Irradiated and unirradiated Young’s modulus of nuclear graphite is usually measured either by an impulse or frequency method, giving the dynamic Young’s modulus (DYM). However, for use in com­ponent stress analysis assessments, the static Young’s modulus (SYM) is required. In addition, the UK irradiation creep modulus was originally defined using SYM, so there is a need to interconvert between the two measurements.

DYM is always higher in value than SYM. Unfor­tunately, SYM has historically been defined in vari­ous ways, that is, as the chord between the origin at zero stress and half, or in some cases two-thirds the failure strength in tension or bending.

There is limited amount of data on the SYM/ DYM ratio for UK nuclear graphite, as given in Figure 48.78 The data is for unirradiated graphite and for graphite irradiated in an inert atmosphere. The ratio for irradiated graphite is higher than for unirradiated graphite. At present, there are no
published data on this ratio for radiolytically oxidized graphite, although safety case requirements have rekindled effort in this area in the United Kingdom.

It has been shown79 that for fine-grained IG-110 nuclear graphite and PGX reflector graphite, as well as ASR-ORB baked carbon, the ratio of static to dynamic Young’s modulus strongly depends on the chord length chosen to define SYM.