Simulation of irradiated graphite

However, some differences are observed and further investigation is required to understand fully the nature of these differences on the atomic scale.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Simulation of irradiated graphite

However, some differences are observed and further investigation is required to understand fully the nature of these differences on the atomic scale.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Simulation of irradiated graphite

A detailed scientific investigation is carried out into the effects of ion irradiation on graphite systems. The results obtained have been compared with analogous radiation induced affects seen in neutron irradiated material. Results indicate that ion irradiation causes structural damage of the graphite lattice, as expected, in a very similar way to neutron irradiation. Such damage is also largely recoverable through thermal annealing. However, certain differences are observed and it seems likely that these are due to differences in the nature of the damage caused, and the recovery of such damage, on the fundamental level. Ultra-high vacuum (UHV) equipment was used to irradiate samples of nuclear grade graphite and highly-orientated pyrolyitc graphite (HOPG) with inert gas ions and external facilities were also used to irradiate with carbon ions at higher energies. After irradiation a variety of scientific techniques were used to analyse the damage caused as a result of the irradiation. Raman spectroscopy was used extensively to study the samples after irradiation and results indicated the presence of significant ion-induced damage through analysis of the Id/Ig ratio. The results were shown to be analogous to neutron induced damage and the amount of damage present was seen to be dependant on ion mass, ion energy and of course the total ion dose. A broad Raman feature was observed at 1500 cm'1, after a critical ion dose and such a feature has been rarely observed in neutron irradiated material, this feature perhaps suggests a fundamental difference between the damage occurring and a difference in the type and extent of defects produced. Raman was also used to effectively monitor damage recovery processes occurring as a result of thermal annealing and such recovery was seen to be analogous to the recovery seen in neutron damages materials. Differential scanning calorimetry (DSC) was used to detect any exothermic release, when heating the sample after irradiation, occurring as a result of a build-up of Wigner-like energy, a phenomenon commonly seen in neutron irradiated graphite. In one case of high energy He+ ion irradiation such an exotherm was observed at 226 °C, the expected temperature range for Wigner-like release, however this result was very difficult to reproduce thus suggesting a fragile balance of ion irradiation parameters resulting in the necessary defects required to observe any Wigner release. Transmission electron microscopy (TEM) showed clearly that the atomic structure of nuclear grade graphite material was markedly altered as a result of ion irradiation, leading to an amorphous-like structure. These results are in agreement with published TEM work on neutron irradiated material and again highlight a similarity between the two forms of radiation. X-ray diffraction (XRD) was also utilised and samples were analysed before and after irradiation. Contrary to XRD patterns of neutron irradiated graphite, there seemed to be little change in the XRD patterns after irradiation and the expected broadening and peak shift of the peak was not seen, suggesting a difference in the damage caused to the system. Further work is required here to understand why this is the case and again a deeper understanding of the underlying damage mechanisms would be beneficial

Thermal Oxidation of Nuclear Graphite: A Large Scale Waste Treatment Option

This study has investigated the laboratory scale thermal oxidation of nuclear graphite, as a proof-of-concept for the treatment and decommissioning of reactor cores on a larger industrial scale. If showed to be effective, this technology could have promising international significance with a considerable impact on the nuclear waste management problem currently facing many countries worldwide. The use of thermal treatment of such graphite waste is seen as advantageous since it will decouple the need for an operational Geological Disposal Facility (GDF). Particulate samples of Magnox Reactor Pile Grade-A (PGA) graphite, were oxidised in both air and 60% O2, over the temperature range 400-1200°C. Oxidation rates were found to increase with temperature, with a particular rise between 700-800°C, suggesting a change in oxidation mechanism. A second increase in oxidation rate was observed between 1000-1200°C and was found to correspond to a large increase in the CO/CO2 ratio, as confirmed through gas analysis. Increasing the oxidant flow rate gave a linear increase in oxidation rate, up to a certain point, and maximum rates of 23.3 and 69.6 mg / min for air and 60% O2 respectively were achieved at a flow of 250 ml / min and temperature of 1000°C. These promising results show that large-scale thermal treatment could be a potential option for the decommissioning of graphite cores, although the design of the plant would need careful consideration in order to achieve optimum efficiency and throughput

A Raman investigation into the effect of temperature on ion-induced damage of graphite

Samples of highly-orientated pyrolytic graphite (HOPG) have been irradiated with 5 keV He+ and Xe+ ions at temperatures of 298 K and 100 K. Irradiation-induced damage has been monitored using Raman spectroscopy through analysis of the intensity of the D-band with respect to the G band (ID/IG ratio). A significant increase in the rate of damage accumulation was observed after ion-irradiation at 100 K, leading to a very broad Raman spectrum. The emergence of a broad feature at ∼1500 cm−1 is also observed and attributed to a significant breakdown of lattice order and the presence of a more amorphous system. This feature is thought to be due to a carbon-ion interaction

Ion-induced damage in graphite: A Raman study

A Raman investigation has been carried out on samples of ion irradiated highly-oriented pyrolytic graphite (HOPG). Irradiation is carried out under ultra-high vacuum (UHV), at room temperature, with 5 keV He+, Ne +, Ar+ and Xe+ ions so as to create a damaged layer, with the doses administered being higher than those previously reported. Modern Monte Carlo simulations (SRIM 2008) are utilised to provide an insight into the ion-graphite interactions, and the effects of varying ion penetration depths are considered when analysing the observed damage. © 2010 Elsevier B.V. All rights reserved

The data derived from fitting a linear line of best-fit over the various temperature regimes in the Arrhenius plot.

<p>Values of <i>E</i>_<i>a</i> are in kJ/mol.</p