22.08.2015   Comprehensive nuclear materials

Stress Rupture

While irradiation creep is relatively well understood the effect of radiation on thermal creep and thereby

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Figure 86 Torques measured during removal of bolts from French PWRs of the CPO series. Only bolts showing no indication of cracking are included. The results are in agreement with predicted creep relaxation when applied to upper or lower preload values, but the predictions do not include any reloading. A, B, and C denote measurements from three different CPO plants. Reproduced from Massoud, J. P.; Dubuisson, P.; Scott, P.; Ligneau, N.; Lemaire, E. In Proceedings of Fontevraud; 2002; Vol. 5; paper 62, 417.

creep rupture is not as well defined. In general it appears that creep rupture properties are not improved by irradiation and are adversely affected as shown in the example of Figure 87.1 ,

As shown in Figure 88 Ukai and coworkers have compared the reduction in rupture life in air, sodium, and after irradiation in FFTF, demonstrating that the largest influence is due to irradiation.193 There is some evidence that irradiation in neutron spectra that pro­duce high He/dpa ratios will decrease rupture life, especially at higher temperatures, compared to irradi­ation in fast reactors due to the accumulation ofhelium bubbles on grain boundaries and triple points.191,192

It is possible to improve the in-reactor stress rupture properties of a given steel by additions of selected trace elements such as P and B, both of which are known to affect the distribution and stability of carbide phases. An example is shown in Figure 89.194 Fortuitously, such additions also add to the swelling resistance of such steels.