27 August 2009

French utility EDF is gearing up for the pre-service inspection of Flamanville 3, scheduled for the end of 2010. By P. Blin, F. Champigny and L. Berhault

The Flamanville 3 EPR, under construction in Normandy, is the first unit to be subjected to the French Ministerial Orders of 10 November 1999 and of the 13 December 2005 from the design phase. According to these orders, the non-destructive examinations (NDE) planned for the in service inspection (ISI) and for the pre service inspection (PSI) must be operational and formally qualified. The PSI is a complete inspection of the main primary and secondary systems. The PSI’s objective is to perform, before the first core loading, all the NDE planned for the future ISI in the same conditions, in order to have a reliable reference for the detection or for the evaluation of possible damage during the ISI.

The EPR nuclear steam supply system design has been optimised based on feedback from the French N4 unit and the German Konvoi units, so that hardly any area is troubled by damage mechanisms. A large and formal programme of NDT dedicated to manufacturing defects detection is performed in order to ensure the structure integrity of components. The reliable EPR design and manufacturing enhances the level of confidence so that no ISI degradation is expected.

The objective of the qualification process is to prove that an NDE application meets the given performance specifications. Most NDE applications are qualified according to the ‘conventional’ approach (for an unspecified defect). The recording threshold is based on a reference (e.g. notch / hole diameter 2mm) and one of the qualification objectives is the detection sensitivity in the inspected volume. The technical justifications study the influential parameters’ contributions on the performances. Although no postulated defect is expected, the detection capability is assessed on planar defects in mock-ups or by simulation. However, some NDE applications are being qualified with the ‘general’ approach (for postulated defects, such as thermal fatigue). The aim is to demonstrate that the NDE application is able to detect, locate and size the postulated defects.

The underlying philosophy

The ISI programme has been set up based on the safety requirements of the EPR’s efficiency objectives. A plant life management programme takes into account safety requirements, component design, calculations, feedback from operational plants and sensitive areas. This plant life management plan defines the main inspection objectives, but not the details of the NDE techniques. Feedback from ISI in operational nuclear power plants, particularly in-vessel inspections, has been studied. This work includes a total of 85 qualified NDE practices, a large knowledge panel, performance reliability issues and the technical feasibility of inspection specifications. The ISI choices must be in accordance with the major EPR requirements such as the security policy, the collective dose and the outage duration.

The ISI programme is set up with the following principles:

• Launching new developments for the new components (new designs).

• Adapting or upgrading some NDE applications already performed and qualified for France’s 58 operational plants.

• Retaining ultrasonic techniques instead of radiography.

• Retaining automated applications for dosimetry/accessibility situations.

EDF is now consolidating the ISI?programme by developing a panel of mock-ups with notches, involving NDE vendors in the qualification process and organising benchmarking and technical exchanges with Olkiluoto 3 in Finland.

The ISI programme

The ISI programme for the EPR is composed of 40 NDE applications for the main primary and secondary systems. In addition, televisual inspections are needed for the internals of the main components (reactor pressure vessel, vessel head, steam generators, pressurizer etc).

This ISI programme includes, but is not limited to, CRDM housing welds (homogeneous and dissimilar), MCL welds (homogeneous and dissimilar), RPV welds and head closures stud/nuts, SG and pressurizer housing welds, SG tubes and MSL and MFWL welds.

Some of the NDE methods are new and P Blin and specific to the EPR (eg main coolant line, control rod driving mechanism). Half are based on ultrasonic techniques, compared with 35% for the operational nuclear power plants.

Half of the NDE for the EPR is automated using robotic equipment, while the other half is carried out manually by qualified inspectors. Robotic equipment will be the responsibility of specialised vendors. To prevent contamination, a new in-vessel machine is required.

EDF is getting a large amount of feedback coming from the operational nuclear power plant NDE. However, the new designs of some EPR components require new automated NDE equipment to examine these areas, eg the MCL and the CRDM housing welds. The challenge is to ensure the performance for these new components.

Author Info:

F Champigny are from the St Denis location of EDF’s CEIDRE engineering and inspection department. L Berhault is from the Montrouge location of EDF’s CNEN nuclear power engineering department, which has a particular focus on the EPR.

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