Based upon the evidence from UK and US creep experiments, Davies and Bradford49,68 suggest the following:

• The strain induced change in CTE is not a func­tion of secondary creep strain, but saturates after a dose of-30 x 1020 n cm 2 EDN (-3.9 dpa).

• There is evidence, from both thermal and irradia­tion annealing, for a recoverable strain several

 

times that of primary creep, and a lower associated secondary creep coefficient that has been previ­ously assumed. • The dose at which the recoverable strain saturates bears a striking similarity to that of the saturation of the CTE change. Davies and Bradford49,68 proposed a new creep model (the M2 model) without the term reflecting changes in CTE due to creep, but containing one additional term, recoverable creep:

 

Figure 19 Comparison of predicted apparent creep strain (from eqn [25]) and the experimental creep strain data for irradiation creep at 600 °C under a compressive stress of 13.8 MPa. The true creep strain is calculated from eqn [13]. From Burchell, T. D. J. Nucl. Mater. 2008, 381, 46-54.

 

♦ Experimental creep strain “ ■ True creep strain — — — CTE correction strain —— Predicted apparent creep strain

 

Figure 20 Comparison of predicted apparent creep strain (from eqn [25]) and the experimental creep strain data for irradiation creep at 600 °C under a compressive stress of 20.7 MPa. The true creep strain is calculated from eqn [13]. From Burchell, T. D. J. Nucl. Mater. 2008, 381, 46-54.

where ec is the total creep strain; s, the applied stress; l, X, and b, are the empirical fitting parameters; ki and k2, the primary and recoverable dose constants respectively; and W is the oxidation change factor (with respect to Young’s modulus) and is analogous to the structure factor. The terms in the eqn [26] are proportional to esu and the effects of structural changes and radiolytic oxidation (gasification of graphite by an activated species that occurs in CO2 cooled reactors) are also included. The rates of satu­ration of the primary and recoverable creep compo­nents are controlled by the dose constants ki and k2. The first and last terms in eqn [26] are primary and secondary creep as in the prior UK creep model, with the middle term being recoverable creep.

Primary creep is still fast acting, but in the AGR temperature range of 400-650 °C, appears to act on a longer fluence scale equivalent to that associated in the United Kingdom with the Young’s modulus pinning,69 ki = 0.1, and saturates at 1 esu (a = 1). The irrecoverable creep is synonymous with second­ary creep, but with a coefficient, b, derived from the irrecoverable strain postthermal anneal, as

0. 15 per 1020 n cm 2 EDN (-1.3 dpa) in the AGR temperature range. The lateral creep strain ratios for primary and recoverable creep are assumed to be the Poisson’s ratio and secondary creep is assumed to occur at constant volume.

Figure 24 shows the performance of the M2 models applied to some high dose ATR-2E tensile creep data52 when irradiated at 500 °C in high flux reactor (HFR), Petten. The prediction matches the observed data well up to significant fluence of -160 x i020ncm~2 EDN (-21 dpa). Only beyond
this fluence does the new model prediction deviate from the data with a delay in the increase in creep strain at high doses that is often referred to as the ‘tertiary’ creep phase.

Figure 25 shows the corresponding compressive creep data,52 irradiated at 550 °C. The model over predicts the data slightly but follows the trend remark­ably well up to a significant fluence of -i60 x i020 n cm~2 EDN (-21 dpa). Beyond this fluence, the compressive prediction also indicates a ‘tertiary’ creep, but the data does not extend into this region. The data52 also indicates a possible difference between tensile and compressive creep (seen more clearly in Figure 18).

Saturation of CTE with creep strain as reported by Davies and Bradford49,68 is not however in agree­ment with other published data. Gray70 reported CTE behavior with creep strain (up to 3%) for three different graphites at irradiation temperatures of 550 and 800 °C. Saturation of the CTE in the

manner described by Davies and Bradford49,68 for UK AGR graphite was not observed.