Multiple-scale modelling of damage and failure of nuclear graphite

Lead Research Organisation: University of Manchester
Department Name: Mechanical Aerospace and Civil Eng

Abstract

Nuclear graphite is used in Advanced Gas-cooled Reactors (AGR) with structural and moderating functions and is considered as an option for next generation high-temperature reactors. The AGR core is a structure of interlocking graphite bricks, which determines the positions and ensures the mobility of nuclear fuel and control rods. The core integrity is a life-limiting factor for AGRs, because failure of the core to ensure rods' mobility would lead to permanent reactor closure with associated economic consequences. The core integrity might be compromised due to changes in bricks' geometric and mechanical properties, as graphite is exposed to high temperature and radiation during service. Nuclear graphite belongs to the class of quasi-brittle materials, which exhibit mechanical behaviour that is intermediate between very brittle, such as glasses, and ductile, such as metals. It has a complex microstructure, with higher-density filler particles and lower-density matrix where both phases have their own manufacturing defects - pores and micro-cracks. Better understanding of how damage of graphite develops during service, how this affects the behaviour of individual bricks, and how changes in bricks' behaviour impact the core integrity is therefore crucial for high-fidelity assessment of AGRs' fitness for service, required for supporting continuous safe operation and life-extension decisions.
The aim of this project is to develop a multiple-scale methodology for calculation of whole-core behaviour by considering the relations between the core and its constituent bricks, and between individual bricks and the underlying graphite microstructure. The first objective is to establish a new whole-core model by investigating two options for assembly analysis: Smoothed Particle Hydrodynamics (SPH), where individual bricks are collections of particles, core is assembly of bricks, and the particles experience different inter-brick and intra-brick interactions; and Finite Element Analysis (FEA), where individual bricks are special finite elements interacting via connector elements. The second objective is to develop a deformation and failure model for individual bricks by investigating two options for failure modeling: continuum damage mechanics and fracture mechanics via cohesive elements. This single-brick model will inform the whole-core model on changes in intra-brick interactions (SPH) or behavior of brick elements (FEA). The third objective is to derive damage law parameters or cohesive element properties for the single-brick model by exploring microstructure-informed discrete models of nuclear graphite. The work involves software development to implement selected models, in-situ imaging of damage evolution (4D imaging) using X-ray Computed Tomography (XCT) to validate graphite models, and validation of single-brick models with existing experimental data. The outcomes from this work are of significant academic interest and industrial importance. The project is inter-disciplinary and offers excellent opportunities for furthering our understanding at theoretical and application levels.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509565/1 01/10/2016 30/09/2021
1808334 Studentship EP/N509565/1 22/09/2016 30/09/2019 Ahmadreza Farrokhnia