Over the last several years, an increasing number of PWR plants have experienced stress corrosion cracking in the area of their control rod drive mechanism (CRDM) nozzles and their accompanying welds. The problem first was discovered in France, and lately highly susceptible US plants are being required by the NRC to perform inspections and make repairs if problems are found.

Framatome ANP has supplied more replacement RV heads with improved design features than any other manufacturer

Framatome ANP was involved with this problem since its inception. As a result, the company has the most experience and has developed specific inspection and repair technologies, tailored to each plant type, to address the problem. Of the 27 inspections performed to date in the US, Framatome ANP completed 24 and has another seven scheduled for spring 2002. This past fall, inspections were performed on eight RV heads, representing three different designs: four Westinghouse plants, three Babcock & Wilcox plants and one Combustion Engineering (CE) plant.

Because of this in-depth experience, Framatome ANP can perform not only the inspections and repairs, but also assist customers in analyzing the factors that must be taken into account to make sound decisions about which option, including replacement, makes the most economical and operational sense.

Minimal Dose Inspections for All Types of Plants

During the fall 2001 outages, different technologies were used, sometimes in combination to complete the inspections. Innovative remote tooling enabled low-dose bare head visual and head penetration nozzle ultrasonic (UT) inspections. Rotating UT, delivered from under the RV head, supported nozzle flaw characterization.

In spring 2002, the company will introduce an ultrasonic inspection technique that verifies the existence of the leak path forming at the interface between the outer surface of the CRDM nozzle and the reactor head base material adjacent to the nozzle outer surface. Based on empirical data taken from Framatome ANP's database of 128 nozzle inspections from seven plants, the leak path UT technique is equivalent to or better than the bare head visual inspection performed to identify leaking nozzles. This innovation will save dose and time.

Speedy Repairs Reduce Outage Time

In addition, specialized equipment was designed to reduce dose levels by automating the procedures for repairing CRDM nozzles. In the fall outage alone, 24 CRDM nozzles and eight thermocouples were repaired at four different plants using the remote ID temperbead repair technique. To maximize the life of the repair, Framatome ANP uses a remediating technique on the weld heat-affected zone.

Using a Midland head mock-up at Framatome ANP's testing facility in Lynchburg also enabled the company to design specialized tools and techniques for both inspection and repair as well as train personnel prior to deployment. For example, a method to raise the insulation of the RV head at one customer's plant enabled a bare head inspection and eliminated the need to perform an underhead inspection.

Replacement: A Viable Option For Long-Term Operations

Companies experiencing the highest number of flaws are seriously considering replacing their RV heads. Since the delivery time for a replacement RV head is longer than one cycle, Framatome ANP, working in partnership with the customer, develops innovative solutions that reduce lead time for replacement heads.

Framatome ANP was contracted to supply three RV head replacements in the US. These will be fabricated in the Chalon Saint Marcel plant. This plant is fully qualified and has never missed a delivery schedule in over 35 years of continuous manufacturing of heavy nuclear components. To meet customer requirements, the facility can manufacture either single or two-piece heads and fit up is warranted to mate with existing plant equipment with no impact to the reactor vessel flange, RV stud or CRDM operations. Most importantly, of the 113 RV heads manufactured and delivered, none has experienced crack initiation in the CRDM J-groove weld. For that reason, Framatome ANP offers a warranty on all replacement RV heads.

RV Head Design Improvements
• Inconel 690 for base and weld materials improves corrosion resistance
• Decreased volume J-groove reduces residual stress
• Polishing the inside surface of the CRDM nozzles induces compression stresses
• Improved CRDM nozzle fabrication techniques increases yield strength
• Integrated Head Assemblies (IHA) reduce head disassembly/reassembly time, enabling shorter outages

  RV head replacement can include CRDM nozzle repair or replacement