The hardening and embrittlement of reactor pressure vessel steels is of great concern in the actual nuclear power plant life assessment. The embrittlement is caused by irradiation-induced damage, like vacancies, interstitials, solutes and their clusters. But the reason for the embrittlement of the material is not yet totally known. The real nature of the irradiation damage should thus be examined as well as its evolution in time. Positron annihilation spectroscopy has been shown to be a very good method for analyzing these defects. Both vacancy type clusters and precipitates can be visualized by positrons. To be able to compare the results obtained by the positron studies, with those of other techniques (such as transmission electron microscopy, atom probe tomography and small angle neutron scattering), quantitative estimations of the size and density of the annihilation sites are needed. Using the approach proposed by A. Vehanen et al., an attempt is made to calculate the needed quantities in model alloys that were neutron irradiated to different doses. The results obtained will be discussed highlighting the difficulties in defining the annihilation centers in spite of using both lifetime and Doppler broadening measurements in the same samples, even in this simple model alloys.

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