Modelling long timescale effects of irradiation damage of nuclear graphite

Lead Research Organisation: Loughborough University
Department Name: Chemistry

Abstract

There are currently 14 Advanced Gas-cooled Reactors (AGRs) in the UK which are operated by EDF. The reactor cores in these AGRs are composed of several tonnes of a synthetic form of graphite, commonly called nuclear graphite. This nuclear graphite serves two purposes: to moderate (slow down) neutrons to enhance nuclear fission, and to form the main structure of the reactor core.

Nuclear graphite is not a single crystal but is instead composed of a highly complex microstructure consisting of many mis-orientated graphite grains. This microstructure can be tailored by the manufacturing conditions.

A key problem is that the high temperature and high irradiation conditions of a nuclear reactor core causes the graphite bricks to swell considerably (dimensional change). This induces stress in the bricks which eventually results in cracks. Cracked and crumbling bricks would eventually lead to blocked channels in the reactor core which would prevent the insertion or removal of fuel rods and control rods. Therefore, materials (and particular microstructures) that are resistant to radiation damage and do not readily swell or crack under irradiation are desirable since they would lead to longer run times of future generation 4 nuclear reactors.

The key aim of this project is to develop understanding of how the microstructure of a graphite brick influences its properties (such as dimensional change) in the high temperature and high irradiation conditions of a reactor core.
This understanding will be gained by the following project objectives:

(1) Model the interaction of small and large atomic defect structures with a view to understanding the main causes of dimensional change in graphite. Uncover the key mechanisms for dimensional change by studying the motion of vacancy and interstitial atoms within graphite as well as the buckling of the graphene layers. We will use advanced computer simulation techniques that can model these graphite structures with atomic level detail. Long timescale techniques will be used to extend the simulation time of our atomistic simulations.

(2) Model the mechanical properties of micrometre sized thin graphite wafer structures containing realistic densities of defects in graphite. Computer simulations of indentation and scratching processes will be performed and compared with experimental results.

(3) Investigation of the thermal properties of graphite structures containing realistic distributions of defects and grain boundaries. Non-equilibrium molecular dynamics simulations will be performed on a range of these structures to determine the effect of the microstructure upon the thermal conductivity of the graphite. The coefficient of thermal expansion will also be simulated for a range of representative graphite structures. These results will also be compared to experimental results.

(4) Computer simulations are only ever as good as the underlying approximations made in fitting a usually limited data set in the model. The data generated in this project will be used to assess the suitability of existing models for graphite available in the literature. These existing models will be improved upon by identifying the best components of each model and combining them into a new hybrid model capable of accurately predicting a wide range of graphite material properties.
 
Description The microstructure and properties of graphite depend upon the atomic-scale defect structures, including crystal defects like dislocations (planes of atoms sliding with respect to one another). These defects lead to the formation of ripples and folds in the material (known as the 'ruck and tuck' defect or 'ripplocation' in the literature. We find that these ripples and folds can readily form within graphite, and have published detailed works on their properties.

The computational nature of the study involves using approximate methods within density functional theory (DFT) to predict graphite properties. Testing various DFT methods, the research finds that van der Waals corrections are essential for accurate modelling graphite due to the layered nature of the material. Standard methods like Local-density approximations (LDA) and generalized gradient approximation (GGA) fail to predict experimental values, and the study provides detailed information on the calculations and settings used.
Exploitation Route The outcomes of this research have practical applications for various stakeholders. The understanding of atomic scale defects within graphite is crucial for developing synthetic graphites for next-generation nuclear reactors (gen 4). Graphite manufacturers and nuclear power plant operators, such as EDF, can leverage the developed computational modelling protocols to accurately model and predict these properties. The energy barrier calculation results from the research papers can be utilised in creating more coarse-grained models, allowing for more accurate modelling of larger structures. This information is valuable for enhancing the design, safety, and operation of advanced reactors, providing a practical tool for those involved in graphite production and nuclear power plant management.
Sectors Energy
 
Title Supplementary information files for Modelling of partial basal dislocation dipoles in bilayer graphene and graphite 
Description © the authors, CC BY 4.0Supplementary files for article Modelling of partial basal dislocation dipoles in bilayer graphene and graphiteBasal dislocations are the most common type of dislocation in bilayer graphene and graphite, and are critical to understanding the physics of graphite because they play a crucial role in the plastic deformation of damaged material. In this study, we utilise molecular dynamics calculations to investigate the properties of basal dislocations in bilayer graphene and graphite. We analyse the dislocation formation energy per unit length of flat and buckled partial basal dislocations, as well as the dissociation and buckling of perfect basal dislocations. We also examine the partial dislocation core widths and formation energies by analysing the atomic disregistry and strain distribution across dislocated supercells. Our findings suggest that buckling is primarily initiated by the edge component of basal dislocations, which controls the degree of hydrostatic strain perpendicular to the dislocation line. Finally, we investigate the buckling of dislocated multilayer supercells and explore how these results relate to the structural deformation of bulk graphite. 
Type Of Material Database/Collection of data 
Year Produced 2023 
Provided To Others? Yes  
URL https://repository.lboro.ac.uk/articles/dataset/Supplementary_information_files_for_Modelling_of_par...
 
Title Supplementary information files for Stacking-mediated diffusion of ruthenium nanoclusters in bilayer graphene and graphite 
Description Supplementary files for article Stacking-mediated diffusion of ruthenium nanoclusters in bilayer graphene and graphite The diffusion, penetration and intercalation of metallic atomic dopants is an important question for various graphite applications in engineering and nanotechnology. We have performed systematic first-principles calculations of the behaviour of ruthenium nanoclusters on a graphene monolayer and intercalated into a bilayer. Our computational results show that at a sufficiently high density of single Ru atom interstitials, intercalated atoms can shear the surrounding lattice to an AA stacking configuration, an effect which weakens with increasing cluster size. Moreover, the interlayer stacking configuration, in turn, has a significant effect on cluster diffusion. We therefore find different trends in diffusivity as a function of cluster size and interlayer stacking. For monolayer graphene and an AA graphene bilayer, the formation of small clusters generally lowers diffusion barriers, while the opposite behaviour is found for the preferred AB stacking configuration. These results demonstrate that conditions of local impurity concentration and interlayer disregistry are able to regulate the diffusivity of metallic impurities in graphite. 
Type Of Material Database/Collection of data 
Year Produced 2022 
Provided To Others? Yes  
URL https://repository.lboro.ac.uk/articles/dataset/Supplementary_information_files_for_Stacking-mediate...