Various proposed graphite irradiation models, includ­ing the UKAEA irradiation creep models, require the pinning and structural terms to be separated. Origi­nally, the pinning and structure terms for Gilsocarbon were separated by ‘eye,’ which is subjective. Recently, more sophisticated statistical, pattern recognition and curve fitting methods have been used.60

Having separated out the pinning term, the struc­ture term is a function of irradiation temperature and dose, whereas the pinning term is only a function of temperature. The pinning term is assumed to not affect creep rate, whereas the structure term affects it. In some models, the structure term is also assumed to be a function of radiolytic weight loss and to be correlated to dimensional change.