Multiscale Simulation Study For Mechanical And Thermal Properties Of UO₂-FeCrAl Fuel-cladding System

There is an urgent demand of new generation of nuclear fuel and cladding materials with accident tolerant abilities.In order to reduce the development period and cost,the theoretical calculations are applied to reveal the relationship between material microstructure and thermo-physical properties in nuclear material development.Since the behavior of materials are generally determined by the characteristics from different scales,multiscale calculations have been recognized to be the most reliable and computationally affordable way based on the current computing power.In this study,we focus on the simulation of uranium dioxide fuels and FeCrAl cladding material.For UO₂ fuels,the heat transfer properties are calculated as well as the influence of microstructure.For FeCrAl alloy,we pay more attention on the mechanical properties.The key problem of multiscale simulations is the design of coupling and data transfer between different simulation methods.Take FeCrAl alloy mechanical property prediction as example,a hierarchicalmultiscale simulation system which couples nanoscale first principle,microscale molecular dynamics,mesoscale phase field methodand macroscale finite element method,is built to reveal the influence of element component,temperature and defects on the alloy macro mechanical properties.This study focus on the role of molecular dynamics?MD?in the whole multiscale simulation system,the designs of coupling method of MD with other scale simulation method are also given and discussed.The thermal conductivity of uranium dioxide fuel are calculated by MD method as well as the influence of crystal orientation,grain boundary and fission gas.The crystal orientation has limited influence on the conductivity,and a first order thermal conductivity tensor could accurately describe the conductivity changes with orientation.The grain boundaries have poor conduct ability,which is 0.1⁰.2 times of single crystal,and the low conductivity rang around grain boundary are about 1.5nm;the coupling method for finite element method polycrystal simulation is given.In addition,the solute fission gas Xe has devastating influence on UO₂ thermal conductivity.For FeCrAl alloy,the potential force field is absence and the particle swarm optimization?PSO?methods is used for fitting parameters in potential formula.The potential of SiC with fission gas He,Kr,Xe is firstly developed with Buckingham or Lennard-Jones formula to validate the reliability of PSO grograms.A more complex embedded atom method based FeCrAl ternary potential is next developed and shows great agreement in mechanical property predictions.The PSO method shows effective searching ability and excellent portability during fitting process,which provides reliable method for potential development for new materials.Via the developed potential,the thermal-mechanical properties of cladding material FeCrAl alloys are calculated,including elastic constant,thermal expansion coefficient and heat capacity.The influence of component and temperature on the properties is predicted and the results are agreement with experiment data.Simulation results show that the mole fiction of Al needs to be controlled below 10%to ensure a good ductility for FeCrAl alloy.For multiscale simulation coupling design,the crystal plasticity phase field method?CP-PFM?and crystal plasticity finite element method?CP-FEM?are introduced as well as the coupling with other scale simulations.Take the iron-based alloy as example,the data transfer from MD method to PFM and FEM are realized.The grain boundary energy and dislocation slip critical shear stress are the important input parameters for PFM and FEM simulation,respectively.The MD models for calculating these 2properties are introduced and the results are close to the previous works or experiment data.