In the United Kingdom, to extend the creep law to higher fluence and to account for radiolytic oxidation, the following approach was taken by the UKAEA. Creep strain, decr/dg, was assumed to be defined by

"df = a(-T)Y texp(~bg)] + b(r )Ef N

where a(T), b(T), and b are temperature-dependent functions equal to 1.0, 0.23, and 4.0, respectively in the AGR and Magnox temperature ranges, and a and g are stress and irradiation fluence, respectively. The need for the temperature dependence outside this range was defined by data for HTRs obtained in the United States and Russia (Figure 63).

The ‘creep modulus’ in the UKAEA model was defined as

Ec = EoSE{ox [56]

where E0 is the unirradiated SYM and S is the irradi­ation temperature — and fluence-dependent struc­ture term derived from the irradiated modulus data (Section 4.11.17.4). To account for radiolytic weight loss, E[ox] is a modulus weight loss term defined as E/E0 = exp(—lx) where l is an empirical constant equal to about 4.0 and x is the fractional weight loss.

There are no rigorous observational data to underpin this model other than a few data points for preoxidized graphite given in Figure 64. It should be noted that in Figure 64, the only two

Longitudinal creep strain (%)

(b) Longitudinal creep strain (%)

1E-24

о c о (D

Russian graphite American graphite (EGCR) American graphite (CGB)

1E-25

100 200 300 400 500 600 Temperature (^C)

700 800 900 1000

samples with a significant amount of weight loss are irradiated to a relatively low fluence. Similarly, there are some, even less convincing, data on creep sam­ples initially irradiated to high fast neuron fluence before loading.92

The main criticisms of the UKAEA creep model, for inert conditions, is that it gives a very poor fit to the high fluence creep data obtained in Germany and the United States, as discussed in the next section.

4.11.20.6.1 German and US creep model

This model was devised for helium-cooled HTR applications where radiolytic oxidation was of no concern. The form of some of the US and German data is given below (Figure 65). There appears to be a difference between tension and compression at high fluence. However, this is the only data that shows this and it is not clear if it is a real effect.

It was assumed that microstructural changes at medium to high fluence would modify the creep rate and account for the shape of these curves. It was assumed that this could be accounted for by modifying the secondary creep coefficient in the UK creep law by the following expression:

/ s A

ecr(secondary) K

where K is the UK creep coefficient, AV/V0 is the change in volume (which is a function of fluence and
irradiation temperature), (AV/V0)m is the volume change at volumetric ‘turnaround,’ and m is a graphite grade and temperature-dependent variable.