Large Break Research Articles

3D Analysis of Hydrogen Distribution and Its Mitigation Using Passive Autocatalytic Recombiners (PARs) Inside VVER-1000 Containment

Hydrogen is a flammable gas that can generate thermal and mechanical loads which could jeopardise the containment integrity upon combustion inside nuclear power plants containment. Hydrogen can be generated from various sources and disperses into the containment atmosphere, mixing with steam and air following a loss of coolant accident and its progression. Therefore, the volumetric hydrogen concentration should be examined within the containment to determine whether a flammable mixture is formed or not. Codes with 3D capabilities could serve this examination by providing detailed contours/maps of the hydrogen distribution inside containment in view of the local stratification phenomenon. In this study, a 3D VVER-1000 as-built containment model was sketched in AutoCAD and then processed into GOTHIC nuclear containment analysis code for hydrogen evaluation. The model was modified to a great extent by installing 80 passive autocatalytic recombiners and locating hydrogen sources to evaluate the performance of the hydrogen removal system inside the containment on maintaining the hydrogen concentration below the flammability limit during a large break loss of coolant accident. 2D profiles and 3D contours of volumetric hydrogen concentration with and without PARs are presented as the simulation outcome of this study. The results were validated against the results of the Final Safety Analysis Report, which also demonstrates the effectiveness of the hydrogen removal system as an engineered safety feature to keep the containment within a safe margin. Detailed 3D contours of hydrogen distribution inside containment can be employed to evaluate the local hot spots of hydrogen, rearranging and optimising the number and location of PARs to avoid the hydrogen explosion inside containment.

Investigating the effect of spray system on the containment condition of VVER1000/V446 NPP during LBLOCA and TLOFW accidents

The containment system serves as the last barrier against the release of fission products. However, over pressurization or hydrogen detonation can compromise its effectiveness, as evidenced by the Fukushima nuclear power plant (NPP) disaster. This study describes the efficacy of the spray system in reducingcontainment pressure andtemperature, as well as,itsimpacton hydrogen mitigation during large break loss of coolant accident (LBLOCA) and total loss of feed water (TLOFW) accident in the VVER-1000/V446NPP. The evaluation is conductedusing theMELCOR 1.8.6code.The results of the LBLOCA analysis demonstrate that the spray system significantly reduces the pressure and temperature of the containment. It also enhances hydrogen mitigation by increasing the amount of recombined hydrogen by around 4% and reducing the onset time of hydrogen recombination by 90,000 s. In the TLOFW accident scenario, the spray system is effective in reducing the containment pressure from 0.41 MPa to 0.27 MPa. The investigation of hydrogen behavior in the reactor containment atmosphere reveals that the simultaneous operation of the spray and the hydrogen recombiner systems is the most effective strategy for hydrogen mitigation. However, activating the spray system without the passive autocatalytic recombiners (PARs) function can lead to an increase in the hydrogen mole fraction in all containment compartments, thereby increasing the likelihood of hydrogen combustion.

Patterns of Esophageal Motility and Bolus Transit in Children With Repaired Esophageal Atresia

Background: Esophageal dysmotility is common after esophageal atresia (EA) repair. Esophageal manometry (HRM) is the gold standard to investigate motility. We aim to characterize esophageal dysmotility and bolus transit for liquid and solids in children with repaired EA. Method: Retrospective analysis of HRM from children with repaired EA. Five liquid and 5 solid swallows per child were assessed for the standard manometric measurement and peristalsis pattern. Impedance tracings were used to assess bolus transit. Results: Ninety measurements were assessed from 9 patients, 44% Females. Mean age was 10.5 (±4.6) years. All children had dysphagia and anastomotic strictures were excluded. None of the children had intact peristalsis, 44.4% of the liquid swallows were failed, 33.3% fragmented and 22.2% ineffective. 44.4% of the solid swallows were failed, 44.4% fragmented and 11.1% ineffective. 33.3% had incomplete bolus transit in the whole esophagus, 44.4% in the upper and 11% in the lower esophagus. Mean peristalsis break for liquid was 7 cm (±2.1) and for solid was 5.9 cm (±3.2) P .025. Conclusion: Impaired esophageal motility is universal post EA repair. Failed and ineffective peristalsis predominate with incomplete bolus transit and large peristalsis breaks after liquid and solid swallows. HRM can provide clinical guidance, diagnostic and prognostic information post EA repair.

