Data on Russian pressure vessel steels in unirradiated, irradiated and annealed conditions are presented. The experimental methods applied included impact testing, fractography, electron microscopy and Auger spectroscopy. Contributions from different mechanisms to radiation embrittlement were evaluated. These studies enabled us to conclude that the major contribution to radiation induced ductile-to-brittle transitions temperature shift in Russian pressure vessel steels is delivered by mechanisms other than radiation hardening.

The actual values of the weld metal characteristics required for determination of the RPV lifetime, that is the ductile-to-brittle transition temperature (DBTT) in the unirradiated condition (Tk0), the phosphorus and copper contents (that are known to affect the radiation stability of steel), were not measured during manufacturing of old VVER-440 reactors. The small samples (templets) were cut out of the inside surface of the old Russian VVER-440 reactor pressure vessels (RPV) to assess the actual condition of the RPV material before and after post-irradiation annealing and after re-irradiation. Calculations have shown that it is not admissible to cut out samples of the size appropriate for machining of fullsize Charpy specimens necessary to comply with Russian Guide for determination of RPV steel DBTT. An application of subsize specimens for estimation of irradiation embrittlement of RPV steel was substantiated. In order to substantiate application of reconstituted subsize specimens made from broken subsize specimens for assessment of irradiation embrittlement of RPV materials, the correlation dependencies between the DBTT values of reconstituted and standard subsize specimens were developed. With this aim reconstitution of broken 5×5×27.5 mm specimens was performed by the stud-welding method. Both unirradiated and irradiated to high fluence steels were used in this program. The influence of reconstitution on the correlation dependence of DBTT for standard fullsize and subsize impact specimens was studied. Irradiation and re-irradiation kinetics of RPV steels were studied using subsize specimens machined from templets of operating units.

This paper presents the results of a study on degradation due to irradiation occurring in WWER-440 reactor pressure vessel (RPV) steel, using subsize Charpy specimens (5×5×27.5 mm3). An application of subsize specimens for estimation of irradiation embrittlement of RPV steel has been substantiated. Comparison between ductile-to-brittle transition temperatures (DBTT) for full-size and subsize Charpy specimens has been carried out. The relation between these two values has been clarified. The results of testing trepans and templates cut out from WWER-440 reactor pressure vessels are considered. The main results of the program TACIS-91/1.1. are discussed.