Plant life management
The big swap-out22 May 2009
In 1999, Beaver Valley’s new owner embarked on an ambitious update project that included the feat of replacing boilers and the vessel head in the same outage.
Beaver Valley features two three-loop pressurised water reactors (PWRs) designed by Westinghouse with a combined capacity of 1815MW. The plant is owned by FirstEnergy Nuclear Operating Company, which also owns the Davis-Besse and Perry nuclear plants in Ohio.
Beaver Valley is commonly referred to as the birthplace of United States commercial nuclear power generation. It is the successor to the Shippingport Atomic Power Station, which was built in 1957 as the first commercial nuclear plant in the USA. The plant operated successfully for more than 25 years before becoming the first US plant to be decommissioned in 1984.
The two reactors at Beaver Valley power station went online in 1976 and 1987. In 1999, FirstEnergy Corp. completed an asset transfer with Duquesne Light to gain full ownership and to become operator of the facility.
When FirstEnergy began operating the Beaver Valley plant, the leadership team created a programme to raise the plant’s productivity. This eight-year programme became known as the ‘Full Potential Program’ and focused on equipment, safety and engineering initiatives to bring systems and procedures up to date.
The Program upgraded many major equipment items while improving plant and personnel safety. In addition, Beaver Valley completed an extended power uprate during that time.
FirstEnergy began a thorough assessment of the health of Beaver Valley when it obtained ownership of the plant in late 1999. The assessment revealed the thermal rating of both unit 1 and unit 2 lagged that of other equivalent Westinghouse three loop PWRs. Further examination uncovered several areas for improvement.
For example, the sub-atmospheric containment design challenged the plant staff. Personnel were required to use supplemental breathing equipment to perform routine operations and maintenance activities.
In addition to sub-atmospheric challenges, Beaver Valley unit 1 had some of the oldest equipment in the industry. For example, the steam generators were of the Westinghouse Model 51 vintage. Alloy 600 cracks inside the generator tubes needed to be plugged; as the situation worsened steam power was lost. In addition, the reactor coolant pump motors were lacking upgrades and the reactor vessel was nearing a critical age.
Besides equipment ageing issues, the plant safety analysis was becoming out-of-date and was not keeping in step with the latest design and techniques. Initiatives such as improved standard technical specifications and licence renewal were not even under consideration until FirstEnergy assumed operations.
The challenge was to develop and implement a programme that would not only raise the standards of Beaver Valley with the rest of the industry, but also to set the standards in plant operation, operations and maintenance cost, safety and radiological dose. FirstEnergy formed a steam generator management team as a collaborative asset management process that focused exclusively on steam generator maintenance. This team and its process became the genesis for the site’s Program.
The Program had multiple initiatives to improve operating efficiency and profits. The most significant initiatives were the replacement of the three steam generators at unit 1, the replacement of the reactor vessel closure head and simplified head upgrade at unit 1, and the first extended power uprate approved for a US pressurised water reactor for both unit 1 and unit 2.
During the Program, concern in industry developed about the susceptibility of a nickel-chromium iron alloy, Alloy 600, and its welds, to pressurised water stress corrosion cracking over the long term. The team determined that this issue needed to be addressed at Beaver Valley as well, by replacing the reactor vessel head. The Program team used the same collaborative processes used in steam generator maintenance. The processes ultimately lead to integrating unit 1’s reactor vessel head replacement with the replacement of its three steam generators. This integration culminated in a record-setting steam generator and reactor head replacement outage in 2006 at the Beaver Valley Power Station.
Additional initiatives included:
• Best estimate loss of coolant accident and alternative source term: An upgrade to the safety analysis of the risks of fission product release from the reactor core into containment through a reanalysis and incorporation of the latest analytical techniques, including radiological alternative source term, which aims to estimate radiological doses in case of an accident.
• Atmospheric containment conversion: A significant analytical,
licensing, and plant modificationprogramme utilising advanced, first-of-a-kind analytical technology was undertaken to convert the containment from a sub-atmospheric to atmospheric design. This provided both safety analysis margin and enhanced personnel safety.
