flow simulation and prediction, yet it remains a challenge due to the complex geometries and high Reynolds number flow in wire-wrapped rod channel. Previous efforts towards this goal exclusively adopts full-order modeling (FOM), which is prohibitively computation-intensive. This work reports the first reference on modal analysis of the turbulent lead-bismuth eutectic (LBE) flow in a single wire-wrapped rod channel by a data-driven dynamic mode decomposition (DMD) method. The spatio-temporal data of fluid velocities and pressure were modeled and collected by using the Reynolds-averaged Navier–Stokes equations (RANS) approach with stress-omega RSM turbulence model. Modal analysis was performed over the snapshot matrices and DMD modes and corresponding eigenvalues were calculated. The DMD modes and eigenvalues were then used to construct a reduced-order modeling (ROM) system for fast fluid field reconstruction and short-term forecasting. The correctness and feasibility of the method was demonstrated and compared with ground-truth CFD simulation results. Our method offers a fast and reliable approach for ROM of turbulent flow in wire-wrapped rod fuel assemblies. We publicize all the data and codes via https://github.com/XJTU-Zhou-group/Wire-wrapped-fuel-pin-CFD-DMD.

Structural basis of calmodulin modulation of the rod cyclic nucleotide-gated channel

Calmodulin (CaM) regulates many ion channels to control calcium entry into cells, and mutations that alter this interaction are linked to fatal diseases. The structural basis of CaM regulation remains largely unexplored. In retinal photoreceptors, CaM binds to the CNGB subunit of cyclic nucleotide-gated (CNG) channels and, thereby, adjusts the channel's Cyclic guanosine monophosphate (cGMP) sensitivity in response to changes in ambient light conditions. Here, we provide the structural characterization for CaM regulation of a CNG channel by using a combination of single-particle cryo-electron microscopy and structural proteomics. CaM connects the CNGA and CNGB subunits, resulting in structural changes both in the cytosolic and transmembrane regions of the channel. Cross-linking and limited proteolysis-coupled mass spectrometry mapped the conformational changes induced by CaM invitro and in the native membrane. We propose that CaM is a constitutive subunit of the rod channel to ensure high sensitivity in dim light. Our mass spectrometry-based approach is generally relevant for studying the effect of CaM on ion channels in tissues of medical interest, where only minute quantities are available.

Using cGMP analogues to modulate photoreceptor light sensitivity.

For many hereditary degenerative diseases, like retinitis pigmentosa, the primary degeneration of rod photoreceptors leads to a secondary loss of cones and eventually to blindness. The pathophysiological mechanism is very often characterized by an increase of cGMP level in rods. Thus, rod degeneration is mainly triggered by an over-activation of cyclic nucleotide-gated (CNG) channels and excessive Ca2+ influx. While targeting of CNG channels was already proposed for therapeutic purposes, thus far, it has not been possible to selectively inhibit rod CNG channels without compromising cone function. Here, we present a novel strategy, based on a combination of cGMP analogues, which enables the selective modulation of rod photoreceptor activity. This approach was first tested by means of the patch-clamp technique on retinal CNG channels, heterologously expressed in Xenopus oocytes and further confirmed by micro-electroretinography recordings on wild-type mouse retina. The cGMP-analogues mixture contained a CNG-channel inhibitor (Rp-8-Br-PET-cGMPS) and a CNG-channel activator (8-pCPT-cGMP). While Rp-8-Br-PET-cGMPS, when applied alone, behaved as a partial agonist and inhibited both cone and rod CNG channels equally, 8-pCPT-cGMP behaved as a very potent agonist and showed a strong concentration-dependent cone selectivity. Co-application of both compounds inhibited rod CNG channels and preserved cone functionality under both physiological and pathological cGMP levels. While the kinetics of rod channels was not influenced by this treatment, the deactivation of cone channels was slowed down. Surprisingly, this aspect did not affect the physiological cone responsiveness to light, indicating possible intracellular compensatory mechanisms. In conclusion, by delaying primary rods degeneration, this strategy shields the healthy cones and thus may either postpone the onset or slow down the disease development. Beyond the treatment of retinal diseases, using cGMP analogues with desired properties may elegantly address the isoform-specificity problem in future pharmacological therapies of related diseases.

