Irradiated creep experiments were carried out between 350 and 650 °C on both PGA and Gilsocarbon graph­ite.91 Some low fluence experiments were also carried out in Calder Hall86 which were used to define the so-called ‘primary creep.’ Creep strain data ecr was normalized to elastic strain units (esu) by dividing by the applied stress (s) and multiplying by the

unirradiated SYM (E0) as given below:

E0 ecr

esu =

а

Surprisingly, they found that by doing this, the creep data for these two types of graphite, with very differ­ent microstructures, could be fitted to a simple ‘creep equation’ of the form

аа

ecr = [exp(-4g)] + °.23 g [54]

E0 E0

where g is the fast neutron dose. This is illustrated in Figure 57.

The primary creep strain is assumed to be recov­erable on removal of the load while still under irradi­ation. Some evidence for this came from out-of-pile measurement experiments such as the FLACH experiments.9 However, if the specimens had been left unloaded for longer duration, more than 1 esu may have been recovered. In addition to this, an experiment carried out on precrept samples, that is,

samples of PGA and Gilsocarbon irradiated in a creep experiment in the BR-2 reactor and then irra­diated with the load removed in DIDO and DFR respectively, exhibited more than 1 esu (a recovery in the region of 6 esu in the case of Gilsocarbon in DFR).