Uncertainty Assessment in the Prediction of Selected Fission Products in the Case of a Postulated Severe Accident Scenario at VVER-1000

The aim of the paper is to present the results and insights gained from the analysis of the postulated severe accident scenario for the Nuclear Power Plant (NPP) equipped with a VVER-1000, focusing on the uncertainty assessment in the prediction of selected Fission Products (FPs). To this end, the simulation of the Large Break Loss of Coolant Accident (LB LOCA) along with a Station Blackout (SBO) scenario was carried out. The capabilities of the ASTECv2.2.0.1 [1] severe accident (SA) code along with the uncertainty quantification tool – SUNSET [2] in the coupled mode was used. The analysis was carried out for the eight input variables and ten output variables. The uncertainty quantification method incorporated within the SUNSET statistical tool was used in order to conduct the present analysis. Based on the Wilks’ formula, the number of ASTEC code runs was determined. The main statistical data are obtained for the selected fission products as figures of merit. The main steps of the analysis are outlined. Furthermore, a brief overview of the SUNSET/ASTECv2.2.0.1 coupling procedures [3] is also presented and discussed. This work demonstrates the application of the uncertainty quantification methodology in the analyses of the severe accidents in the Nuclear Power Plants and provides the specific recommendations of its use. The main outcomes from the assessment provided will be useful and may be a base for the future analyses related to the evaluation of the safety of the VVER-1000.

Uncertainty and sensitivity analysis of iodine release in severe accidents of advanced pressurized water reactors based on the Latin Hypercube method and Grey Correlation Coefficients

Based on an HPR1000 pressurized water reactor (PWR), a large break loss of coolant accident (LBLOCA) model was created in ASTEC software to simulate several accident scenarios. By varying the initial conditions, the release of iodine was evaluated to analyse the influencing factors. It was shown that the break size contributes to the release rate of iodine, while in-vessel retention (IVR) and the containment spray system (CSS) play significant roles in inhibiting the release of iodine into the environment, and more than 90% of the iodine is distributed in the containment when a break occurs in a hot leg or surge pipeline. With the use of the Latin hypercube method, grey correlation coefficients were calculated to quantitatively estimate the input parameters. The results showed that the main factors affecting iodine release vary in the different phases, and the temperature of the cooling water in external vessel reactor cooling (EVRC) displays the highest correlation coefficients, with values greater than 0.85. Overall, IVR is an important measure to prevent iodine release into containment and environmental settings.

Implementation of chrome oxidation model into best-estimate system code for accident tolerant fuel

A chrome oxidation model was developed/implemented into the MARS-KS 1.5 code to improve the prediction of accident tolerant fuel with chrome-coated cladding. Initially, a multi-layer cladding model was implemented, followed by investigation of the peak cladding temperature (PCT) during large break loss-of-coolant accidents with varying coating thicknesses. Results showed that increasing the coating thickness led to a decrease in blowdown PCT, while reflood PCT did not show a clear tendency. The chrome-coated cladding oxidation model was then developed/implemented, and validated by comparing weight gain due to oxidation using experimental data. The Parisi et al. (2020) model showed substantially lower weight gain, while the Wu et al. (2020) model predicted relatively well against the experimental data. However, there were still some scatters in oxidation data, indicating the need for developing a specific oxidation model for each Cr-coated cladding candidate for better prediction.

The large-break LOCA uncertainty analysis in a VVER-1000 reactor using TRACE and DAKOTA

An uncertainty analysis is inevitable in simulating reactor performance, particularly in the accident analysis for the estimation of safety borders. In this paper, the application of the uncertainty analysis of a Large Break Loss of Coolant Accident (LBLOCA) from main coolant pipeline at the reactor inlet and outlet, and also pressurizer (PRZ) connecting pipe are presented. First, the model developed in TRACE is verified in steady-state conditions and LBLOCA using the Final Safety Analysis Report (FSAR) of Bushehr nuclear power plant. By the verified model, the uncertainty analysis is made respecting uncertainty in boundary conditions. Finally, the analysis is achieved using the surrogate models and Wilks’ method in DAKOTA. The 95/95 uncertainty band of the peak cladding temperature (PCT) and main reactor parameters are obtained and analyzed. The results indicate that the PCT uncertainty in LBLOCA at the reactor inlet is large, the PCT varies in the range of 910 to 970 °C. The drop temperature time of fuel cladding is in the range of 270 to 600 s for LBLOCA at the reactor inlet. The PCT uncertainty in LBLOCA at the reactor outlet and PRZ connecting pipe is low.

