Steam generator replacement and repair technology continues to advance
By Bala Nair, Westinghouse Electric Corp., Energy Systems Business Unit
Nuclear pressurized water reactors (PWR) utilize tubular heat ex changers–or steam generators (SG)–for converting the thermal power produced by nuclear fission into steam for driving turbines to produce electricity. SGs were originally expected to last for the licensed life of nu clear power plants–40 years. However, during the past 25 years al most every PWR in the United States has reported problems with their SGs. These problems have ranged from less efficient heat transfer to, in very few cases, tube rupture.
PWR SG tubes, generally made from mill-annealed Alloy 600, can be subjected to operating conditions that result in tube wall degradation in the form of stress corrosion cracking. Periodic inspections of SG tubes are performed using nondestructive examination techniques, and rigorous acceptance limits are defined for establishing continued tube operability through specific tube plugging limits. If those limits are exceeded, the tube must be removed from service by plugging, or otherwise brought back into compliance by repairing it.
While there have been dramatic advancements in maintenance and repair technologies in recent years, there may be times when it proves more prudent for a utility to replace its SGs. The economic tradeoff between major repairs and replacement may justify new SGs. Or, it may be necessary to upgrade a plant`s SG technology for operational, regulatory or political reasons.
A primary concern is ensuring that the new SG design is compatible with existing station conditions and that the replacement SG is manufactured to exact design- and plant-specific criteria. These compatibility issues include margin management, upgrading and regulatory compliance, as well as stress, metallurgical and thermal-hydraulic analysis. These design considerations must meet both short-term and long-term plant operational goals. Westinghouse engineers have designed replacement SGs to meet the diverse objectives and plant-specific requirements of 16 operating nuclear stations. Table 1 shows the replacement SGs designed at the Westinghouse Pensacola plant over the last decade.
A second important factor affecting the decision to replace the SG and driving recent design enhancements and technical advances is the need to address many of the problems experienced by older units. Today`s replacement generators incorporate design features developed through many years of operating experience and extensive testing.
These recent advancements include:
z thermally treated advanced alloy tubing;
z stainless steel, flat-broached contact tube supports and baffle plates;
z hydraulic expansion tube sheet joints; and
z “minimum gap” U-bend construction.
Thermally treated Alloy 690 for SG tubing provides maximum corrosion resistance–far greater than the Alloy 600 or Alloy 800 tubing materials used in the past. This is the same advanced alloy used for SG sleeving. Alloy 690 sleeves have been in service since 1977 in 17 operating Westinghouse SGs and have achieved excellent operating and performance records. In addition, laboratory testing demonstrates this superior alloy`s resistance to primary water stress corrosion cracking.
Current replacement SG designs have flat-contact broached tube support and baffle plates made of Type 405 ferritic stainless steel. The broached tube support plate permits greater steam/water flow in the open areas adjacent to the tube. The flat contact geometry provides dryout margin, and the broached lobes limit the width of the flat contact zone. These features reduce sludge deposition and the potential for tube corrosion.
Full-depth hydraulic expansion of the tubes in the tubesheets to close the crevice produces the fewest residual stresses. In fact, years of laboratory testing and in-plant experience have confirmed that hydraulic expansion produces the lowest residual stresses of any tubesheet joint closure process. There has been no indication of excessive tube stress or subsequent corrosion in any of the 101 Westinghouse SGs manufactured since 1980, with more than 1 million hydraulically expanded tubesheet joints.
All Westinghouse SGs manufactured since 1986 have 0.08 mm nominal U-bend gaps. Earlier models had much larger 0.51 mm nominal U-bend gaps. Experience shows that this narrower gap, in conjunction with tightly controlled U-bend tubing ovality and an enhanced retainer ring design, increases tube wear margin by a factor of 40.
These and other SG design enhancements, such as stainless steel antivibration bars with a rectangular configuration and tighter control of insertion depths, have improved SG operating reliability.
Inspection and maintenance
Utilities regularly perform SG tube inspections using eddy current (EC) probes capable of detecting and characterizing defects at an early stage. SG inspection and additional maintenance activities (such as tube plugging, removal of tubes for laboratory examinations, tube stabilization, chemical decontamination, etc.) or repair activities (such as sleeving) can have a major impact on the outage schedule. Technology advances are beginning to change the way tube integrity is viewed. Laser-welding technology, in particular, promises to revolutionize the repair of damaged areas in tube walls.
