Fast Transients Research Articles

Species-specific responses of Antarctic terrestrial microalgae to salinity stress. Comparative study in Klebsormidium sp. and Stigeoclonium sp.

We studied the changes in PSII photochemical processes in the cells of Antarctic algae Klebsormidium sp. and Stigeoclonium sp. exposed to salinity stress (0 – 3M NaCl) for 3 h. Salinity stress induced a decrease in the potential (FV/FM) and effective quantum yield of PSII electron transport (FPSII). Salinity stress induced a decrease in vitality index (Rfd, relative decrease of chlorophyll fluorescence). Analyses of the polyphasic fast chlorophyll fluorescence transients (OJIP) showed that with the increase in salt concentration, the chlorophyll fluorescence signals recorded at the phases J, I, and P declined, and the transient flattened with increaseing NaCl concentration reaching close to zero ChlF values at salt concentration of 3 M NaCl after 180 min. exposition. Klebsormidium sp. was found more salinity stress resistant than Stigeoclonium sp.

The Phenomena of Bursts by Opening Low-Voltage Relay

The article deals with the issue of electromagnetic disturbances caused by electromechanical relays. Electromagnetic compatibility (EMC) is important for the reliable work of electrical and electronic devices. Our research focused on the low-voltage relay, which is the source of electrical fast transients (EFT)/burst disturbances. The voltage and current waveforms were tested on the constructed stand while opening the relays’ contacts. We investigated one-circuit topology with varied element parameters in order to observe their influence on the burst phenomenon. The new technologies in the measurement equipment allow us to observe many detailed aspects of the burst phenomenon, which were not reported up to now. In order to analyze the disturbances, they have been fragmented into restoration, ignition and arc times. The number of disturbance cycles differs depending on the value of the circuit capacitance on the load side. The effect of capacity on times of restoration, ignition and burning arc was also observed. The division into cycles allowed us to observe changes in the studied phenomenon along with the increased distance between the relay contacts during its opening. There were also discrepancies between the presentation of the phenomenon in the standard defining the method of measuring the resistance to interference EFT/burst and the measurement results.

Fast optical flares from M dwarfs detected by a one-second-cadence survey with Tomo-e Gozen

Abstract We report on a one-second-cadence wide-field survey for M-dwarf flares using the Tomo-e Gozen camera mounted on the Kiso Schmidt telescope. We detect 22 flares from M3–M5 dwarfs with a rise time of 5 s ≲ trise ≲ 100 s and an amplitude of 0.5 ≲ ΔF/F⋆ ≲ 20. The flare light-curves mostly show steeper rises and shallower decays than those obtained from the Kepler one-minute cadence data and tend to have flat peak structures. Assuming a blackbody spectrum with a temperature of 9000–15000 K, the peak luminosities and energies are estimated to be 1029 erg s−1 ≲ Lpeak ≲ 1031 erg s−1 and 1031 erg ≲ Eflare ≲ 1034 erg, which constitutes the bright end of fast optical flares for M dwarfs. We confirm that more than $90\%$ of the host stars of the detected flares are magnetically active based on their Hα-emission-line intensities obtained by LAMOST. An estimated occurrence rate of detected flares is ∼0.7 per day per active star, indicating they are common in magnetically active M dwarfs. We argue that the flare light-curves can be explained by the chromospheric compression model: the rise time is broadly consistent with the Alfvén transit time of a magnetic loop with a length scale of lloop ∼ 104 km and a field strength of 1000 gauss, while the decay time is likely determined by the radiative cooling of the compressed chromosphere down near to the photosphere with a temperature of ≳ 10000 K. These flares from M dwarfs could be a major contamination source for a future search of fast optical transients of unknown types.

