Retrofit Turbines Increase Rotor Inspection Interval
IN A LETTER dated February 3, 1998, the Nuclear Regulatory Commission (NRC) granted PECO Nuclear, the operators of Limerick Generating Station (LGS) permission to extend the inspection, maintenance and replacement intervals of the plant`s steam turbines from six to 10 years. Permission was also granted to extend test frequencies for main turbine stop valves and combined-intermediate valves from once a week to once every three months and to increase the test frequency of the main turbine control valves from once every month to every three months.
This milestone, which is likely to save PECO millions of dollars in inspection costs, was a key objective of the company`s plan to retrofit the high and low-pressure turbines on each of Limerick`s two units with Siemens Power Corp. (SPC)-design rotors and blades. In the submittal to the NRC, Siemens provided a methodology for analyzing missile generation that proved to the commission that the rotors could support turbine inspection intervals to 100,000 operating hours (approximately 11.5 years) without increasing the risk of failure. According to the NRC, “The basic principles of the SPC methodology are the same as those used in previous installations that have been accomplished at Grand Gulf, Comanche Peak, and Connecticut Yankee nuclear power stations.”
The turbine modifications, which are scheduled to be completed during outages in April 1998 and April 1999, are expected to mitigate a number of problems including stress corrosion cracking (SCC) in the turbine disk key-ways, SCC at blade attachment points, and erosion of rotating and stationary components. Increased steam path efficiencies and enhanced net turbine output cap the list of improvements Limerick will experience once the retrofit project is completed.
The six disk LP turbine rotor design significantly reduces the probability of a turbine missile in the following ways:
a) Fewer disks.
b) Disks 2 and 3 are not keyed, which reduces the stress concentration factor, which in turn reduces the probability of disk burst due to SCC.
c) Significant compressive stresses are introduced in the disk by heat treatment of the disks and shot peening of surfaces including the blade area.
d) Number 1 disks are keyed on the downstream side of steam flow, which reduces the probability of burst due to reduced SCC growth rates at reduced steam temperature. The keyways are located downstream of the steam flow where the metal temperature is higher than the surrounding steam temperature. This reduces condensation and reduces SCC potential.
e) Increased residual compressive stresses due to larger disk sizes, reduces the probability of disk burst due to SCC.
f) SPC specifies material yield strength range for the disks to be within the range of 113-129 ksi (780-890 MPa–well below the value at which the material becomes susceptible to hydrogen induced SCC.
g) The first 20 percent of the inlet of disk number 1 hub is tapered, while the shaft is profiled to give a relatively large cavity eliminating the shrink fit for the area.
In the NRC Safety Evaluation documentation a number of specific design features were noted as significantly reducing the probability of a turbine missile failure event. “SPC has used the same general blade roo