Experimental study on boiling two-phase of liquid sodium along a 7-rod bundle – Part Ⅱ: Heat transfer characteristics

Sodium boiling will occur in some hypothetical accident cases (such as additional reactivity introduction and heat discharge capacity deterioration, and Local fault propagation). Thus sodium boiling experiments are necessary for the safety analysis and the development of serious accident programs in sodium-cooled fast reactors. In the present study, the boiling two-phase heat transfer experiments of liquid sodium were carried out on the boiling liquid sodium test loop. The experimental results showed that the entire boiling process can be divided into four stages. The heat transfer coefficient of the boiling two phases in the rod bundle channel increases gradually with the increase of heat flux and grows very slowly with the increase of system pressure. Based on the 54 groups of rod channels obtained, the new correlation of transfer heat coefficient of boiling two-phase liquid sodium was developed in a 7-rod bundle. The new correlation can well predict the experimental data of boiling two-phase liquid sodium obtained by other scholars, and the prediction error is within ±50\\%. Finally, mean relative deviation (MRD), absolute mean relative deviation (MARD), and root mean square deviation (RMSRD) were introduced to evaluate the accuracy of the correlation quantitatively.

Retinal Cyclic Nucleotide-Gated Channel Regulation by Calmodulin.

Retinal cyclic nucleotide-gated (CNG) ion channels bind to intracellular cGMP and mediate visual phototransduction in photoreceptor rod and cone cells. Retinal rod CNG channels form hetero-tetramers comprised of three CNGA1 and one CNGB1 protein subunits. Cone CNG channels are similar tetramers consisting of three CNGA3 and one CNGB3 subunits. Calmodulin (CaM) binds to two distinct sites (CaM1: residues 565-587 and CaM2: residues 1120-1147) within the cytosolic domains of rod CNGB1. The binding of Ca2+-bound CaM to CNGB1 promotes the Ca2+-induced desensitization of CNG channels in retinal rods that may be important for photoreceptor light adaptation. Mutations that affect Ca2+-dependent CNG channel function are responsible for inherited forms of blindness. In this review, we propose structural models of the rod CNG channel bound to CaM that suggest how CaM might cause channel desensitization and how dysregulation of the channel may lead to retinal disease.

A mathematical framework for evaluation of column shape profiles for an advanced gas-cooled reactor core model subject to seismic excitation

The safe shutdown of Advanced Gas-cooled Reactor (AGR) nuclear power stations in response to a seismic event is vital to their safety case. The tubular graphite bricks used to moderate the neutrons within an AGR core are arranged in columns whose bores provide channels for either fuel or control rods. Earthquake-induced distortion of the channels could impede the insertion of the control rods and compromise the safe shut-down, maintenance and servicing of the reactor. This paper presents a mathematical framework, utilising Euler mechanics, to evaluate the column shape displacement profiles of fuel and control rod channels within a state-of-the-art quarter-sized physical model of an AGR core when subjected to seismic loading. The data obtained from sensors installed within the model bricks, and configured to monitor interface displacements, are used to infer the global behaviour of a multi-stacked brick column subject to seismic excitation. Directly measured displacements of the top of the brick columns, obtained using a motion capture vision system, are compared with the displacements calculated using the framework presented, verifying the validity of the procedure. Statistical analysis is employed to quantify and characterise the performance of the Euler mechanics method. For multiple model build configurations, which represent different brick-cracking scenarios in an aged core, the Pearson correlation factor between the direct and indirect measurements is evaluated for the top of the column displacements giving an average value of 0.96 in the direction of the input motion. This shows that good agreement is achieved for the column shape displacement time-histories. The seismic responses are shown to be significantly larger in amplitude in the presence of large numbers of cracked bricks.

Chemical shift assignments of calmodulin bound to the β-subunit of a retinal cyclic nucleotide-gated channel (CNGB1)

Rod cyclic nucleotide-gated (CNG) channels are formed by two protein subunits (CNGA1 and CNGB1). Calmodulin (CaM) binds to the cytosolic regulatory domain of CNGB1 and decreases the open probability of CNGA1/CNGB1 channels. The CaM binding site within bovine CNGB1 (residues 679–702) binds tightly to Ca2+-bound CaM, which promotes Ca2+-induced inactivation of CNGA1/CNGB1 channels in retinal rods. We report complete NMR chemical shift assignments of Ca2+-saturated CaM bound to the CaM-binding domain of CNGB1 (BMRB no. 51222).

Modal Analysis of Lead-Bismuth Eutectic Flow in a Single Wire-Wrap Rod Channel

Modal Analysis of Lead-Bismuth Eutectic Flow in a Single Wire-Wrap Rod Channel

Numerical investigation on thermohydraulic performance of high temperature hydrogen in twisted rod channels

Nuclear thermal propulsion (NTP) is one of the most promising approaches for deep space exploration and manned interstellar voyage. As the propellant and coolant of NTP reactors, hydrogen provides higher specific impulse. In this paper, the thermohydraulic performance of high temperature hydrogen flowing through the twisted rod bundles were investigated numerically by contrasting with the cruciform rod bundles and the circular rod bundles. The heat transfer models were validated by literature data and the relative errors are within 20%. Twisted rods could enhance lateral mixing and heat transfer since the twisted lobes and valleys could enhance radial heat transfer and produce a strong rotational flow near the walls disturbing boundary layer, which reduces hot spot factors and assures the thermal safety. The largest relative difference of the JF factor between the twisted and other non-twisted channels are 28.0% and 6.1% respectively in this model.