Magnetar Engines in Fast Blue Optical Transients and Their Connections with SLSNe, SNe Ic-BL, and lGRBs

We fit the multiband lightcurves of 40 fast blue optical transients (FBOTs) with the magnetar engine model. The mass of the FBOT ejecta, the initial spin period, and the polar magnetic field of the FBOT magnetars are respectively constrained to , , and . The wide distribution of the value of B p spreads the parameter ranges of the magnetars from superluminous supernovae (SLSNe) to broad-line Type Ic supernovae (SNe Ic-BL; some are observed to be associated with long-duration gamma-ray bursts), which are also suggested to be driven by magnetars. Combining FBOTs with the other transients, we find a strong universal anticorrelation of , indicating they could share a common origin. To be specific, it is suspected that all of these transients originate from the collapse of extremely stripped stars in close binary systems, but with different progenitor masses. As a result, FBOTs distinguish themselves by their small ejecta masses with an upper limit of ∼1 M ⊙, which leads to an observational separation in the rise time of the lightcurves of ∼10 days. In addition, FBOTs together with SLSNe can be separated from SNe Ic-BL by an empirical line in the M peak–t rise plane corresponding to an energy requirement of the mass of 56Ni of ∼0.3M ej, where M peak is the peak absolute magnitude of the transients and t rise is the rise time.

Extragalactic Millimeter Transients in the Era of Next-generation CMB Surveys

The next generation of wide-field cosmic microwave background (CMB) surveys are uniquely poised to open a new window into time-domain astronomy in the millimeter band. Here, we explore the discovery phase space for extragalactic transients with near-term and future CMB experiments to characterize the expected population. We use existing millimeter-band light curves of known transients (gamma-ray bursts, tidal disruption events, fast blue optical transients (FBOTs), neutron star mergers) and theoretical models, in conjunction with known and estimated volumetric rates. Using Monte Carlo simulations of various CMB survey designs (area, cadence, depth, duration) we estimate the detection rates and the resulting light-curve characteristics. We find that existing and near-term surveys will find tens to hundreds of long-duration gamma-ray bursts (LGRBs), driven primarily by detections of the reverse shock emission, and including off-axis LGRBs. Next-generation experiments (CMB-S4, CMB-HD) will find tens of FBOTs in the nearby universe and will detect a few tidal disruption events. CMB-HD will additionally detect a small number of short gamma-ray bursts, where these will be discovered within the detection volume of next-generation gravitational wave experiments like the Cosmic Explorer.

XMM-Newton and Swift observations of supergiant high mass X-ray binaries

Wind-fed supergiant X-ray binaries are precious laboratories not only to study accretion under extreme gravity and magnetic field conditions, but also to probe the still highly debated properties of massive star winds. These include clumps, originating from the inherent instability of line driven winds, and larger structures. In this paper we report on the results of the last (and not yet published) monitoring campaigns that our group has been carrying out since 2007 with both XMM-Newton and the Swift Neil Gehrels observatory. Data collected with the EPIC cameras on board XMM-Newton allow us to carry out a detailed hardness-ratio-resolved spectral analysis that can be used as an efficient way to detect spectral variations associated with the presence of clumps. Long-term observations with the XRT on board Swift, evenly sampling the X-ray emission of supergiant X-ray binaries over many different orbital cycles, are exploited to look for the presence of large-scale structures in the medium surrounding the compact objects. These can be associated either with corotating interaction regions or with accretion and/or photoionization wakes, and with tidal streams. The results reported in this paper represent the outcomes of the concluded observational campaigns we carried out on the supergiant X-ray binaries 4U 1907+09, IGR J16393−4643, IGR J19140+0951, and XTE J1855−026, and on the supergiant fast X-ray transients IGR J17503−2636, IGR J18410−0535, and IGR J11215−5952. All results are discussed in the context of wind-fed supergiant X-ray binaries and ideally serve to optimally shape the next observational campaigns aimed at sources in the same classes. We show in one of the Appendices that IGR J17315−3221, preliminarily classified in the literature as a possible supergiant X-ray binary discovered by INTEGRAL, is the product of a data analysis artifact and should thus be disregarded for future studies.

