LONGLIFE

Treatment of long term irradiation embrittlement effects in RPV safety assessment

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 Obiettivo del progetto (Objective)

'In view of the increasing age of the European NPPs and envisaged life time extensions up to an EOL of 80 years, there is a need for an improved understanding and prediction of RPV irradiation embrittlement effects connected with long term operation (LTO). Effects caused by high neutron fluences such as the possible formation of Late Blooming Phases and as yet unknown defects must be considered adequately in safety assessments. However, the surveillance database for long irradiation times (>20 years) and low neutron flux is sparse, which leads to uncertainties in the treatment of LTO irradiation effects. In this context microstructural data are essential for the understanding of the involved mechanisms. The proposed project aims at: 1) improved knowledge on LTO phenomena relevant for European reactors; 2) assessment and proposed improvements of prediction tools, codes and standards; 3) elaboration of best practice guidelines for irradiation embrittlement surveillance. The suggested scope of work is: i) Summary of boundary conditions for LTO and systematic (re)evaluation of the international prediction procedures of irradiation embrittlement ii) Generation of microstructural data of irradiated representative or original RPV materials; demonstration that damage models are (or not) consistent with the mechanisms of irradiation damage in the LTO (use of PERFORM60 prediction tools) iii) Investigation of specific LTO relevant phenomena like late blooming phases and flux effect from available results; role of Cu, Ni, Mn, P, Si under the aspect of LTO; iv) Correlation of microstructural data with mechanical properties and identification of the most important influencing factors (link with PERFORM60) v) Influence of high neutron fluences on fracture toughness curves shape vi) Comparison of embrittlement results from decommissioned plants with surveillance data vii) Elaboration of recommendations for RPV embrittlement surveillance under LTO conditions'

Introduzione (Teaser)

Scientists are conducting careful microstructural analyses of highly irradiated nuclear reactor materials. Outcomes will inform predictive tools, codes and standards for ageing nuclear power plants and new ones under construction.

Descrizione progetto (Article)

Light water reactors (LWRs) are behind the majority of the world's nuclear power generation. Their steel reactor pressure vessels (RPVs) contain coolant water at high temperatures and pressures. They are subject to strict regulations regarding probability of failure under normal operating conditions and in the case of potential accidents.

European nuclear power plants are ageing yet the surveillance database for long-term operation (LTO, more than 40 years) is limited. As such, it is necessary to enhance the knowledge of LTO behaviours regarding neutron irradiation embrittlement of the steel vessels in order to validate or correct existing prediction tools, codes and standards.

Better characterisation of microstructural behaviours of irradiated RPV materials will facilitate safe extension of the end-of-life (EOL) of existing reactors as well as improved safety of new reactors under construction. EU-funded scientists are addressing this aim in the context of the project 'Treatment of long term irradiation embrittlement effects in RPV safety assessment' (LONGLIFE).

Investigators are interested in enabling the microstructural characterisation of defects and damage induced by neutrons. As such, they are evaluating current existing irradiation embrittlement data and identifying gaps in microstructural and mechanical parameters for LTO. Highly irradiated RPV materials are being used to identify the conditions for late-blooming effects (LBEs), in which the onset of embrittlement occurs only after the neutron influence exceeds a certain threshold. An important question in establishing appropriate regulations and codes is whether or not RPV materials subjected to accelerated irradiation tests demonstrate the same behaviours as those in service. Scientists have already produced several reports based on the outcomes of the testing and analysis protocols. These include guidelines on the reuse of tested specimens, the transferability of test reactor results to real LWR conditions, and for monitoring radiation embrittlement during life extension periods of older reactors.

LONGLIFE final results are expected to enhance the safety of both existing LWRs and of new reactors under construction. The implementation of guidelines will demonstrate the dedication of the EU to superior safety monitoring and encourage public support for carbon-free nuclear energy of the highest safety standards.