ULTRASONIC EXAMINATION OF HYDROGEN FLAKING IN LARGE FORGINGS: FROM VALIDATION TO SITE INSPECTION D. Moussebois, Laborelec, Belgium

During 2012 RPVs inspections, nearly-laminar indications were detected in the low er and upper shells of Doel 3 and Tihange 2 RPVs. As a consequence, the Doel 3 and Tihange 2 NPPs have to stay in cold shutdown until it has been proved that they can be safely operated. The performance and capability of the applied UT technique to detect and charact erize DDH flaws have to be confirmed. To achieve this objective, an investigation has been launched on a block of limited si ze extracted from a steam generator shell known for containing hydrogen flaking. The paper describes the UT measurements performed on the block and the results of t he destructive tests which enable to demonstrate the capability of a straight ultrasonic beam to detect and size laminar and nearly laminar indications and to correctly identify ligament s b tween two adjacent indications. It also address the in-service inspection techniques and pr oce ures used for the site inspections and the lessons learned from the experiments. RPV INSPECTIONS IN BELGIUM The ASME Code Section XI, Edition 1992, is fully applicable to the regular in-service insp ction of all Belgian units. The current in-service inspection program starts from this ASME framework, but in addition, national and international operating experience is constantly being integrated in order to continuously improve the inspections programs. This implies that the inspection planning and methods ar e reevaluated at regular intervals. The in-service inspection program of a ten-year interval covers the volum etric examination of all welds, studs and bolts as well as the visual inspection of the inner surface of the vessel. It must be completed by the end of the interval. The inspection procedures are qualified by the licensee’s qualification body (EQB). The work is supervised by the safety authorities. Equipment and techniques for RPV shell inspection Since the 1982 Pre-service Inspection of the Doel 3 RPV, regulatory ultrasonic exami nations have been carried out by Intercontrôle (France) using the automated MISB (Machine d’Inspection en Service Belge). The MIS-B equipment uses the acoustic beam focusing technique, concentrating sound energy in a restricted volume. The RPV wall thickness is divided into three depth ranges, to be insonified by different ultrasonic transducers. For the shell welds inspection, one 0°L probe and four 45°T shear wave transducers, orthogonally oriented, are dedicated to each depth range. All transducers are immersed within the RPV and are actuated in pulse-ec ho mode without contact with the examination surface. The acoustic beam focusing technique generates narrow acoustic beams, which require, in o rder to assure full material coverage, small scanning steps and increments between emissi on of acoustic pulses. 710 M or e In fo a t O pe n A cc es s D at ab as e w w w . n dt . n et /? id = 18 54 8 Scanning steps of 2 mm and increments of 2 or 5 mm, depending on flaw density, have been used in the hydrogen flaking inspection. Indications dimensions were measured by the standard 6 dB amplitude drop technique. Figure 1 MISB 3G In Service Inspection Machine DOEL 3 2012 RPV core zone inspection During the 2012 outage at Doel 3 NPP, specific ultrasonic in-service inspect ions were performed to check for underclad cracking in the reactor pressure vessel. No underclad defec ts w re found but a large number of nearly-laminar indications were detected mainly in the lower and upper shells. Figure 2 MIS shell and core zone inspection Tool