AREVA NP developed the axial blade probe, pictured here, to perform reactor vessel head penetration inspections. Photo courtesy of AREVA.
When it comes to plant inspection and maintenance, surprises happen. At no other time in the commercial nuclear power industry’s history has this been truer than since serious material degradation issues were first identified at pressurized water reactor (PWR) plants.
Because of potential problems identified by the industry during the past two decades, PWR vessel closure head (RVCH) nozzle penetrations are meticulously inspected at all nuclear power plants. Reactor designer AREVA NP has performed numerous plant equipment material degradation inspections. The company has an extensive materials R&D program, global industry knowledge base and is committed to finding solutions to industry problems. Sometimes, solving complex engineering and material problems requires quick decision making and creativity to maintain a plant’s outage schedule and save money.
Such was the case during a scheduled refueling outage at Dominion Nuclear Connecticut’s Millstone nuclear power station in late 2007. The ultrasonic examination of RVCH penetration adapter nozzles revealed surface discontinuities on nine penetration nozzles. This posed quite a challenge for engineers, technicians and their testing equipment. Because of the irregular surfaces, ultrasonic transducer elements could not couple properly with these particular nozzles to perform the procedure.
Finding a solution was critical. The identified discontinuities were oriented around the nozzles above the J-grove weld, which prevented compliance with Nuclear Regulatory Commission (NRC) examination requirements for the most safety significant flaw: a circumferential crack above the weld.
No other PWR plant had reported this occurrence and no other NSSS vendor performing these examinations had previously identified this phenomenon. There was no available method for interrogating this region in the absence of a uniform, prime surfaced geometry without removing the thermal sleeves. It seemed the only solution would be to remove the sleeves and perform a combination of direct measurements and ultrasonic examinations. This would have been costly in more ways than one.
AREVA NP and Millstone engineers worked together to develop a first-of-a-kind (FOAK) remote examination and quantification technique, so the required inspection could be completed without removing the thermal sleeves. By avoiding sleeve removal, this FOAK technique resulted in a labor and materials cost savings of $6.8 million, prevented the addition of 15 outage days and resulted in a dose savings of 16 person-rems.
In addition to nozzle discontinuities, wear was observed on thermal sleeves in a circumferential area on the same lateral plane as the nozzle ends. Based on this observation, plant engineers theorized that centering tabs on the sleeves had worn away some of the nozzle base material. The wear had created the surface discontinuity, which prevented the ultrasonic transducer elements from coupling with the nozzles.
Essentially 360 degree wear on some nozzles indicated that normal fluid turbulence in the upper head area had caused the sleeves to rotate within the nozzle creating the circumferential wear. The existence of thermal sleeve wear had been reported at other PWR plants; however, movement of the thermal sleeve as a cause for nozzle base material degradation had previously gone undetected.
The primary inspection probe used to perform the ultrasonic examination is an axial blade probe. Under normal conditions, the examination involves inserting the thin stainless steel blade between the thermal sleeve and the nozzle inside diameter surface. At the end of the blade 5MHz transducer elements are operated in a “pitch-catch” configuration mounted on one end. These elements are oriented circumferentially with respect to the nozzle. The thin gap between the transducers and the material is filled with a medium capable of transferring sound energy with a minimum amount of attenuation. The perfect medium is a thin stream of water, which is delivered to the surface of the nozzle at the transducers to provide coupling and data retrieval.
Using this proven technique, complete ultrasonic coverage surprisingly could not be obtained on nine of the 78 nozzles. The area where complete coverage was not obtained using the axial blade probe was consistent with the location of thermal sleeve centering tabs. Analysis of ultrasonic data indicated that the surface of the nozzle was separating from the transducer elements just prior to coverage loss.
To avoid sleeve removal and augment ultrasound data, Millstone and AREVA NP engineers and technicians separately developed examination techniques to provide the additional coverage needed. Individually, the techniques fell just short of complying with inspection requirements. Combining the two techniques proved to be the creative solution that was needed.
One technique involved rescanning the nine nozzles found to have discontinuities with a circumferential blade probe to obtain the required volumetric coverage. The circumferential blade probe had transducer elements oriented in the axial direction with respect to the nozzle, which enabled the transducers to be coupled on either side of the surface irregularity. With the transducer elements oriented axially, the circumferential probe bridged the “gap” created by material loss and provided the necessary coverage.
In the other technique, eddy current coils were attached to the end of the blade probe and passed over the inside diameter surface. As electric current was induced into the area of interest, the magnetic field from the current was monitored. Data collected were compared to eddy current data retrieved from a reference specimen. The eddy current examination provided additional quantitative and qualitative information.
Data collected from this FOAK examination technique provided plant engineers complete confidence that circumferential cracks did not exist in the nozzle base material. Although developed for the examination of PWR reactor vessel closure head penetration nozzles, this technique can be applied to any interrogation of near-surface discontinuities.
The new technique earned Millstone employees a Top Industry Practice (TIP) award at the Nuclear Energy Assembly in May. The Nuclear Energy Institute (NEI) annually recognizes and rewards members who on their own or working with NSSS vendors create new ways to improve safety, economics and plant performance.
The TIP awards help the industry share innovations to build a stronger nuclear energy industry in a competitive environment. TIP awards are part of an industrywide benchmarking program that provides an avenue for promoting best practices. Information is also shared through programs at EPRI, INPO, WANO and the NRC.
Aging degradation of nickel-chromium-iron Alloy 600 in RVCH penetrations were first identified in France in 1991. French researchers had been studying the effects of boric acid corrosion on nickel alloys for some four decades. The industry’s first material degradation issue with RVCH penetrations was identified at EDF’s Bugey 3 in 1991. When a serious problem involving RVCH penetration leakage was identified at Davis-Besse in Ohio in March 2002, AREVA NP was prepared to share its knowledge and solutions with its customers and the U.S. nuclear power industry.
Today, the effects of boric acid corrosion on nickel alloy components in a nuclear power plant operating environment are much better understood. The industry’s concerted effort to identify and mitigate potential material problems, as exemplified by the Millstone-AREVA NP team effort, will help ensure that nuclear energy will play an important role in meeting our nation’s monumental energy and environmental challenges.