ML061080570
| ML061080570 | |
| Person / Time | |
|---|---|
| Site: | Quad Cities  |
| Issue date: | 04/10/2006 |
| From: | Exelon Nuclear |
| To: | Office of Nuclear Reactor Regulation |
| banerjee M, NRR/ADRO/DORL, 301-415-2277 | |
| References | |
| %dam200606 | |
| Download: ML061080570 (41) | |
Text
1 Quad Cities Unit 2 (QC2)
Dryer Update April 10, 2006
2 Introduction Patrick Simpson Licensing Manager
3 Agenda
- Background
- Dryer Inspection Results
- Preliminary Cause
- Repair Strategy
- Conclusions/Outage Status Tim Hanley Tim Hanley Roman Gesior Tim Hanley Randy Gideon
4 Background
Tim Hanley Director Midwest Operations Projects
5 Background
Skirt Skirt Base Ring Dryer Upper Support Ring Lifting Eye (1 of 4)
Cut Out for Reactor Vessel Support Lug (1 of 4)
Drain Channel Lifting Rod (1 of 4)
Dryer Guide Slots Mounting Block for Latch Assembly (1 of 4)
Dryer Configuration Dryer Banks (A-F)
(Vane Assemblies)
6 Background (cont.)
QC2 replacement dryer was installed in May 2005
- During fabrication some ovality was created in the skirt section of the dryer During the installation, an interference was encountered with the separator guide rods that prevented the dryer from fully seating During the removal of the dryer to correct the interference the dryer impacted the dryer support lugs that are attached to the inside of the reactor vessel at the 140°, 220°, and 320° locations
- Cause: lack of clearance between the skirt base ring/reactor vessel support lugs and excessive clearance between dryer guide rods/dryer Indications of deformation at 140o, 220o, and 320o locations were observed at this time Subsequent inspections during the current refueling outage revealed additional indications at these locations
- Deformation of the skirt base ring on the ID at 140° and 220° locations
- Skirt panels dimpled at 140° and 220°
- Neither of these conditions were observed at the 320° location
7 Background (cont.)
Original Deformation No Damage Deformation
8 Background (cont.)
- Deformed areas were non-destructively (PT) examined on the OD and skirt base ring with no indications identified and dispositioned as acceptable for use for one cycle
- Repairs, modifications, and inspections of these areas were planned to be implemented during current refueling outage
- QC1 replacement dryer was installed in May 2005
- Based on QC2 lessons learned avoided ovality and installation issues
9 Dryer Inspection Results Tim Hanley Director Midwest Operations Projects
10 Dryer Inspection Results
- QC2 dryer inspections have been completed
- Inspection scope encompassed and exceeded the requirements of BWRVIP-139
- Inspections a combination of VT-1 and VT-3
- General exterior visual examination
- Locations potentially subject to fatigue
- Outer structural welds including:
- Hoods, vanes, skirt, upper support ring, skirt base ring
- Inner structural welds including:
- Cross beams (to upper support ring and support castings)
- Drain channels and tie bar welds
- Scope expansion:
- Skirt plates
- Skirt base ring cut-outs, gussets
- Latch boxes
11 Dryer Inspection Results (cont.)
- 14 total indications documented in Indication Notification Reports (INRs)
- 7 indications were minor in nature with no structural significance:
- 2 documented the re-inspection of the deformed areas at the 220o and 320o location with no changes noted
- 2 documented surface anomalies that were classified as non-relevant
- 1 documented a small piece of debris that is captured in the dryer internals (less than 2/10 in length)
- 1 documented damage to the 20o area at the separator guide rod cut out that required minimal repair
- 1 documented deformation to perforated plates
12 Dryer Inspection Results (cont.)
- The following 7 indications will be discussed:
- 3 documented small cracks in the end vane in three vane assemblies (INRs 06-06, 06-08, and 06-10)
- 1 documented the rotation of the lifting eyes and damage to the lifting rod threads (INR 06-01)
- 1 documented a crack in the latch box located at the 220o location (INR 06-29)
- 1 documented a crack in the vane assembly end plate near the 320o location (INR 06-04)
- 1 documented a large crack in the dryer skirt and base plate at the 140o location (INR 06-02)
13 20 o
200 o
45 o
225 o 140 o
40 o
220 o
320 o
0 o
180 o
90 o
270 o
Separator Guide Cutout (2 Locations)
Lifting Eyes INR 06-01 (4 Locations)
Reactor Vessel (4 Locations) 135 o
315 o
Vane Bank Crack Skirt Crack INR 06-02 Latch Box Crack Dryer Inspection Results (cont.)
