Slow Transients Research Articles

Ladder-Type Soil Model for Dynamic Thermal Rating of Underground Power Cables

This paper presents an optimal RC ladder-type equivalent circuit for the representation of the soil for dynamic thermal rating of underground cable installations. This is useful and necessary for their optimal and accurate real-time operation. The model stems from a nonuniform discretization of the soil into layers. The resistive and capacitive circuit elements are computed from the dimensions and physical parameters of the layers. The model is perfectly compatible with the International Electrotechnical Commission thermal–electric analog circuits for cables. The optimum model order is determined, for fast and slow thermal transients, from a comprehensive parametric study. It is shown that an exponential distribution of the soil layers leads to accurate results with differences of less than 0.5 °C with respect to transient finite-element simulations. An optimal model with only five layers that delivers accurate results for all practical installations and for all time scenarios is presented. The model of this paper is a simple-to-use and accurate tool to design and analyze transient operation of underground cables. It represents a relevant improvement to the available operation and monitoring tools. For illustration purposes, a step-by-step model construction example is given. The model has been validated against numerous dynamic finite-element simulations.

Foreshock activity related to enhanced aftershock production

AbstractForeshock activity sometimes precedes the occurrence of large earthquakes, but the nature of this seismicity is still debated, and whether it marks transient deformation and/or slip nucleation is still unclear. We here study at the worldwide scale how foreshock occurrence affects the postseismic phase and find a significant positive correlation between foreshock and aftershock activities: earthquakes preceded by accelerating seismicity rates produce 40% more aftershocks on average, and the length of the aftershock zone after 20 days is 20% larger. These observations cannot be reproduced by standard earthquake clustering models that predict the accelerating pattern of foreshock occurrence but not its impact on aftershock activity. This strongly suggests that slow deformation transients, possibly related to episodic creep, could initiate prior to the main shock and extend past the coseismic phase, resulting in compound ruptures that include a very long period (up to tens of days) component.

Kinetic modeling of NO selective reduction with C3H6 over Cu-SSZ13 monolithic catalyst

• Kinetic model for lean NO reduction with C 3 H 6 on Cu/CHA monolith is developed. • Model includes the key intermediates detected by DRIFTS. • Model predicts steady state C 3 H 6 and NO conversions vs. temperature quantitatively. • Model predicts transient behavior for C 3 H 6 + O 2 and C 3 H 6 + NO + O 2 reaction systems. • Model provides useful insights into non-ammonia pathways for lean NO x reduction. A mechanistic-based kinetic model is developed for selective catalytic reduction of NO with C 3 H 6 on Cu chabazite (Cu-SSZ13) monolithic catalyst based on bench scale flow reactor studies and in-situ DRIFTS measurements. The SCR mechanism involves reaction between oxygenates (partially oxidized hydrocarbon species) and NO to form isocyanates species ( NCO), detected by DRIFTS, which are further reduced to N 2 . We have shown in an earlier study that reaction intermediates, most likely surface isocyanates, poison the active sites on the catalytic surface resulting in the inhibition of other reactions (Raj et al., 2013 [7]). The kinetic model was developed in steps, starting with steady-state CO oxidation, followed by C 3 H 6 oxidation, and then the C 3 H 6 + NO + O 2 reaction system. This approach ensured consistency in the parameter estimation and resulted in a more robust model. The models for CO + O 2 , C 3 H 6 + O 2 and C 3 H 6 + NO + O 2 were also validated using a new set of steady state experiments. The monolith model predicts the observed negative order with respect to C 3 H 6 and positive order with respect to O 2 for the C 3 H 6 oxidation reaction. For the C 3 H 6 + NO + O 2 reaction system, the predicted NO and C 3 H 6 conversions agree well with both steady state and transient experimental data. The model captures the significant shift of C 3 H 6 light-off to higher temperature in the presence of NO due to formation of the inhibiting NCO surface species as well as slow transients associated with surface intermediates and their effect on the extent of reactions.

Reply to comments on the article by S. A. Mallios and V. P. Pasko “Charge transfer to the ionosphere and to the ground during thunderstorms”

Davydenko and Mareev [2014] question some considerations that were made in order to model and quantify the contribution of lightning discharges to the Global Electric Circuit (GEC), and the application of the model to the worldwide lightning activity as presented byMallios and Pasko [2012]. In this reply we address their points believing that the reply along with the comment will contribute to a better understanding of the studied problem.

