This paper presents the experience feedback and views of the French Regulatory Authority (ASN) and of the technical support institute (IRSN) on PWSCC prevention since the initiation in 1989 of the "Inconel Zones Review" requested by ASN to Electricité de France (EDF), the national operator of a fleet of 58 PWRs.
This proactive requirement, launched before the discovery, in September 1991, of the only CRDM nozzle leak in France, on Bugey unit 3, was then triggered by the recurrence of many alloy 600 rapid degradations and leaks, world wide, and also in France in the late 1980s, particularly on steam generator tubes and on some pressurizer penetrations.
Thus, the ASN requested that EDF, perform a comprehensive (generic) proactive assessment on all the nickel-base alloy components and parts of the main primary circuits, which of course included vessel head penetrations and bottom vessel head penetrations, and some other zones as a first priority.
This proactive "review" did, a minima, include the following tasks and actions:
- Update and complete, by an extensive R&D program, the understanding and characterization of the Ni base alloys prone to PWSCC,
- Analyze the various materials, metallurgical features, mechanical stresses, and physicochemical conditions of the parts exposed to primary water, in order to predict the occurrence of PWSCC initiation and propagation,
- Provide a prioritization of the zones to be inspected,
- Implement by improved NDE techniques a practical inspection program on the 58 PWRs,
- Prepare and implement any needed mitigation actions as a result of the components conditions assessment.
The present paper relates the main features of the French regulatory experience over more than 13 years and recalls the main principles of the assessment, which were applied by ASN. These principles, which are formalized in the current regulation rules revised in 1999, are briefly listed hereunder:
- It is based on avoiding and preventing any leaking on the main primary circuit.
- In service inspections (ISI), including volumetric and surface NDE, have been agreed upon between ASN and EDF for all vessel head penetrations, with a re-inspection schedule.
- The preexisting regulatory hydraulic testing program was carefully implemented, which implied the removal of thermal insulation on the vessel heads.
- A comprehensive R&D program had to be conducted by EDF, the main progress reports and presentations had to be regularly submitted to DGSNR and IRSN staff.
- The assessment and the ranking of the sensitivity of the different nickel base alloy zones, derived from R&D and empirical models, would have to be confirmed by a comprehensive ISI program, including bottom head penetrations, steam generator partition plates, and more specific weld metal zones.
- ASN reviewed the various mitigations and preventive measures proposed by EDF, either temporary, such as leak detection systems, anti-ejections devices, interim repairs, or long term commitment of the French operator to replace in due time the vessel heads comprising the most affected CRDM penetrations.
This paper also presents the ASN's follow up of the domestic and international feedback, such as the occurrence of PWSCC cracking (initiation and propagation) in the weld materials, whose occurrence is rather limited in France.
Guy TURLUER, Gérard CATTIAUX, Bernard MONNOT, (IRSN/DES), Fontenay-aux-Roses
David EMOND, Jacques REUCHET, Philippe CHARTIER (DGSNR/BCCN), Dijon