INR 06-29 INR 06-04 A
B C
D E
F INRs 06, 08, 10 Support Lugs
14 Dryer Inspection Results (cont.)
- INRs 06-06, 06-08, and 06-10 document minor indications in the vane bank assemblies for dryer banks F, D, and B, respectively Arrows indicate areas of cracking Vane spacer outside diameter 0.625 Dryer Vane Bank Assembly
15 Dryer Inspection Results (cont.)
- Small cracks in the end vane in various vane assemblies (INRs 06-06, 06-08, and 06-10)
- Characterized as fatigue cracking
- Located in the end vane in their respective vane assembly which is wrapped around and welded to the end plate (non-structural)
- Indications appear to have initiated from the hole in vane that accepts 5/8 tie rod protector
- Dispositioned as no repair required - will be inspected during the next refueling outage
- Missing material cannot be confirmed to be lost in the vessel -
conservatively captured by lost parts program
- Applicability to QC1
- Not a concern due to the small size of the cracks and lack of a driving force at that location
- Cracks expected to be self relieving
- Will be inspected during the upcoming planned outage
16 Dryer Inspection Results (cont.)
- INR 06-01 documented lifting eye rotation and lifting rod thread damage 2.75 Rod Guide Fixture 3 Lift Rod Gap Dryer Lifting Eye Assembly Setscrew Hole Dryer Lifting Eye Assembly
17 Dryer Inspection Results (cont.)
Rotation of the lifting eyes and damage to the lifting rod threads (INR 06-01)
- Four lifting eyes were found rotated out of alignment
- Damage to the threads in the lifting rod at the 45o location Cause - installation issues with the setscrew in addition to an inadequate setscrew design that allowed the eyes to rotate with setscrew in place and tack weld intact Corrective action - unthreaded stock at the top of the lifting rod is being ground flat and a longer set screw is being added to ensure positive engagement Applicability to QC1
- Lifting eyes on QC2 dryer were removed after initial attempt to engage lifting rig and reinstalled - not required on QC1
- Lifting eye unlikely to separate from lifting rod due to unthreaded area at the top of lifting rod
- In the unlikely event the lifting eye came off of the lifting rod, lost parts analysis has concluded that it would not prevent a safety function
- Will be inspected during the upcoming planned outage
18 Dryer Inspection Results (cont.)
- INR 06-29 identified crack in latch box Cracking at corner Interior view of Latching Box Cover Latch Box
19 Dryer Inspection Results (cont.)
- Crack in the latch box at 220o location (INR 06-29)
- Function of the latch box is to limit bypass flow from the inside of the dryer skirt to the downcomer region
- Non-structural component
- Located on the same azimuth that was stressed during the impact event
- Characterized as fatigue cracking
- Other three latch boxes and welds were inspected with no issues identified
- Repair consists of excavating cracked area and re-welding
- Applicability to QC1
- QC1 dryer did not experience an impact event
- Non-structural
- Will be inspected during the upcoming planned outage
20 Dryer Inspection Results (cont.)
Vane spacer bar outside diameter 0.625 INR 06-04 Vane Assembly 140° Location 320° Location 320° Location 320° Location
21 Dryer Inspection Results (cont.)
Crack in the vane assembly end plate near the 320° location (INR 06-04)
- Located on same azimuth of the dryer that experienced skirt base ring damaged during the impact event
- Crack is in the bottom 2 inches of a ~6 foot weld
- Characterized as fatigue cracking with a combination of bending and torsional loading
- Inspection identified an abrupt change in weld thickness at the crack location
- Similar locations inspected with no cracks identified nor were any other cases of abrupt changes in weld thickness identified
- Not a loose parts concern due to attachment to other dryer members
- Dispositioned as no repair required - re-inspect during the next refueling outage Applicability to QC1
