•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 paper focuses on chloride corrosion of steel rebar in alkaline solutions. The effect of chloride content on corrosion initiation was assessed through cyclic voltammetry (CV) tests at pH values ...between 12.6 and 13.8. The potential value E1,1st was derived by CV tests to define the pitting initiation condition. The effect of inhibitor addition on the critical chloride content was also assessed. The performances of two innovative organic-acid-based inhibitors were compared to that of sodium nitrite, which is the most used for chloride corrosion prevention in concrete; the parameter f* was proposed for comparison purposes; this parameter defines the efficiency of the substance compared to alkalinity to prevent pitting initiation.
The results confirm the role of the chloride-alkalinity ratio in corrosion initiation; when the value of the ratio exceeds 0.6, the probability increases, confirming the literature data. The addition of nitrites causes an increase in the critical chloride threshold, as expected. The efficiency of sodium nitrite is similar to the effect of alkalinity; the value of the coefficient f* is equal to 0.84. Sodium aspartate exhibited the highest effect at pH values approaching 13.7; lower pH values resulted in lower efficiencies, nearly reaching zero at pH 12.6. Sodium lactate showed the lowest efficiency (f*=0.01).
Additive manufacturing (AM) technologies are gaining increasing attraction for biomedical applications due to the granted customizability and optimal surface topography for osteointegration. ...Ti-6Al-4V is one of the most promising materials due to its biocompatibility. However, excessive ions release can occur, leading to a relevant immunologic response in the surrounding tissues. Despite the corrosion behavior of the conventionally-manufactured material being well known, it should be assessed for AM-processed components, as the effect of the unique superficial and microstructural features granted by the process is still quite unknown. The aim of this paper is the electrochemical evaluation of the passive current density of the laser powder bed fusion (LPBF)-processed Ti-6Al-4V alloy via potentiostatic tests, carried out at typical in-service potentials for biomedical implants. This parameter is correlated with the ion release rate of the alloy, a fundamental phenomenon to address to prevent possible inflammations caused by the implant. Different manufacturing conditions (surface finishing, heat treatment) and exposure time (0, 60 and 6000 h) were considered. The importance of performing these measurements over a long period (> 8 months) was demonstrated. In fact, despite the initial current densities being significantly affected by the surface and microstructural differences, the ion release rates converged for long-time exposures. The results also underlined the good corrosion resistance of the material. Poor corrosion performances, alongside with significant current densities development, were observed in the as-built condition. A pickling treatment demonstrated to mitigate such effect without compromising the unique surface finishing granted by the manufacturing technology.
The paper focuses on the reuse of crushed asphalt (GA) as a partial replacement (up to 20%) of natural aggregates for concrete manufacture. Addition of GA aggregates produced a positive effect on ...workability loss. The GA mixes, however, showed a significant tendency to bleed and segregate at the highest replacement percentage applied. GA led to a decrease of compressive strength in concrete (with respect to that of the reference concrete) up to 50% due to the weakness of the cement paste / recycled aggregate interface. To compensate for this negative effect, a reduction of w/c for the GA concretes was necessary. A decrease of w/c allowed the GA concretes to show drying shrinkage values substantially similar to those of reference concrete with the same cement factor. The experimental results confirmed the possibility of partial substitution (max. 15%) of natural aggregates with crushed asphalt for making concrete.
•Corrosion resistance of as-built AlSi10Mg lower than that of polished samples.•Chloride concentration played a key role in the initiation of localized corrosion.•Selective dissolution of the α-Al ...phase observed, especially at melt pool edges.•Si-rich particles acted as cathodes to promote preferential attack of Al.
Potentiodynamic tests were performed on AlSi10Mg alloy produced using laser powder bed fusion. As corrosion depends on the surface conditions, we compared the rough as-built surface with that formed after mechanical polishing and passivation in air. The as-built samples showed lower pitting potentials than polished samples. Corrosion initiation depended heavily on the chloride concentration. The higher potential of Si particles compared to the Al matrix provided the driving force for selective dissolution of the α-Al phase. The potential difference between the Si particles and α-Al phase was higher at the melt pool edges, resulting in preferential attack of these regions.
