Multiphysics phase-field modeling of quasi-static cracking in urania ceramic nuclear fuel

This work presents phase-field modeling of quasi-static cracking in urania (UO₂) ceramic nuclear fuel under neutron radiation at high temperatures. Considering the tightly coupled multi-physics processes within the fuel during reactor power operation, a diffusion model including Fickian and Soret effects is used to describe the oxygen hyper-stoichiometry (x in UO_2+x), and the temperature field is given by a thermal model involving non-uniform fission-generated heat source and heat flow across fuel pellet, pellet-cladding gap and cladding to the outside heat sink. Both temperature and irradiation effects are taken into account for the basic thermo-mechanical properties and irradiation behaviors of the nuclear fuel. Especially, the acceleration of fuel thermal creep by oxygen hyper-stoichiometry is included. The fracture due to the above physical processes is approximated by a scalar phase-field variable based upon a cohesive phase-field fracture method. A granite fracture experiment is simulated to validate the thermo-fracture coupling approach. For the first time, the diffusion-thermo-mechanical-fracture coupling model is applied to UO₂ fuel pellet cracking during reactor startup, power ramp and reactor shutdown. UO₂ creep is found to play an important role on the fuel pellet fragmentation. The developed capability supports interpretation of experimental data and can guide material design of ceramic nuclear fuels.     

A molecular model for carbon black primary particles with internal nanoporosity

A molecular model of primary particles of porous carbon black has been developed. Using the hexagonal graphite sheets as building units, we simulated formation of carbon particles consisting of a core–shell structure. Several structural properties of carbon were examined. Graphite layers arrange in a concentric fashion in the shell region near the external surface of carbon. This trend gradually diminishes toward the center of carbon particles, resulting in an amorphous characteristic in the core region. In line with XRD experiments, our simulations show that about half of the graphite sheets in the carbon shell form microcrystalline domains typically consisting of 2–5 layers with a broad interlayer spacing of 0.34–0.4 nm. Starting from nonporous carbon particles with a high density of 2 g/cm³, a ‘digging’ approach was further developed to particularly model the internal nanoporosity of mesoporous carbon materials that are often obtained by the silica templating technique. The validity of the modeling technique to generate pores inside carbon particles is discussed in view of reproducing targeted PSDs.     

Numerical simulation and sensitivity analysis of effective parameters on natural convection and entropy generation in a wavy surface cavity filled with a nanofluid using RSM

ABSTRACT

In this work, a 2-D numerical investigation and a sensitivity analysis have been done on the natural convection heat transfer in a wavy surface cavity filled with a nanofluid. For this purpose, the effects of three parameters, the Rayleigh number (10³?≤Ra?≤?10⁵), nanoparticles volume fraction (0.00 ≤??≤?0.04), and the shape of the nanoparticles (spherical, blade, and cylindrical), are studied. Discretization of the governing equations is performed using a finite volume method (FVM) and solved with the SIMPLE algorithm. The effective parameters analysis is processed utilizing the Response Surface Methodology (RSM). Comparison with previously published work is performed and the results are found to be in good agreement. The results showed that increasing the Rayleigh number and ? increases the mean Nusselt number and the total entropy generation. Also, the nanofluids with spherical- and cylindrical-shaped nanoparticles have the highest and lowest Nusselt numbers and entropy generations, respectively. The sensitivity of the mean Nusselt number and entropy generation ratio to Ra and ? is found to be positive, whereas it is predicted to be negative to nanoparticles shape.     

Numerical investigation of heat and mass transfer flow under the influence of silicon carbide by means of plasma-enhanced chemical vapor deposition vertical reactor

The effect of characteristics flow (contour of velocity), mass transfer (Sherwood number) and heat transfer (Nu number) on the growth rate of silicon carbide by means of plasma-enhanced chemical vapor deposition vertical reactor is investigated. The species transport and thermal fluid transport with chemical reaction are taken into account. The steady-state laminar fluid flow and gas flow having ideal behavior are considered. A mixture of silane and propane (2% molar) as main reactant gases and hydrogen (96% molar) as propellant gas are injected into the reactor. Four different diameters of shower head, three different substrate rotation speeds and five different temperatures of the substrate are used. The finite volume method is employed to solve the problem. The governing equations are solved by upwind differencing scheme. The assumption of speed–pressure coupling leads to use of semi-implicit method for pressure-linked equations to solve the governing equation. It is found that the deposition rate reduces with the shower head diameter and value of substrate temperature and enhances with rotational speed of the substrate. Furthermore, the best shower head diameter to achieve maximum rate of deposition is 1 mm. At the end, a comparison as a limiting case of the considered problem with the existing studies is made. Comparing the results of this experiment with prior studies has shown acceptable consistency.

     

Making vinyl ester resin greener: Succinic acid–glycerol-derived reactive diluent as an alternative to styrene

A bio-based reactive diluent (BRD) was synthesized from succinic acid and glycerol, and successfully copolymerized with epoxy acrylate (EA). Chemical structure of BRD was studied by nuclear magnetic resonance and Fourier transform infrared. The performance of vinyl ester resin compositions has been examined through differential scanning calorimetry, thermogravimetric analysis, and dynamic mechanical analysis, as well as tensile and flexural tests. Results demonstrated good compatibility between EA and the BRD. Compared to styrene, the BRD contribution of 50 wt% enhanced the elastic modulus (~40%). Samples copolymerized with BRD or styrene, have shown a similar thermal stability. Mechanical properties of cured blends, containing up to 25 wt% of BRD, found to be superior than styrene-diluted compartments. Viscosities of EA–BRD blends were in range of 3.25–0.43 Pa/s at 30°C. Inexpensive bio-based source, good thermomechanical and rheological properties, and great compatibility with EA are of advantages of these BRD-containing formulations.