- QC1 dryer did not experience an impact event
- Isolated to this one location in QC2
- Not a lost parts concern
- Will be inspected during the upcoming planned outage
22 Dryer Inspection Results (cont.)
INR 06-02 140 Degree Support Lug Cutout Dryer Skirt
23 Dryer Inspection Results (cont.)
- Crack in the dryer skirt and base ring at the 140° location (INR 06-02)
- Approximate 6 foot crack extends from a cutout in the base ring into the adjacent skirt
- Crack affected area was below the normal reactor water level
- Boat samples taken in four locations for analysis
- Preliminary cause indicates impact event key contributor
- Applicability to QC1
- QC1 dryer did not experience an impact event
- Ovality issues were addressed prior to manufacturing
- Lost parts analysis concluded that if a lost part were generated would not compromise a safety function
24 Preliminary Cause Roman Gesior Corporate Programs Director
25 Preliminary Cause
- Multi-disciplined team used to determine cause of dryer damage
- Tools being used to determine cause of failure
- Metallurgical failure analysis
- Dryer inspection results and observations
- Stress analysis
- Failure Modes and Effects Analysis (FMEA)
- Event and causal factor chart
- Conclusions
- Load on skirt base ring during impact event induced damage while plastically deforming skirt base ring and skirt plate
- Reduced the fatigue endurance
- Residual stresses from fabrication were also a contributor
- Operating pressure oscillation loads from Main Steam Line (MSL) acoustics resulted in skirt/base ring stresses that when combined with the reduced fatigue endurance was adequate to propagate cracking
26 Preliminary Cause
- Other causes investigated:
- Design
- Adequacy of FEA
- Applied Loads/damping
- New design
- Base ring cutouts
- Fabrication
- Fit-up/sequence of fabrication
- Base ring distortion
- Materials issues
- Installation
- Decision making after fit-up
- Impact with separator guide pins
- Load cell not functioning
- Operating transients
27 Preliminary Cause (cont.)
Metallurgical Analysis 19.125 36.125 5.75 3.75 10.125 11.625 3.75 8.75 Sample 1 Sample 2 Sample 3 Sample 4 140° Location
28 Preliminary Cause (cont.)
Metallurgical Analysis
- Sample analysis results
- Scanning Electron Microscope (SEM) results show surfaces are characteristic of fatigue
- Fracture surface near the ID of the skirt base ring is consistent with torsional fatigue
- No evidence of cold work induced stress corrosion cracking
- No evidence of ductile tearing
- Initiation site has not been identified
- Secondary cracks that connect with the main fracture emanate out of weld root
29 Preliminary Cause (cont.)
Metallurgical Analysis Weld details require ID and OD penetration of 0.16 with 0.1 fillet reinforcement As built configuration has significant fillet reinforcement and weld size not cause of failure
30 Preliminary Cause (cont.)
Metallurgical Analysis
- Crack in the dryer skirt and base ring (140° location)
- Impact event induced a large torsional load (>47,000 pounds) in the skirt base ring and bending load in skirt plate
- Load on base ring at reduced section (due to cut-out area) resulted in localized high stress
- The load resulted in plastic deformation of skirt base ring and skirt plate (dimples on skirt plate)
- The plastic deformation of the base ring and skirt plate reduced the fatigue endurance limit of the material
- Pressure oscillation loads from MSL acoustics provided cyclic stress necessary to propagate crack
- The operating loads would also produce a torsional load on the base ring through the support gusset
31 Preliminary Cause (cont.)
Dryer Pressure and Strain Measurements
- Pressure oscillation loading alone at 140° location would not have initiated a fatigue crack
- Other azimuths of dryer (MSLs C and D) with less plastic deformation had no cracking
- Similar configuration
- Higher pressure loading than the side that cracked (MSL B)
- Skirt flat plate adjacent to MSLs has significantly higher pressure loads - however no cracking
- Based upon in plant measurements
- Area of skirt base ring cracking was the most significantly deformed during the impact event
- Examination of the skirt at this location indicates dimpling
- The impact event resulted in residual stresses that reduced the endurance of the dryer skirt/base ring plate
