SBO analysis of a generic PWR-900 with ASTEC and MELCOR codes

Abstract After the Fukushima accident, the interest of the public to nuclear safety has growth and the international technical nuclear community has increased his attention in the investigation and the characterization of Severe Accident (SA) scenarios. In order to simulate the different, complex and multi-physical phenomena involved in a SA, computational tools, known as SA codes, have been developed in the last decades. In order to give some insights on the modelling capabilities of these tools and the differences in the calculation results, also related to the user-effect, an analysis of an unmitigated Station Black Out (SBO) occurring in a generic Western three-loops PWR 900 MWe has been carried out by the authors in the framework of the NUGENIA TA-2 ASCOM project. The simulation results of ASTEC code (study carried out with ASTEC V2, IRSN all rights reserved, [2019]), developed by IRSN, and MELCOR 2.2 code, developed by SANDIA for USNRC, have been compared and analyzed. The SBO scenario considered takes into account the intervention of the accumulators as only accident mitigation strategy. Several figures of merits related to the thermal-hydraulic (e.g. primary pressure, cladding temperature, etc.) and to the core degradation (e.g. hydrogen production, etc.) have been considered to describe the accident evolution until the vessel failure, for the two codes comparison

An Adjoint Method for the Optimal Boundary Control of Turbulent Flows Modeled with the Rans System

In recent years, the optimal control in fluid dynamics has gained attention for the design and the optimization of engineering devices. One of the main challenges concerns the application of the optimal control theory to turbulent flows modeled by the Reynolds averaging Navier-Stokes equations. In this work we propose the implementation of an optimal boundary control problem for the ReynoldsAveraged Navier-Stokes system closed with a two-equations turbulence model. The optimal boundary velocity is sought in order to achieve several objectives such as the enhancement of turbulence or the matching of the velocity field over a well defined domain region. The boundary where the control acts can be the main inlet section or additional injection holes placed along the domain. By minimizing the augmented Lagrangian functional we obtain the optimality system comprising the state, the adjoint, and the control equations. Furthermore, we propose numerical strategies that allow to solve the optimality system in a robust way for such a large number of unknowns

Validation on a new anisotropic four-parameter turbulence model for low Prandtl number fluids

This work aims to validate a new anisotropic four-parameter turbulence model for low-Prandtl number fluids in forced and mixed convection. Traditional models based on the gradient-diffusion hypothesis and Reynolds analogy are inadequate to simulate the turbulent heat transfer in low-Prandtl number fluids. Additional transport equations for thermal variables are required to predict the characteristic thermal time scale. In a four-parameter turbulence model, two additional transport equations are solved for the temperature variance and its dissipation rate. Thus, it is possible to formulate appropriate characteristic time scales to predict the near-wall and bulk behaviour of mean and turbulent variables. The isotropic version of the four-parameter model has been widely studied and validated in forced and mixed convection. We aim to extend the model validity by proposing explicit algebraic models for the closure of Reynolds stress tensor and turbulent heat flux. For the validation of the anisotropic four-parameter turbulence model, low-Prandtl number fluids are simulated in several flow configurations considering buoyancy effects and numerical results are compared with DNS data

Thermal-hydraulic and neutronic codes coupling for the analysis of a Lead Fast Reactor

In this work the thermal-hydraulics and neutronics behavior of a Lead Fast Reactor (LFR) core is investigated evaluating the power generation distribution taking into account the local temperature field. The temperature field is evaluated using the CFD finite element code FEMuS and exchanged with the multiscale neutron code DONJON-DRAGON, which interpolates the macroscopic cross-sections according to the local temperature field and local lead density distribution. As a result, the neutron flux changes and defines a new power density distribution which is used to update the temperature field into the CFD code. The coupling between neutron and CFD codes is achieved through their inclusion into the numerical platform SALOME. The numerical libraries MED, included into the SALOME platform, are used to exchange data run-time between FEMuS and DONJON

Optimal Pressure Boundary Control of Steady Multiscale Fluid-Structure Interaction Shell Model Derived From Koiter Equations

The fluid-structure interaction (FSI) problem has been extensively studied, and many papers and books are available in the literature on the subject. In this work, we consider some optimal FSI pressure boundary control applications by using a membrane model derived from the Koiter shell equations where the thickness of the solid wall can be neglected and the computational cost of the numerical problem reduced. We study the inverse problem with the aim of achieving a certain objective by changing some design parameters (e.g. forces, boundary conditions or geometrical domain shapes) by using an optimal control approach based on Lagrange multipliers and adjoint variables. In particular, a pressure boundary optimal control is presented in this work. The optimality system is derived from the first-order optimality condition by taking the Fréchet derivatives of the Lagrangian with respect to all the variables involved. This system is solved by using a finite element code with mesh-moving capabilities. In order to support the proposed approach, we perform numerical tests where the pressure on a fluid domain boundary controls the displacement that occurs in a well-defined region of the solid domain