Neutrino Emission from Luminous Fast Blue Optical Transients

Mounting evidence suggests that luminous fast blue optical transients (LFBOTs) are powered by a compact object, launching an asymmetric and fast outflow responsible for the radiation observed in the ultraviolet, optical, infrared, radio, and X-ray bands. Proposed scenarios aiming to explain the electromagnetic emission include an inflated cocoon, surrounding a jet choked in the extended stellar envelope. Alternatively, the observed radiation may arise from the disk formed by the delayed merger of a black hole with a Wolf–Rayet star. We explore the neutrino production in these scenarios, i.e., internal shocks in a choked jet and interaction between the outflow and the circumstellar medium (CSM). If observed on axis, the choked jet provides the dominant contribution to the neutrino fluence. Intriguingly, the IceCube upper limit on the neutrino emission inferred from the closest LFBOT, AT2018cow, excludes a region of the parameter space otherwise allowed by electromagnetic observations. After correcting for the Eddington bias on the observation of cosmic neutrinos, we conclude that the emission from an on-axis choked jet and CSM interaction is compatible with the detection of two track-like neutrino events observed by the IceCube Neutrino Observatory in coincidence with AT2018cow, and otherwise considered to be of atmospheric origin. While the neutrino emission from LFBOTs does not constitute the bulk of the diffuse background of neutrinos observed by IceCube, the detection prospects of nearby LFBOTs with IceCube and the upcoming IceCube-Gen2 are encouraging. Follow-up neutrino searches will be crucial for unraveling the mechanism powering this emergent transient class.

An Integrated GaN Overcurrent Protection Circuit for Power HEMTs Using SenseHEMT

GaN power high-electron-mobility transistors (HEMTs), with their fast switching transients and poor overcurrent tolerance, require overcurrent protection (OCP) circuits that can respond in under <inline-formula><tex-math notation="LaTeX">$\text{0.5}\, \mu \text{s}$</tex-math></inline-formula>. Monolithically integrating digital and analog circuits with GaN power devices is enabled by recent advancements in high-voltage GaN integrationplatforms. Thus, integrated OCP designs can be used to reduce the protection response time, the area penalty, and the assembly complexity. This work presents the first fully integrated, senseHEMT-based GaN OCP integrated circuit (IC) with an adjustable current limit and programmable blanking time, suitable for a wide range of power applications. The IC, implemented on a 200-V GaN-on-Silicon-on-Insulator (GaN-on-SOI) process, contains a power HEMT with a senseHEMT for current sensing, a totem-pole-based gate driver, an analog comparator, and a set of logic circuits to enable high-speed closed-loop OCP. The fabricated design is tested in both a clamped switching setup and a 12&#x2013;48 V boost converter. The minimum OCP response time is estimated to be 36 ns with <inline-formula><tex-math notation="LaTeX">$dI_{D}/dt\geq \text{30}\,\text{A}/\mu \text{s}$</tex-math></inline-formula>, one of the fastest amongst state-of-the-art OCP circuits for GaN.

Wettability and CHF limits of Accident-Tolerant nuclear fuel cladding materials in light water reactor conditions

We present the results of experimental investigations aimed at evaluating the thermal–hydraulic performance of chromium-coated zircaloy, i.e., one of the most promising accident tolerant fuel (ATF) cladding material for light water nuclear reactors. Precisely, we investigate the wettability and critical heat flux (CHF) limits of chromium-coated and conventional zircaloy surfaces in prototypical reactor conditions. For both surface types, we measure the contact angle in a vapor-saturated atmosphere from ambient pressure to the operating pressure of pressurized water reactors (PWRs), i.e., ∼15 MPa. We measure the ambient-pressure steady-state flow boiling CHF with a spatially uniform heat flux. We measure the high-pressure steady-state flow boiling CHF with a cosine shape heat flux (up to 20 MPa) and with a uniform heat flux (up to 15 MPa), also exploring the effect of CRUD deposits on the chromium-coated surface. Our results reveal that the chromium surface and the bare zircaloy surface have similar wettability and both become super-hydrophilic in PWR conditions, and that there is practically no difference in the steady-state CHF limits, both at low-pressure and high-pressure conditions, also when the chromium-coated surface is covered by a CRUD deposit. However, while the chromium-coating does not improve the CHF compared to the bare zircaloy surface, it improves the post-CHF behavior. The chromium coating prevents the reaction between zircaloy and steam, which results in the formation of a brittle zirconium oxide through the surface of the cladding. We also measure the transient CHF under exponentially escalating heat flux inputs of a nano-smooth and a rough surface mimicking a chromium-coated zircaloy cladding. Interestingly, the results of the transient heat flux tests suggest that the CHF limit for very short periods (i.e., fast transients) is independent of the surface finish, being the same for a rough chromium surface or a nano-smooth surface.

