PGA Graphite Research Articles

The optical texture of PGA, Gilsocarbon, NBG-18, and IG-110 nuclear graphite

Graphite will be used as a structural and moderator material in high-temperature gas-cooled reactors. While the microstructure of nuclear graphite, such as filler particles and porosity, has been intensively studied, little attention has been paid to the optical texture. The optical texture, which means the appearance of carbon under polarized light, describes the arrangement of graphite crystallites in terms of the shape, size, and orientation of the domains. It controls the properties of coke and is a classic concept in the field of carbon materials. This study reports the optical texture characterization of nuclear graphite: PGA, Gilsocarbon GCMB, NBG-18, and IG-110. Under polarized light, some previously unnoticed structural features, such as flour particles in PGA and Gilsocarbon GCMB, binder barrier with fused coarse mosaic in Gilsocarbon GCMB, granular domains, and isotropic carbon matrix in NBG-18 are reported and discussed. Point-counting method was applied to polarized light images to quantitatively determine the volume faction of each structural component in nuclear graphite. Updated structural models based on quantitative optical texture analysis will provide a better understanding of the structure-property relationship of nuclear graphite.

The release of carbon-14 from irradiated PGA graphite by thermal treatment in air

The United Kingdom Magnox reactors have all ceased generation and await decommissioning. This fleet of reactors contains 50,000–60,000 tonnes of irradiated graphite categorised as Intermediate Level Waste, which will require long-term storage/disposal either in a proposed Geological Disposal Facility (England and Wales) or in an on-site near surface storage facility (Scotland). Carbon-14 (C-14) is one of the principal long-lived radionuclides in the graphite that determines this waste categorisation. Graphite from some Magnox reactors is known to contain carbonaceous surface deposits. Recent studies have shown that the C-14 activities (expressed as Bq per gramme of material) in these deposits are significantly higher compared to those in the underlying graphite. The mechanism for this phenomenon has not yet been explained but the adsorption of the C-14 pre-cursor nitrogen points to a possible production pathway. Such C-14-rich deposits could potentially lead to some alleviation on waste categorisation through their removal by thermal treatment in air. Historical aqueous leaching studies have further shown that there is a small but relatively rapid initial release of C-14 from irradiated graphite, leaving a much more significant non-leachable fraction. If this mobile C-14 fraction were associated with carbonaceous deposits or even graphite surfaces that had been exposed to nitrogen, thermal treatment in air could render the material more radiologically inert even if removal provided no alleviation on waste categorisation, thereby benefiting packaging and storage options. The study presented here broadens previous thermal treatment investigations of C-14 in carbonaceous deposits to a larger number of core samples from two Magnox reactors with differing operational histories. Thermal treatment in air has also been used to investigate C-14 distributions in the underlying graphite. In addition, a test experiment is presented to investigate any possible link between the mobile fraction of C-14 observed in leaching studies and C-14-rich carbonaceous deposits.

Synthesis of carbon-13 labelled carbonaceous deposits and their evaluation for potential use as surrogates to better understand the behaviour of the carbon-14-containing deposit present in irradiated PGA graphite

The present work has used microwave plasma chemical vapour deposition to generate suitable isotopically labelled carbonaceous deposits on the surface of Pile Grade A graphite for use as surrogates for studying the behaviour of the deposits observed on irradiated graphite extracted from UK Magnox reactors. These deposits have been shown elsewhere to contain an enhanced concentration of 14C compared to the bulk graphite. A combination of Raman spectroscopy, ion beam milling with scanning electron microscopy and secondary ion mass spectrometry were used to determine topography and internal morphology in the formed deposits. Direct comparison was made against deposits found on irradiated graphite samples trepanned from a Magnox reactor core and showed a good similarity in appearance. This work suggests that the microwave plasma chemical vapour deposition technique is of value in producing simulant carbon deposits, being of sufficiently representative morphology for use in non-radioactive surrogate studies of post-disposal behaviour of 14C-containing deposits on some irradiated Magnox reactor graphite.

Evaluation of surface deposits on the channel wall of trepanned reactor core graphite samples

Samples have been trepanned from the fuel and interstitial channel walls of PGA graphite reactor cores of two Magnox gas cooled power stations after a period of service. These samples have been considered explicitly for the presence of deposits on the channel facing surfaces. A combination of focused ion beam milling and imaging has been used to determine the presence of such deposits and where present to make measurements of the thickness. These thicknesses vary from a few nanometres to tens of micrometres. In addition, both the chemical composition and chemical state have been investigated using energy dispersive X-ray microanalysis in a scanning electron microscope and Raman spectroscopy respectively.EDX measurements showed that surface deposits found on the channel walls of one of the reactors contained increased concentrations of oxygen, iron, chromium and sulphur compared with the underlying material. Raman spectroscopy also suggested that the deposit had a smaller crystallite size than PGA graphite.

