A Synchrotron X-ray Study on the Wigner Effect of the Irradiated Nuclear-grade Graphite (original) (raw)
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This study simulates the Wigner Effect of nuclear-grade graphite in a High Temperature Gas-cooled Reactor (HTGR). The graphite was artificially irradiated with 3 MeV C 2+ ions to mimic the fast neutronradiation damage of the HTGR core environment. The irradiation temperatures were controlled between the range of 500-800°C in a high vacuum environment of 10 À7 torr. This high-dosage radiation creates enormous amounts of Frenkel pairs, which induce lattice swelling. These Frenkel vacancies and interstitials generate new strain fields and, hence, store energy in the distorted crystalline structure. The structural integrity of nuclear grade graphite was quantified using high-resolution transmission electron microscopy (HRTEM). The microstructure was estimated by the fast Fourier transform of HRTEM images. Within the samples irradiated with 10 dpa at 600°C, the d-spacing of {0 0 0 2} expanded from 0.336 nm to 0.396 nm accompanying with the greatest distorted graphite microstructure. The c-axis of graphite swelled approximately 18% and the disorder coefficient was 1.10 ± 0.17 (1/nm). The synchrotron X-ray experimental results, gauged from 500 lm 3 volume, suggesting that the ion-implanted graphite only deformed locally and epitaxially. This study also presents possible mechanisms.
An understanding of lattice strain, defects and disorder in nuclear graphite
Carbon
In this study, microstructural parameters, such as lattice dimension, micro-strain and dislocation density, of different neutron-irradiated graphite grades have been evaluated using the diffraction profiles of X-ray diffraction (XRD) and the scattering profiles of Raman spectroscopy. Using Gen-IV candidate graphite samples (grade PCEA, GrafTech), subjected to neutron irradiation at 900°C to 6.6 and 10.2 dpa, and graphite samples of similar grain size and microstructure taken from the core of the British Experimental Pile Zero reactor, which have been irradiated at 100-120 °C to 1.60 dpa, an investigation on the effect of irradiation dose and temperature on the aforementioned microstructural parameters is presented. Coefficients of variation for the lateral crystallite size, micro-strain and dislocation density, as acquired from XRD and Raman spectroscopy, are at approximately 13%, 17% and 38%, respectively. Quantification of microstructural parameters as a function of neutron dose from the two complementary techniques are in agreement and imply that the quantified results are reasonable. Supporting evidence for the microstructural information obtained is provided by direct observations made using high-resolution transmission electron microscopy. Damage mechanisms are reviewed and discussed in relation to results presented.
Microstructure, Stress and Damage Characterisation in Nuclear Graphite
Nuclear graphite is employed as a neutron moderator or reflector in high temperature reactors. Irradiation causes dimensional change (shrinkage and swelling), creep, variation in thermal expansion properties and thermal conductivity, and changes in strength and elastic modulus. In CO 2 gas-cooled reactors, there is also an additional effect of radiolytic oxidation on these properties. The rate and magnitude of these property changes with neutron fluence and temperature are sensitive to the microstructure of the nuclear graphite. The safe design and operation of graphite moderated reactors is therefore dependent on mechanistic understanding of the processes of irradiation damage, materials test reactor (MTR) data and monitoring data obtained during the reactor operation. New build of high temperature reactors will require new nuclear graphites, with improved resistance to irradiation damage and oxidation. This will need an improved understanding of the relationships between microstructure and properties. New MTR and monitoring programmes may benefit from improved techniques for the measurement of structure and properties. This paper reports results from a programme of research into nuclear graphite structure-property relationships in Manchester. This is part of a larger activity which ranges from modelling stress development in irradiated components to the study of new disposal methods for irradiated graphites.