Ultra-incompressibility and anomalous shear deformation resistance of transition metal polynitrides TMN8 (TM = Ti, Zr, Hf)

The recently proposed transition metal polynitride, tetragonal TiN8 featured infinitely armchair-like N chains, has attracted much attention for its larger high-energy density value than the well-known TNT explosive. However, as a typical transition metal nitrogen-rich compounds with outstanding mechanical properties, the electronic properties and structure-mechanical relations remain to be further explored. Here, we performed first principles calculations that investigate the thermodynamic stabilities of tetragonal TMN8 (TM = Ti, Zr, Hf), and fully studied the mechanical and elastic anisotropic behavior of these materials. In particular, the stress-strain relations and the related evolutions of chemical bonding were systematically studied. The uniaxial ultra-incompressible nature of these TMN8 has been demonstrated by the calculated elastic moduli, originated from the strong N–N covalent bonds along the c-axis. Under (110) [11‾0] shear direction, the TMN8 exhibits an abnormally large breaking shear strain that produces a strong shear strength exceeding 40 GPa. This unusual behavior stems from the strongly and stably 3D structure framework consisted of strong N–N covalent bonds in infinite N chains connected by Ti atom through Ti–N covalent bonds in Ti–N hexahedrons. These obtained findings explicate the crystal structural configurations and chemical bonding characters that are responsible for the mechanical properties of TMN8 and provide insights for understanding other transition metal polynitrides.

Multiphysics analysis of fuel fragmentation, relocation, and dispersal susceptibility–Part 3: Thermal hydraulic evaluation of large break LOCA under high-burnup conditions

Increasing the peak rod average burnup of pressurized water reactor (PWR) fuel beyond 62 GWd/tU may increase fuel fragmentation, relocation, and dispersal (FFRD) susceptibility during a large break loss of coolant accident (LBLOCA). TRACE thermal hydraulic (TH) LBLOCA analyses were performed for a realistic 24-month high-burnup PWR equilibrium cycle, to inform subsequent transient BISON high-burnup FFRD susceptibility evaluations. Realistic LBLOCA systems behavior was first established by configuring to and comparing with the BEMUSE OECD LBLOCA benchmark. Fuel and operating conditions were then applied from high-burnup VERA depletion calculations. LBLOCA simulations were performed for 281 selected high-burnup rods, for which transient TH boundary conditions were collected for later use in BISON. The TRACE results indicated that rod linear heat rate (rather than burnup) is the main predictor of peak cladding temperature (PCT) during the event. PCT typically occurred at a local burnup lower than the rod-average burnup, especially for twice-burned fuel.

The effects of cladding thermo-physical and thermo-chemical properties on the coping time during a PWR unmitigated LB-LOCA

To evaluate quantitatively the effects of individual thermo-physical and thermo-chemical cladding properties on the coping time, an unmitigated large break loss of coolant accident (LB-LOCA) was simulated using Modular Accident Analysis Program (MAAP) code. For this purpose, hypothetical claddings were created to decompose the influence of each property enhancement. The variations in the cladding properties could not affect the main conditions of the reactor coolant system such as pressure, reactor core water level, etc., but led significantly different variations in the cladding temperature. When the coping time was defined as the time elapsed between departure from normal operation and the moment at which cladding melts, the specific heat capacity (enthalpy) and melting temperature of cladding materials were identified as properties extending the coping time most. The improved oxidation resistance could extend moderately the coping time. On the other hand, the cladding thermal conductivity had no effect on the coping time.

