ML20207G226
| ML20207G226 | |
| Person / Time | |
|---|---|
| Site: | Nine Mile Point  |
| Issue date: | 06/07/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20207G222 | List: |
| References | |
| NUDOCS 9906110066 | |
| Download: ML20207G226 (8) | |
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,j NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 30666 4 001 O. . . . . /g 1
! SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION
, MODIFICATION OF CORE SHROUD TIE ROD UPPER SPRING ASSEMBLIES j i .
NINE MILE POINT NUCLEAR STATION. UNIT NO.1 l
f DOCKET NO. 50-220 1
1.0 INTRODUCTION
By letter dated May 21,1999, as supplemented by letter dated May 28,1999, Niagara Mohawk Power Corporation (NMPC or licensee) proposed a modification to each of the four core shroud stabilizers (a.k.a., tie rod assemblies) for implementation during the current 1999 refueling
- outage at Nine Mile Point Nuclear Station, Unit 1 (NMP1). NMPC provided additional information to the U.S. Nuclear Regulatory Commission (NRC) staff during a telephone call on l May 27,1999 (see summary of telephone conversation dated June 3,1999).
l Pursuant to 10 CFR 50.55a(g)(4), the components must meet the requirements of Section XI of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code).
Because the proposed modification is not included under the definition of repair or replacement l
specified in Section XI of the ASME Code, the proposed modification was submitted to the NRC staff for review and approval as an alternative repair pursuant to Title 10 of the Code of Federal Reaulations (10 CFR) Section 50.55a(a)(3)(i).
2.0 BACKGROUND
The proposed modification results from NMPC's visual examination of the tie rod assemblies during the current refueling outage (RFO15). The examination revealed that a 3/8-inch cap screw connecting the spring bracket to the upper spring in one of the tie rod assemblies had l failed and that the broken portion had dislodged from the upper spring assembly. In addition, NMPC's visual examination of the reactor pressure vessel (RPV) cladding revealed areas of scratches and some evidence of wear at the locations where the upper spring of each of the four tie rod assemblies contacts the RPV cladding. NMPC determined that the scratches and extent of wear were acceptable.
The root cause of the cap screw failure was intergranular stress-corrosion cracking (IGSCC) in the alloy X-750 material driven by large, sustained differential thermal expansion stress due to fastening of dissimilar materials with the cap screw. A potential contributing cause was the sustained stresses that were attributed to the torquing of the cap screw associated with the Enclosure
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9906110066 990607 l
F original assembly of the tie rods. A second potential contributing cause was the stresses associated with friction between the vessel wall and the upper spring contact points.
The proposed modification would replace the design function of the failed cap screw and other
. cap screws that have the potential for future failure. The modification includes changes to each end of the upper spring for each of the four tie rods and contains provisions to prevent the creation of loose parts in the RPV due to the potential for failure of the subject cap screws. In addition, the modification includes rounding off the leading edges of the upper wedge and l
upper contact of the upper spring for the four tie rods as a preventative measure to reduce stresses on the tie rod assemblies and to reduce the likelihood of future wear on the cladding of the RPV.
The proposed modification to the upper spring, including the tie rod assemblies, are illustrated in Figures 1,2,3, and 4 attached to this safety evaluation (SE). The upper spring modification, shown in Figures 2,3, and 4, involves the addition of a clamp fabricated from 300 series stainless steel material, secured to attach the existing upper spring bracket to the upper spring by two bolts, consisting of XM-19 material, and installed perpendicular to the plane of maximum l
thermal expansion. Currently, four 3/8" cap screws secure the upper spring bracket to the upper spring. The two XM-19 bolts replace the function of the four existing 3/8" cap crews. 1 The XM-19 threaded bolts are staked in place by type 316/316L locking pins. The clamp also functions to prevent the existing 3/8" cap screws from loosening or dislodging and becoming loose parts.
) 3.0 EVALUATION 3.1 Structural Evaluation NMPC stated that a structural evaluation of the proposed modification was conducted. NMPC's !
analysis and conclusions were reviewed by the NRC staff, and are summarized in the following three paragraphs.
The significant loads on the bolts of the upper tie rod spring assembly consist of loads due to differential thermal expansion loads caused by fastening of dissimilar materials, loads due to installation torque, and loads due to friction or potential binding between the RPV and the contact wedges.