Effect of Low and Sub-Freezing Temperature on the PEFC Performance of Unsupported Pt-Ni Aerogel Cathode Catalyst Layers

Polymer electrolyte fuel cells (PEFCs) have received considerable attention on account of their ability to use green hydrogen as an environmentally benign fuel. For fuel cells to replace entirely combustion engines in automotive applications, though, their durability must be enhanced and the manufacturing costs must be reduced. The latter can be accomplished by alloying platinum with other metals such as Ni, as to increase its oxygen reduction reaction (ORR)-activity and decrease the required loading of this expensive metal. Complementary, novel catalysts are needed that can sustain the corrosion of the carbon support used in the state-of-art platinum nanoparticle (Pt/C) ORR- catalysts, which is induced by the high potentials ( > 1.5 V) reached upon PEFC start-up and/or shut downs.To meet the demands mentioned above, in our previous work we presented a novel cathode catalyst layer (CL) based on an un-supported, bimetallic Pt-Ni nanochain network referred to as aerogel. This material displayed a PEFC ORR-activity ≈2.5-fold larger than a commercial Pt/C at 80 °C and 100 % relative humidity (RH), while preserving 90 % of this initial performance after an accelerated stress test that mimicked PEFC startup/shutdown (vs. only 40 % for Pt/C) [1]. Such CLs had been prepared by hand-spraying, which is highly sensitive to the handler and prone to irreproducibility. Thus, in the first part of this contribution we will show how a careful optimization of an automated spraying procedure allowed us to produce aerogel catalyst coated membranes (CCMs) that display a PEFC performance (at 80˚C, 100 % RH and 1.5 barabs) similar to our previous best results in Ref. 1.On top of this, in automotive applications PEFCs are exposed to a wide range of temperatures (including < 0°C) [2], dry conditions, and fast transients to high current densities. Such operative modes can imply problems in the evacuation of product liquid water caused by the limited water storage capacity (i.e., void volume) of the pores in such ultra-thin CLs (≈ 1.5 μm). Specifically, the water in the fuel cell could freeze when the temperature drops below 0˚C resulting in ice formation, and thus in a blockage of the pathways through the pores in the catalyst layer, thereby preventing the supply of reactants and leading to a cell failure. Thus, the second part of this contribution will feature our results with Pt3Ni CLs exposed to PEFC tests at ≤ 60 °C including sub-freezing temperature and start-up. Specifically, our PEFC tests include electrochemical impedance spectroscopy (EIS) measurements from which we estimate the effect of the temperature on the proton transport resistance through the aerogel CL, thus allowing us to quantify the mass transport losses along this broad range of conditions [3]. Based on these results, specific temperature and RH conditions were selected to conduct additional current up-transient failure tests [4, 5].In summary, this contribution will portray our efforts at demonstrating the applicability of aerogel CLs in PEFC cathodes operated under application-relevant conditions.