Laser powder bed fusion (LPBF) is one of the most widespread additive manufacturing (AM) technologies for metals, in which the components are built additively layer upon layer along the z-axis (the ...building direction) perpendicular to the building platform (xy-plane). Here, we evaluated the effect of post-processing heat treatments at 200, 300, and 400 °C on the corrosion resistance of AlSi10Mg alloy manufactured by LPBF for two orientations in the building chamber (XY – parallel to the building plane or XZ – perpendicular to the building plane). Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests were conducted at the constant chloride ion concentration of 0.02 M and 23 °C for the as-built and heat-treated specimens. All specimens were polished on the surface and displayed similar behaviors in the EIS tests. The pitting potential results were widely scattered and therefore analyzed using a statistical approach. Statistical data analysis based on analysis of variance technique (ANOVA) was performed. The results confirmed that the population significantly differs only by considering the heat treatments and the building direction plays only minor role. The cumulative distribution curves of pitting potentials showed a decrease in pitting resistance as the temperature of heat treatment increases.
Effect of heat treatment temperature on the pitting potentials of AlSi10Mg obtained by means of laser powder bed fusion. Display omitted
•Pitting potentials of AlSi10Mg present a wide experimental variability.•Cumulative frequency of pitting potentials shows a deleterious effect of the temperature of post-processing heat treatment.•Corrosion attack take place at the edge of melt pool up to 300 °C of heat treatment temperature.
The paper deals with the effect of heat treatments on corrosion resistance of an AlSi10Mg alloy obtained by means of Direct Metal Laser Sintering. The tests were performed on as-processed alloy and ...after different post-heat treatment, covering stress relieving, annealing at high temperature and water quenching.
Potentiodynamic and electrochemical impedance spectroscopy tests were carried out in aerated Harrison solution. The results show localized corrosion and selective penetrating attack at the border of melt pools on untreated or only stress relieved specimens. The selective attack was not observed after high temperature annealing. Modification of EIS plot evidenced galvanic coupling with silicon coarse particles that extensively precipitate during high temperature annealing. The morphology of precipitates is described in order to evidence their relevance in terms of rising selective penetrating attack and galvanic coupling.
•Thiosulfates inhibit the reactions between oxygen and sulfites.•Thiosulfates and sulphites can promote an increase in the corrosion rate of steel.•Sulphites and thiosulfates modify mainly cathodic ...potentiodynamic curves.•Corrosion leads to the formation of thick scales of mackinawite.
The paper deals with the general corrosion of carbon steel in geothermal plants for conveying and re-injecting condensates. The results of electrochemical and weight loss tests are discussed in function of sulphite and thiosulphate concentrations typical of Italian geothermal power plants. Thiosulphates can inhibit the reactions between oxygen and sulphites and increase the corrosion rate, but they control the acidification resulting from the reactions of sulphites with oxygen. In anaerobic solutions, both species modify mainly the cathodic process. In the absence of oxygen, the corrosion process leads to the formation of mackinawite scales, while iron sulphide forms due to the reduction of thiosulphate and sulphite.
Friction stir welded butt joints were performed on 8mm thick sheets made of AA6060 T6 aluminum alloy by means of a CNC machine tool, at feed rates between 117 and 683mm/min and tool rotational speed ...between 838 and 1262rpm. Tensile tests, metallographic analyses and micro-Vickers tests were carried out to evaluate the mechanical properties of the joints as a function of the process parameters. The fatigue behavior was studied by means of crack growth tests performed according to ASTM E647 standard on CT specimens, with propagation in the middle of joint along the weld nugget. The results show the influence of welding process parameters on mechanical properties and fatigue behavior. Reduction of UTS of about 20–30% with respect to base material occurred with rupture in the softened zone of welding, usually HAZ. In this range, slight variations of joint efficiency were observed with f/S ratio, while the width of the softening area increases for decreasing values of this parameter. Fatigue crack growth was always slower than that in the base material at low ΔK below 12MPam1/2. The effect of non-optimal welding parameters was evident at intermediate and high ΔK, due to defects, such as tunnels, that cause dramatic increase of propagation rate up to five times higher than the base material.
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