ML060600119
| ML060600119 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 08/10/2005 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| Download: ML060600119 (4) | |
Text
AUGUST 10. 2005 CONFERENCE CALL
SUMMARY
REPLACEMENT STEAM GENERATORS ARKANSAS NUCLEAR ONE. UNIT 1 DOCKET NO. 50-313 On August 10, 2005, the Nuclear Regulatory Commission (NRC) staff participated in a conference call with Arkansas Nuclear One (ANO), Unit 1 representatives regarding their replacement steam generators which are scheduled to be installed in October 20,D5. The call focused on the design of the replacement steam generators and the actions taken by the licensee in response to inspection findings at Oconee Unit 1. Oconee Unit 1 and ANO Unit I both use once-through steam generators. A summary of the information discussed during the call along with some background information is provided below.
Oconee 1 has two once through steam generators designed and fabricated by Babcock and Wilcox International (BWI). These steam generators were put into service during refueling outage (RFO) 21 in 2003 (i.e., the original steam generators were replaced in 2003).
Each steam generator has 15,631 thermally treated Alloy 690 tubes which have an outside diameter of 0.625-inch and a nominal wall thickness of 0.038-inch. The tubes were manufactured by Sumitomo. The tubes are arranged in a triangular pattern with a spacing of approximately 0.875-inch. The heat transfer surface area is 134,600 ft2. The total length of a tube is 674.1-inches with a heat transfer length of 629-inches.
At Oconee 1, the tubes were hydraulically expanded into the tubesheet for 13-inc:hes from the tube-end. The tubesheets are 22-inches thick. The tubes are supported by 15 tube support plates constructed from 410 stainless steel. The tube support plate has trifoil shaped holes through which the tubes pass. The trifoils have an hour glass profile to improve hydraulic resistance (i.e., reduce the pressure drop across the plate), provide a flat contact surface for the tube, facilitate tubing of the steam generator, and provide better accessibility for water lancing and chemical cleaning. The trifoil land is 0.16-inch wide. The spacing between the tube supports was slightly modified in the replacement steam generators with the bottom and top tube support plate being moved closer to the tubesheet. The open tube lane that was present in the original Oconee 1 steam generators is tubed in the current steam generators.
Prior to placing the steam generators into operation, one tube in each of the two steam generators was plugged.
The first inservice inspection of the steam generator tubing following replacement of the original steam generators started in April 2005. All of the inservice tubes were inspected full length with a bobbin coil. As a result of these inspections, approximately 11.5% of the tubes in steam generator A and 9.6% of the tubes in steam generator B had indications of wear at the tube support plate elevations. Most of the indications were located between the ninth (009) and eleventh (011) tube supports. In addition, most of the indications are shallow (less than 20%
through-wall) and all of the tubes had adequate structural and leakage integrity. Some of the tubes had multiple indications at the same tube support elevation and some tubes had indications at multiple support plate elevations. Most of the indications are in the periphery of ATTACHMENT
the tube bundle; however, there are indications spread throughout the interior poition of the tube bundle. The largest indications are located approximately 5 tubes in from the periphery.
Due to the large number of tubes with degradation, a root cause investigation was commenced by Oconee and BWI representatives to determine the extent of condition and the factors that led to the large number of tubes affected by wear. This root cause investigation is expected to continue through the end of 2005. Additional details concerning the findings at Oconee are available in the NRC document management system, ADAMS, under Accession Number ML051940482.
In October 2005, the original once-through steam generators at ANO 1 are scheduled to be replaced. The replacement once-through steam generators were fabricated by Areva in France. The replacement steam generators arrived at ANO 1 on August 9, 2005.
Representatives from ANO 1 have been following the investigations at Oconee 1. At Oconee 1, several probable causes of the wear were identified (refer to ML051940482). These causes include: (1) tube vibration induced by motion of the steam generator; (2) excessive compressive loads in the tubes; (3) high wear rate coefficient for Alloy 690 against the 410 stainless steel tube support plates; (4) acoustically induced tube vibration; (5) broached hole surface finish roughness; and (6) divergent nozzle created by the hourglass broached hole design. Each of these probable causes is discussed below.
Tube vibration due to the motion of the steam generator is not a significant concern for the ANO 1 replacement steam generators since the support skirt used in the original steam generator was retained in the design of the replacement steam generator. The Oconee replacement steam generators used a pedestal design which is less resistant to rocking motion.
The ANO 1 representatives also indicated that the Oconee Unit 1 original steam generators also had more wear than any of the other original once through steam generators,. The reason for this increased propensity for tube wear in the original steam generator is not known.
