A. Ulbricht

SANS response of VVER440-type weld material after neutron irradiation, post-irradiation annealing and reirradiation

It is well accepted that the reirradiation behaviour of reactor pressure vessel (RPV) steel after annealing can be different from the original irradiation behaviour. We present the first small-angle neutron scattering (SANS) study of neutron irradiated, annealed and reirradiated VVER440-type RPV weld material. The SANS results are analyzed both in terms of the size distribution of irradiation-induced defect/solute atom clusters and in terms of the ratio of total and nuclear scattering intensity in a saturation magnetic field (A ratio). The measured A ratio is compared with calculations performed on the basis of the cluster composition reported for a similar weld material investigated by means of three-dimensional atom probe field ion microscopy. The observed deviation between both estimates and possible reasons for the discrepancy are discussed. Special emphasis is placed on the differences between the materials response to the original irradiation and to reirradiation after annealing. The results indicate that reirradiation-induced clusters are slightly different in their average composition and their formation saturates at a lower volume fraction than in the case of the original irradiation.

Irradiation effects on toughness behaviour and microstructure of VVER-type pressure vessel steels

Abstract The irradiation sensitivity and the annealing behaviour were studied on seven different heats from VVER 440 and VVER 1000-type reactor pressure vessel steels. The specimens were irradiated at the Rheinsberg prototype VVER 2 reactor to mean neutron fluences between 43 and 127.6×10 18 n/cm 2 [E>0.5 MeV ] . Toughness and strength properties were determined and the microstructure was analysed using the small angle neutron scattering (SANS) technique. There is an obvious correlation between the irradiation-induced changes of transition temperature, hardness and volume fraction of microstructural features of radii up to 2 nm. The main parameters of influence are the neutron fluence and the nickel content. The nickel-containing VVER 1000-type pressure vessel steel is more sensitive to irradiation than the VVER 440-type steel which has a low nickel content. For the latter, the sensitivity to radiation embrittlement depends on the copper and phosphorus contents. Annealing at 475°C (100 h) reduces the irradiation effect but not completely in every case.

Cluster Dynamics Simulation of Reactor Pressure Vessel Steels under Irradiation

The coupling between copper rich precipitates (CRP) and point defects in neutron irradiated iron alloys and VVER steels was investigated by means of cluster dynamics (CD) simulations. The consideration of the strain energy effect on CRP kinetics as well as the application of the regular solution model for the case of different fixed copper contents of CRP provides a good agreement between the simulation results and experimental data for complex iron based alloys with small (0.015 wt%) and high (0.42 wt%) copper content. It was found that the CD simulation is applicable to irradiated VVER steel with 0.07 wt% of copper.

Nanoindentation of ion-irradiated reactor pressure vessel steels – model-based interpretation and comparison with neutron irradiation

Abstract Ion-irradiation-induced hardening is investigated on six selected reactor pressure vessel (RPV) steels. The steels were irradiated with 5 MeV Fe2+ ions at fluences ranging from 0.01 to 1.0 displacements per atom (dpa) and the induced hardening of the surface layer was probed with nanoindentation. To separate the indentation size effect and the substrate effect from the irradiation-induced hardness profile, we developed an analytic model with the plastic zone of the indentation approximated as a half sphere. This model allows the actual hardness profile to be retrieved and the measured hardness increase to be assigned to the respective fluence. The obtained values of hardness increase vs. fluence are compared for selected pairs of samples in order to extract effects of the RPV steel composition. We identify hardening effects due to increased levels of copper, manganese-nickel and phosphorous. Further comparison with available neutron-irradiated conditions of the same heats of RPV steels indicates pronounced differences of the considered effects of composition for irradiation with neutrons vs. ions.

Application of Positron Annihilation Spectroscopy to the Study of Irradiated Fe-Cr Alloys

The aim of this work was to investigate the effect of Cr on the microstructure of neutron-irradiated Fe-Cr alloys. Neutron irradiation-induced damage at its early stage was simulated by ion implantation. Positron Annihilation Spectroscopy was applied to identify irradiation-induced defects depending on the Cr-content in Fe-Cr alloys. Different irradiation scenarios were used to investigate the influences of irradiation step by step.

Flux dependence of cluster formation in neutron irradiated weld material - small-angle neutron scattering experiments and rate theory simulation

Small-angle neutron scattering was applied to investigate the size distribution of irradiation-induced defect-solute clusters in a reactor pressure vessel weld material containing 0.22 wt% copper. In order to identify flux effects the material was exposed to neutron irradiations at two different levels of neutron flux in such a manner that the same value of neutron fluence was accumulated. We observed a pronounced effect of neutron flux on the cluster size, whereas the total volume fraction of the irradiation-induced clusters was found to be insensitive to the level of flux. The result is compatible with a rate theory model according to which the range of applied fluxes covers the transition from a flux-independent regime at lower fluxes to a regime of decelerating cluster growth. The issue of the effect of flux on the mechanical properties is also addressed.