32 Preliminary Cause (cont.)
Dryer Pressure and Strain Measurements Skirt pressure transducers
- P22, P24, and P25 Skirt strain gages
- S8, S1, and S2 A Hood pressure transducers P3, P12, P15, and P17 140 Deg Mounting Block Skirt cracking B Hood pressure transducers P20 and P21 Instrument psi (rms)
Max-Min psi P3 (90°)
0.631 3.704 P12 (70°)
0.690 3.976 P15 0.547 3.192 P17 0.232 1.550 P20 (250°)
0.499 3.201 P21 (290°)
0.883 4.360 P22 0.422 2.622 P24 0.225 1.595 P25 0.344 2.436
33 Preliminary Cause (cont.)
Dryer Pressure and Strain Measurements B
A C
D Dryer Orientation Main Steam Lines Banks A - F
34 Preliminary Cause (cont.)
Dryer Pressure and Strain Measurements
- Dryer pressure load becomes smaller moving down from steam nozzles
- Lowest outer hood pressure of 3.2 psi is greater than largest skirt pressure of 2.6 psi
- Skirt pressure drops from 2.6 psi to 1.6 psi from P22 to P24 lower on skirt
- Pressure loads also drop when moving circumferentially away from the nozzles
- Circumferential trend away from MSL P12 = 3.98 psi to P15 = 3.19 psi to P17 = 1.6 psi
- P3 (MSL B) = 3.7 psi is less than P12 (MSL A) = 3.98 psi and P21 (MSL D) = 4.36 psi
35 Preliminary Cause (cont.)
Dryer Pressure and Strain Measurements
- Pressure measurements alone would indicate that the skirt is more susceptible at a different location
- Cracking occurred at location of low measured pressure
- Indicates that the residual stress due to impact event is a larger contributor than the acoustic pressure oscillation
- Therefore, cracking occurred at 140° location due to increased plastic deformation and residual stress
- Condition of high stress locations on dryer with no cracks supports applied loads are conservative
- Number of fatigue cycles at EPU operation (>200 days) with 155 Hz load would have resulted in cracks at this location if stresses exceeded endurance limit
- No dryer degradation at 40° azimuth where dryer did not get hung up on RPV wall support
36 Preliminary Cause (cont.)
Finite Element Model
- 2005 evaluation of impact event
- Stress identified in the evaluation was extremely low and therefore the deformation residual stress was not considered an issue
- Stress levels under-predicted due to simplistic modeling of dryer skirt base ring plate
- Cutout not included
- Skirt base ring support gusset was not included
- Recent model update with solid elements including cut-out and gussets indicate that the cycle operating stresses are low (<30% of endurance limit)
- PT inspections of the deformed areas did not include the dryer ID
- An analysis was not performed to characterize the stress level or dryer loading due to the dryer being hung up
- PT inspection results provided condition assessment that material tensile stresses were not exceeded
37 Preliminary Cause (cont.)
Conclusions
- Load on skirt base ring during impact event induced damage while plastically deforming skirt base ring and skirt plate
- Reduced the fatigue endurance
- Residual stresses from fabrication were also a contributor
- Operating pressure oscillation loads from MSL acoustics resulted in skirt/base ring stresses that when combined with the reduced fatigue endurance was adequate to propagate cracking
38 Repair Strategy Tim Hanley Director Midwest Operations Projects
39 Repair Strategy
- 140o and 220o locations
- Removed a portion of skirt base ring and skirt panel and replaced with plates of the original dimensions
- Cutout size: ~26 by ~40 for the 140o skirt section
- Similar repair was made at the 220o location; however, height is only 12 inches, which removes all deformation that was measured in the dryer skirt
- Restores dryer to as close to original design configuration as possible
- 320o location
- Major portion of the deformed base plate was already being removed at the 320o location as part of the original modification to address the cause of the impact event
- Removing skirt ring gussets at all four locations
- These changes to the dryer have been independently reviewed by a third party
40 Conclusions/Outage Status Randy Gideon Plant Manager
41 Conclusions
- Replacement dryer design is robust
- Cracking that occurred in the skirt and base ring would not have occurred without the impact event
- Remainder of indications are not structurally significant
- Design enhancements and repairs have been analyzed and independently reviewed
- Dryer inspection results demonstrate replacement dryer design is sufficient to accommodate EPU operation