On Photoelectrical Properties of 6H-SiC Bulk Crystals PVT-Grown on 6H- and 4H-SiC Substrates

Free carrier lifetimes and diffusion coefficients were determined in 6H-SiC bulk crystals grown by PVT on 6H-and 4H-SiC seeds varying the temperature from 300 K to 650 K and at excess carrier densities ΔN0 from 1017 cm-3 to 1019 cm-3. Carrier generation was achieved by using a single or two-photon absorption of picosecond pulses at 351 and 532 nm, respectively. Fast and slow recombination transients revealed the decay time of free carriers and the presence of deep acceptor traps. The thermal trap activation energy Ea = 0.33 eV was determined in the 6H/4H sample and ascribed to the boron, while the presence of deeper traps is suggested in the 6H/6H sample. At room temperature and reaching conditions of trap saturation regime (ΔN0 1019 cm3), both crystals revealed the bipolar diffusion coefficient Da 4 cm2/s. For comparison, we also determined the photoelectrical parameters in commercial 6H-SiC crystals grown by the Lely and PVT techniques.

Optimization of Survey Strategies for Detecting Slow Radio Transients

AbstractWe investigate the optimal tradeoff between sensitivity and field of view in surveys for slow radio transients using the event detection rate as the survey metric. This tradeoff bears implications for the design of surveys conducted with upcoming widefield radio interferometers, such as the ASKAP VAST survey and the MeerKAT TRAPUM survey. We investigate (i) a survey in which the events are distributed homogeneously throughout a volume centred on the Earth, (ii) a survey in which the events are homogeneously distributed, but are only detectable beyond a certain minimum distance, and (iii) a survey in which all the events occur at an identical distance, as is appropriate for a targetted survey of a particular field which subtends Npoint telescope pointings. For a survey of fixed duration, Tobs, we determine the optimal tradeoff between number of telescope pointings, N, and integration time per field. We consider a population in which the event luminosity distribution follows a power law with index − α, and tslew is the slewing time between fields or, for a drift scan, the time taken for the telescope drift by one beamwidth. Several orders of magnitude improvement in detection rate is possible by optimization of the survey parameters. The optimal value of N for case (i) is Nmax ~ Tobs/4tslew, while for case (iii) we find Nmax = (Lmax/L0)2[(3 − α)/2]2/(α − 1), where Lmax is the maximum luminosity of a transient event and L0 is the minimum luminosity event detectable in an integration of duration Tobs. (The instance Nmax > Npoint in (iii) implies re-observation of fields over the survey area, except when the duration of transient events exceeds that between re-observations of the same field, where Nmax = Npoint applies instead.) We consider the balance in survey optimization between telescope field of view, Ω, and sensitivity, characterised by the minimum detectable flux density, S0. For homogeneously distributed events (i), the detection rate scales as NΩS−3/20, while for targetted events (iii) it scales as NΩS1 − α0. However, if the targetted survey is optimised for N the event detection rate scales instead as ΩS−20. This analysis bears implications for the assessment of telescope designs: the quantity ΩS−20 is often used as the metric of telescope performance in the SKA transients literature, but only under special circumstances is it the metric that optimises the event detection rate.

Validation of a Dynamic Model of the Brindisi Pilot Plant

Abstract In this work, a dynamic model of the Brindisi CO2 capture pilot plant is implemented in K-spice general simulation tool. The model is used to simulate relevant step changes performed during a pilot plant campaign conducted in the EU project Octavius in May and June 2013. Model results are compared to dynamic pilot plant data and it shows good transient agreement to the experimental results. The model is therefore able to capture the main process dynamics. An offset is, however, observed in some cases, especially during the initial simulation time. This is most likely caused by the fact that the model was given a steady state starting point, while the pilot plant was not necessarily completely at steady state when the step change was introduced. It is challenging to ensure steady state conditions prior to dynamic tests in a pilot plant, especially for one that is connected to a real power production unit as this one. Power production variations will act as disturbances to the capture unit, and due to slow transients in the solvent inventory of the capture unit, it will take several hours to ensure steady state conditions with stable inlet flue gas conditions.

