Electron irradiation of nuclear graphite studied by transmission electron microscopy and electron energy loss spectroscopy
Introduction
Over 80% of the UK’s current nuclear reactors are graphite-moderated Advanced Gas Cooled Reactors (AGR) or Magnox reactors [1]. In addition to moderating the energies of neutrons in the fission process, the graphite core provides structural support, contains the fuel and control rods and allows for coolant flow. The graphite blocks are subject to high levels of neutron irradiation resulting in chemical and physical property changes, which in turn affect neighbouring reactor components. The lifetime of such reactors is therefore primarily limited by the performance of the irreplaceable graphite within the working reactor, so an accurate measure of its condition is essential for economic success and plant safety.
Nuclear graphite is a synthetic material produced from pitch and petroleum coke particles, with a high degree of crystallinity following thermal treatment at high temperatures (graphitization) [2]. 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 μm) to micro- and macro-pores around domains and particles (Fig. 1) [3].
For over 70 years, a considerable body of evidence has been assembled to understand the behaviour of irradiated graphite [2], [4], [5], [6]. The bulk properties of damage features have been thoroughly investigated and theoretical models of induced structural changes derived [7]. Although this has allowed property changes in the irradiated bulk to be partly understood and accounted for in current and future graphite based reactor designs, the mechanisms of such processes at the nanoscale still remain uncertain. This work investigates the effect of electron irradiation on nuclear grade graphites within a transmission electron microscope (TEM) in an attempt to understand the fundamental processes involved in radiation damage.
Early work, by Mitchel et al., investigated the effects of electron irradiation through stored energy release following irradiation at high temperature [8]. In 1972 the effects of electron irradiation to graphite were examined by Ohr et al. for the first time, who reported a displacement threshold accelerating voltage of below 120 kV [9].
Throughout this paper we will compare the effects of neutron and electron irradiation therefore it is important to understand the key differences between the two. The atomic displacement rate of the carbon atoms in the graphite is measured in displacements per atom (dpa) and is dependent on the kinetic energy of the incident particle [2]. According to calculations by Thrower and Mayer [10] a 1 MeV electron and neutron produce an average of 1.6 and 500 atomic displacements, respectively. It is generally understood that cascades of atomic displacements are the most common route for large scale structural disturbances and models have been developed to calculate the number of atoms involved in cascade events resulting from different incident energies [11].
When mimicking the effects of neutron irradiation damage with electron irradiation in the TEM, it is important to account for both the higher dose rate of electrons compared to neutrons in a nuclear reactor (greater by about 104) and the reduced displacements per atom from electrons due to the lower mass [12]. Electron irradiation causes point defect damage whereas the higher mass and lower dose rate of neutrons causes cascade damage. However, the relatively wide spacing of graphite’s basal planes results in a low density of the cascade events, and the low neutron dose rates (10−7 dpa s−1) and high temperatures (∼450 °C) in the nuclear reactor allow damage to partially anneal out between cascade events [13], [14], [15]. Interstitial and vacancy defects created during irradiation can behave independently or coalesce into clusters and gradually deform the crystal lattice ultimately resulting in both chemical and physical changes. Damage accumulation at temperatures below 200 °C increases the Wigner energy of the graphite, due to a lack of atomic diffusion [16]. It is widely agreed that single vacancies become mobile at 100–200 °C whereas interstitial atoms become mobile at temperatures of 500 °C [17]. Thus only at higher temperatures such as those in the Gen IV graphite moderated Very High Temperature Gas Reactors (>300 °C) does stored energy dissipation occur by diffusion driven atomic re-ordering and the problem is addressed in the short term. Longer term exposure to a high temperature environment (>400 °C) however, gives rise to creep and dimensional change [18], [19], [20].
The key observed changes in nuclear grade graphite as a result of neutron irradiation are micro-crack closure resulting from expansion in the c-direction and dimensional change from irradiation induced creep, both of which depend on the overall level of initial crystallinity [16], [21]. 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 small-angle 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]. Initially, the expansion is largely accommodated for in cracks and pores created during the manufacturing process; Mrozowski cracks arise from the anisotropy in graphite thermal expansion coefficients and lie perpendicular to the c-direction hence the initial accommodation of expansion, so that the initial macroscopic response is a net shrinkage in the a-direction [23]. Upon further irradiation and once the cracks and pores are fully closed, irreversible net macroscopic expansion occurs. The transition between contraction and expansion is referred to as ‘turnaround’ [24], [25].
Transmission electron microscopy is an established tool for characterising both electron and neutron irradiated graphite [15], [26]. There are however, very few detailed TEM-EELS studies on nuclear graphites but a significant volume of work on graphitizing and non-graphitizing carbons [27], [28]. In this work we will focus on quantitative analysis of atomic lattice imaging and EEL spectroscopy to elucidate the nanoscale changes that occur in irradiated graphite.
Section snippets
Sample preparation
Virgin Pile Grade A (PGA) graphite sourced from the University of Manchester was chosen for inspection. PGA is a medium to coarse grain anisotropic nuclear graphite of typical density 1.74 g cm−3. The anisotropy of this particular graphite comes from the tendency of the needle-like grain particles in the filler to align in the extrusion direction during the manufacturing process. Samples were crushed using an agate pestle and mortar and mixed with acetone before being dispersed onto a holey
TEM
Four areas of thin (<50 nm) PGA graphite were subjected to an average electron fluence of 4.2 × 1018 electrons cm−2 s−1 (2.4 × 10−4 dpa s−1). Images of the basal planes and electron energy loss spectra were recorded periodically throughout. The micrographs and their corresponding selected area electron diffraction (SAED) patterns shown in Fig. 2 are typical of the damage produced by a 200 kV electron beam. In particular, the tortuosity (or curvature) of the (0 0 2) planes can be seen to increase, the
Final discussion and conclusions
We have presented a new methodology to quantitatively analyse TEM micrographs of irradiation damaged graphite. Following electron irradiation at 200 keV, a decrease in the graphite (0 0 2) fringe length and an increase in tortuosity and relative misorientation was observed indicating a reduction in the alignment of basal planes. Analysis of the low and core loss of several EEL spectral series indicates little or no change in valence electron density, a decrease in planar sp2 content (to levels
Acknowledgements
This paper was written by Brindusa Mironov and Helen Freeman. Funding was provided by the National Nuclear Laboratory and EPSRC (grants EP/J502042/1 and EP/I003312/1). We acknowledge Abbie Jones of the University of Manchester, UK, for the provision of polarised light micrographs and Anne. A. Campbell at the University of Michigan, U.S.A, for the provision of the Graphite Anisotropy Analysis Program (GAAP). Fred S. Hage would like to acknowledge Magnus Kristofer Nord (Norwegian University of
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