Microcracks in nuclear graphite and highly oriented pyrolytic graphite (HOPG)

Wen, K; Marrow, James; Marsden, Barry

doi:10.1016/j.jnucmat.2008.07.012

Cited by 92 publications

(63 citation statements)

References 15 publications

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“…30 and the irradiation creep curves are comparable to those shown in Fig. 30b, along with the UKAEA creep law defined by equation (23). The ATR-2E creep curves given in Fig.…”

Section: Implications For Irradiation Creep In Graphitesupporting

confidence: 72%

Dimensional change, irradiation creep and thermal/mechanical property changes in nuclear graphite

Marsden

Haverty

Bodel

et al. 2016

International Materials Reviews

110

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Since the start of the 'nuclear age' graphite has been employed as a moderator in around 100 nuclear reactors, and today there are still some 30 graphite-moderated reactors operating and there are plans for new Generation IV high-temperature reactors. Many of the graphite moderator reactors now producing power are operating beyond their original design life. Therefore in some cases, to aid the reactor operators and designers, the existing graphite irradiation databases need to be extended either to a higher temperature or higher neutron fluence. Furthermore, data are needed for the different grades of graphite that are available at present. This can either be achieved by expensive, time consuming irradiation programmes or by improving the understanding of the mechanisms and processes which lead to irradiationinduced dimensional and property changes in the graphite core components. This review looks at three of the most important graphite properties which change with exposure to irradiation, namely dimensional change, irradiation creep and thermal expansion. The behaviour of UK AGR, Magnox and an experimental grade of German reactor graphite are explored in some detail. First graphite reactor core design is briefly discussed, giving examples of typical graphite components and core arrangements. Issues related to aging graphite component and core behaviour are illustrated through examples of component internal and thermal stress generation, and issues related to whole core behaviour are also outlined. Second the manufacture and microstructure of different nuclear graphite grades are discussed, highlighting how the choice of raw materials and manufacturing technique influences the graphite properties. Third the coefficient of thermal expansion, dimensional change and irradiation creep are analysed using microstructural and averaging methods which are used to relate crystal to bulk properties by accounting for graphite crystal orientation and porosity. These techniques, which were first applied to nuclear graphite in the 1960s, are extended and discussed with the aim of trying to lend some understanding to the role the microstructural crystallite and porosity distributions play in defining the dimensional stability and properties of virgin graphite, irradiated graphite and stressed graphite.

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“…30 and the irradiation creep curves are comparable to those shown in Fig. 30b, along with the UKAEA creep law defined by equation (23). The ATR-2E creep curves given in Fig.…”

Section: Implications For Irradiation Creep In Graphitesupporting

confidence: 72%

Dimensional change, irradiation creep and thermal/mechanical property changes in nuclear graphite

Marsden

Haverty

Bodel

et al. 2016

International Materials Reviews

110

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“…Dimensional change is determined in a number of ways, such as directly measuring specimens before and after irradiation, using X-ray diffraction to assess crystallite behaviour, and measuring changes in cracks and porosity with electron and light microscopy and smallangle neutron scattering [18,19,22]. The fundamental dimensional changes are known to involve crystallographic expansion in the c-direction and contraction in the a-direction [3].…”

Section: Irradiation Of Nuclear Graphitementioning

confidence: 99%

“…When the graphitization process is complete, two main features can be distinguished: the majority filler particles and a minority binder phase, both of which are formed by domains of aligned individual crystallites and appear as a single colour in a polarised light micrograph. Both features have potentially inter-and intra-structural porosity ranging from Mrozowski cracks between crystallites (50 nm -10 m) to micro-and macropores around domains and particles ( Figure 1) [3].…”

Section: Introductionmentioning

confidence: 99%

Electron irradiation of nuclear graphite studied by transmission electron microscopy and electron energy loss spectroscopy

Mironov

Freeman

Brown

et al. 2015

Carbon

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Structural and chemical bonding changes in nuclear graphite have been investigated during in-situ electron irradiation in a transmission electron microscope (TEM); electron beam irradiation has been employed as a surrogate for neutron irradiation of nuclear grade graphite in nuclear reactors. This paper aims to set out a methodology for analysing the microstructure of electron-irradiated graphite which can then be extended to the analysis of neutron-irradiated graphites. The damage produced by exposure to 200 keV electrons was examined up to a total dose of approximately 0.5 dpa (equivalent to an electron fluence of 5.6x 10 21 electrons cm -2 ). During electron exposure, high resolution TEM images and electron energy loss spectra (EELS) were acquired periodically in order to record changes in structural (dis)order and chemical bonding, by quantitatively analysing the variation in phase contrast images and EEL spectra.

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“…However the microstructure of nuclear graphite is complex and at low doses, it is suggested that c-axis expansion is accommodated by the microcracks [3,6] leaving solely to shrinkage along the a-axis which leads to the observed net volume shrinkage. With increasing dose during reactor operation, the rate of contraction or shrinkage reduces and eventually the cracks are presumed to close and c-axis expansion is no longer accommodated producing net volume expansion, the critical point at which this reversal occurs is known as "turnaround" which can vary with operating temperature [7].…”

Section: Introductionmentioning

confidence: 99%

On the nature of cracks and voids in nuclear graphite

Freeman

Jones

Ward

et al. 2016

Carbon

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Microcracks in neutron-irradiated nuclear grade graphite have been examined in detail for the first time using a combination of transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), energy dispersive X-ray (EDX), and energy filtered TEM (EFTEM). Filler particles from both unirradiated Pile Grade A (PGA) and three irradiated British Experimental Pile 'O' (BEPO) graphite specimens were investigated with received doses ranging from 0.4 to 1.44 displacements per atom (dpa) and an irradiation temperature of between 20-120°C. We suggest that the concentration and potentially the size of microcracks increase with increasing neutron irradiation and show that disordered carbon material is present in a range of microcracks (of varying size and shape) in all specimens including unirradiated material. EFTEM and EELS data showed that these cracks contained carbon material of lower density and graphitic character than that of the surrounding bulk graphite. The presence of partially filled microcracks has potentially significant implications for the development of microstructural models for the prediction of radiation-induced dimensional and property changes in nuclear graphite.

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Microcracks in nuclear graphite and highly oriented pyrolytic graphite (HOPG)

Cited by 92 publications

References 15 publications

Dimensional change, irradiation creep and thermal/mechanical property changes in nuclear graphite

Dimensional change, irradiation creep and thermal/mechanical property changes in nuclear graphite

Electron irradiation of nuclear graphite studied by transmission electron microscopy and electron energy loss spectroscopy

On the nature of cracks and voids in nuclear graphite

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