1 Krr(T)

 

[52] ox

In recent years, it has been realized that the use of the product rule is simplistic, and most probably, only applicable for low irradiation data, up to a fluence not far beyond dimensional change turnaround and only for relatively low weight loss. Therefore, there has been a recent trend to use empirical fits to reactor or MTR data where available.

By the late 1940s, it was known that graphite compo­nents, when subjected to fast neutron irradiation, suffered significant dimensional change. It was thought that, because of the flux gradient across the brick section, these dimensional changes would gen­erate significant stresses in hollow graphite modera­tor blocks and that this would lead to significant

г
—— ——
№ я. • ■
350
Gilsocarbon (300-400 °C) □ Petroleum coke (560°C) О Unidentified coke (560 °C) О Gilsocarbon (560 °C) a Pitch coke (300-440 °C) E/E0 ■ Petroleum coke (300-440 °C) • Unidentified coke (300-440 °C)———————————————————————————— V(E/E0)

Figure 52 Change in strength of irradiated Gilsocarbon graphite compared to change in modulus. Modified from Brocklehurst, J. E. Chem. Phys. Carbon 1977, 13, 145-272.

Porosity

component failures within a few years of reactor operation. Therefore, the fuel channels in the early reactors, such as the Windscale Piles, were designed to avoid the buildup of stress.

By the 1950s, it was realized that there was an irradiation-induced mechanism that was relieving

stresses generated by dimensional change and the term ‘irradiation induced plasticity’85,86 was coined to describe this mechanism. Later, around 1960, the term ‘irradiation creep’87 started to be used for the difference between dimensional change in loaded and unloaded graphite specimens irradiated to the same dose.