A Case of Full-thickness Macular Hole Formation Secondary to Laser Retinopexy

Purpose: To report a case of full-thickness macular hole (FTMH) formation secondary to demarcation laser retinopexy in a retinal break with localized retinal detachment patient.Case summary: A 59-year-old male visited our clinic with ocular discomfort in both eyes. Uncorrected visual acuity (UCVA) was 0.63 in right eye. Large retinal break in 1:30 o/c, localized retinal detachment and laser marking scars all around the right eye were found in fundoscopic exam. Posterior-vitreous detachment or vitreomacular traction was not observed in optical coherence tomography (OCT). Demarcation laser retinopexy was performed around the margin of retinal detachment and peripheral degenerative lesions. Three months after demarcation laser retinopexy, UCVA in right eye of the patient was decreased to 0.16 and full thickness macular hole was observed on OCT examination. Pars planar vitrectomy, internal limiting membrane peeling, and SF6 gas tamponade were performed in the right eye. One month after the surgery, closure of FTMH was observed. Three months after surgery, there were no recurrence of FTMH in the right eye.Conclusions: Demarcation laser photocoagulation of localized retinal detachments may predispose to FTMH formation. Even though it can be occurred rarely, follow-up check-up is necessary in consideration of the possibility of FTMH, which can cause serious visual loss.

The modeling of passive containment cooling system and sensitivity analysis using the SPRUCE code

Passive Containment Cooling Systems (PCCSs) have attracted wide attention to improve the inherent safety of nuclear power plants. This study summarizes a system analysis method of a closed-loop natural circulation of the PCCS via separately evaporation and condensation. A lumped-parameter model is built using SPRUCE, a code of system simulation of severe accidents, to investigate the natural circulation characteristics and heat-removal efficiency of the PCCS. The analysis of heat-removal capacity of the PCCS is carried out under typically thermal conditions of the containment, such as pressure from 0.28 to 0.4 MPa, and the results are validated by comparison with the experimental data obtained from a single-loop testing facility of the PCCS in full scale. The SPRUCE model agrees with the data, despite slightly underestimating the rate of condensation heat transfer. It conservatively satisfies the requirement of design capacity. To further verify the effectiveness of mitigation on containment overpressure, a representative scenario of LBLOCA (Large Break Loss of Coolant Accident) is implemented using the code. Meanwhile, the effects of non-condensable gas are also presented, as a reference for optimization of PCCS design.

Structural study of wild-type and phospho-mimic XRCC4 dimer and multimer proteins using circular dichroism spectroscopy

Purpose To investigate the structural features of wild-type and phospho-mimicking mutated XRCC4 protein, a protein involved in DNA double-strand break repair. Materials and methods XRCC4 with a HisTag were expressed by E. coli harboring plasmid DNA and purified. Phospho-mimicking mutants in which one phosphorylation site was replaced with aspartic acid were also prepared in order to reproduce the negative charge resulting from phosphorylation. The proteins were separated into dimers and multimers by gel filtration chromatography. Circular dichroism (CD) spectroscopy was performed in the region from ultraviolet to vacuum-ultraviolet. The CD spectra were analyzed with two analysis programs to evaluate the secondary structures of the wild-type and phospho-mimicked dimers and multimers. Result and Discussion The proportion of β-strand in the wild-type dimers was very low, particularly in their C-terminal region, including the five phosphorylation sites. The secondary structure of the phospho-mimic hardly changed in the dimeric form. In contrast, the β-strand content increased and the α-helix content decreased upon multimerization of the wild-type protein. The structural change of multimers slightly depended on the phospho-mimic site. These results suggest that the β-strand structure stabilizes the multimerization of XRCC4 and it is regulated by phosphorylation at the C-terminal site in living cells. Conclusion An increase in the β-strand content in XRCC4 is essential for stabilization of the multimeric form through C-terminal phosphorylation, allowing the formation of the large double-strand break repair machinery.

