Radiation Damage in Graphite

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 

A general theory of dimensional change in graphite due to Simmons [62] has been extended by Brocklehurst and Kelly [17]. A detailed account of the treatment of dimensional changes in graphite can be found in Kelly and Burchell s analysis of H-451 graphite irradiation behavior [63]. 

The elastic modulus and strength are related by a Griffith theory type relationship. 

Graphite will creep under neutron irradiation and stress at temperatures where thermal creep is normally negligible. The phenomenon of irradiation creep has been widely studied because of its significance to the operation of graphite moderated fission reactors. Indeed, if irradiation induced stresses in graphite moderators could not relax via radiation creep, rapid core disintegration would result. The observed creep strain has traditionally been separated into a primary reversible component (e,) and a secondary irreversible component (e2), both proportional to stress and to the appropriate unirradiated elastic compliance (inverse modulus) [69], The total irradiation-induced creep strain (ej is thus  

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The process responsible for initiating RES follows from the earlier discussion of radiation damage in graphite. Specifically, in a displacement event a Frenkel pair... 

J.W.H. Simmons, Radiation Damage in Graphite, Pergamon Press, (1965 ). 

Fig. 6. Radiation damage in graphite showing the induced crystal dimensional strains. Impinging fast neutrons displace carbon atoms from their equilibrium lattice positions, producing an interstitial and vacancy. The coalescence of vacancies causes contraction in the a-direction, whereas interstitials may coalesce to form dislocation loops (essentially new graphite planes) causing c-direction expansion.

Simmons, J.H.W., Radiation Damage in Graphite, Pergamon Press, Oxford, 1965. Kelly, B.T. and Burchell, T.D., Structure-related property changes in polycrystallinc graphite under neutron irradiation, Carbon, 1994, 32, 499 505. 

Porous Structure and Adsorption Properties of Active Carbons, M. M. Dubinin Radiation Damage in Graphite, W N. Reynolds... 

Fig. 4.26. Radiation damage in graphite showing the induced crystal dimensional strains.

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