Sleeving allows degraded tubes to remain in service. In the sleeving process, a length of tubing (sleeve) is inserted into the degraded tube to bridge the affected area. The sleeve`s upper and lower ends are then bonded to the tube with a leak-tight seal.
Westinghouse has developed a method of welding sleeves into place that uses a high-powered neodymium-YAG laser and a fiber-optic delivery system. Laser-welded sleeving can be applied at the tubesheet and the tube support plate locations. Since first implemented in 1992, laser-welded sleeving has built an impressive performance record: None of the sleeves installed has failed in service.
Westinghouse has elected to use laser welding instead of prior mechanical and conventional fusion bonding processes such as brazing and gas tungsten arc welding because it provides the following advantages:
z hermetically tight sleeve/tube joint;
z high degree of process control that is less sensitive to SG secondary-side conditions such as sludge, metallic deposits and moisture;
z tolerance to field variability in sleeve/tube fitup;
z minimum distortion of the parent tube; and
z excellent weld surface finish that is crucial to verifying weld integrity using ultrasonic test and EC inspections.
Thermally treated Alloy 690 was selected as the sleeve material for its high resistance to stress corrosion. A computer-controlled hydraulic expansion process is used to expand the sleeve in place prior to making the laser-welded joint. This process ensures that the tube bulge is limited to typically 0.001 inch, providing for minimum tube stress.
The full-length tubesheet sleeve`s lower ends and the elevated tube sheet sleeve, which are mechanical interference-fit hard-rolled, provide a practically leak-tight joint in the tubesheet region without welding. All welds are provided with a heat treatment stress relief to reduce welded joint stresses caused by weld shrinkage, thereby significantly improving corrosion resistance.
Westinghouse has implemented a comprehensive test to investigate residual stresses in a sleeved tube due to welding and heat treatment under locked tube conditions (assuming the tube is locked at the tube support plates due to corrosion deposits). The program also evaluates the effect of those stresses on corrosion performance. The test program results were used to optimize the heat treatment process parameters and to estimate laser-welded sleeves` future corrosion. The results support laser-welded sleeving`s use as a long-term solution to degradation and a viable option to SG replacement.
Acceptance testing of the welded joint is performed by ultrasonic test inspection of the joint to verify weld width adequacy at the sleeve/tube interface. A baseline EC inspection is also performed to verify that the weld is defect-free and to provide a benchmark for future in-service inspections.
Since it was first implemented in the United States at Farley 2 five years ago, laser-welded sleeving has become the technology of choice in the industry and captured a dominant market share. A total of about 32,200 sleeves have been installed in 10 separate campaigns in seven nuclear plants. They include a record-setting 16,429 sleeve campaign at Maine Yankee in 1995. Not one of these sleeves has experienced any in-service weld joint degradation–an unblemished record unmatched by any other sleeving process. Table 2 presents the field experience with laser-welded sleeves.
Development efforts following the laser-welded sleeving campaign at Farley 2 have focused on productivity improvements. They included refinements to the weld head design and welding process, attention to equipment logistics within plant containment and equipment reliability. Those efforts led to a steady improvement in sleeving productivity with each successive sleeving campaign. While production rates in the very small initial Farley campaigns were about 10 sleeves per day per SG, they improved to 34 sleeves per day per SG at Doel 4 and to 38 sleeves per day per SG at Maine Yankee. During subsequent campaigns that involved less than 1,000 sleeves, the production rate was 30 sleeves per day per SG. Weld acceptance rates were 98.6 percent at Doel 4, 99.06 at Maine Yankee, 99.7 percent at Byron 1 and 100 percent in the three sleeving campaigns completed during 1996. These production rates and tube recovery successes have led to significant reductions in outage schedules and have made laser-welded sleeving an economically attractive repair option even for very small sleeving campaigns.
The highly efficient laser-welded sleeving process in combination with Westinghouse`s advanced ROSA-III robot-based delivery system has also resulted in impressive reductions in radiation exposure. The total exposure at Doel 4 for the 11,232 sleeve campaign was only 67 man REM, while on a per-sleeve basis, the exposures at Maine Yankee were even lower.