• Upgraded and improved standard technical specifications: By leveraging the safety analysis effort for the power uprate, improved standard technical specifications were created, resulting in the technical specification and licensing basis being clarified and aligned for consistency.
the Power Uprate
The Program included planning and executing the first extended power up-rate approved for a three-loop US pressurised water reactor. This initiative required a complete analysis and regulatory approval from the Nuclear Regulatory Commission (NRC), which resulted in an uprating of the plant’s thermal rating by 9.4%.
The extended power uprate was undertaken to increase operating and design margins and optimise station output. It included a measurement uncertainty recapture modification. The uprate resulted in an increase in reactor thermal power from 2652MWt to 2900MWt, and an electrical output increase of approximately 150MWe. The new power level was chosen based on a comparison with similar Westinghouse PWRs in the USA.
The strategy to gain licensing approval from the NRC was unique. Several licensing submittals were sequenced to support the entire Program.
FirstEnergy performed the reanalysis of the containment using the Electric Power Research Institute (EPRI) computer code. The code was a first kind application for containment integrity analysis and was tailored for design basis analysis.
All of the plant’s safety analyses were upgraded. The large break loss of coolant accident (LOCA) analysis was redone using the Westinghouse “best-estimate” methodology in order to gain margin. In addition, all safety analyses were revised and operator action time requirements were defined and validated.
The 9.4% uprate was implemented in stages to align with the timeframe for plant modifications. A 1.4% modification for instrument uncertainty in both units took place in 2001. Phase one of numerous plant modifications was completed in 2006, permitting a 3% uprate at both units. Finally, phase two permitted a 5% uprate at unit 1 in March 2007 and a 5% uprate at unit 2 in June 2008. The delay in uprating unit 2 was to allow for the installation of a new high-pressure turbine rotor.
The implementation of the uprates in phases was viewed to be a strength by both the NRC and the Institute of Nuclear Power Operations (INPO). The phase implementation process was in the plant’s licensing submittal and thus became an NRC commitment.
modifications in detail
The containment conversion directly led to improved personnel safety through the elimination of supplemental breathing apparatus for containment entries. By eliminating the 40 pound SCUBA equipment, employee manoeuvrability was increased, preventing worker fatigue, which in turn reduced heat stress. This also reduced stay times to perform tasks which reduced radiation exposure to workers.
Personnel safety was also enhanced by having increased access to equipment. This lead to increased equipment monitoring, which ensures equipment is running optimally and degraded conditions are found earlier.
Finally, modifications such as increasing openings inside the containment internal walls, in addition to redesign of the reactor vessel head and its external components, have enhanced the safety environment for workers while performing outage activities.
Other modifications included:
• High head safety injection (HHSI) pump replacement and upgrade: The charging pump impellers were replaced to allow for increased safety injection flow under accident conditions. This results in a lower peak clad temperature during a small-break LOCA.
• Installation of fast acting feedwater isolation valves: Valves were installed in each of the unit 1 feedwater lines to isolate main feedwater flow under postulated feedwater or main steam line breaks. This lowers energy input to the containment, which in turn lowers peak containment accident pressure. These valves provide redundancy to the feedwater control valves.
• Addition of AFW cavitating
venturis: The cavitating venturis provide two benefits; they limit energy input to the containment under accident conditions, and protect the AFW pumps from run-out and damage.
• Reactor cavity drain addition: This modification provides a drainage path to the containment sump for water that would otherwise remain trapped in the reactor cavity.
This modification increases available net positive suction head (NPSH) to the pumps drawing water from the sump.
• Replacement of steam generators: This affords both analytical and operating margin. With the new steam generators, the analytical tube plugging levels are decreased, which affords additional safety analysis margin. There are other aspects of this particular replacement job. The use of Alloy 690 tubes reduces their susceptibility to primary water stress corrosion cracking. Stainless steel tube support plates were used in lieu of carbon steel, reducing their susceptibility to outside diameter stress corrosion cracking. Additional antivibration bars were employed to reduce susceptibility to vibration-induced fatigue. Performance margin was gained with the use of a main steam line restricting nozzle, which lowers break flow under postulated main steam line breaks. An elevated feedwater ring was used to eliminate the potential for thermal stratification-induced cracking on the feedwater inlet lines. Top feed-ring J-nozzles were used to eliminate the potential of draining the header and inducing water hammer.
• Replacement of the reactor vessel head with new control rod drive mechanisms (CRDMs): A high-polished cladding finish helps reduce radiation exposure when performing reactor vessel head inspections. A new thermal sleeve configuration assists in the ultrasonic testing of ‘J’ welds. Also, the new CRDMs have one less weld seam which reduces risk to canopy seal leakage.
