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Application of rate theory modelling to neutron-irradiated low-Cu and Cu-enriched Fe-based model alloys

Bergner, F.
Rate theory (RT) is a mean-field approach focussed on the long-term evolution of precipitates or clusters of defects and/or solute atoms in the present case. Interactions between any one particular cluster and particular cluster-forming species are replaced by average interactions. Therefore, any space variables drop out and the temporal evolution of the size distribution is at the centre of interest. On the one hand, the assumptions may not be completely valid in the case of displacement cascades and special attention has to be paid to the initial conditions and to the source term. On the other hand, RT allows the evolution of the cluster size distribution to be followed over time intervals corresponding to the operation of a reactor pressure vessel up to end-of-life within reasonable computation time. As a second advantage, cluster size distributions measured by means of small-angle neutron scattering (SANS) are well suited for the experimental validation of RT models.

Application of RT to three particular systems exposed to neutron irradiation will be considered in the presentation:
• the coupled evolution of the concentrations of single vacancies and single self-interstitial atoms (SIAs) in Fe,
• the evolution of the size distribution of vacancy clusters in Fe,
• and the evolution of pure coherent Cu clusters in dilute Fe-Cu.
Special emphasis will be placed on the aspect of experimental validation using SANS results obtained for low-Cu and Cu-enriched Fe-based model alloys. The problem of multi-component clusters will be touched in the outlook.

The first maximum at cluster sizes of about 0.3 nm is caused by the direct generation of small clusters due to neutron irradiation. After some ten days of irradiation a second peak appears, which becomes more and more dominant. It is observed that:

• the shape of the size distribution is rather independent of the model parameters,
• the rate of cluster growth is essentially influenced by sink strength for vacancies and interstitials only,
• and the volume fraction at a given instant of time is influenced by both sink strength and surface tension of Fe.

From these findings a strategy that allows for fitting the calculated size distribution to SANS data is worked out. However, it turns out that even in low-Cu Fe-based alloys except pure Fe the average cluster detected by means of SANS is not a pure vacancy cluster. Comparability is therefore limited and a concept to deal with two- or multi-component clusters is required.
  • Invited lecture (Conferences)
    International School on Experimental Quantification of Irradiation Damage (ISQUID), 25.-29.09.2006, Rochehaut sur Semois, Belgium
  • Contribution to proceedings
    International School on Experimental Quantification of Irradiation Damage (ISQUID), 25.-29.09.2006, Rochehaut sur Semois, Belgium
    Proceedings on CD, Mol: SCK-CEN, 1-47

Permalink: https://www.hzdr.de/publications/Publ-8895
Publ.-Id: 8895