Performance Considerations in Replacement of Low-Pressure Turbine Rotors for Nuclear Power Plants: A Case Study

In recent years, the nuclear power industry has witnessed profound changes in terms of renewal of operating licenses and power uprates. Renewal of operating licenses for an additional 20 years beyond the original licensed period of 40 years entails several considerations relating to aging management, performance, reliability, availability, and maintainability. Power uprates range from a low of up to 2% due to improved techniques in feedwater flow measurement to a high of up to 20% for extended power uprates. Since the limitations of power uprates are generally encountered in design of the turbine cycle, the impact upon the performance, reliability, availability and maintainability of the equipment and components in the turbine cycle may vary from low or moderate to significant. Several nuclear power plant owners have already replaced the low-pressure turbine rotors of their nuclear units with improved designs to mitigate blade failures and forced outages due to stress corrosion cracking, to reduce inspection intervals and maintenance, achieve higher output due to improved efficiency, etc. Others are either embarking upon or planning similar initiatives to confront aging, performance, availability, reliability and maintainability concerns stemming from renewal of operating licenses as well as the need to accommodate higher pressures and flows accompanying the proposed power uprates. Typically, the original low-pressure turbine designs utilizing built-up rotors with shrunk-on disks are being replaced with monoblock rotors with fully integral disks, couplings, blading and shrouds. The last stage blading is also longer resulting in a larger annulus area. Since these replacement programs involve significant expenditures, several factors need to be considered in order to ensure that the objectives of the rotor replacement programs are met. Using a case study, this paper examines the various considerations involved in replacing the low-pressure turbine rotors for a nuclear power plant. Design, performance and test considerations that need to be addressed before and after the low-pressure turbine rotors are replaced are discussed. The use of performance modeling tools in evaluating performance gains from low-pressure turbine rotor replacements is reviewed. Finally, the paper provides recommendations for ensuring that the objectives of a low-pressure turbine rotor replacement program are met.