Compressive stress increases, and tensile stress decreases, the irradiation-induced dimensional change of graphite as illustrated in Figure 54. In these experiments, two matching graphite samples, a loaded specimen and an unloaded ‘control’ specimen, were irradiated adjacent to each other in an MTR, and dimensional change in the direction of load was measured.

However, as well as change in dimension in the load direction, there are also dimensional changes perpen­dicular to the load direction as shown in Figure 55.

An irradiation creep curve can be simply obtained by subtraction of the unloaded dimensional change curve from the crept dimensional change curve, as illustrated in Figure 56. However, for practical use in assessments, this would require data for a range of temperatures and fast neutron fluences covering all of the expected operating conditions. Also, in the case of carbon dioxide-cooled reactors, the effect of (a)

the rate of radiolytic oxidation on creep rates would have to be quantified and understood. In addition, changes to the CTE and Young’s modulus with irra­diation creep have been observed, which further complicate assessment technology.