Postirradiation examination (PIE) of graphite samples trepanned from the UK reactor cores has been carried out for more than 50 years. Due to the nature of the material, there are sample size and geometry restrictions and no standard test methods to cover measurements on this material. Nevertheless, these measurements are used to support the continued operation of the UK reactor cores, and hence a large program of trials is carried out to provide confidence that each method is accurate and reproducible. These trials typically involve a study of size effects using virgin graphite and simulant materials for the irradiated and oxidized graphite, but a corresponding study with irradiated samples is usually not possible. This paper combines the work of two UK studies to investigate the size effect of the PIE test methods on irradiated graphite on the basis of the characterization of graphite used in the advanced gas-cooled reactors (AGRs) and Magnox reactors. The AGR study focused on the static Young's modulus, three-point bend strength of unnotched and notched beams, and the work of fracture. The Magnox study focused on the coefficient of thermal expansion, diametral compression, and flexural strength. The two studies used large irradiated graphite samples from “installed sets” (i.e., precharacterized graphite samples installed in the reactor cores before the start of operation for monitoring purposes). Large Magnox samples that were trepanned from the reactor core after shutdown were also tested. The purpose of these investigations was to relate the graphite measurements normally undertaken on small trepanned samples to property values obtained using standard test methods on irradiated material. The sample selection was such that it covered as wide a range of dose and radiolytic weight loss as possible. This paper outlines the methodology, results, and conclusions for each of the studies and provides some guidelines for similar studies on new graphites.

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 Generation-IV candidate graphite samples (grade PCEA, GrafTech), subjected to neutron irradiation at 900 degrees 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 degrees C to 1.60 dpa, an investigation is presented on the effect of irradiation dose and temperature on the aforementioned microstructural parameters. Using two complementary techniques in Raman spectroscopy and XRD, which produced agreeable results, the average lateral crystallite size of the two graphites tested was found to decrease with increasing exposure to fast-neutron irradiation or at lower irradiation temperatures. Conversely, dislocation densities and micro-strains were found to increase following the same changes in irradiation conditions. Supporting evidence for the microstructural information obtained is provided by direct observations made using high-resolution transmission electron microscopy. These images demonstrate the presence of irradiation-induced prismatic edge dislocations as well as features that indicate the presence of basal dislocations. They also provide supporting evidence for the progressive deterioration of the graphitic planes via damage mechanisms as proposed in the literature. (C) 2017 Elsevier Ltd. All rights reserved.

We present our findings after scaled-down drop-weight tests, performed under relatively low loading conditions and employing a small-scale spherical indenter as a projectile, to boost the strain rate and energy density of the impact, resulted in the generation of a cavity of measurable depth on the surface of a pure, fully dense, alumina ceramic. We demonstrate that activated dislocations are a main contributor in the formation of the residual impression with an estimated maximum density of ~4.02×1014 m−2.

We have used Hertzian and Vickers indentation to investigate contact damage in sintered SiC ceramics, one consisting of uniform, fine-grains and the other a coarse, elongated grain structure. Cracking-resistance measured by Hertzian indentation, showed no discernible difference, nor did the Vickers hardness. However, numerical analysis of the Vickers indentation size effect, performed using the proportional specimen resistance model, indicates 77.3% greater surface energy, mostly realised through cracking, is experienced by heterogeneous SiC per unit area of indentation impression. This is typified by an observable increase in the number of radial cracks generated around Vickers impressions, which has been found to artificially increase the KIC determined by Vickers indentation fracture. Quantitative measurements of pre-existing flaws by Hertzian indentation show that heterogeneous SiC retains a higher density of larger flaws. Relationships between the differences in cracking around Vickers indents and the pre-existing flaw populations of these two SiC ceramics are discussed.

We report herein, a scalable method for the preparation of alumina (Al 2 O 3 )-phosphate ceramics reinforced with carbon nanotubes (CNTs).All composites were manufactured by direct on-site growth of CNTs on ceramic particles via catalytic chemical vapour deposition.Introduction of catalyst metals to the substrate was achieved through two simple approaches, drip-coating and vacuum filtration, both of which have been reviewed.Transmission electron microscopy was utilised to investigate the interface between the Al 2 O 3 surface and the in-situ CNTs.Resultant ceramics were produced by impregnating phosphoric acid into the Al 2 O 3 +CNT nanocomposite powder followed by die-pressing.In order to maintain the integrity of the CNTs, dehydration/curing was performed at 130-150 ○ C. Scanning electron microscopy was elected to comparatively characterise the microstructure of this type of ceramic nanocomposite against its monolithic equivalent.Possible mechanisms by which specific features have formed are discussed.

Vickers indentation was employed to measure the microhardness of monolithic alumina and six alumina-based nanocomposites consisting of variable silicon carbide nanoparticle volume percentages of 0.3% to 20%. Indentation tests were performed over a broad range of loads from 0.5N to 40N. The resultant hardness-load curves exhibit cumulative increases in the apparent hardness based on the silicon carbide content and reveal each sample suffers from a prominent indentation size effect (ISE). Herein, we present a comprehensive analysis of this data using Meyer’s Law, the proportional specimen resistance model (PSR) and the modified proportional specimen resistance model (MPSR) and employ TEM imagery to detail potential mechanisms by which silicon carbide nano-reinforcements influence the “true hardness” and the ISE.