Simulation of the microfluidic mixing and the droplet generation for 3D printing of nuclear fuels

This paper presents the computational fluid dynamics modeling of an additive manufacturing process that is candidate for the production of Gen IV nuclear reactor fuels. The modeled process combines the internal gelation to produce metal hydrous oxides with the 3D ceramic printing to create a green body from these gelled oxides as described by Pouchon (2016). The objective of the simulations is to optimize the process parameters: microfluidic mixing of the internal gelation reagents and generation of droplets of the mixed solutions. The simulations were performed using the OpenFOAM software, and to perform these simulations with the correct solution parameters, the properties of the fluids of interest were measured. The results show that a thorough mixing of the metal solution and the methenamine and urea mixture in a microfluidic mixer can be achieved in tens of milliseconds by either winding the mixing channel to create secondary flows or splitting the solutions inlets to yield additional diffusion interfaces. The optimal droplet size is achieved by using a mechanically vibrating 3D printing head that leads to a frequency-following Rayleigh instability. The results of the simulations suggest the parameters (micromixer geometry, flow rate, vibration frequency and others) that will optimize the mixing efficiency in a microfluidic mixer and the droplet generation process from a 3D printing head.

Study of Microfluidic Mixing and Droplet Generation for 3D Printing of Nuclear Fuels

Internal gelation and 3D printing processes were proposed to combine in a process that is promising to produce nuclear fuels with simplified production route and enhanced in-core performance [1]. The process relies on in-situ mixing of feed solutions that bear actinides and uranium nitrates with the methenamine and urea mixture. These feed solutions should be thoroughly mixed and kept at low temperature up to an inkjet printing head, where an inkjet printing process is used to deposit controlled volumes of the mixed solution on the pellet under-fabrication. Microwave and/or laser-heating are then used to trigger the gelation reaction and to solidify these printed dots and later to drive out the excess water and pre-sinter the gelled oxides. The current study is an initial approach to optimize the upfront parameters of the 3D printing process. The feed solutions need be mixed in a passive microfluidic mixer, where the first optimization parameter is the minimum mixing time in the microfluidic mixer that will yield an acceptable mixing level. To do so, CFD modelling using OpenFOAM for different designs of microfluidic mixers was conducted to select the most efficient conception. To validate the CFD results, simulation of microfluidic mixers similar to some literature cases were performed and compared. The uncertainty of selected parameters in the CFD models was used to evaluate a conservative retention time and length. Secondly, the next printing step was modeled with OpenFOAM, which is the droplet generation process. The objective from these models was to correlate the feed parameters like the flow rate and the usage of an acoustic mechanical vibrator to the printed droplets size, generation rate, and the required stand-off distance of the printing head. To perform these CFD simulations with the right solution parameters, rheology and diffusivity measurements were conducted. A proposed empirical approach was investigated for the evaluation of the diffusivity of multi-species solutions. The resultant parameters from the current study are the mixing times and lengths for given microfluidic mixers that are industrially feasible, also, the rate and volume at which the droplets of the broth mixture are generated. These results can be used as guidelines for designing the inkjet printing head and to highlight areas where further investigations are required for an efficient production process.

EPFL2017

Dynamics of isolated confined air bubbles in liquid flows through circular microchannels: an experimental and numerical study

Navid Borhani, Sepideh Khodaparast, Mirco Magnini, John Richard Thome

Experimental and numerical studies are performed to characterise the dynamics of isolated confined air bubbles in laminar fully developed liquid flows within channels of diameters d = 0.5 mm and d = 1 mm. Water and glycerol are used as the continuous liquid phase, and therefore, a large range of flow capillary numbers 10(-4) < Ca < 10(-1) and Reynolds numbers 10(-3) < Re < 10(3) are covered. An extensive investigation is performed on the effect of bubble size and flow capillary number on different flow parameters, such as the shape and velocity of bubbles, thickness of the liquid film formed between the bubbles and the channel wall, and the development lengths in front and at the back of the bubbles. The micro-particle shadow velocimetry technique (mu PSV) is employed in the experimental measurements allowing simultaneous quantification of important flow parameters using a single sequence of high-speed greyscale images recorded at each test condition. Bubble volume and flow rate of the continuous liquid phase are precisely determined in the post-processing stage using the mu PSV images. These parameters are then used as initial and boundary conditions to set up CFD simulations reproducing the corresponding two-phase flow. Simulations based on the volume of fluid technique with the aim of capturing the interface dynamics are performed with both ANSYS Fluent v. 14.5, here augmented by implementing self-defined functions to improve the accuracy of the surface tension force estimation, and ESI OpenFOAM v. 2.1.1. The present approach not only results in valuable findings on the underlying physics involved in the problem of interest but also allows us to directly compare and validate results that are currently obtained by the experimental and computational methods. It is believed that similar methodology can be employed to rigorously investigate more complex two-phase flow regimes in micro-geometries.

Springer Verlag2015

The EQL0D fuel cycle procedure and its application to the transition to equilibrium of selected molten salt reactor designs

Boris Aviv Hombourger, Jiri Krepel, Andreas Pautz

In recent years, Molten Salt Reactors (MSRs), one of the Generation IV concepts, have noticeably gained in interest. Several molten salt reactor types use fuel in liquid form, bringing various advantages, notably in terms of fuel cycle. However, simulating the evolution of liquid fuel under irradiation necessitates specific tools and methods, for example to simulate the removal of insoluble fission products from the salt mixture or continuous refueling of the reactor. For this purpose, the Serpent 2-based procedure EQL0D has been developed. It is designed for both equilibrium and finite-time burn-up calculations in liquidfueled molten salt reactors. This paper first details the most important methods implemented in the procedure to simulate liquid-fuel systems, after which the adequacy of the procedure is verified by comparing benchmark results with a comparable code. Examples of applications of this tool to the start-up and transition to closed fuel cycle of the historical single- and two-fluid Molten Salt Breeder Reactors as well as the more recent Molten Salt Fast Reactor, a fast-spectrum concept, are then presented. The obtained results show the challenge faced by thorium-cycle breeder MSRs to transition from available nuclear fuels to a closed thorium cycle, as many candidate fuels prove to be unusable for the transition to a closed cycle. (C) 2020 Elsevier Ltd. All rights reserved.

PERGAMON-ELSEVIER SCIENCE LTD2020

Dynamics of isolated confined air bubbles in liquid flows through circular microchannels: an experimental and numerical study

Navid Borhani, Sepideh Khodaparast, Mirco Magnini, John Richard Thome

Springer Verlag2015

Study of Microfluidic Mixing and Droplet Generation for 3D Printing of Nuclear Fuels

EPFL2017