ENERCON completed an analog to digital upgrade of the speed control system for the safety related Nuclear Emergency Feedwater (EFW) Turbine. The original Terry Turbine analog control system was obsolete and subject to maintenance challenges including difficulty in obtaining spare and replacement parts.
The upgraded control system includes a modern, digital, commercial-grade dedicated Woodward governor controller to replace the original analog controller. It also includes a new electro-mechanical governor valve actuator with digital amplifier to replace the legacy analog system which used a mechanical linkage between the valve stem and a lube oil servo piston.
ENERCON reviewed the installed location of all new equipment to assess concerns related to flooding, fire hazards, combustible loading, room heat load, and impacts to probabilistic risk assessment (PRA) and safe shutdown analyses (SSA). The engineering firm also evaluated the capabilities of the upgraded system to ensure it was seismically and environmentally qualified for the worst-case conditions in which it must operate. All component designs requiring commercial grade dedication were reviewed to ensure they were adequately qualified for use in the safety related EFW system.
ENERCON performed a 50.59 evaluation for the modification. In support of this evaluation, the ENERCON Digital I&C Group prepared a Qualitative Assessment of the new digital equipment to evaluate the likelihood of failure due to a latent software error or common cause failure. The Qualitative Assessment was developed in accordance with NRC Regulatory Issue Summary 2002-22, Regulatory Guide 1.187, NEI 01-01, and NEI 96-07. It concluded the likelihood of failure was sufficiently low as not to affect the results of the Updated Final Safety Analysis Report (UFSAR). The modification was successfully installed, and the new system is currently operational. The complexity of existing regulatory guidance for safety related digital upgrades such as this has challenged the industry and commonly resulted in cost overruns and regulatory risk. ENERCON has proven the ability to successfully provide the needed design and licensing requirements for such upgrades and is currently completing the equivalent design for other systems.
