Microstructure alterations in the base material, heat affected zone and weld metal of a 440-VVER-reactor pressure vessel caused by high fluence irradiation during long term operation; material: 15 Ch2MFA approximate to 0.15 C-2.5 Cr-0.7 Mo-0.3 V

Within the scope of the EC research project Tacis '91 ('RPV-Embrittlement') , trepans were taken from the highly irradiated circumferential RPV-weld of the Novovoronesh power plant unit-2 of the type VVER-440/230. The cumulate d fast fluence level in this position reaches up to 6.5 x 10(19)/cm(2) (E > 0.5 MeV). In a joint research work, the mechanical properties, the chemica l composition, and the microstructure of the base material, the heat affect ed zone (HAZ), and the weld metal have been investigated in order to study the influence of irradiation, and of post irradiation heat treatment (475 d egrees C, 560 degrees C) on the properties. The examination of the microstr ucture performed by analytical transmission electron microscopy (200 kV) sh ows the existence of dislocation loops ('black dots'), irradiation induced precipitates, and segregation of copper in the carbides. These changes in m icrostructure, which are due to service affection (neutron irradiation, tem perature) have occurred more pronounced in the weld metal and the HAZ than in the base material. (C) 1999 Elsevier Science S.A. All rights reserved.