I Postulated dynamic events during accident conditions do not impose any additional loads. l However, for the purpose of checking the bearing at the contact wedges, seismic loads were included along with the upset thermal condition loads. The detailed analysis for the bolts was performed for the normal and upset operating conditions. The analysis of the bolts was based upon the use of X-750 and XM 19 materials for the bolt. Although both materials were found acceptable, XM-19 was selected. In the evaluation, it is assumed that the currently remaining j bolts (X-750) are ineffective. The calculations were performed using strength of material ;
equations, thermal and mechanical properties, and allowable values in accordance with the ASME Code, Section lil, Subsection NG.
1 The tensile stress in ths bolt was determined to be 13.7 ksi (the larger of the normal and l thermal upset conditions). This is well within the allowable stress of 35.4 ksi which is 0.9 Sy at i l :
. the upset temperature for the XM-19 material bolt. The allowable stresses for the XM-19 material were based upon annealing at a temperature of 2025-2075 *F. Bolt head shear stress was evaluated for the upper contact bolts and shown to be acceptable. Fatigue was also assessed in the analysis. The allowable fatigue cycles were determined to be 700,000 cycles.
The actual number of thermal cycles (startup and shutdown) was conservatively assumed as 200, which is much smaller than the allowable 700,000 cycles. Thus, fatigue stresses were
, determined to be insignificant. The bolts are threaded into the nut plate. The only significant l
loading on the nut plate is due to the tensile load in the bolt. The thread shear in the nut plate due to the bolt load was determined to be less than 3.5 ksi, which is well within the allowable value of 11.3 ksi, which is 0.6 Sy at the thermal upset temperature for the plate material. l l
l The NRC staff has reviewed NMPC's structural evaluation and finds it to be acceptable. On the basis of this review, the NRC staff concludes that the design of the repair modification satisfies '
the structural requirements of the ASME Code and, therefore, provides an acceptable level of .
quality and safety. l 3.2 Materials Evaluation i
in its submittal to the NRC staff, NMPC states that the clamps will be fabricated from type 316L stainless steel materials and the bolts from XM-19 materials. NMPC indicated that the XM-19 materials for the bolts were selected because of material availability and because its thermal expansion coefficient is similar to that of the type 316L stainless steel parts to which it is adjoined, thereby minimizing the thermal expansion tensile stresses. The locking pins used to prevent the loosening of pre-torqued XM-19 bolts will be made from type 316/316L stainless steel materials. All the components will be fabricated from materials in a solution-annealed condition and the materials will be tested to ensure that they were not sensitized. To minimize I susceptibility to IGSCC due to surface cold work, requirements for cold work control in ,
machining the threads are included in the fabrication specification. To eliminate potential stress l corrosion cracking due to surface contamination, chemical controis are imposed during the process of fabrication to ensure that there is no surface contamination such as by heavy metals and chlorine or sulfur compounds. <
The NRC staff has determined that the materials (type 316/316L and XM-19) selected for the i
repair of components are ecceptable because they are ASME Code-approved materials. The solution annealed condition of these materials, together with the machining and chemical controls imposed during fabrication, will provide the repair of components with adequate l l resistance to IGSCC. Various BWR internals and other repair components made of these materials have demonstrated successful service experience in the BWR environment.
3.3 Loose Parts Considerations 4 In its submittal, NMPC states that the repair uses locking methods for threaded connections.
l_ All parts will be captured and held in place by a method that will last for the design life of the repair. The modified upper springs are designed to minimize the potential for loose parts inside the RPV, and the addition of the clamps are intended to prevent the existing 3/8" cap screws from becoming loose parts. The threaded bolts are captured and held in place by interference-fit locking pins that stake the bolts. The locking pins are prevented from backing out of their installed holes by reducing the diameter of the hole edge after installation of the pins.
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4-Therefore, the NRC staff finds that the proposed modification includes acceptable provisions to preclude the occurrence of loose parts. -
4.0 CONCLUSION
On the basis of its review of the analysis discussed above, the NRC staff finds that the structural design of the repair modification satisfies the structural requirements of the ASME Code, Section lil, Subsection NG, " Core Support Structures," and is, therefore, acceptable.
The NRC staff finds that the materials selected for component repair are specified as approved materials by the ASME Code and are, therefore, acceptable. Acceptable provisions are included to preclude the occurrence of loose parts. Accordingly, the NRC staff concludes that the proposed modification provides an acceptable level of quality and safety. Accordingly, it is 1 authorized in accordance 10 CFR 50.55a(a)(3)(i). ]
Attachments: b 1
- 1. Figure 1, Core Shroud Stabilizers
- 2. Figure 2, Upper Spring
- 3. Figure 3, Upper Clamp installation
- 4. Figure 4, Lower Clamp Assempty PrincipalContributors: D. Hood {
J. Rajan '
W. Koo Date: June 7,1999 l
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