ANO-1 solves operating problems
After 22 years of operation, reliability and performance at Arkansas Nuclear One, Unit 1 (ANO-1) had deteriorated significantly. But the modular installation of redesigned condenser tube bundles and waterboxes has given the plant a new lease of life.
Entergy Operations, Inc.,
Arkansas Nuclear One,
Russelville, Arkansas, USA;
Thermal Engineering International,
Los Angeles, California, USA
Arkansas Nuclear One, Unit 1 (ANO-1), an 883 MWe PWR, was licensed for commercial operation in 1974. A condition assessment of the original condenser was performed in 1995 which revealed the condenser had experienced several types of tube degradation during the 22 years of previous service.
The original Westinghouse turbine/condenser system consisted of a single-pressure, dual exhaust LP turbine exhausting to two, single pressure, single pass surface condensers. The failure mechanisms found in the condenser included ID erosion, steam impingement, ammonia grooving, inter-granular corrosion, vibration damage, circumferential cracking, localized corrosion, ID pitting, and mechanical damage.
The most significant of these was the circumferential cracking at or near mid-span between support plates. This was primarily caused by increased tube vibration due to uniform ID erosion. During this condition assessment, the average wall loss of the condenser tubes was measured to be 34 per cent less than the originally supplied tube wall thickness. Even more significant, the erosion rate was estimated to be 1.7 per cent annually.
In addition, copper deposition throughout the secondary system from the Admiralty condenser tubing was detrimental to the integrity of the steam generator tubing. Also, the secondary chemistry pH suitable for the Admiralty tubing resulted in higher secondary corrosion rates and more iron transport to the steam generators. Finally, the adverse effects of circulating water contamination was a threat to secondary chemistry limitation and damaging to the steam generators.
After an exhaustive evaluation of all viable alternatives, it was decided that the existing Admiralty condenser tubes and Muntz tube sheets should be replaced with a new tube bundle design utilizing titanium tubes and tube sheets. This evaluation considered numerous other non-copper alloys ranging from 316L stainless steel to AL6XN.
The primary factors used to evaluate each material were material compatibility with the site`s brackish cooling water, initial and future maintenance costs, plant performance, industry operating experience and probable failure modes (corrosion, under deposit pitting, biological fouling, etc.). It was further determined that this replacement would be accomplished using shop fabricated tube bundles and waterboxes to minimize required installation time.
Each tube bundle was to be completely shop assembled and thoroughly tested before being shipped to ANO-1 for installation. In addition to compensating for the difference in the thermal conductivity between Admiralty and Titanium, the replacement condenser design was required to optimize condenser performance at the contemplated eight per cent power uprate condenser duty. Table 1 shows comparative performance requirements.
Also, the physical size and weight of the tube bundles (13.5 m long; 4 m wide and nearly 5.5 m high and weighing 88450 kg) presented the project with many transportation and installation challenges. These challenges called for an extraordinary level of coordination between the various entities involved in this condenser tube bundle and waterbox replacement project. The limited installation window available with the scheduled refuelling outage further enhanced this level of coordination.
Titanium bundle design
The design optimization programme had to consider the required eight per cent power uprate utilizing the existing circulating water system while producing an efficient turbine output. The difference between the heat-transfer coefficients of the original Admiralty tubing and the new rolled/welded 1 inch (25 mm) diameter, 24 BWG and 22 BWG titanium tubing (ASTM B338, Gr. 2) provided a significant challenge in the design of the new condenser bundles.
With the external envelope of the condenser intact, the condenser design optimization had to consider several key elements. Assessment of the existing circulating water pump capacity against condenser back-pressure with the new bundle design was of the utmost importance. Multiple cases of thermal/hydraulic design combinations were analyzed, balancing the circulating water system, optimizing pumping efficiency, cooling water usage, megawatt output, and hardware cost. Evaluation of the space constraints within the existing condenser provided the final interface requirements with the new modules. Spacing of the new support plates was reduced considerably with the lighter gauge titanium tubing in order to alleviate tube vibration during full load operation with one bundle out of service.
New condenser waterboxes were designed to facilitate installation, improve flow distribution to the tubes, and minimize pressure drop. This was the critical interface piece between the new condenser bundles and the existing circulating water piping in the plant. A sophisticated Finite Element Analysis programme verified the structural integrity of the new waterboxes, and subsequent shop hydrostatic testing confirmed the results. Waterboxes were equipped with an impressed current cathodic protection system and internal epoxy coating to prevent galvanic corrosion of the carbon steel surfaces.
The new titanium tube bundles were more erosion resistant, with better deaeration capabilities. However, their lighter weight in spite of a more compact tube field necessitated a thorough uplift analysis to evaluate the integrity of the existing anchor bolts and the necessity for additional anchoring.
