September-October 2018 NPJ

CEDM High Temperature Coil Design By Joshua Gates and Charlie Fox, Arizona Public Service Electric Company. Joshua Gates Josh Gates, Engineer Sr., has worked at Palo Verde Generating Station for 10 years. As a component engineer, he specializes in Instrumentation and Control (I&C), Control Element Drive Mechanism Control System (CEDMCS), Programmable Logic Controllers (PLCs), and cranes. Gates earned his Bachelor of Science in Electrical Engineering from DeVry University. The Nuclear Energy Institute’s Top Innovative Practice Awards highlight the nuclear industry’s most innovative techniques and ideas. This innovation won the 2018 Westinghouse Combustion Engineering Vendor Award. The team members who participated included Joshua Gates, Senior Engineer, Palo Verde Generating Station; Charlie Fox, Senior Engineer, Palo Verde Generating Station; Gregory E. Falvo, WEC Principal Design Engineer, Reactor Mechanicals Products II; William S. Struzenski, WEC Principal Design Engineer, Reactor Mechanicals Products I; Bruce F. Allen, WEC CRDM Engineering Technical Lead, Fellow Engineer Reactor Mechanical Products. In 2009, Palo Verde began installing a new simplified reactor head assembly in each of its units. Through monitoring after the installation, Palo Verde discovered that the Combustion Engineering (CE) Control Element Drive Mechanism (CEDM) Gripper Coil temperatures were operating near, and in some cases above, their design temperature limits. In addition, there was a coil temperature delta across the reactor core that did not exist before. The station took actions to reduce coil operating temperatures but in many cases coils still operated at or above design temperatures. In addition to the elevated temperatures, all of the plant’s CE CEDMCS coils were from original startup, so they were approaching end of life. Palo Verde and Westinghouse collaborated on an innovative approach: taking the proven Westinghouse coil design and making it work as a CE coil. The Westinghouse High Temperature (HT) CEDM Gripper Coil was developed at the request of Palo Verde Generating Station (PVGS) and funded by the plant. It is a design which uses all of the materials of construction of the robust Westinghouse Control Rod Drive Mechanism (CRDM) coil and repackages them into the original CE CEDM design that can handle high temperature over a long period of time. Innovation The original CE CEDM coil was developed by CE in the early 1970’s. Since this CEDM uses five (5) coils, a standard design was chosen for the coils to keep the total cost as low as possible. In this design solid copper wire, which has a baked polyimide enamel coating, is wound into a coil using a similar wet polyimide enamel varnish to hold it together. After winding, the coil is baked to evaporate the solvent out of the wet enamel and then increasingly higher temperatures cure the enamel. The coil is then wrapped in fiberglass tape and potted in a Room Temperature Vulcanization (RTV) type of silicone potting compound. In the CE coil design, the only electrical insulation between the turns of copper wire is the polyimide enamel. The coil design has a rated service life of 40 years at 350°F which is due entirely to the thermal life of the polyimide enamel. If the temperature rises to above approximately 400 to 450°F, the polyimide varnish can start to carbonize with air and form elemental carbon, which is very conductive. As long as internal coil temperature remains below 400°F, the life of the coil will be as predicted. The essential failure mechanism for the CE coil design is as follows: 1. The center winding of the coil becomes overheated due to insufficient air cooling, or an unplanned control system over- voltage excursion. This creates a small location of carbonized polyimide material in the center winding and a few turns of wire become shorted to each other. This in turn, causes the overall coil electrical resistance to drop slightly. 2. The CE rod control system operates on a constant voltage, so the decreased resistance causes an increased current in the coil, and consequently the coil will operate at a higher temperature. 3. The cascading effect of both of the above actions can quickly lead to a very high temperature in the center of the coil, more carburization, and may progress to the point where there are no longer sufficient turns of intact copper wire for the coil to generate enough magnetic flux to operate the CEDM motor properly. 4. At continued high temperature, the CE coil can go from being good to failure in only a few months of operation. The Westinghouse CRDM coil was developed in the mid-1960s for the Westinghouse CRDM and all Westinghouse plants have used this design for over 40 years worldwide. The design is also used in the new Westinghouse AP1000 CRDM. Only a handful of coils, out of the thousands in operation, have ever failed over this time period. The Westinghouse CRDM coil uses extensive amounts of fiberglass insulation material to improve reliability. It is conservatively rated for 60 years 42 NuclearPlantJournal.com Nuclear Plant Journal, September-October 2018