RPV Integrity Assessment: Comparison Between 3 Loops and 4 Loops Studies

For the French utility (Electricite de France–EDF), Nuclear Energy represents 75% of generation of the total electric energy in France. Total nuclear electricity were generated mainly from Nuclear Power plants stations, 34 PWR NPPs 3-loop Reactors- 900 MWe, 20 PWR NPPs 4-loop Reactors- 1300 MWe and 4 PWR NPPs 4-loop Reactors- 1450 MWe. The 3-loop Reactor Pressure Vessel (RPV) integrity assessment, applied on 34 PWR NPPs Reactors, involved the verification of the integrity of the component under the most severe conditions of situation, and the result obtained was the justification of the 900 MWe RPV life management for at least 40 years and to prepare the projection beyond 40 years. Since 2000, in the continuity of these results, the studies were carried out on the 20 PWR NPPs 4-loop 1300 MWe Reactor Pressure Vessels, and the recent results obtained show the demonstration of the integrity of the RPV, in the most severe conditions of loading in relation with RTNDT (Reference Nil Ductility Transition Temperature), and other major parameters. This approach is based on specific mechanical safety studies on the RPV to demonstrate the absence of the risk of failure by brittle fracture. For these mechanical studies the major input data are necessary: 1 - the fluence distribution and the values of 3-loop and 4-loop RPV, 2 - RTNDT during the lifetime in operation, 3 - the temperature distribution values in the downcomer and the PTS evaluation. The main results must show significant margins against initiation of brittle fracture for all of 3-loop and 4-loop RPV. The flaws considered in this approach are shallow flaws beneath the cladding (subclad flaws) or in the first layer of cladding. The major tasks and expertises engaged by EDF are: • more precise assessment of the fluence calculations, • better knowledge of the vessel material properties, including the effect of radiation, • the NDE inspection program on the core zone. The comparison of the results are developed in this paper: • for the fluence evaluation and the optimisation of the fuel management, • the data gathered from radiation specimen capsules, removed from the vessels (radiation surveillance program), • and the thermal-hydraulic and mechanical calculations based on finite element thermal-hydraulic and 3D elastic-plastic mechanical computations.