Visual inspections during a recent refueling outage identified circumferential cracking in the core spray piping inside the reactor vessel in a US boiling water reactor (BWR) nuclear power plant. The core spray system is a critical emergency core cooling system in BWR plants that provides water flow to the nuclear fuel during postulated accident conditions. The extent of observed cracking in the core spray piping was beyond the critical length for which the structural integrity of the piping could be assured by analysis. Unless alternate, more sophisticated inspection methods could show that visual inspections over-characterized the degree of cracking, a repair would be needed prior to restart of the plant.

The plant owner contacted AREVA Nuclear Power in Lynchburg, Virginia to provide a repair for the piping, and AREVA in turn contacted MPR, their strategic partner for BWR reactor vessel internals repairs. MPR was responsible for design and analysis of the repair while AREVA had responsibility for fabrication and installation.

MPR Clamp Design. This 3-D model shows the clamp design that MPR and AREVA developed to repair the cracked core spray piping.

Typically, projects to design, analyze, fabricate and install such a repair are initiated nine to twelve months before the outage when the repair is to be installed. The emergent nature of this core spray pipe cracking required that AREVA and MPR complete in days a scope of work that typically requires months. This urgency was particularly important to the plant owner because the core spray pipe cracking could potentially extend their outage by several weeks, with a generation revenue loss of several million dollars.

To repair the core spray pipe cracking, MPR designed a clamp which bolted over the existing core spray piping. The clamp is designed to be installed using remote long-handled tooling because during refueling operations, the reactor vessel is flooded with water to provide shielding for workers on the refuel floor. The location on the core spray piping where the clamp had to be installed was approximately 60 feet below the water surface. To install the clamp, AREVA developed the needed long-handled, remote tooling, and trained and qualified workers on its use.

The MPR-designed clamp maintains the functionality of the core spray piping, even if the crack becomes throughwall around the entire circumference of the pipe. To assure that the core spray system could provide the required cooling flow rate, the clamp design had to structurally replace the pipe and limit the amount of leakage flow out of the gap resulting from an assumed 360°, throughwall crack. The repair developed by MPR involved a clamshell clamp that was bolted together over the pipe and spanned the crack. The upper and lower clamp halves each included two pins, one on each side of the crack. The pins were designed to fit into holes cut by Electrical Discharge Machining (EDM) into the pipe. The diametral clearance between the hole and pin is less than 1/32 inch. Forces and moments on the pipe resulting from seismic loads, thermal anchor motions, fluid drag, and pressure loads are transferred from the pipe to the clamp via the pins. The very small gap between the pins and the holes in the pipe limits the amount that the crack could open and minimizes leakage from the core spray piping.

To ensure that the holes in the pipe matched the locations of the pins in the clamp, MPR designed the clamp to act as a template for EDM cutting. The clamp was installed on the pipe with the EDM equipment attached to the clamp at the pin locations. The EDM equipment cut through a small amount of the clamp, and then the pipe. The clamp was then removed from the vessel and the pins were installed. Using this approach, it was certain that the clamp pins would also fit into the EDM holes in the core spray pipe.

The prospect of a severe financial impact associated with extending the outage required that MPR and AREVA perform the work on a highly expedited basis. After receiving authorization from the plant owner, AREVA and MPR immediately mobilized teams to perform alternate inspections (which confirmed the need for a repair), design the clamp, and develop installation tooling and procedures.

MPR completed the clamp design and key calculations to demonstrate that the design satisfied its design requirements in four days. The AREVA installation tooling development team worked in parallel, and the tooling was completed one week later, when mockup testing was initiated. Testing, transport to the site and installation were completed in the subsequent week. Altogether, the repair was designed, fabricated, qualified, tested and installed in less than three weeks.

Core Spray Piping Repair Schedule. Despite a short deadline, MPR and AREVA worked together to meet the development and installation schedule, which was less than three weeks.

The ability of the AREVA/MPR team to develop and implement a first-of-a-kind core spray piping repair in 18 days was a major success that brought considerable benefit to the plant owner. The repair installation had minimal impact on the outage critical path and, importantly, avoided extending the outage.

This project is an example of the benefits MPR provides in the area of BWR reactor internals repair. Not only was the AREVA/MPR team able to respond immediately to urgent conditions, but MPR and AREVA were able to apply their extensive experience in BWR internals repair and inspection to implement and complete this repair quickly. MPR was able to adapt previous repair concepts to the plant’s core spray piping and AREVA was able to adapt existing inspection and installation tooling to implement this modification. This core spray piping repair project is the eleventh BWR internal repair or replacement component designed by MPR to be installed in a BWR in the US or Japan. It was also the fourth project where the AREVA/MPR team partnered to fully design and install a BWR internals repair. The AREVA/MPR team is currently completing an additional internal core spray piping repair to be installed at another BWR in 2007.

For further information on this article, a copy of the latest MPR Profile or our engineering services, contact Larry Cundy.