Reactor Pressure Vessel: EDF R&D Program to Support Lifetime Management

The Reactor Pressure Vessel (RPV) is with the concrete containment one of the two components of a NPP whose replacement is not considered as reasonably feasible. The RPV lifetime has thus an important impact on the lifetime of the whole NPP. One of the key issues concerning RPV lifetime is the radiation effect on the RPV steel in the core zones. The vessel steel becomes indeed more brittle in the RPV core region where radiation is high. Margins have been included at design and manufacturing stages taking into account the material’s embrittlement. Moreover, operating measures have been taken to manage ageing of RPV in order to extend lifetime. The challenge is to preserve high margins and to provide the safety studies showing these margins. A large R&D program has been developed to support lifetime extension. The objective of the program is to develop tools and provide input data for the demonstration of the safe operation of the reactor pressure vessel significantly over a 40-year lifetime. The aim of the paper is to present an overview of the R&D program to support lifetime management on the fields of materials, mechanics and thermalhydraulics. Experiments are indeed performed on irradiated material in order to improve the knowledge on embrittlement for high fluences and to be able to determine embrittlement correlations for materials representative of French RPV. Actions are also planned to improve evaluation of the RPV mechanical behaviour and to describe physical phenomena such as crack arrest or warm pre-stressing effect. Last, studies are realized to improve the thermal loadings evaluations under hypothetical accidental scenarios. These studies are supported by thermalhydraulic numerical simulations whose validation is obtained by comparison to experimental results from experimental hydraulic loops representative of French RPV.