GRB Orphan Afterglows in Present and Future Radio Transient Surveys

AbstractOrphan Afterglows (OA) are slow transients produced by Gamma Ray Bursts seen off–axis that become visible on timescales of days/years at optical/NIR and radio frequencies, when the prompt emission at high energies (X and γ rays) has already ceased. Given the typically estimated jet opening angle of GRBs θjet~ 3°, for each burst pointing to the Earth there should be a factor ~ 700 more GRBs pointing in other directions. Despite this, no secure OAs have been detected so far. Through a population synthesis code we study the emission properties of the population of OA at radio frequencies. OAs reach their emission peak on year-timescales and they last for a comparable amount of time. The typical peak fluxes (which depend on the observing frequency) are of few μJy in the radio band with only a few OA reaching the mJy level. These values are consistent with the upper limits on the radio flux of SN Ib/c observed at late times. We find that the OA radio number count distribution has a typical slope − 1.7 at high fluxes and a flatter ( − 0.4) slope at low fluxes with a break at a frequency–dependent flux. Our predictions of the OA rates are consistent with the (upper) limits of recent radio surveys and archive searches for radio transients. Future radio surveys like VAST/ASKAP at 1.4 GHz should detect ~ 3 × 10− 3OA deg− 2yr− 1, MeerKAT and EVLA at 8.4 GHz should see ~ 3 × 10− 1OA deg− 2yr− 1. The SKA, reaching the μJy flux limit, could see up to ~ 0.2 − 1.5 OA deg− 2yr− 1. These rates also depend on the duration of the OA above a certain flux limit and we discuss this effect with respect to the survey cadence.

Spontaneous stabilization of HTGRs without reactor scram and core cooling—Safety demonstration tests using the HTTR: Loss of reactivity control and core cooling

It is well known that a High-Temperature Gas-cooled Reactor (HTGR) has superior safety characteristics; for example, an HTGR has a self-control system that uses only physical phenomena against various accidents. Moreover, the large heat capacity and low power density of the core result in very slow temperature transients. Therefore, an HTGR serves inherently safety features against loss of core cooling accidents such as the Tokyo Electric Power Co., Inc. (TEPCO)’s Fukushima Daiichi Nuclear Power Station (NPS) disaster. Herein we would like to demonstrate the inherent safety features using the High-Temperature Engineering Test Reactor (HTTR). The HTTR is the first HTGR in Japan with a thermal power of 30MW and a maximum reactor outlet coolant temperature of 950°C; it was built at the Oarai Research and Development Center of Japan Atomic Energy Agency (JAEA). In this study, an all-gas-circulator trip test was analyzed as a loss of forced cooling (LOFC) test with an initial reactor power of 9MW to demonstrate LOFC accidents. The analytical results indicate that reactor power decreases from 9MW to 0MW owing to the negative reactivity feedback effect of the core, even if the reactor shutdown system is not activated. The total reactivity decreases for 2–3h and then gradually increases in proportion to xenon reactivity; therefore, the HTTR achieves recritical after an elapsed time of 6–7h, which is different from the elapsed time at reactor power peak occurrence. After the reactor power peak occurs, the total reactivity oscillates several times because of the negative reactivity feedback effect and gradually decreases to zero. Moreover, the new conclusions are as follows: the greater the amount of residual heat removed from the reactor core, the larger the stable reactor power after recriticality owing to the heat balance of the reactor system. The minimum reactor power and the reactor power peak occurrence are affected by the neutron source. The greater the strength of the neutron source, the larger the minimum reactor power.