Dirichlet boundary control of a steady multiscale fluid-structure interaction system

This work aims to extend the techniques used for the optimal control of the NavierStokes systems to control a steady multi-scale FSI system. In particular, we consider a multiscale fluid-structure interaction problem where the structure obeys a membrane model derived from the Koiter shell equations. With this approach, the thickness of the solid wall can be neglected, with a meaningful reduction of the computational cost of the numerical problem. The fluid-structure simulation is then reduced to the fluid equations on a moving mesh together with a Robin boundary condition imposed on the moving solid surface. The inverse problem is formulated to control the velocity on a boundary to obtain a desired displacement of the solid membrane. For this purpose, we use an optimization method that relies on the Lagrange multiplier formalism to obtain the first-order necessary conditions for optimality. The arising optimality system is discretized in a finite element framework and solved with an iterative steepest descent algorithm, used to reduce the computational cost of the numerical simulations

A new projection method for Navier-stokes equations by using Raviart-thomas finite element

Computational Fluid Dynamics codes usually adopt velocity-pressure splitting to reduce the computational effort in the solution of the Navier-Stokes equations. In standard projection methods, the finite element approximations show difficulties to find a solution with discrete free-divergence velocity field in all space points. In this work, a new velocity-pressure method for Navier-Stokes equations that projects the velocity field inside the discrete free-divergence velocity space is presented. This algorithm computes the velocity field on the discrete free-divergence space by using Raviart-Thomas finite elements. The projection is obtained by the minimization of the distance, over the discrete free-divergence space, between the auxiliary field and the desired Raviart-Thomas interpolation space. The Raviart-Thomas discretization is based on the quadrilateral and hexahedral finite element space and therefore the divergence mimetic computational approach is used to avoid the well-known degradation of the divergence term convergence. The auxiliary velocity field is obtained by solving the velocity-pressure split system used in the classical Chorin­Temam algorithm. The pressure is recovered by the orthogonal space to the projection on the Raviart-Thomas interpolation space. The method is investigated with an explicit and semi-implicit treatment of the pressure terms. The issues on boundary conditions and the errors in the reproducibility of the tangential components are investigated. Several numerical examples are reported to support this new projection method

Simulation of TALL-3D experimental facility with a multiscale and multiphysics computational platform

This work details the development of a computational platform in joint collaboration between the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (enea) and the University of Bologna (unibo). The platform is based on the open-source SALOME software that integrates the CATHARE system code for nuclear safety, FEMUS and OpenFOAM CFD codes in a unique framework, with efficient methods for data exchange. The computational platform has been used to simulate complex multiscale and multiphysics systems, such as the tall-3d facility, with a defective boundary condition approach on overlapping domains. The tall-3d experimental facility has been realized with the purpose of providing reference results to be used for both standalone and coupled System Thermal-Hydraulic (STH) and Computational Fluid Dynamic (CFD) code validation. The transient phenomenon of unprotected loss of lead-bismuth eutectic (LBE) flow that has been experimentally simulated at tall-3d is here studied. The system code is used to simulate the tall-3d apparatus while the CFD code is used to get a better insight into the fluid streaming occurring in the main tank component and improve the system code predictions. A flow transition from forced to natural convection is used to validate the codes and the platform ability to reproduce the experimental data

FEMuS-Platform: a numerical platform for multiscale and multiphysics code coupling

Nowadays, many open-source numerical codes are available to solve physical problems in structural mechanics, fluid flow, heat transfer, and neutron diffusion. However, even if these codes are often highly specialized in the numerical simulation of a particular type of physics, none of them allows simulating complex systems involving all the physical problems mentioned above. In this work we present a numerical framework, based on the SALOME platform, developed to perform multiscale and multiphysics simulations involving all the mentioned physical problems. In particular, the developed numerical platform includes the multigrid finite element in-house code FEMuS for heat transfer, fluid flow, turbulence and fluid-structure modeling; the open-source finite volume CFD software OpenFOAM; the multiscale neutronic code DONJON-DRAGON; and a system-scale code used for thermal-hydraulic simulations. Efficient data exchange among these codes is performed within computer memory by using the MED libraries, provided by the SALOME platform