Graphite irradiation effects

Fig. 8. Neutron irradiation induced dimensional changes for GraphNOL N3M graphite irradiated a 600 or 875 °C [61]. Note that the radial dimensional changes exceed the axial changes due to textural effects.

Kennedy, C.R. and Woodruff, E.M., Irradiation effects on the physical properties of grade TSX graphite, WCH-EP-0211, Westinghouse Hanford Company, Richland, Washington, 1989. 

Irradiation Effects on Thermophysical Properties of Graphite and Carbon... 

Fig. 10.5. D concentration vs damage in various types of graphite irradiated with 6MeV C+ [33], 200 keV C+ [20] and neutrons [33]. Neutron and ion damage have similar effects on D retention. D retention increases with damage at low damage levels, but saturates at 0.1-1 dpa. The line represents a fit to the POCO and...

A. L. Pinter, Irradiation Effects on Graphite, Report BNWL-SA-2468, Battelle Pacific Northwest Laboratories, Richland, WA, 1969. 

There are multiple barriers to the release of fission products from an HTGR core the fuel kernel, the particle coatings, the fuel rod matrix, and the fuel element graphite. The effectiveness of the individual barriers to fission product release may depend upon a number of factors Including the chemistry and half-llfes of the various fission products, temperature, and Irradiation effects. These barriers are described briefly below. 

During normal steady-state operation, the graphite elements of the core are subjected to stresses induced by the strain gradients resulting from the varying temperature and fluence fields. In addition to stresses created by thermal and irradiation effects, the fuel elements are subject to mechanical loads of gravity, fluid forces and seismic events. Of these mechanical loads only the seismic loads are significant, and even those are small in relation to those due to thermal and irradiation effects. 

Experimental data for the mechanical and physical properties of the selected graphite candidates (IG-110, IG-430, NBG-18, and NBG-25) were produced. In addition, the fracture and oxidation behaviors were estimated. To understand the radiation effects in nuclear-grade graphite, an atomistic stmctural change in IG-110 irradiated with 3 MeV H" " and gamma—irradiation effects were characterized (Kim et al., 2009b Hong et al., 2012 Corwin et al., 2008). 

W. J. Gray, Neutron Irradiation Effects on Carbon and Graphite Cloths and Fibres, BNWL-2390, BatteUe, Pacific Northwest Laboratories, 1977. 

Table 2. Effect of neutron irradiation on some graphite or CFC materials studied for fusion applications [12]...

The third term in Eq. 7, K, is the contribution to the basal plane thermal resistance due to defect scattering. Neutron irradiation causes various types of defects to be produced depending on the irradiation temperature. These defects are very effective in scattering phonons, even at flux levels which would be considered modest for most nuclear applications, and quickly dominate the other terms in Eq. 7. Several types of in-adiation-induced defects have been identified in graphite. For irradiation temperatures lower than 650°C, simple point defects in the form of vacancies or interstitials, along with small interstitial clusters, are the predominant defects. Moreover, at an irradiation temperatui-e near 150°C [17] the defect which dominates the thermal resistance is the lattice vacancy. 

Fig. 10. The effect of annealing on the normalized thermal conductivity of irradiated graphite and graphite composites.

If two irradiations are undertaken in similar neutron spectra to the same total number of atomic displacements and at the same temperature, but at different rates (i.e., over different time intervals), the graphite sample with the shorter exposure time will show more damage (i.e., a flux or rate effect). This is because the net observed damage is a function not only of the total damage produced (dependant on the neutron dose), but also on the extent of annealing of that damage, which is... 

Fig. 7. High-temperature neutron irradiation a-axis shrinkage behavior of pyrolytic graphite showing the effects of graphitization temperature on the magnitude of the dimensional changes [60].

In support of the development of graphite moderated reactors, an enormous amount of research has been conducted on the effects of neutron irradiation and radiolytic oxidation on the structure and properties of graphites. The essential mechanisms of these phenomena are understood and the years of research have translated into engineering codes and design practices for the safe design, construction and operation of gas-cooled reactors. 

Ishiyama, S., Burchell, T.D., Strizak, J.P. and Eto, M., The effect of high fluence neutron irradiation on the properties of a fine-grained isotropic nuclear graphite, J. Nucl. Mater., 1996, 230, 1 7. 

High-resolution transmission electron microscopy (HREM) is the technique best suited for the structural characterization of nanometer-sized graphitic particles. In-situ processing of fullerene-related structures may be performed, and it has been shown that carbonaceous materials transform themselves into quasi-spherical onion-like graphitic particles under the effect of intense electron irradiation[l 1],... 

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