The structural integrity of the RPV is an essential issue for the plant safety. At the design stage, the demonstration is required with material properties at end of life, to ensure the adequacy of the design with the expected operating transients in all conditions. During operation, the integrity assessment is updated every ten years with new existing knowledge and feedback of operating experience, in particular in service aged material data coming from Irradiation Surveillance Program ISP, fluence evaluations taking into account the effective in service core arrangements in each vessel, in service detected flaws plus a postulated subclad crack whose detection cannot be guaranteed by the qualified ISI program. The final assessment showed that the regulatory criteria are met until the fourth decennial outage for 900 MW RPV. The analysis is performed in accordance with French regulations (use of safety coefficients) and follows a deterministic approach in which the input parameters and uncertainties are taken into account conservatively. For the future demonstration beyond 40 years, a multidisciplinary effort is committed to improving knowledge in order to reduce uncertainties in data and in methods. This extensive program involves in particular: - Thermohydraulic analysis and description of transients: temperatures and heat exchange coefficients; - Mechanical analysis: warm pre-stress effect and crack arrest. In addition, a complementary study using a probabilistic approach to rationalize the level of conservatism of input data is launched. In this report, the French deterministic approach and the main results for 40 years duration are presented and the new developments for the future.
The French fleet comprises 58 PWR. The integrity of the reactor pressure vessel is one of the main considerations regarding plant life extension. The limiting damage is the irradiation embrittlement associated with brittle fracture risk. During the operation, integrity assessment is updated every 10 years taking in account the knowledge acquired by the feedback. In addition steps have been taken to reduce irradiation. Moreover all the research and development have also led to improvements. The purpose of the paper, based on the latest updated demonstration of the 3 loops 900 MW vessels for the period 30–40 years of service, is to develop the strategy to be implemented to go beyond 40 years and the progress in each topic. On one hand the demonstration by analysis is the privileged axis of justification for an operating life of 60 years. - First, by improving at the maximum deterministic approach with regard to the input data and methods to reduce uncertainties. - Second, by a complementary probabilistic study with the aim of showing the conservatism of the deterministic approach. On the other hand, the arrangements already taken during operation for monitoring and mitigating irradiation embrittlement are maintened and reinforced: irradiation surveillance program, in service inspection, limitation and surveillance of fluence, not to mention heating the safety injection water which remains a possibility after the improvements of data and methods.
Structural integrity of the Reactor Pressure Vessel (RPV) is one of the main considerations regarding safety and lifetime of Nuclear Power Plants (NPP) since this component is considered as not reasonably replaceable. Brittle fracture risk associated with the embrittlement of RPV steel in irradiated areas is the main potential damage. In France, deterministic integrity assessment for RPV is based on the crack initiation stage. The stability of an under-clad postulated flaw in the core area is currently evaluated under a Pressurized Thermal Shock (PTS) through a fracture mechanics simplified method. One of the axes of EDF’s implemented strategy for NPP lifetime extension is the improvement of the deterministic approach with regards to the input data and methods so as to reduce conservatisms. In this context, 3D finite element elastic-plastic calculations with flaw modelling have been carried out recently in order to quantify the enhancement provided by a more realistic approach in the most severe events. The aim of this paper is to present both simplified and 3D modelling flaw stability evaluation methods and the results obtained by running a small break LOCA event.
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