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Staff Accomplishments

Catching cracks: Remote inspection systems put to the test

June 2004
The stress and corrosion inside a nuclear reactor can cause areas in and around the vessel head penetrations, called nozzles, to crack. Inspection systems find these cracks using state-of-the-art nondestructive methods. By studying the results of these inspections systems at two nuclear power plants, Battelle is learning which cracks were detected and which, if any, were not detected during routine inspections. With these results, Battelle, which manages the Pacific Northwest National Laboratory for the U.S. Department of Energy, will also help determine how to improve future inspections. When eight cracked nozzles were removed from the pressure vessel heads of reactors at Davis-Besse in Ohio and North Anna 2 in Virginia, the U.S. Nuclear Regulatory Commission and the Electric Power Research Institute saw it as an opportunity to evaluate nondestructive inspection methods and to study the cracking that is occurring in the vessel head area. The removed nozzles were sent to the Radiochemical Processing Laboratory, which is managed by PNNL, as part of a joint program between the NRC and EPRI. The RPL was selected because of its radiochemical capabilities and the ease with which private companies can be safely brought onto the site. At the RPL, researchers decontaminated the nozzles to a level that could be safely handled with minimum protective equipment. Finally the laboratory space was arranged to mimic the height and orientation of the nozzles when the reactor pressure vessel head is located in its storage stand during reactor refueling. Four companies who perform inspections for nuclear utilities, Framatome ANP, WesDyne International, R. Brooks Associates (teamed with Southwest Research Institute), and Pacific Gas & Electric, brought their inspection systems to the RPL where the nozzles were set up for inspection. Each vendor spent a week using their sophisticated and expensive inspection systems to hunt for cracks. In addition to the U.S. vendors, EDF Group from France also spent a week conducting special silastic mold measurements using a system they developed. Each vendor will provide their results, so an assessment can be made of the effectiveness of the nondestructive measurements. The measurements will be compared with the inspection results when the nozzles were still in the reactor's pressure vessel head and will be used to help guide planned destructive testing. "The schedule for this project was very aggressive," said project manager John Abrefah. "All of this work was done in a little less than 10 months. Our staff worked nights, weekends, and even holidays to get everything ready." In addition, PNNL will send two decontaminated nozzles to offsite vendors for destructive testing to learn more about the cracking process. The two testing companies were selected by EPRI's Materials Reliability Program. The remaining six nozzles will undergo further testing at PNNL using advanced nondestructive inspection techniques. The information gained from the inspection studies at the RPL along with the destructive testing will be compared and provided to the NRC. The NRC will determine if improvements to existing requirements and standards are needed, and if so, they will upgrade inspection standards for the nation's 103 nuclear reactors. In addition, this information may be shared with other countries to help them promote better inspections and improved nuclear power safety. For utilities, these tests provide additional information to help make decisions about the timing, effectiveness, and activities required during reactor maintenance. This research may also enhance training and improve the nondestructive inspection methods for identifying cracks in a nuclear plant. Contacts: Project managers: Steven Doctor, (509) 375-2495, steven.doctor@pnl.gov; John Abrefah, (509) 373-0927, john.abrefah@pnl.gov; Allan Pardini, (509) 375-2525, allan.pardini@pnl.gov Product line manager: Wally Weimer, (509) 375-6922, walter.weimer@pnl.gov

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