Correcting High-Resolution Borehole Strainmeter Data from Complex External Influences and Partial-Solid Coupling: the Case of Trizonia, Rift of Corinth (Greece)

High-resolution borehole strainmeters are usually installed in tectonically active regions in order to detect slow-slip events, and to estimate slow transients related to earthquake swarms. However, they are also sensitive to other numerous influences, internal or external. Furthermore, the quality of their coupling to the rock through cementation, and the mechanical properties of the rock mass around them, have a critical influence on their records. Many of the existing strainmeters present such problems, and the correction for these effects often remains a challenge. In this paper, we present the analysis of the records of a high-resolution borehole dilatometer (Sacks–Evertson), located in the seismically active rift of Corinth (Greece) (station TRZ in the Trizonia island). We show that the instrument suffers from an only partial-solid coupling, and that the nearby sea tides have a direct (through elastic response) and indirect (through pore-pressure diffusion) effect on the dilatation signal, which adds up to the solid tidal strain source. We propose a methodology that allows, in a first step, to better separate the internal (solid tide) from the external (air pressure, sea level) influences, by calculating a frequency-dependent transfer function outside the range of the tidal periods. We then extrapolate this function, in particular at the tidal periods. In a second step, the resulting variation with frequency of the coupling coefficients with sea level led us to estimate the proportion of instrument not solidly cemented to rock (thus in contact with water pore pressure), which is about 90 % of the total height. Despite the small proportion of solid coupling, the sensor resolution remains very good up to a few tens of hours of a time period, thanks to the confining effects of the rocks on the local pore pressure. These results allow us to correct for the external effects, and reduce the associated variance by 80–90 % (in the period range of minutes to days). The empirical correction of the sea level effect could be explained using a simple Boussinesq’s approximation and 1D pore-pressure diffusion model, which contributed to better constraint of some of the poro-elastic parameters in the vicinity of the instrument. After correction, the solid tidal signal at the 24-h period is almost anti-correlated with those of the theoretical solid tide. This surprising result is consistent with a similar anti-correlation observed for the longest period surface waves (200 s) comparing the TRZ dilatometer signals to the strain measured by a nearby borehole strainmeter (MOK, 15 km). This could be related to the presence of a shallow fault close to the instrument, which would creep in response to seismic wave-related stress.

Design-Related Studies for the Preliminary Safety Assessment of the Molten Salt Fast Reactor

Molten salt reactors are liquid fuel reactors so that they are flexible in operation, but they are very different from solid fuel reactors in the approach to safety. This study concentrates on the specific concept named Molten Salt Fast Reactor (MSFR). Since this new nuclear technology is in development, safety is an essential point to be considered all along the research and development studies. After a short description of the MSFR systems, necessary to device accidental scenarios, this paper will focus on the decay heat evaluation of such a reactor. Among different contributions, the decay heat of fission products in the MSFR is evaluated to be low (3% of nominal power), mainly due to the reprocessing during the reactor operation. As a result, the contribution of the actinides is significant (0.5% of nominal power). However, the decay heat of the fission products is important, and among the different uncertainty sources, the fission yield uncertainties are pointed out. The unprotected loss of heat sink transients are studied in this paper. It appears that slow transients are favorable (>1 min) to minimize the temperature increase of the fuel salt. This work will be the basis of further safety studies as well as an essential parameter for the design of the draining system.

Representative Source Terms and the Influence of Reactor Attributes on Functional Containment in Modular High-Temperature Gas-Cooled Reactors

Modular high-temperature gas-cooled reactors (MHTGRs) offer a high degree of passive safety. The low power density of the reactor and the high heat capacity of the graphite core result in slow transients that do not challenge the integrity of the robust TRISO fuel. Another benefit of this fuel form and the surrounding graphite is their superior ability to retain fission products under all anticipated normal and off-normal conditions, which limits reactor accident source terms to very low values. In this paper, we develop estimates of the source term for a generic MHTGR to illustrate the performance of the radionuclide barriers that comprise the MHTGR functional containment. We also examine the influence of initial fuel quality, fuel performance/failure, reactor outlet temperature, and retention outside of the reactor core on the resultant source term to the environment.

On-line reactivity calculation using Lagrange method

In this paper, a novel multi-step method is proposed for solving the inverse point kinetics problem using Lagrange polynomial method. By use of this approach, the need for nuclear power history or the Laplace transform is vanished. Furthermore, the accuracy of the method is of order hn for the (n+1)-point formula, where h is the computational time-step.The three- and five-point formulas of the Lagrange method are used for on-line reactivity calculations and results are benchmarked against reference solutions for different nuclear power forms. Moreover, results for different computational time-steps are compared in each case.The results show the accuracy of the proposed method in all benchmarking cases. For slow transients (large reactor periods), it is shown that time-steps of up to 1s lead to highly reliable reactivity calculations. However, the optimal time-step in almost all cases is shown to be 0.1s. The main advantage of the proposed approach, in contrast with previous numerical methods, is its stability and convergence in large time-step calculations.The proposed method can be used as real time reactivity meter in all nuclear reactors without limitation of nuclear power form.