Infrared feedback weld monitoring
A major process enhancement was implemented for the first time at Byron 1 and in all subsequent sleeving campaigns. The enhancement uses welding process infrared feedback monitoring to verify the weld`s adequacy and to detect weld head mirror degradation. It also detects weld head rotational anomalies and the presence of weld perforations.
Early detection of such problems permits corrective actions to be taken, such as mirror or weld head replacement. This has resulted in fewer weld repairs, along with associated productivity improvement and shorter outage schedules.
Laser-welded, direct tube repair
As an extension of its laser-welded sleeving technology, Westinghouse has developed laser-welded, direct tube repair (DTR). With the DTR process, the degraded tube region is restored by applying an overlapping, spiral, filler wire weld on the inside tube surface, using the laser beam`s focused energy. The parent tube is melted to a depth of 60 to 80 percent of the wall thickness. In addition, a buildup of 0.020 inch in wall thickness is created on the tube inside diameter. The repaired zone is typically 1 inch long. The use of filler wire (Alloy 672) enables melted tube wall composition and buildup to approach Alloy 690`s composition.
DTR process` major advantage over sleeving is its ability to implement the repair above existing sleeved locations in the tube. Multiple sleeve installations in the same tube increase the tube`s hydraulic resistance and limit the sleevings` overall effectiveness. DTR process implementation at multiple elevations in the same tube produces an insignificant effect on hydraulic resistance and heat transfer because of the larger tube inner diameter at the repair and the relatively short repair zone (1 inch compared to a minimum sleeve length of 12 inches).
The process has currently been qualified for 7/8 inch tubing at the tube support plate locations. An American Society of Mechanical Engineers Boiler and Pressure Vessel, Section XI Code Case has been approved for the DTR process. A licensing database review by the U.S. Nuclear Regulatory Commission is expected to be initiated in early 1997. A process application in a U.S. plant is targeted for spring 1998.
In-situ leak and pressure testing
In recent years utilities have increasingly used in-situ leak and pressure testing for directly evaluating degraded SG tubing strength and leak rate properties. The combination of alternate repair criteria for tubes with outside diameter stress corrosion cracking required in creased leak rate data. Also, the use of EC inspection data in combination with analytical and semiempirical calculations resulted in overly conservative tube integrity and leak rate assessments. In-situ leak and pressure testing was determined to be more economical and provided more meaningful results than pulled tube data.
During tube integrity demonstrations, burst strength and total leak rate under postulated accident conditions are key regulatory requirements. The in-situ leak and pressure test system is capable of applying the required pressure loadings to the degraded zone in the tube and accurately measuring any leakage. A specially designed mandrel with inflatable bladders isolates the degraded region to be tested and introduces water under pressure at that location.
The Westinghouse in-situ leak and pressure testing system, which has been qualified for both 7/8 inch and 3/4 inch tubing, was implemented successfully at Farley 2 in 1996. A system for testing the full tube length has been developed and is awaiting qualification.
The Westinghouse nuclear service robot, ROSA-III, is another example of technology that has been developed to improve inspection and maintenance methods and shorten outage duration. The primary function of the ROSA-III robot is to perform a full spectrum of SG primary-side inspection, maintenance and repair services. Representing the third generation of the ROSA service robots, ROSA-III has advanced, fully integrated robot and end-effector controls that are programmable for flexibility and easy upgrades. High reliability, zero entry into the channel head and automatic end-effector loading combine to maintain the lowest radiation exposures in the industry.
The robot employs harmonic drives and high torque dc motors that, combined with an efficient robot arm configuration, provide high service productivity and availability for improving outage schedules. The portable and modular control system offers a full six axes of robot motion for manway to tubesheet delivery, with an 80 pound payload capacity.
The state-of-the-art computer control system includes online instruction, diagnostics and recovery prompts, system integrity reports, tube tracking and logbook functions, collision detection and automatic service execution capability. A single operator, using advanced man-machine interface systems, controls both the robot and end effector functions as far as 7,000 feet from the SG, using advanced fiber-optic networking.
Robot-based services include EC acquisition, including automatic bobbin and rotating pancake coil acquisition, mechanical plugging, plug removal, laser-welded sleeving, laser-welded direct tube repair, tube pull, shot peening, stabilizer installation, and in-situ leak testing. z
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Westinghouse`s ROSA III robot, with sleeving end-effector, inside the SG.
Westinghouse`s laser-weld head.