Modelling the air-sea-land interactions responsible for the direct trigger of turbidity currents by tropical cyclones

Tropical cyclones directly trigger turbidity currents in submarine canyons as a consequence of storm surges, high waves, onshore blowing winds and extreme currents. The resultant supply of sediment at the heads of the canyons plays a crucial role in the genesis of turbidity currents and thus is key in understanding frequency and duration of their flows. Here we present a single numerical framework capable of modelling turbidity currents driven by cyclone-induced winds and waves through resolved, quasi-3D hydrostatic equations. Our simulations predict the occurrence of a canyon-confined turbid underflow induced by a tropical cyclone that caused a seafloor pipeline to shift. Turbidity current occurrence is shown to be related not just to tropical cyclone intensities but also to their tracks with respect to the canyon head. The modelled underflow is well approximated by similarity profiles from laboratory and field observations which demonstrates the reliability of the model in capturing the structure of turbidity currents. The proposed triggering of turbidity currents off the centre of coastal embayments is likely to occur when the abrupt rotation of incoming winds induced by a passing cyclone remains always coastal-bound all across the cyclone’s waxing and waning stages. Our results show that these conditions can give rise to simultaneous, opposite alongshore currents and eventually result in offshore-bound rip currents. Conversely, it is unlikely that turbidity currents will be triggered by cyclone-induced rip currents when the cyclonic rotation results in peak offshore winds (coming from the land), as no fetch is available for generating large breaking waves to induce simultaneous, opposite alongshore currents. Nevertheless, the sole presence of strong alongshore currents deflected at the headlands of a coastal embayment (or delta) is likely to trigger sediment-gravity flows and eventually result in turbidity currents offshore the edge of the embayment without the aid of rip currents.

Introducing shrinkage in heavy-tailed state space models to predict equity excess returns

AbstractWe forecast excess returns of the S &P 500 index using a flexible Bayesian econometric state space model with non-Gaussian features at several levels. More precisely, we control for overparameterization via global–local shrinkage priors on the state innovation variances as well as the time-invariant part of the state space model. The shrinkage priors are complemented by heavy tailed state innovations that cater for potential large breaks in the latent states, even if the degree of shrinkage introduced is high. Moreover, we allow for leptokurtic stochastic volatility in the observation equation. The empirical findings indicate that several variants of the proposed approach outperform typical competitors frequently used in the literature, both in terms of point and density forecasts.

Excited States, Symmetry Breaking, and Unphysical Solutions in State-Specific CASSCF Theory.

State-specific electronic structure theory provides a route toward balanced excited-state wave functions by exploiting higher-energy stationary points of the electronic energy. Multiconfigurational wave function approximations can describe both closed- and open-shell excited states and avoid the issues associated with state-averaged approaches. We investigate the existence of higher-energy solutions in complete active space self-consistent field (CASSCF) theory and characterize their topological properties. We demonstrate that state-specific approximations can provide accurate higher-energy excited states in H2 (6-31G) with more compact active spaces than would be required in a state-averaged formalism. We then elucidate the unphysical stationary points, demonstrating that they arise from redundant orbitals when the active space is too large or symmetry breaking when the active space is too small. Furthermore, we investigate the singlet-triplet crossing in CH2 (6-31G) and the avoided crossing in LiF (6-31G), revealing the severity of root flipping and demonstrating that state-specific solutions can behave quasi-diabatically or adiabatically. These results elucidate the complexity of the CASSCF energy landscape, highlighting the advantages and challenges of practical state-specific calculations.

Application of Moving Particle Simulation Method on Water-skipping

Inspired by the phenomenon of stone-skipping in life, the problem of continuous water-skipping in trans-media vehicles is widely concerned, which is characterized by multiphase flow-solid coupling, dynamic boundary, liquid surface fragmentation and high impact loads. Based on the Moving Particle Semi-implicit numerical method, this paper investigates the disc’s motion characteristics and liquid surface variation, focusing on the influence of the water entry parameters on the sloshing phenomenon of the water-skipping. The simulation results indicate that the disc water skipping process can be divided into three stages: water entry, sliding and re-flying, during which the disc is with large impact loads and the water surface is with large scale breaking and droplet splashing. With certain rotation velocity, the disc could maintain a smooth water skipping, while there will be a lateral displacement due to the Magnus effect. The larger the water-entry velocity and angel is, the larger impact load is, the faster velocity decay rate is, the larger vacuole area is, and larger the water surface broking area and the droplet splashing is. Predicted water-skipping phenomenon agrees well with the experimental result obtained from high-speed photography.