The condenser bundles were fabricated to high quality standards in a controlled shop environment. Several shop mock up tests were performed to determine the optimum tube pull out load, and the required wall reduction for the rolled tubes. Prior to rolling and welding, 100 per cent base line eddy current testing of the tube bundles was completed. Results of this test were stored on several tapes and were submitted to ANO. Future in-service eddy current examination can be made and compared to base line results to determine wear.
All tube ends were rolled and welded using automatic welding machines. Tube welds were checked for integrity using dye penetrant testing, and also vacuum leak testing to verify the integrity of the tube joints and the entire tube length. Once pronounced sound, condenser modules were wrapped in specially designed traps and shipped to the site.
With the massive size and weight of each titanium tube bundle, transportation logistics became a significant issue.
The Departments of Transportation in several states had to be contacted to assure that roads, highways, and bridges would be available for this transport. Special permits and full time escorts were required from the manufacturing plant to the job site to assure safety and security of all involved. Limited height was a critical element throughout the transportation route. Rail transportation was out of the question due to oversize weight and dimensions, and truck transportation was the most viable alternative at the time of shipment.
The trunnions, welded into the bundles, were supported by the longitudinal “I” beams. Essentially it could be called a “double-pole” trailer where two specially designed “I” beams almost 4.6 m (15 ft) apart supported the tube bundle suspended between them. The special trunnions supported the tube bundle, reducing its overall height above the road as much as possible. Since only one such special trailer was built, it had to make four round trips from the manufacturing facility to ANO to complete the delivery project. Waterboxes were shipped separately to maintain the overall schedule and to keep bundle transportation within allowable size and weight limitations.
The installation phase of the condenser tube bundle and waterbox replacement project presented numerous challenges for the project team.
The first challenge was completing the condenser replacement within the scheduled refuelling outage duration of 42 days and 1 hour. This limitation affected every decision made on the project from the beginning of the project.
The second challenge was the physical location of the condenser, which was 5.8 m below ground level. The only existing access to the area was through a condenser tube pull pit, which was approximately 3 m shorter than the condenser tube bundles. To provide access to the existing condenser shells, a large opening was made in the side of the turbine building, which included the removal of structural steel, interference piping and electrical interferences.
The third challenge was the proximity location of several interferences outside the turbine building which greatly restricted access to the condenser area. These interferences include the main transformers and startup transformer #2 and associated electrical buss ducts and 500 kV and 261 kV overhead power lines. These interferences precluded the use of a large crane to rig the tube bundles into the tube pull pit. A specially designed gantry crane capable of lifting, translating and rotating each tube bundle was designed and assembled over the tube pull pit (Figure 6). The gantry included a specially designed lift frame that allowed the load to be shifted while still suspended to locate centre of gravity and ensure that the tube bundles were kept level during the rigging operation. The gantry also included a hydraulic turntable for better control of the tube bundles while the bundles were rotated.
The fourth major challenge was the limited access to the condenser inside the turbine building. To get the new tube bundles and waterboxes into the condenser, a large track was assembled in the turbine building basement and extending into the condenser shell. Specially designed hydraulically powered rollers were used in a load equalizing arrangement to ensure that each roller shared 25 per cent of the total bundle load and to prevent damage to the tube bundles. Also, specially designed carts were fabricated to allow the replacement waterboxes to be moved into position through the condenser shell using the rigging assembly available in the condenser hotwell.
The fifth challenge was the removal of the existing condenser tube bundles and waterboxes. After an extensive evaluation of all available alternatives, it was decided that the existing condenser tube bundles would be cut in half inside the condenser shell and removed with waterbox attached. However, to move the tube bundle halves with waterbox attached, each bundle would have to be stiffened to allow the bundle to be both rolled out of the condenser shell using the hydraulic rollers and lifted out of the tube pull pit with the gantry crane.
The final challenge was estimating, planning, scheduling and managing the resources and manpower required to complete a construction project of this size with the available refuelling outage window.
The ANO-1 condenser project was successful in all phases of the project including: initial condition assessment, project feasibility study, competitive bid process, condenser re-design, shop fabrication, bundle transportation and field installation. The complete installation of the redesigned condenser tube bundles and waterboxes was accomplished during the 14th refuelling outage of ANO-1. The duration of the refuelling outage was 43 days and 15 hours.
The circulating water outage required for the condenser replacement was 33 days and 21 hours. Although a full ASME PTC 12.2 performance test was not completed following the condenser tube bundle replacement, the condenser backpressure for the redesigned condenser was measured using plant instrumentation and the results indicated an improvement of 0.19 inHg to 0.23 inHg when compared to the backpressure of the original condenser at the same circulating water temperature and condenser duty.
To date, the replacement condenser tube bundles have not experienced any in-service tube leaks and unit has been operating reliably and efficiently.