Radio Scattering Horizons for Galactic and Extragalactic Transients

Radio wave scattering can cause severe reductions in detection sensitivity for surveys of Galactic and extragalactic fast (∼ms duration) transients. While Galactic sources like pulsars undergo scattering in the Milky Way interstellar medium (ISM), extragalactic fast radio bursts (FRBs) can also experience scattering in their host galaxies and other galaxies intervening in their lines of sight. We assess Galactic and extragalactic scattering horizons for fast radio transients using a combination of NE2001 to model the dispersion measure and scattering time (τ) contributed by the Galactic disk, and independently constructed electron density models for the Galactic halo and other galaxies’ ISMs and halos that account for different galaxy morphologies, masses, densities, and strengths of turbulence. For source redshifts 0.5 ≤ z s ≤ 1, an all-sky, isotropic FRB population has simulated values of τ (1 GHz) ranging from ∼1 μs to ∼2 ms (90% confidence, observer frame) that are dominated by host galaxies, although τ can be ≫2 ms at low Galactic latitudes. A population at z s = 5 has 0.01 ≲ τ ≲ 300 ms at 1 GHz (90% confidence), dominated by intervening galaxies. About 20% of these high-redshift FRBs are predicted to have τ > 5 ms at 1 GHz (observer frame), and ≳40% of FRBs between z s ∼ 0.5–5 have τ ≳ 1 ms for ν ≤ 800 MHz. Our scattering predictions may be conservative if scattering from circumsource environments is significant, which is possible under specific conditions. The percentage of FRBs selected against from scattering could also be substantially larger than we predict if circumgalactic turbulence causes more small-scale (≪1 au) density fluctuations than observed from nearby halos.

MUSSES2020J: The Earliest Discovery of a Fast Blue Ultraluminous Transient at Redshift 1.063

In this Letter, we report the discovery of an ultraluminous fast-evolving transient in rest-frame UV wavelengths, MUSSES2020J, soon after its occurrence by using the Hyper Suprime-Cam (HSC) mounted on the 8.2 m Subaru telescope. The rise time of about 5 days with an extremely high UV peak luminosity shares similarities to a handful of fast blue optical transients whose peak luminosities are comparable with the most luminous supernovae while their timescales are significantly shorter (hereafter “fast blue ultraluminous transient,” FBUT). In addition, MUSSES2020J is located near the center of a normal low-mass galaxy at a redshift of 1.063, suggesting a possible connection between the energy source of MUSSES2020J and the central part of the host galaxy. Possible physical mechanisms powering this extreme transient such as a wind-driven tidal disruption event and an interaction between supernova and circumstellar material are qualitatively discussed based on the first multiband early-phase light curve of FBUTs, although whether the scenarios can quantitatively explain the early photometric behavior of MUSSES2020J requires systematical theoretical investigations. Thanks to the ultrahigh luminosity in UV and blue optical wavelengths of these extreme transients, a promising number of FBUTs from the local to the high-z universe can be discovered through deep wide-field optical surveys in the near future.

Electromagnetic Interference Filter Design for a 100 kW Silicon Carbide Photovoltaic Inverter Without Switching Harmonics Filter

The design of electromagnetic interference (EMI) filter with smaller size and lightweight for a high efficiency and high power density 100 kW SiC five-level T-type (5LT2) photovoltaic (PV) inverter becomes more challenging due to the increased EMI noise resulted by high switching frequency, fast switching transients, and the removal of LCL filter. This paper proposes an unbalanced Wheatstone bridge and a two-stage notch filter to attenuate the increased common mode (CM) and differential mode (DM) noise respectively. The conventional Wheatstone bridge will suffer size and weight penalty for 100 kW PV converter due to the high parasitic capacitance of the 100 kW PV panel. An unbalanced Wheatstone bridge with a one-stage LC filter is therefore developed to address this issue and reduce the total CM inductance. The proposed two-stage notch filter is designed to achieve high attenuation, solve its parameter variation issue and help reduce the size and weight of DM filter. The total weight of the designed EMI filter for this 100 kW PV inverter is 3.54 kg. The measured conducted EMI spectrum of the 100 kW SiC PV inverter with and without proposed EMI filter are provided to validate the effectiveness of the EMI filter design.