Bootstrapping multi-field inflation: non-Gaussianities from light scalars revisited

Primordial non-Gaussianities from multi-field inflation are a leading target for cosmological observations, because of the possible large correlations generated between long and short distances.These signatures are captured by the local shape of the scalar bispectrum. In this paper, we revisit the nonlinearities of the conversion process from additional light scalars into curvature perturbations during inflation.We provide analytic templates for correlation functions valid at any kinematical configuration, using the cosmological bootstrap as a main computational tool. Our results include the possibility of large breaking of boost symmetry, in the form of small speeds of sound for both the inflaton and the mediators.We consider correlators coming from the tree-level exchange of a massless scalar field.By introducing a late-time cutoff, we identify that the symmetry constraints on the correlators are modified. This leads to anomalous conformal Ward identities, and consequently the bootstrap differential equations acquire a source term that depends on this cutoff.The solutions to the differential equations are scalar seed functions that incorporate these late-time growth effects. Applying weight-shifting operators to auxiliary “seed” functions, we obtain a systematic classification of shapes of non-Gaussianity coming from massless exchange.For theories with de Sitter symmetry, we compare the resulting shapes with the ones obtained via the δN formalism, identifying missing contributions away from the squeezed limit. For boost-breaking scenarios, we derive a novel class of shape functions with phenomenologically distinct features in scale-invariant theories.Specifically, the new shape provides a simple extension of equilateral non-Gaussianity: the signal peaks at a geometric configuration controlled by the ratio of the sound speeds of the mediator and the inflaton.

Guide to anomaly-mediated supersymmetry-breaking QCD

We present a careful study of the chiral symmetry breaking minima and the baryonic directions in supersymmetric QCD ($SU({N}_{c})$ with ${N}_{f}$ flavors) perturbed by anomaly mediated supersymmetry breaking (AMSB). For the s-confining case of ${N}_{f}={N}_{c}+1$ and most of the free-magnetic phase (${N}_{f}\ensuremath{\le}1.43{N}_{c}$) we find that naive tree level baryonic runaways are stabilized by loop effects. Runaways are present, however, for the upper end of the free magnetic phase (${N}_{f}\ensuremath{\gtrsim}1.43{N}_{c}$) and into conformal window, signaling the existence of incalculable minima at large field values of $\mathcal{O}(\mathrm{\ensuremath{\Lambda}})$. Nevertheless, the chiral symmetry breaking points are locally stable, and are expected to continuously connect to the vacua of QCD for large SUSY breaking. The case of ${N}_{f}={N}_{c}$ requires particular care due to the inherently strongly coupled nature of the quantum modified moduli space. Due to the incalculability of critical K\"ahler potential terms, the stability of the chiral symmetry breaking point along baryonic directions cannot be determined for ${N}_{f}={N}_{c}$. With the exception of this case, all theories to which AMSB can be applied (${N}_{f}<3{N}_{c}$) possess stable chiral symmetry breaking minima, and all theories with ${N}_{f}\ensuremath{\lesssim}1.43{N}_{c}$ (aside from ${N}_{f}={N}_{c}$) are protected from runaways to incalculable minima.

Cosmological imprints of SUSY breaking in models of sgoldstinoless non-oscillatory inflation

In supergravity, the dynamics of the sgoldstino – superpartner of the goldstino superfield associated with the breaking of supersymmetry at low energy – can substantially modify the dynamics of inflation in the primordial Universe. So-called sgoldstinoless models assume the existence of a nilpotency constraint S 2 = 0 that effectively removes the sgoldstino from the theory. Such models were proposed to realise non-oscillatory inflation scenarios with a single scalar field, which feature a long period of kination at the end of inflation, and therefore a non-standard post-inflationary cosmology. Using effective operators, we propose models in which the sgoldstino is stabilized close to the origin to reproduce the nilpotent constraint. We show that small sgoldstino fluctuations may lead to a sizeable back-reaction on the cosmological history. We study the effect of this back-reaction on the inflation observables measured in the cosmic microwave background and confront the model to a series of constraints including limits on ΔN eff. We show that the peculiar form of the potential in the large supersymmetry breaking scale limit can generate peaks in the scalar power spectrum produced from inflation. We study how certain perturbation modes may re-enter the horizon during or after kination and show that a large supersymmetry breaking scale may lead to the formation of primordial black holes with various masses in the early Universe.