By Brian Schimmoller, Contributing Editor
The aging nuclear fleet, the rising cost of new generation capacity and the growing popularity of zero- or low-carbon emitting generation sources are refocusing utility attention on existing assets. In colloquial terms, nuclear plant owners are assessing how much blood can be squeezed from the turnip. The May edition of this column focused on structural stability issues associated with power uprates. Here we examine the steam turbine retrofits that accompany many power uprates.
In June, Exelon signed an agreement with Alstom to retrofit steam turbines at the Quad Cities and Dresden plants in Illinois and at the Peach Bottom plant in Pennsylvania. The three plants each include two boiling water reactors. The retrofits are expected to increase output from each unit by about 40 MW. The additional 240 MW in capacity across the three plants is equivalent to $1,750/kW, one-third to one-quarter the current estimated cost of new nuclear capacity.
The graying of the nuclear fleet is driving interest in steam turbine retrofits. More than 40 percent of the U.S. nuclear fleet is over 30 years old and more than 90 percent of the fleet is on the other side of 20. While some units have had steam turbine retrofits, the majority has not. Over time, materials used in nuclear power plants become increasingly susceptible to stress corrosion cracking (SCC) due to age and exposure to radiation fields. In steam turbines, the disc bores and blade attachment areas of low-pressure turbine rotors are susceptible to SCC.
At the Exelon plants, the low-pressure steam paths will be replaced with equipment to increase steam flowresulting in higher outputand to address mechanical reliability issues with the existing steam turbines. Amir Shahkarami, senior vice president of engineering and technical services for Exelon Nuclear, said the low-pressure turbines suffer from erosion/corrosion of the inner casings and stress corrosion cracking of the blade attachment areas of the rotors.
The retrofit scope includes new low-pressure rotors, rotating and stationary blades, inner casings and blade carriers:
- High-efficiency, integrally shrouded, reaction-type blading for the front stages.
- Longer last-stage (L-0) rotating blades to reduce the energy content of the steam leaving the turbine, thereby increasing turbine output. For Quad Cities and Dresden, the last-stage blades will increase from 38 inches to 47 inches; for Peach Bottom, the last-stage blade will increase from 43 inches to 57 inches.
- To provide consistent and predictable vibration characteristics, snubbers at approximately three-quarter height will continuously interconnect the last-stage rotating blades and the second-to-last-stage (L-1) blades will be linked by integral tip shrouding.
- Reduced stage leakage due to improved sealing and enhanced reaction characteristics over the length of the blade.
- Careful selection of materials to provide enhanced erosion/corrosion characteristics.
The new low-pressure rotors at the Exelon plants will feature welded-rotor technology. Compared to monobloc rotors, welded rotors permit the use of different materials, each specifically optimized to suit the conditions seen by that area of the rotor, said Alan Holmes with Alstom’s steam turbine retrofit engineering group. “Welded rotors can eliminate stress corrosion cracking, increase the time between inspections and reduce the work to be performed during these inspections.” The weight of individual forgings used to manufacture a welded rotor is small compared to the weight of an equivalent monobloc rotor. The welded low-pressure rotors for Quad Cities and Dresden will be made up of seven forgings weighing 55,000 pounds each and giving a welded rotor weight of 260,000 pounds.
The smaller forgings allow for a higher quality forging and more thorough inspection. If nondestructive evaluation of a forging reveals an unacceptable defect, then that forging can be readily replaced. If the same defect were found in a monobloc rotor forging, the entire forging would have to be replaced, at significant cost and with a longer delivery schedule. Further, because the large forgings required for nuclear rotors are available from only a few facilities worldwide, the use of smaller forgings reduces cost and supply risks.
A more recent driver for steam turbine retrofits involves the direct tie being made between uprates and greenhouse gas emission reductions. When Exelon announced its low carbon roadmap in July, the company identified some 350 MW of potential additional capacity through nuclear uprates by 2014.
“The new turbines will increase the megawatt output due to efficiency improvements in the new design and ensure reliability over the life of the units,” Shahkarami said. “What’s more, 350 MW in nuclear uprates will reduce annual carbon dioxide emissions by an estimated 2 million metric tons.”
While these retrofits will not receive any undue regulatory favor due to the carbon dioxide emission reductions, increased public favor is quite possible. Still, steam turbine retrofits are and will remain site-specific projects that must pass muster as cost-effective capital improvement initiatives, yielding long-term economic benefits while maintaining safe and reliable plant operation. Full-scope steam turbine retrofits are not always viable and each machine has to be evaluated according to its particular circumstances. At the Exelon plants, for example, the performance benefits associated with retrofitting the high-pressure cylinders of the steam turbines were not needed at the current time, according to Holmes.
Increasingly, however, partial and full-scope steam turbine retrofits can present a sound business case. And in today’s economic and environmentally sensitive business culture, the blood squeezed from the steam turbine turnip often flows a welcome green.