Irradiation thermal stress

Graphite will creep imder 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 ( ,) and a secondary irreversible component (Ej), both proportional to stress and to the appropriate unirradiated elastic compliance (inverse modulus) [69]. The total irradiation-induced creep strain (ej is thus ... 

Expansion of the fuel pellets due to high internal temperatures, cracking due to thermal stresses, and irradiation-induced swelling may lead to contact of the fuel with the cladding. 

For the cooldown analysis, only the l/4t location needs to be examined. During cooldown, the thermal stresses are tensile at the inside surface and compressive at the outside. Again, membrane stresses due to pressure are always positive. As a result, the total stress will always be greater at the inside than the outside surface. Since the maximum allowable stresses, taking into account irradiation effects, will decrease more at the inside than outside surface, it is clear that the total stress at the inside surface will approach the maximum allowable before those at the outside surface. Therefore, only the l/4t location needs to be examined for the cooldown transient. 

Zhigilei, L. V. Garrison, B. J. Microscopic mechanism of laser ablation of organic solids in the thermal stress confinement irradiation regime. J. Appl. Phys. 2000, 88, 1-18. 

Figure 18. Thermal cycling and irradiation effects on the failure stress of composite materials. (Reproduced from reference 9.)...

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