The differential thermal expansion between the tubes and the steam generator shell can result in excessive compressive loads on the tubes. This is a result of the tubes wanting to expand more than the shell, but being restricted from expanding by the tubesheet. Compressive tube loading can result in an increase in flow induced vibration and tube wear. As a result, the tubes are placed into tension at room temperature during fabrication. Alloy 690 expands more than Alloy 600 (i.e., the coefficient of thermal expansion for Alloy 690 is greater than that of Alloy 600). Due to temperature variations along the length of the tube, AREVA spent resources on optimizing the preload put into the tubes during fabrication to achieve the optimum preload during operation. In the replacement steam generator design at ANO 1, the goal was to put a slightly higher preload in the tubes than in the original steam generators. In the Oconee documentation (ML051940482), only one preload was identified in the replacement steam generator design rather than a range of preloads so comparison with Oconee is difficult.
With the right amount of tension at room temperature, the tubes will stay in tension or be slightly in compression during normal operation. In the ANO 1 replacement steam generators, the tubes were placed into tension by hydraulically expanding the tubes into the lower tubesheet for the full length of the tubesheet with the upper tubesheet end free to move, followed by hard rolling and welding of the tube then hydraulically expanding the tubes into the upper tubesheet for the full length of the tubesheet. This final hydraulic expansion produces 2
the desired tensile preload. At Oconee, the tubes were hydraulically expanded into both the upper and lower tubesheet at the same time. The tubes were only expanded for half the thickness of the tubesheet (i.e., expanded for 13-inches).
High wear rate coefficients between Alloy 690 and 410 stainless steel have been reported by EPRI. The wear rate coefficients could be a factor of 2 or 3 higher than in the original steam generator design. In the replacement steam generators at ANO 1, the 410 stainless steel received a specialized heat treatment to reduce this wear coupling. Additional discussions are planned between ANO 1 and AREVA to discuss this issue.
With respect to the potential for acoustically induced tube vibration, the ANO-1 representatives indicated that the steam aspirator port in the replacement steam generators at Oconee was changed to have a tapered edge. This is a potential area for vortex shedding. The steam aspirator port in the replacement steam generators at ANO-1 is not tapered, but it is 0.5-inch larger than in the original steam generators. The spacing of the tube supports was also changed in the replacement steam generators at Oconee to provide more even spacing.
This moved the nearest tube support farther away from the steam aspirator port. In the original steam generators there was approximately 13.5-inches from the bottom of the aspirator port to the nearest tube support. In the replacement steam generator design for Oconee, there is approximately 18-inches from the aspirator port to the nearest tube support. The irregular spacing of the tube supports in the original steam generators was retained in the replacement steam generators at ANO-1.
The surface finish roughness of the tube support plates is not a significant concern for the ANO 1 replacement steam generators. In the ANO 1 replacement steam generators, the tube supports are 1.18-inches thick (compared to 1.5-inches in the original steam generators and in the Oconee replacement steam generators, have broached trifoil-shaped holes (similar to Oconee), were fabricated from 410 stainless steel (similar to Oconee), and were fabricated to a finish of 30RA. The tube support lands at ANO 1 do not have an hourglass shape (unlike Oconee). There are still some drilled hole tube support locations at the upper tube support plates.
ANO 1 also reviewed the differential thermal expansion between the tube supports and the tubesheet. The tube supports (410 stainless steel) expand less than the tubesheet (carbon steel). This differential expansion could result in bowing the tube in the lower and upper spans of the tube (i.e., between the tubesheet and the nearest tube support). The extent to which this condition occurs depends, in part, on the amount of preload in the tube. If the tube comes into contact with one land, this could increase the wear rate. AREVA has indicated that is could occur, but the effects are not considered significant.
The licensee for ANO 1 also discussed several other design changes between their original and replacement steam generators. For example, the height of the upper shroud was increased to get it closer to the tubesheet (for better support). The range of clearances between the tube and the tube support plate were increased from 0.010- to 0.021-inch in the original design to 0.015-to 0.032-inch in the replacement steam generator design. AREVA expects the larger gap will increase the damping and result in less tube wear. This is based on test data.
Discussions between AREVA and ANO 1 representatives are still on-going on this issue (Oconee expects more wear for larger clearances).
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The tube support plates in the ANO 1 replacement steam generators are supported differently than that in the Oconee replacement steam generators. At ANO 1, the tube support plates are supported by stay (or support) rods. In the replacement steam generators, the number of stay rods was increased compared to the original design. The tube supports are free to slide in the replacement steam generators at ANO 1. In the Oconee replacement steam generators, the edges of the tube supports are restricted from movement by blocks welded to the shroud.
Because of differential thermal expansion, this could result in "cupping" of the tube support which may lead to more wear in the periphery of the tube bundle. A combination of tube bowing or tube support plate "cupping" would be consistent with the observation of wear in the periphery and tapered wear scars near the edge of the tube support.
The ANO 1 replacement steam generators have the same tube diameter, thickness, and pitch as the original steam generators. The open tube lane was removed in the replacement steam generator design. The preservice inspection will be done on site with the steam generators in a horizontal position.
The ANO 1 representatives are following the root cause investigation into the Oconee findings and are conducting their own assessments to ensure no safety significant issues exist with their replacement steam generators.
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