Extragalactic fast X-ray transient candidates discovered byChandra(2000–2014)

Context.Extragalactic fast X-ray transients (FXRTs) are short flashes of X-ray photons of unknown origin that last a few seconds to hours.Aims.Our ignorance about their physical mechanisms and progenitor systems is due in part to the lack of clear multiwavelength counterparts in most cases, because FXRTs have only been identified serendipitously.Methods.We develop a systematic search for FXRTs in theChandraSource Catalog (Data Release 2.0; 169.6 Ms over 592.4 deg2, using only observations with |b|&gt; 10° and before 2015), using a straightforward X-ray flare search algorithm and incorporating various multiwavelength constraints to rule out Galactic contamination and characterize the candidates.Results.We report the detection of 14 FXRT candidates from a parent sample of 214 701 sources. Candidates have peak 0.5–7 keV fluxes between 1 × 10−13and 2 × 10−10erg cm−2s−1andT90values from 4 to 48 ks. The sample can be subdivided into two groups: six “nearby” FXRTs that occurred withind ≲ 100 Mpc and eight “distant” FXRTs with likely redshifts ≳0.1. Three distant FXRT candidates exhibit light curves with a plateau (≈1–3 ks duration) followed by a power-law decay and X-ray spectral softening, similar to what was observed for the previously reported FXRT CDF-S XT2, a proposed magnetar-powered binary neutron star merger event. After applying completeness corrections, we calculate event rates for the nearby and distant samples of 53.7−15.1+22.6and 28.2−6.9+9.8deg−2yr−1, respectively.Conclusions.This novel sample ofChandra-detected extragalactic FXRT candidates, although modest in size, breaks new ground in terms of characterizing the diverse properties, nature, and possible progenitors of these enigmatic events.

Analytic Light Curves of Dense CSM Shock Breakout and Cooling

Dense circumstellar material (CSM) is thought to play an important role in observed luminous optical transients: if such CSM is shocked, e.g., by ejecta expelled from the progenitor during core-collapse, then radiation produced by the shock-heated CSM can power bright UV/optical emission. If the initial CSM has an “outer edge” where most of the mass is contained and at which the optical depth is large, then shock breakout—when photons are first able to escape the shocked CSM—occurs near it. The rather thin shell of shocked CSM subsequently expands, and in the ensuing cooling-envelope phase, radiative and adiabatic losses compete to expend the CSM thermal energy. Here we derive an analytic solution to the bolometric light curve produced by such shocked CSM. For the first time, we provide an analytic solution to the cooling-envelope phase that is applicable starting from shock breakout and until the expanding CSM becomes optically thin. In particular, we account for the planar CSM geometry that is relevant at early times and properly treat radiative losses within this planar phase. We show that these effects can dramatically impact the resulting light curves, particularly if the CSM optical depth is only marginally larger than c/v sh (where v sh is the shock velocity). This has important implications for interpreting observed fast optical transients, which have previously been modeled using either computationally expensive numerical simulations or more simplified models that do not properly capture the early light-curve evolution.

Calculation of voltage distribution along the transformer winding using the wide band transformer model

Electrical devices in transmission and distribution networks are submitted to fast front transients. These transients are often a cause of a failure in the power system. To simulate these phenomena, it is necessary to detailly model all the components of the power system. This paper concentrates on studding transformer internal failures due to voltage stress that can occur when fast front electromagnetic wave travels through the transformer. To prevent a failure, an internal insulation of the transformer has to be dimensioned to sustain these transient phenomena. Therefore, it is necessary to use advanced wideband transformer models for its dimensioning. In this paper, a wide band transformer model based on limited transformer geometry is used to simulate evaluate the stresses on internal transformer insulation in the case of fast front transients. The model is validated for calculation of internal overvoltages with the test case from CIGRE brochure.

