Neutron Embrittlement of VVER 1000 and 440/213 RPVs: Learning From EC Projects on RPV Integrity
Authors: Rieg Claude , Ahlstrand Ralf , Bieth Michel , Debarberis Luigi , Sevini Filippo , Taylor Nigel , Karzov Georgy , Margolin Boris , Dragunov Yuri , Piminov Vladimir , Kryukov Alexander , Shtrombakh Yaroslav 
Published: August 2008

Since 1991 the European Commission has financed a significant number of Technical Assistance Projects to the Commonwealth of Independent States (TACIS) and EURATOM R&D actions addressing the main safety issues on RPV material embrittlement and integrity assessment. Since the VVER 440 reactors of the reference series 213 are made from recognised neutron embrittlement resistant materials and include comprehensive surveillance programmes, a standard plant life management procedure can be applied to address long-term concerns, mostly aiming at reducing uncertainties in the assessment techniques. Therefore, the open issues (flux effect, upgrading of surveillance results, implementation of toughness measurements and relevant acceptance criteria, behaviour of the cladding) are quite universal. The efficiency of late annealing (at fast [E>0.5 MeV] neutron doses over 1020n/cm2) and the re-embrittlement after annealing remain key issues for any final decision for their operational lifetime. The more recently developed VVER 1000 reactors have some well-known features arising from the original design and manufacturing process (high nickel content in the core weld, location of the surveillance specimens), which have to be carefully considered if appropriate mitigation measures are to be implemented during operation. A precise identification of the issues related to the surveillance programme has been achieved thanks to research on dosimetry evaluation, representativeness (temperature and flux effect) of the specimens and optimisation of the evaluation of their results. Nickle, just as copper and phosphorus, is now recognised as having a detrimental effect on neutron embrittlement in synergy with other elements (e.g. manganese). The analyses of available data for CrNiMn steels do not show a significant effect for fast [E>0.5 MeV] neutron doses below 7.1019n/cm2, but their consistency and relevance might be questionable. A way has already been pioneered which shows how valuable results can be obtained using the existing surveillance programmes specimens. A systematic application on the Russian & Ukrainian plants is now planned in order to get updated figures on design end of life (EOL) integrity assessment. This includes updated dosimetry assessment, multiple specimen testing (reconstitution, impact and static toughness tests) and advanced integrity analyses. An optimised database of representative surveillance results (up to the design end of life) is expected, which should provide a sound basis for further understanding and setting up of relevant prediction tools, considering at the same time any other specific R&D results. The global integrity assessment will also provide for preparing and implementing adequate mitigation measures in due time, if necessary. The paper will report about the knowledge on RPV embrittlement effects, providing evidence of recent contributions to solve shortcomings of the VVER 440/213 and 1000 units. The current state-of-the-art and the remaining open issues have been assessed recently by a group of international experts. The planned R&D activities and the detailed scope of the latest TACIS projects are described.