Control room operators at the Susquehanna nuclear power plant in Pennsylvania began shutting down the Unit 2 reactor on Wednesday in order to inspect and replace turbine blades that are suspected to have developed cracks.
“We have been monitoring turbine performance closely for the last several years, and continue to work with the manufacturer to address conditions that are associated with cracks developing,” said Timothy S. Rausch, senior vice president and Chief Nuclear Officer. “We continue to take appropriate actions based on the monitoring data we are collecting to ensure long-term safety and reliability. That’s why we decided to shut down Unit 2 now to inspect blades and replace any that are found to have developed cracks.”
It is suspected that vibrations are a primary contributor to the blade crack growth.
The results of monitoring at the Unit 1 reactor has shown that after replacing the old blades with newly designed blades last month, the vibration levels have been significantly reduced.
Workers will monitor the performance of the Unit 1 turbine blades over the course of the next year, and plans to install the newly designed blades in the Unit 2 turbine during the next scheduled refueling outage in the spring of 2015.
PPL, operators of the Susquehanna nuclear power plant, have repeatedly attempted to repair the turbines at both the Unit 1 and Unit 2 reactors in recent years.
Turbine retrofits at BWR units, like those at Susquehanna, are complicated by the presence of radioactive contamination and health physics issues for the installation teams.
The Unit 2 turbine was replaced over 10 years ago, and PPL attempted to repair it during the last refueling outage in April 2013.
The Susquehanna plant has had problems with the turbines dating back nearly to the start of commercial operation.
During the first four years of operation, the Susquehanna plant dealt with problems of disc cracking in the keyway areas on the turbine rotors. The disc inside diameter cracking in the keyway areas caused by stress corrosion on some disc designs is a well-known problem in the industry. The Susquehanna plant also had to repair severe erosion on the original LP turbine rotors and inner casings.
The problem was addressed by replacing the monoblock rotors, but subsequent issues arose with dovetail cracking and inner casing erosion. Dovetail cracking is a type of stress corrosion cracking that is found in the attachment root for the turbine blades.
In 2001, Siemens Power Generation was awarded a contract to carry out a turbine modernization project to replace the LP turbines and the HP turbine of both Unit 1 and Unit 2 at Susquehanna.
The Unit 2 turbine was installed in March 2003 and the Unit 1 turbine was installed the next year in March 2004.