A Grid Fundamental and Harmonic Component Detection Method for Single-Phase Systems

Single-phase grid-connected converters are widely used in many applications such as photovoltaics, fuel cells, active power filters, etc. An important topic for the development of their control schemes is ac signal detection, such as grid phase detection for grid-interfacing inverters, and harmonic detection for harmonic compensation devices. Since only one signal is available, the task is more difficult than in three-phase systems. Among the existing methods, the frequency-domain ones are known to have a one-cycle delay and heavier computational burden. Meanwhile, the time-domain methods often rely on phase-locked loop, quadrature signal generation, and complex filtering techniques; the resulted multiple-looped system may suffer from slow transients and stability issues. This paper proposes a new detection method based on anticonjugate harmonic decomposition and cascaded delayed signal cancellation. The method uses constant zero as the quadrature signal, and has a completely open-looped structure. The resulted detection system is very simple and robust. The fundamental and harmonic detection transients can be as short as 0.47 cycle in most cases, or 1.5 cycles for cases with considerable frequency variations. Meanwhile, zero steady-state error can be guaranteed in complicated harmonic scenarios, including all typical single-phase system harmonics. The performance of the proposed detection method is verified by experiments.

Methodology for the selection of long‐medium‐voltage power cable configurations

Single conductor cables are widely used in several types of installations over the last years, including the interconnection of wind parks to the power grid. Owing to the complexity of cable installations each configuration must be treated individually, regarding the use of sheath cross-bonding, the influence of grounding resistances and the use of sheath voltage limiters. In this study, a systematic investigation of the performance of a long-medium-voltage cable configuration is presented. Sheath voltages and currents are calculated during steady-state and short-circuit conditions for varying sheath arrangements using the ATP/EMTP software. Results are validated by field measurements. The analysis is also extended to include the investigation of overvoltages in the installation, during slow and fast transients, according to the basic insulation level for the cable system.

VAST: An ASKAP Survey for Variables and Slow Transients

AbstractThe Australian Square Kilometre Array Pathfinder (ASKAP) will give us an unprecedented opportunity to investigate the transient sky at radio wavelengths. In this paper we present VAST, an ASKAP survey for Variables and Slow Transients. VAST will exploit the wide-field survey capabilities of ASKAP to enable the discovery and investigation of variable and transient phenomena from the local to the cosmological, including flare stars, intermittent pulsars, X-ray binaries, magnetars, extreme scattering events, interstellar scintillation, radio supernovae, and orphan afterglows of gamma-ray bursts. In addition, it will allow us to probe unexplored regions of parameter space where new classes of transient sources may be detected. In this paper we review the known radio transient and variable populations and the current results from blind radio surveys. We outline a comprehensive program based on a multi-tiered survey strategy to characterise the radio transient sky through detection and monitoring of transient and variable sources on the ASKAP imaging timescales of 5 s and greater. We also present an analysis of the expected source populations that we will be able to detect with VAST.

ASGARD: A LARGE SURVEY FOR SLOW GALACTIC RADIO TRANSIENTS. I. OVERVIEW AND FIRST RESULTS

Searches for slow radio transients and variables have generally focused on extragalactic populations, and the basic parameters of Galactic populations remain poorly characterized. We present a large 3 GHz survey performed with the Allen Telescope Array (ATA) that aims to improve this situation: ASGARD, the ATA Survey of Galactic Radio Dynamism. ASGARD observations spanned 2 years with weekly visits to 23 deg^2 in two fields in the Galactic Plane, totaling 900 hr of integration time on science fields and making it significantly larger than previous efforts. The typical blind unresolved source detection limit was 10 mJy. We describe the observations and data analysis techniques in detail, demonstrating our ability to create accurate wide-field images while effectively modeling and subtracting large-scale radio emission, allowing standard transient-and-variability analysis techniques to be used. We present early results from the analysis of two pointings: one centered on the microquasar Cygnus X-3 and one overlapping the Kepler field of view (l = 76{\deg}, b = +13.5{\deg}). Our results include images, catalog statistics, completeness functions, variability measurements, and a transient search. Out of 134 sources detected in these pointings, the only compellingly variable one is Cygnus X-3, and no transients are detected. We estimate number counts for potential Galactic radio transients and compare our current limits to previous work and our projection for the fully-analyzed ASGARD dataset.

