The unirradiated stress-strain behavior of graphite is nonlinear and exhibits hysteresis and permanent set. It is different in tension to compression (Figure 45) and graphite is also much stronger in compression than in tension.

Similar curves can be found for Gilsocarbon.74 On irradiation, in an inert atmosphere, there is a rapid and significant increase in modulus attributed to pinning of dislocations in the basal plane. Also, the stress-strain behavior becomes almost linear (Figure 46). This increase soon saturates, but there is a secondary increase attributed to structure tight­ening (or closure of porosity due to high crystal strain). Finally, at very high fluence, there is a rapid fall in modulus due to the degeneration of the graphite microstructure. Brown75 also showed that the Vickers hardness of isotropic graphite was con­siderably increased by irradiation. Young’s modulus is significantly reduced by radiolytic oxidation.

Graphite Young’s modulus increases with increas­ing temperature (Figure 47), which is attributed to the tightening of the structure, presumably because of the closure of microcracks; Maruyama et a/.76 tested samples in vacuum to avoid thermal oxidation.

 

Kq(30) K0(30) Ki(30) Kirr(30, 30)

 

f [45]

 

Substituting in the previous equation then gives

 

1 Klrr(T)

 

[46]

 

Thus, the irradiated thermal conductivity can be pre­dicted at any temperature for graphite irradiated in an inert atmosphere. The effect of weight loss and high fast neutron fluence is dealt with by a version of the ‘product’ rule leading to

 

1 Krr(T )

 

[47]

 

where Sk is the high dose reduction in thermal con­ductivity and [K0/K]ox is the reduction in thermal conductivity due to radiolytic oxidation.

 

Figure 45 Stress-strain curves of unirradiated pile grade A graphite. (a) Tension and (b) Compression. Modified from Losty, H.; Orchard, J. In Fifth Conference on Carbon; Pergamon, 1962; pp 519-532.

However, note the significant difference in strength at room temperature between samples tested in air and in vacuum. Several other authors have reported similar findings, attributing the difference to adsorbed moisture.77 The increase in strength with temperature is significant above 600 °C, making it of interest only for HTR reactor components. Also of interest in Figure 47 is the correlation between modulus and strength.