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We propose viscoelastic smoothed particle hydrodynamics (SPH) with extended boundary conditions as a new method to model the extracellular matrix (ECM) in contact with a migrating cell. The contact mechanics between a cell and ECM is modeled based on an existing boundary method in SPH that corrects for the well-known missing kernel support problem in Fluid Structure Interactions (FSI). This boundary method is here extended to allow the modeling of moving boundaries in contact with a viscoelastic solid. To validate the method, simulations are performed of tractions applied to a viscoelastic solid, Stokes flow around an array of square pillars, and indentation of a viscoelastic material with a circular indenter. By drop out of the inertial terms in the SPH equations of motion, the new SPH formulation allows to solve problems in a low Reynolds environment with a timestep independent of the particle spacing, permitting to model processes at the cellular scale (i.e. μm-scale). The potential of the method to capture cell-ECM interactions is demonstrated by simulation of a self propelling object that locally degrades the ECM by fluidizing it to permit migration. This should enable us to model and understand realistic cellâ matrix interactions in the future. ispartof: Computer Methods in Applied Mechanics and Engineering vol:322 pages:515-540 status: published

A new shutdown system that does not rely on absorber rods, is being developed at SCK.CEN and UCL for application in Liquid Metal Reactors (LMR). The system consists of tubes filled with absorber particles. During normal operation, these particles are kept above the active core by means of a metallic melt seal. In case of an accident, the system is activated by the temperature increase in the coolant. This leads to melting of the metal seal, releasing the absorber particles into the core. The resulting flow of the particles has been studied both experimentally, and with Discrete Element Method (DEM) simulations. This paper focuses on the second important aspect of the safety system, being the melting and flowing of the metallic seal in interaction with the solid absorber particles moving through the molten seal.

It is widely recognized that the control of the activity of dissolved oxygen is essential for the use of lead–bismuth eutectic (LBE) as primary coolant for nuclear systems. A lead oxide mass exchanger (PbO MX) comprised of a packed bed of lead oxide spheres is a promising solution to regulate the dissolved oxygen activity in the LBE. In order to design and operate a PbO MX properly, the dissolution kinetics of PbO in flowing LBE should be determined. A high-fidelity Computational Fluid Dynamic (CFD) model of solid-to-liquid mass transfer has been developed to determine the mass transfer coefficient of a PbO MX in flowing LBE. We validated the developed CFD model against experimental data obtained from the LBE test loop CRAFT, at SCK•CEN. The model is in good agreement with the experimental data. The effect of the channel to particle diameter ratio ( R ) on the mass transfer coefficient was evaluated by using the validated model. A mass transfer correlation for PbO MX was obtained, in terms of the Sherwood number, by fitting the simulation results for the general case of a random packed bed with a porosity of about 0.4.

Calculating contact forces between complex shapes for performing Discrete Element Method (DEM) simulations is a long standing problem with no unique ideal solution. In this work, a new method to calculate interactions between arbitrary rounded bodies is presented. Each body is represented by a triangulated surface mesh, in which each triangle is associated with a unique radius of curvature. Then, normal contact forces are calculated by numerically integrating a (Hertz) contact pressure formulation over the contact area between two contacting particles. This results in a mechanistic contact description that is converging with refinement of a given triangulation and directly uses physical material properties as parameters of the contact model. After showing convergence upon mesh refinement towards the Hertzian solution, the error for non-spherical curvatures is investigated and the new model is compared with an indentation experiment of a pear-shaped object. Finally, the method is demonstrated in a simulation of gravitational packing by simulating packing of spheres, pear-shaped as well as gummy bear-shaped objects. ⃝c 2014 Elsevier B.V. All rights reserved.

One important safety aspect of any reactor is the ability to shutdown the reactor. A shutdown in an ADS can be done by stopping the accelerator or by lowering the multiplication factor of the reactor and thus by inserting negative reactivity. In current designs of liquid-metal-cooled GEN IV and ADS reactors reactivity insertion is based on absorber rods. Although these rod-based systems are duplicated to provide redundancy, they all have a common failure mode as a consequence of their identical operating mechanism, possible causes being a largely deformed core or blockage of the rod guidance channel. In this paper an overview of existing solutions for a complementary shut down system is given and a new concept is proposed. A tube is divided into two sections by means of aluminum seal. In the upper region, above the active core, spherical neutron-absorbing boron carbide particles are placed. In case of overpower and loss of coolant transients, the seal will melt. The absorber balls are then no longer supported and fall down into the active core region inserting a large negative reactivity. This system, which is not rod based, is under investigation, and its feasibility is verified both by experiments and simulations. © 2012 Simon Vanmaercke et al. ispartof: Science and Technology of Nuclear Installations vol:2012 status: published