The contribution made by lattice vacancies to the Wigner effect in radiation-damaged graphite
Journal of Physics: Condensed Matter, 2013
Models for radiation damage in graphite are reviewed and compared, leading to a re-examination of the contribution made by vacancies to annealing processes. A method based on density functional theory, using large supercells with orthorhombic and hexagonal symmetry, is employed to calculate properties and behaviour of lattice vacancies and displacement defects. It is concluded that annihilation of intimate Frenkel defects marks the onset of recovery in electrical resistivity, which occurs when the temperature exceeds about 160 K. Migration of isolated monovacancies is estimated to have an activation energy of E a ≈ 1.1 eV. Coalescence into divacancy defects occurs in several stages, with different barriers at each stage, depending on the path. The formation of pairs ultimately yields up to 8.9 eV energy, which is nearly 1.0 eV more than the formation energy for an isolated monovacancy. Processes resulting in vacancy coalescence and annihilation appear to be responsible for the main Wigner energy release peak in radiation-damaged graphite, occurring at about 475 K.
Dynamic microstructural evolution of graphite under displacing irradiation
Carbon, 2014
Graphitic materials and graphite composites experience dimensional change when exposed to radiation-induced atomic displacements. This has major implications for current and future technological ranging from nuclear fission reactors to the processing of graphene-silicon hybrid devices. Dimensional change in nuclear graphites is a complex problem involving the filler, binder, porosity, cracks and atomic-level effects all interacting within the polygranular structure. An improved understanding of the atomistic mechanisms which drive dimensional change within individual graphitic crystals is required to feed into the multiscale modelling of this system.
Early Damage Mechanisms in Nuclear Grade Graphite under Irradiation
Materials Research Letters, 2013
Using Raman and X-ray photoelectron spectroscopy, we delineate the bond and defect structures in nuclear block graphite (NBG-18) under neutron and ion irradiation. The strengthening of the defect (D) peak in the Raman spectra under irradiation is attributed to an increase in the topological, sp 2 -hybridized defects. Using transmission electron microscopy, we provide evidence for prismatic dislocations as well as a number of basal dislocations dissociating into Shockley partials. The non-vanishing D peak in the Raman spectra, together with a generous number of dislocations, even at low irradiation doses, indicates a dislocation-mediated amorphization process in graphite.
Characterisation of the Deformation and Fracture of Nuclear Graphite using Neutron Diffraction
It has been demonstrated that the initiation and growth of localised, heterogeneously-distributed process zones is associated with the non-linear stress-strain response of graphites used to moderate UK gas-cooled civil nuclear reactors. These graphites, such as Gilsocarbon graphite, have heterogeneous complex polygranular microstructures which contain pores and flaws arising from their fabrication. The macroscopic properties of such nuclear graphites are dictated by their microstructure. Due to the presence of pores and aggregates, the lattice strain is not expected to change 1:1 with the externally bulk strain applied to macroscale specimen. Deformation of the material containing pores and flaws causes localisation of strains and, hence, initiation of inelastic damage. The length-scale at which the localised damage develops during loading can be characterised by the lattice strain in a bulk volume of material. Therefore, in situ neutron diffraction on a Gilsocarbon graphite bend geometry test specimen has been undertaken at the ENGIN-X, ISIS facility. It was found that lattice strain changes linearly with applied bulk strain but with reduced magnitude. The results are discussed with respect to the evolution of characteristic process zones, as deformation is increased, and the associated of microcracking.
Damage tolerance of nuclear graphite at elevated temperatures
Nuclear-grade graphite is a critically important high-temperature structural material for current and potentially next generation of fission reactors worldwide. It is imperative to understand its damage-tolerant behaviour and to discern the mechanisms of damage evolution under in-service conditions. Here we perform in situ mechanical testing with synchrotron X-ray computed micro-tomography at temperatures between ambient and 1,000 °C on a nuclear-grade Gilsocarbon graphite. We find that both the strength and fracture toughness of this graphite are improved at elevated temperature. Whereas this behaviour is consistent with observations of the closure of microcracks formed parallel to the covalent-sp 2-bonded graphene layers at higher temperatures, which accommodate the more than tenfold larger thermal expansion perpendicular to these layers, we attribute the elevation in strength and toughness primarily to changes in the residual stress state at 800–1,000 °C, specifically to the reduction in significant levels of residual tensile stresses in the graphite that are 'frozen-in' following processing.