• Simultaneous hot and cold leg recirculation: The low head safety injection (LHSI) pumps were realigned to provide additional flow to back-flush the reactor to prevent boron precipitation. This modification also allows operators to redirect flow from the control room. It also eliminates the need to rely on the HHSI pumps to split flow between the hot and cold legs, and eliminates the need for in-plant manual operator action to throttle HHSI flow to prevent potential damage due to run-out.
• Weld overlays of Alloy 600 on pressurizer nozzles provide a structural weld overlay on the nozzle connections of the unit 2 pressurizer.
This modification eliminates the highest susceptible location for primary water stress corrosion cracking identified at the plant.
radiation & cost reductions
Beaver Valley has benefited in radiation exposure reductions as the result of: new steam generators; new reactor head and control rod drive mechanisms; head assembly upgrades and modifications to the containment structure; and working platforms around the new steam generators. The one-time savings is estimated to be nearly 350 person/rem for the life of the plant. It is based on lower source range due to new steam generators, zinc injection, reduced inspection scope on new generators and new reactor vessel head and user-friendly access to the new components. Each outage, the refuelling team and steam generator management team expect to realize an additional dose savings of seven person/rem.
In addition, cost savings have come from several areas of the project.
Containment conversion (operating at atmospheric conditions) allows for reduced outage time. It is estimated that a minimum of four to six hours per outage is saved due to the time saved to draw a vacuum in containment to sub-atmospheric conditions.
In order to replace the steam generators, a construction opening in the containment wall was required. An opening was cut in the outer containment concrete vessel and the inside liner plate removed. An interior wall on which the crane operates (crane wall) also required a larger opening. Given that this opening was required to support steam generator replacement, it was decided to use the same opening to replace the reactor vessel head. This was the first time that both a steam generator and vessel head were replaced concurrently. The savings from utilising a single construction opening was approximated to be $50 million.
A simplified reactor head package was incorporated at the time of the head replacement. This simplified head package allows for easier removal of the head apparatus (for example missile shields, ventilation piping), which reduces outage time. It is estimated that two days of critical path time is realized from this modification.
The overall steam generator inspection time is reduced. The previous poor condition of the original Westinghouse Model 51 steam generators required that every steam generator tube needed to be inspected each refuelling outage. The inspection cost savings are estimated at $3.5 million per outage.
The integration of project components also saved costs. The safety reanalysis effort was conducted one time. The analysis effort incorporated changes due to planned equipment modifications, the change in normal and post-accident containment operation, as well as post accident recovery for radiological considerations. For example, all the analysis was conducted at a bounding power level to encompass the planned thermal uprate to the plants. Equipment modifications required to enhance safety analysis margin, such as safety injection pump upgrades, were incorporated in the reanalysis effort.
Implementation costs due to plant impacts, including licensing, were also lowered. (The team realized that converting from a sub-atmospheric to an atmospheric containment may not have been cost-beneficial by itself.)
Beaver Valley estimates approximately $20 million was saved due to the integrated approach.
After having invested more than $550m on the Beaver Valley units 1 & 2 upgrade programme, FirstEnergy is finding greater results than expected. The Program reduced operations and maintenance expense, increased output and increased margins in the areas of shutdown safety, plant safety and operating margin. Beaver Valley’s Institute of Nuclear Power Operations (INPO) performance index rose from 42.75 for unit 1 in 1997 to 100 in February 2009. The performance index at unit 2 also increased from 51.57 in 1997 to its current rating of 98.4.
The Program results also permitted the Beaver Valley Power Station to seek licence extensions for each unit, with positive results. In August 2007, applied to the NRC to extend the operating licences of Beaver Valley unit 1 and unit 2 by an additional 20 years. During 2008, the NRC issued a favourable draft supplemental environmental impact statement and a favourable report on the station’s proposed aging-management programmes. Should the NRC approve the renewal application (expected July-September 2009), unit 1 would continue to operate until 2036 and unit 2 until 2047. Plans are now underway to replace the head and three steam generators at unit 2 in 2017.
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|Beaver Valley plant life programme|
Dec. 1999- FENOC takes ownership of Beaver Valley