On the nature of fast blue optical transients

ABSTRACT Short rise times of fast blue optical transients (FBOTs) require very light ejected envelopes, $M_{\rm ej} \le 10^{-1} \, \mathrm{M}_\odot$, much smaller than of a typical supernova. The detection by Chandra of X-ray emission in AT2020mrf of LX ∼ 1042 erg s−1 after 328 d implies total, overall dominant, X-ray energetics at the gamma-ray burst level of ∼6 × 1049 erg. We further develop a model of Lyutikov and Toonen, whereby FBOTs are the results of a late accretion-induced collapse of the product of double white dwarf (WD) merger between ONeMg WD and another WD. Small ejecta mass, and the rarity of FBOTs, results from the competition between mass-loss from the merger product to the wind, and ashes added to the core, on a time-scale of ∼103–104 yr. FBOTs proper come from central engine-powered radiation-dominated forward shock as it propagates through ejecta. All the photons produced by the central source deep inside the ejecta escape almost simultaneously, producing a short bright event. The high-energy emission is generated at the highly relativistic and highly magnetized termination shock, qualitatively similar to pulsar wind nebulae. The X-ray bump observed in AT2020mrf by SRG/eROSITA, predicted by Lyutikov and Toonen, is coming from the breakout of the engine-powered shock from the ejecta into the preceding wind. The model requires total energetics of just few × 1050 erg, slightly above the observed X-rays. We predict that the system is hydrogen poor.

Three-dimensional transient analysis of Xi’an pulsed reactor by the coupled neutronics and thermal-hydraulics code

In this paper, an integrated code is proposed to simulate the transient thermo-neutronic behavior of the Xi'an Pulsed Reactor (XAPR) core during reactivity insertion accidents (RIA), which is developed by coupling the neutron transport kinetics code DAISY and a parallel-channel thermal-hydraulics code based on the internal (master–slave) coupling scheme. The few-group parameters adopted in the simulations including the seven-group cross-sections and six-group delayed neutron precursors are calculated by the NECP-X code. The XAPR pulse operation data are first employed to validate the ability of the coupled code to perform dynamic analysis, and then transient characteristics of different parameters such as core power and fuel temperature due to the different form and magnitude of reactivity insertion has been further simulated and analyzed. The numerical results indicate that the coupled code is correct and suitable for the transient calculation of reactivity insertion in XAPR and is able to capture the main physics behind fast RIA transients.

Stress relieved Zircaloy-4 recovery and recrystallization during fast anisothermal transients

ABSTRACT Cold-work stress-relieved Zircaloy-4 kinetics of recovery-recrystallization has been studied under dynamic conditions. A specific furnace dilatometer was used to heat Zircaloy-4 specimens up to . Comparison of dynamic results with data of isothermal recovery and recrystallization showed that the process is delayed when the heating rate is increased. A model was proposed to describe the evolution of the material unrecrystallized fraction during rapid anisothermal transients. Comparisons showed a good agreement between the experimental results and the model predictions.

Power Sharing Control Algorithm for Direct Integration of Fuel Cells in a Dual-Inverter Electric Vehicle Drivetrain

Fuel cell (FC)-battery hybrid electric vehicles (EVs) are an alternative to emergent battery EVs. Normally, a dc–dc converter is used to connect the low-voltage dc output of the FC to the high-voltage EV battery and ensure that a slow-changing unidirectional power flow is maintained from the FC. This article proposes to directly integrate the FC as one of the two energy sources in a dual-inverter-based EV drive. A conventional EV battery is used as the second energy source. A power sharing algorithm is introduced, which allows the dual-inverter drive to be modulated such that unidirectional power flow is maintained from the FC. In addition, power flow is controlled such that the minimum FC power to avoid its unnecessary shutdown is maintained under all operating points of an EV drive cycle, including regenerative braking. A key aspect of this algorithm is the injection of motor reactive current as a means to achieve desired FC power transfer during fast mechanical power reduction transients or when low mechanical power is required from the drive. This method allows for the motor to achieve fast mechanical power transients while respecting the slowly changing power reference of the FC.