MRP-139 Recommendations: Inspection of Dissimilar Metal ... · MRP-139 Recommendations: Inspection of Dissimilar Metal Welds in Reactor ... as usual in end of ten-year interval reactor
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Austenitic and Bimetallic Weld Inspection II MRP-139 Recommendations: Inspection of Dissimilar Metal Welds in Reactor Pressure Vessels in Spain J.R. Gadea, A. Willke, J.J. Regidor, Tecnatom, Spain ABSTRACT MRP 139 recommendations for inspection of dissimilar metal (DM) welds have been recently launched in the USA. Although these recommendations are not mandatory in Spain yet, it is expected they will be in the next future; therefore Spanish nuclear power plants (NPPs), being sensitive about this recommendations, are planning the implementation of this type of inspections on the vessel nozzle DM welds. As this type of inspections could have an impact in the critical path duration of the outage, the possibility of using standard manipulators for the vessel inspection is not considered. It is necessary the use of mechanical equipment able to examine the nozzles DM welds in a short vessel occupation time (VOT) with high quality, qualified techniques and minimum requirements of the refuelling platform. In 2001 Tecnatom developed and inspection equipment for implementing the reactor pressure vessel (RPV) nozzles examination with the lower internals in place. This equipment named TENIS was designed in order to accomplish the stringent requirements and the updated examination techniques; it was used for the inspection of the DM welds of Ascó 2 NPP hot nozzles. INTRODUCTION The MRP-139, providing Inspection and Evaluation Guidelines on Primary System Piping Butt Welds, addresses to the NDE examination of these areas, based on several cases of flaw discovered in alloy 600 and 182 in various plants. The phenomenon of Primary Water Stress Corrosion Cracking (PWSCC) has been a recurring problem since mid 80s in Alloy 600, and most recently –second half of 2000- also in Alloy 82/182. According to the previous mandatory requirements prior MRP-139, ASME XI requires that the whole population of dissimilar metal vessel nozzle-to-safe end welds (ASME Category B-F) and dissimilar metal piping welds (ASME Category B-J) be inspected at ten-year intervals. The implementation of MRP-139 requires that utilities conduct essentially equivalent exams earlier and more frequently than required by the current ASME Code for Alloy 82/182 butt weld locations. Although in Spain this new requirements are not mandatory yet, as long as the ASME Code Case that refers to them is not issued, some plants want to be anticipated to this situation and they have begun to program this type of inspections in order to accomplish the examination frequency when the rule is in regular use. It has to say that the scope of the Code Case N-770 is some wider than the MRP-139 scope, because this last one includes mitigation activities taking in consideration several methods (WOL, SWOL, FSWOL, INLAY, MSIP, etc.). Anyhow, this regulation addresses to make examinations pre and post mitigation treatment. Hot legs are more sensitive to PWSCC than cold legs, and operation temperature plays an important role. The implementation of this regulation means: To increase the frequency of inspections (not deferrable at the end of ten-year interval) and to prepare the inspection by using qualified procedures. The inspections referred to in this article are those covering the nozzle-to-safe end dissimilar welds of the reactor vessel. The development of the article shall be focused on the remote ultrasonic examination of such areas from the nozzle bore, using a mechanical equipment that was specifically designed and developed. The inspection described here was carried out in Ascó NPP (Spain) in 2010.
