•Investigation of fuel compressibility effects on the Molten Salt Fast Reactor (MSFR) dynamics.•Modelling of the MSFR helium bubbling system.•Development of a coupled neutronics and fluid dynamics ...model for the MSFR.•Modelling of both liquid fuel and helium bubbles as compressible fluids.•Effects on compressibility due to presence and distribution of helium bubbles are investigated.
Compressible fluid dynamics is of great practical interest in many industrial applications, ranging from chemistry to aeronautical industry, and to nuclear field as well. At the same time, modelling and simulation of compressible flows is a very complex task, requiring the development of specific approaches, in order to describe the effect of pressure on the fluid velocity field. Compressibility effects become even more important in the study of two-phase flows, due to the presence of a gaseous phase. In addition, compressibility is also expected to have a significant impact on other physics, such as chemical or nuclear reactions occurring in the mixture. In this perspective, multiphysics represents a useful approach to address this complex problem, providing a way to catch all the different physics that come into play as well as the coupling between them.
In this work, a multiphysics model is developed for the analysis of the generation IV Molten Salt Fast Reactor (MSFR), with a specific focus on the compressibility effects of the fluid that acts as fuel in the reactor. The fuel mixture compressibility is expected to have an important effect on the system dynamics, especially in very rapid super-prompt-critical transients. In addition, the presence of a helium bubbling system used for online fission product removal could modify the fuel mixture compressibility, further affecting the system transient behaviour. Therefore, the MSFR represents an application of concrete interest, inherent to the analysis of compressibility effects and to the development of suitable modelling approaches. An OpenFOAM solver is developed to handle the fuel compressibility, the presence of gas bubbles in the reactor as well as the coupling between the system neutronics and fluid dynamics. The outcomes of this analysis point out that the fuel compressibility plays a crucial role in the evolution of fast transients, introducing delays in the expansion feedbacks that strongly affect the system dynamics. Moreover, it is found that the gas bubbles significantly alter the fuel compressibility, yielding even larger differences compared to the incompressible approximation usually adopted in the current MSFR solvers.
•A multiphysics model for the MSFR is extended with an SP3 neutron transport module.•The new module is successfully verified against Monte Carlo simulations.•A strong dependence of the void ...coefficient on the bubble distribution is found.•Differences are highlighted compared to a neutron diffusion approach.•The SP3 runtimes are only 17% higher compared to a diffusion module.
The aim of this paper is the extension of a multiphysics OpenFOAM solver for the analysis of the Molten Salt Fast Reactor (MSFR), developed in previous works (Cervi et al., 2017, 2018). In particular, the neutronics sub-solver is improved by implementing a new module based on the SP3 approximation of the neutron transport equation. The new module is successfully tested against a Monte Carlo model of the MSFR, in order to assess its correct implementation. Then, a neutronics analysis of the MSFR is carried out on a simplified axial-symmetric model of the reactor. Particular focus is devoted to the analysis of the MSFR helium bubbling system and its effect on reactivity. The presence of bubbles inside the reactor is handled with a two-fluid thermal-hydraulics module, previously implemented into the solver. The void reactivity coefficient is evaluated on the basis of the bubble spatial distribution calculated by the multiphysics solver. Then, the results are compared to simulations carried out with uniform bubble distributions, highlighting significant differences between the two approaches. The outcomes of this work constitute a step forward in the multiphysics analysis of the Molten Salt Fast Reactor and represent a useful starting point for the optimization of the MSFR helium bubbling system, as well as for the development of appropriate control strategies.
•The stability of the SCWR is studied with the root locus criterion.•The effects of core power and coolant flow rate on stability are assessed.•The SCWR proves to be stable over its entire ...operational range.•A comparison with the BWR stability characteristics is presented.
The Supercritical Water Reactor (SCWR) is a concept for an advanced nuclear reactor operating at high temperature (500 °C average core outlet temperature at nominal power) and at high pressure (25 MPa), which give the SCWR a thermal efficiency of about 45%. However, due to the strong variability of the water properties near the thermodynamic pseudocritical point, concerns are raised towards thermal-hydraulic instabilities. A simulation tool was developed in Matlab® from the perspective of linear systems, aimed at investigating the reactor stability and identifying potential regions of instability through a consolidated and relatively simple approach. A frequency-domain stability analysis of the SCWR is carried out with the root locus criterion, characterizing the system stability features over its entire operating power interval. The impact of the coolant flow rate on the stability is also studied. The results show that the system is stable over the whole investigated operational range. Finally, the dynamic behavior of the SCWR is compared to the Boiling Water Reactor (BWR), pointing out significant differences due to the different working points and design features of the two reactors. The results of this study could be a starting point for further research on the SCWR, providing the designers with important feedbacks for the optimization of the SCWR coolant circuit.