A geochemical model for interpretation of chalk core flooding experiments

We develop a model that takes into account the combined effect of transport and chemical reactions for the study of various flooding experiments with chalk cores where the flooding involves seawater-like brines. The chemical system we consider is based on a selection of the most prominent ions and minerals relevant for the lab experiments in question. We include mechanisms such as advection, diffusion, and water–rock chemistry in terms of precipitation and dissolution, ion exchange, aqueous equilibrium of carbon species, complexes, and charge balance. The aqueous chemistry and the ion exchange processes are assumed to take place instantaneously whereas kinetic is included for the dissolution/precipitation processes. An important motivation behind the development of the model is to bring forth a model that can support the study of water-weakening effects as well as brine-dependent oil recovery in the context of chalk reservoirs.In particular, we test the performance of the proposed model by comparing with some recent flooding experiments involving MgCl2 type of brines and seawater-like brines. The measured effluent concentrations reflect that there typically are some rapid transient effects for small times followed by some slower transients, and finally, convergence to steady state concentrations. For a fixed set of reaction rate constants, various standard thermodynamic constants, flow injection rate, and diffusion coefficient, the proposed model is able to capture main trends of the measured outlet ion concentrations for a set of tests with the above mentioned brines. Consequently, the model shows that the experimental behavior can possibly be understood as a result of an interplay between advection and diffusion, and water–rock interaction in terms of precipitation/dissolution of minerals and ion exchange processes. We use the model to detect the role played by the different components, in particular, the role played by the ion exchange and the dissolution/precipitation which seems important for further investigations of brine-dependent water weakening and oil-recovery mechanisms.

Heterogeneity and independency of unitary synaptic outputs from hippocampal CA3 pyramidal cells

The variation of individual synaptic transmission impacts the dynamics of complex neural circuits. We performed whole-cell recordings from monosynaptically connected hippocampal neurons in rat organotypic slice cultures using a synapse mapping method. The amplitude of unitary excitatory postsynaptic current (uEPSC) varied from trial to trial and was independent of the physical distance between cell pairs. To investigate the source of the transmission variability, we obtained patch-clamp recordings from intact axons. Axonal action potentials (APs) were reliably transmitted throughout the axonal arbour and showed modest changes in width. In contrast, calcium imaging from presynaptic boutons revealed that the amplitude of AP-evoked calcium transients exhibited large variations both among different boutons at a given trial and among trials in a given bouton. These results suggest that a factor contributing to the uEPSC fluctuations is the variability in calcium dynamics at presynaptic terminals. Finally, we acquired triple whole-cell recordings from divergent circuit motifs with one presynaptic neuron projecting to two postsynaptic neurons. Consistent with the independency of calcium dynamics among axonal boutons, a series of uEPSC fluctuations was not correlated between the two postsynaptic cells, indicating that different synapses even from the same neuron act independently.We conclude that the intra-bouton and inter-bouton variability in AP-induced calcium dynamics determine the heterogeneity and independency of uEPSCs.

How Things Get Stuck: Kinetics, Elastohydrodynamics, and Soft Adhesion

We consider the sticking of a fluid-immersed colloidal particle with a substrate coated by polymeric tethers, a model for soft, wet adhesion in many natural and artificial systems. Our theory accounts for the kinetics of binding, the elasticity of the tethers, and the hydrodynamics of fluid drainage between the colloid and the substrate, characterized by three dimensionless parameters: the ratio of the viscous drainage time to the kinetics of binding, the ratio of elastic to thermal energies, and the size of the particle relative to the height of the polymer brush. For typical experimental parameters and discrete families of tethers, we find that adhesion proceeds via punctuated steps, where rapid transitions to increasingly bound states are separated by slow aging transients, consistent with recent observations. Our results also suggest that the bound particle is susceptible to fluctuation-driven instabilities parallel to the substrate.