Experimental investigation on flow oscillation of boiling two-phase flow in a single rod channel

Flow oscillation of two-phase flow has significant effect on the safety of relative industrial application, such as nuclear reactor. Oscillation of mass flux can induce undesirable such as boiling crisis. In this paper, a visual experimental study is carried out in a single rod channel to investigate the mechanism of flow oscillations. The system pressure is from 0.1 MPa to 1.0 MPa with different mass flux and inlet subcooling. According to the characteristics of the flow oscillation combined with bubble behavior, the flow oscillation is divided into slightly flow oscillation with high subcooled and high mass flux condition, boiling onset flow oscillation with bubbly flow and flow patterns transition oscillation with flow pattern transition. In this paper, the effect of thermal conditions on flow oscillation is also analyzed and discussed in detail.

Numerical investigation of the CHF in a vertical round tube and a single rod channel based on the eulerian two-fluid model

The critical heat flux (CHF) is essential to the operational safety of nuclear power plants (NPP), and it plays a vital role in the thermal-hydraulic and structural design of a fuel assembly. In this study, the commercial computational fluid dynamics package STAR-CCM+ was used to predict the departure from the nucleate boiling (DNB) phenomenon. Based on the Eulerian two-fluid model and the extended Rensselaer Polytechnic Institute (RPI) wall boiling model combined with the Weisman-Pei bubble crowding model, the DNB phenomenon was investigated for two coolants (water and R134a) with different geometric structures and working conditions. The results demonstrated that one set of Eulerian two-fluid models could accurately predict the CHF within a certain range of working conditions. The CHF value calculated in this study show good agreement with the experimental measured CHF data, which indicated that the Eulerian two-fluid model had a good application prospect for CHF prediction. The observed mean error was less than 10%. The results of this study may contribute to an advanced boiling model development for CHF prediction in complex geometric structures such as fuel assembly. In addition, these results demonstrated that it was feasible to use the refrigerant R134a, which had a low critical pressure and low latent heat of vaporization, as the modeling fluid to replace water, which had a high latent heat of vaporization and a high critical pressure in the experimental study.

Vesicular Release of GABA by Mammalian Horizontal Cells Mediates Inhibitory Output to Photoreceptors.

Feedback inhibition by horizontal cells regulates rod and cone photoreceptor calcium channels that control their release of the neurotransmitter glutamate. This inhibition contributes to synaptic gain control and the formation of the center-surround antagonistic receptive fields passed on to all downstream neurons, which is important for contrast sensitivity and color opponency in vision. In contrast to the plasmalemmal GABA transporter found in non-mammalian horizontal cells, there is evidence that the mechanism by which mammalian horizontal cells inhibit photoreceptors involves the vesicular release of the inhibitory neurotransmitter GABA. Historically, inconsistent findings of GABA and its biosynthetic enzyme, L-glutamate decarboxylase (GAD) in horizontal cells, and the apparent lack of surround response block by GABAergic agents diminished support for GABA's role in feedback inhibition. However, the immunolocalization of the vesicular GABA transporter (VGAT) in the dendritic and axonal endings of horizontal cells that innervate photoreceptor terminals suggested GABA was released via vesicular exocytosis. To test the idea that GABA is released from vesicles, we localized GABA and GAD, multiple SNARE complex proteins, synaptic vesicle proteins, and Cav channels that mediate exocytosis to horizontal cell dendritic tips and axonal terminals. To address the perceived relative paucity of synaptic vesicles in horizontal cell endings, we used conical electron tomography on mouse and guinea pig retinas that revealed small, clear-core vesicles, along with a few clathrin-coated vesicles and endosomes in horizontal cell processes within photoreceptor terminals. Some small-diameter vesicles were adjacent to the plasma membrane and plasma membrane specializations. To assess vesicular release, a functional assay involving incubation of retinal slices in luminal VGAT-C antibodies demonstrated vesicles fused with the membrane in a depolarization- and calcium-dependent manner, and these labeled vesicles can fuse multiple times. Finally, targeted elimination of VGAT in horizontal cells resulted in a loss of tonic, autaptic GABA currents, and of inhibitory feedback modulation of the cone photoreceptor Cai, consistent with the elimination of GABA release from horizontal cell endings. These results in mammalian retina identify the central role of vesicular release of GABA from horizontal cells in the feedback inhibition of photoreceptors.