The complex chemistry of copper (Cu) in freshwater sediments at low concentrations is not well understood. We evaluated the transformation processes of Cu added to freshwater sediments under suboxic ...and anoxic conditions. Freshwater sediments from three sources in Michigan with different characteristics (Spring Creek, River Raisin, and Maple Lake) were spiked with 30 or 60 mg kg−1 Cu and incubated under a nitrogen atmosphere. After 28-d, each treatment subset was amended with organic matter (OM) to promote anoxic conditions and evaluate its effects on Cu speciation. OM addition triggered a shift from suboxic to anoxic conditions, and sequential extractions showed that Cu accordingly shifted from acid-soluble to oxidizable fractions. Extended X-ray absorption fine-structure (EXAFS) spectroscopy revealed that Cu sulfides dominated all anoxic samples except for Spring Creek 30 mg kg−1, where Cu(I) was predominantly complexed to thiol groups of OM. Covellite and chalcopyrite (CuFeS2) were the predominant Cu species in nearly all anoxic samples, as determined by Raman spectroscopy, scanning electron microscopy, and X-ray absorption near-edge structure (XANES) spectroscopy. Copper reduction also occurred under suboxic conditions: for two of three sediments, around 80% had been reduced to Cu(I), while the remaining 20% persisted as Cu(II) complexed to OM. However, in the third coarsest (i.e., Spring Creek), around 50% of the Cu had been reduced, forming Cu(I)-OM complexes, while the remainder was Cu(II)-OM complexes. Toxicity tests showed that survival of H. azteca and D. magna were significantly lower in suboxic treatments. Anoxic sediments triggered a near-complete transformation of Cu to sulfide minerals, reducing its toxicity.
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•Organic matter triggered anoxic conditions reducing highly oxidized S species.•Cu speciation altered from acid soluble to oxidizable fraction under anoxia.•CuS (covellite) predominated in both suboxic and anoxic fine sediments.•Cu(I)-thiol and Cu(II)-humic acid dominated coarse suboxic sediments.•CuS precipitation reduced acute and chronic Cu toxicity on H. azteca and D. magna.
In this work, complex network theory is applied for the first time in the field of nuclear reactor physics to present a new approach for the evaluation of the multiplication factor of a nuclear ...system. The approach describes the random walk of a neutron in a network representative of the nuclear system. The network is constituted by multiple layers, each one representing a type of reaction (scattering, fission and capture) and each layer is constituted by different nodes, each one representing a different spatial position in the nuclear system. The probability of a neutron to jump from a node to another is governed by the material cross sections.
Good agreement is achieved between the predictions of the proposed method and of Monte Carlo simulation.
The outcome of this work constitutes a starting point for further research on the application of complex network theory to the field of nuclear reactor physics, while, at the same time, the experience retrieved from the application of complex networks may give useful insight for the improvement of classical approaches to nuclear reactor analysis.
The development of next-generation nuclear reactors requires a careful investigation of their stability characteristics and of their overall dynamical behavior. In the current work, a stability ...analysis is carried out from the perspective of linear systems for all the Gen-IV reactor concepts. Linear, zero-dimensional models of the reactors are developed in MATLAB® and the root locus criterion is applied to investigate the stability of the systems over their entire power range. The analysis is carried out for the stand-alone cores, assuming the inlet coolant temperature as a fixed parameter, and also considering the primary coolant circuit of each reactor, in order to evaluate the effect of out-of-core energy dynamics on the systems stability. All the reactors proved to be stable with a large margin over their nominal power. In the last part of the work, a comparison of the dynamic behavior of the Gen-IV reactors is presented, in order to point out the influence of the different geometrical features and of the different materials employed as coolant.
Summary
The multidisciplinary team brief and effective clinical decision‐making are critical to airway surgery. To illustrate this, we present the case of a 58‐year‐old female with papillary thyroid ...cancer invading the trachea. We describe a basic framework that was used to aid planning the management of this patient. Tracheal resection is a complex airway operation requiring the evaluation of airway obstruction risk, the formulation of strategies for complex airway management and lung ventilation during complete resection of the tracheal segment and a handover plan for safe tracheal extubation. We suggest that team performance is facilitated by a standardised structure for consideration of anticipated events and important decisions to be made before the operation. Furthermore, it can provide a platform to engage the team when unanticipated events occur and alternate plans have to be made in a time‐critical manner.
Evidence-based language Allen, J.G.; Cervi, E.
British journal of anaesthesia,
January 2014, 2014-Jan, 2014-01-00, 20140101, Letnik:
112, Številka:
1
Journal Article
•A coupled neutronics-shock physics solver for imploding fissile solids is presented.•A hydrodynamic model is developed to simulate shock propagation in solids.•Different neutron transport models are ...developed to evaluate the neutron flux.•An ALE moving mesh is implemented to describe shock-induced deformations.•All the developed models are verified/validated with many test cases.
The aim of this work is to present a coupled neutronics-shock physics model for the study of shockwave compression of solid fissile materials. The shock-physics solver implements multi-material continuum mechanics balance equations, a hydrodynamic material response model and a dynamic mesh to describe shock-induced deformations in solid bodies. In addition, an Arbitrary Lagrangian-Eulerian (ALE) formulation of the governing equations is adopted, in order to avoid mesh distortion and tangling problems in case of large deformations. As for neutronics, a multigroup SP3 neutron transport model is selected for the estimation of the neutron flux.
Several case studies are presented to validate the developed models, demonstrate the coupling between the two physics and highlight the advantages of the ALE approach.
The proposed model can be a useful tool for the simulation of shock implosion of fissile materials, such as in subcritical plutonium experiments or in reactivity accidents initiated by strong energetic events.
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