Computer Modelling of Crack Propagation in Porous Reactor Core Graphite

This study aims to develop computer models, with a microstructure representative of the PGA graphite, to contribute to the understanding of the relationship between the amount of porosity, the load-displacement behaviour and crack propagation. The project is in two linked parts, the first provides a model of the porous graphite which is then introduced into a lattice type finite element model to provide the load-displacement and crack propagation predictions. Microstructures consisting of matrix and pores with added aligned filler particles, typical of needle coke, were studied. The purpose was to isolate the effect of filler particles on fracture strength and the fracture path. In the paper crack paths and fracture mechanisms are discussed for different amounts of porosity and various filler particle arrangements.

Measuring the Static Modulus of Nuclear Graphite from Four-Point Flexural Strength Tests and DIC

The paper describes a novel method based on Digital Image Correlation (DIC) for measuring the static modulus of active PGA graphite specimens during conventional four-point bend strength tests. DIC has been used in combination with finite element and numerical modelling to monitor the displacement fields developing in the specimen during testing, and calculate representative modulus values. Details of the model and results are presented for two specimen geometries and a range of materials with different levels of exposure and density, tested in the perpendicular and parallel orientations. The calculated static modulus values from the DIC measurements confirm the trend between modulus, flexural strength and density. Comments and observations on the uncertainty in the measurement are also presented.

Influence of oxidation on toughness parameters for two nuclear grade graphites

Single edge notched beams of two nuclear graphites were oxidized in CO2 at 900°C up to 49% burn-off for IM1-24 graphite and up to 12% burn-off for PGA graphite, and fracture properties KIc, γwof and GIc were measured as a function of oxidation. Property decrements as a function of burn-off were fitted to an exponential decay curve (the Knudsen equation) characterized by a decay parameter, b. For both graphites oxidized to the same extent, the decrements in γwof and GIc are less than for the stress-based parameter KIc. The more severe decrements for KIc are probably because critical flaws of high aspect ratio influence this toughness parameter, whereas the energy-based toughness parameters are influenced by a spectrum of pores of different sizes and shapes. For IM1-24 graphite, crack propagation makes an increasing contribution to the total work of fracture upon oxidation due to growth in the process zone size ahead of the crack tip. The anisotropy of PGA graphite is reflected in the values of Q0 for the three properties parallel and perpendicular to the extrusion direction, but there is no clear evidence for anisotropy in the exponential decay parameter, b. This may indicate that processes that occur in the more isotropic binder phase rather than the anisotropic filler particles dominate fracture.

The effect of thermal oxidation on polycrystalline graphite studied by X-ray tomography

The effect of thermal oxidation on PGA graphite samples was investigated by X-ray tomography. This technique allowed an investigation of the evolution of microstructure and local density to be carried out as a function of weight loss. The results show a localised oxidation process which progressively reveals the large coke particles with increasing overall weight loss. The local density measurements arising from the tomographs were used to estimate the weight loss distribution. The reliability of the values were checked by independent thermal conductivity measurements and volume weight loss calculations by immersion technique.

Crack growth resistance in nuclear graphites

Crack growth resistance curves for the non-linear fracture parameters KR, JR and R were measured for unirradiated PGA and IM1-24 graphites that are used as moderators in British Magnox and AGR nuclear reactors respectively. All the curves show an initial rising part, followed by a plateau region where the measured parameter is independent of crack length. JR and R decreased at large crack lengths. The initial rising curves were attributed to development of crack bridges in the wake of the crack front, while, in the plateau region, the crack bridging zone and the frontal process zone, ahead of the crack tip, reached steady state values. The decreases at large crack lengths were attributed to interaction of the frontal zone with the specimen end face. Microscopical evidence for graphite fragments acting as crack bridges showed that they were much smaller than filler particles, indicating that the graphite fragments are broken down during crack propagation. There was also evidence for friction points in the crack wake zone and shear cracking of some larger fragments. Inspection of KR curves showed that crack bridging contributed ~0.4 MPa m0.5 to the fracture toughness of the graphites. An analysis of JR and R curves showed that the development of the crack bridging zone in the rising part of the curves contributed ~20% to the total work of fracture. Energies absorbed during development of crack bridges and steady state crack propagation were greater for PGA than for IM1-24 graphite. These differences reflect the greater extent of irreversible processes occurring during cracking in the coarser microtexture of PGA graphite.

The radiolytic graphite/CO 2reaction: The effect of carbon monoxide at 225–375°C

The effect of CO on the γ-radiation induced reaction between graphite and CO 2 has been investigated at 225–375°C. G(−C) values have been determined using a 14C-labelled PGA graphite specimen for a range of CO concentrations from 10 vpm to 1·3 per cent. The results agree with those of other workers in areas of overlap and fit a previous mechanism for CO inhibition. Equations for the dependence of G(−C) on CO concentrations at 375, 280 and 225°C are derived. The product ratio 14CO: 14CO 2 is not significantly affected by CO concentration, but increases with temperature. The mechanism of the radiolytic gasification of graphite by CO 2 is discussed in the light of these results.