Post on 13-Mar-2022
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Regulatory Activities on Kuosheng Anchor Bolt Issue
Department of Nuclear RegulationAtomic Energy Council, Taiwan
July 1, 2014
2014 AEC-NRC Bilateral Technical Meeting
SkirtSkirt
PedestalPedestal
RPV Support Skirt Anchor Bolts
Inner Bolts(60)Inner Bolts(60) Outer Bolts(60)Outer Bolts(60)
ASTM A-540 Gr. B23 Class 1 ,φ:3 inchL :26 inch
ASTM A-540 Gr. B23 Class 1 ,φ:3 inchL :26 inch
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Background The function of the anchor bolts is to
connect RPV support skirt flange to the pedestal mounting plate. Each bolt was designed with pretension .
One broken outer bolt was found at Kuosheng Unit 2 in October 2011 during scheduled inspection per ISI program.
One broken inner bolt was found to be physically broken . Additional two failures bolts and another four bolts with crack indication were found (by UT) at KuoshengUnit 1 in March 2012 .
0
5
A1 D15 D14D13
D12D11
D10D9
D8D7
D6
D5
D4
D3
D2
D1
A2A4A5A6
A7A8
A9A10
A11
A3
A12
A13A14
A15
B1
B2
B3
B4
B5
B6B7
B8B9
B10B11
B12B13
B14 B15 C1 C2 C3 C4 C5C6
C7C8
C9C10
C11C12
C13
C14
C15
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90270
180
PLAN VIEW FROM ABOVE Failed Bolt(C6)Failed Bolt(C6)
Failed Bolt(D14)Failed Bolt(D14)Physically Broken(A2)Physically Broken(A2)
Indication(C9,D11)
Indication(C9,D11)
Indication(B10,B13)Indication(B10,B13)
UNIT #1UNIT #1
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UNIT #2UNIT #2
A1 D15 D14D13
D12D11
D10D9
D8D7
D6
D5
D4
D3
D2
D1
A2A4A5
A6A7
A8A9
A10A11
A3
A12
A13A14
A15
B1
B2
B3
B4
B5
B6
B7B8
B9B10
B11B12
B13B14 B15 C1 C2 C3 C4 C5
C6C7
C8C9
C10C11
C12
C13
C14
C15
0
90270
180
PLAN VIEW FROM ABOVE
Physically Broken(B1)Physically Broken(B1)
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AEC established a review task force team of experts and AEC staffs to conduct comprehensive technical review and site inspection on Kuosheng anchor bolt issue.
Review areas Root Cause Analysis Corrective Actions Justification for Continued Operation Future monitoring program
Survey on other NPPs
Regulatory Actions
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The root cause of crack initiation:SCC
Improper installation tool :
Concentrated stress or high residual stress
Sensitive material
Corrosive environment during construction
Initial Crack (SCC)
Root Cause Analysis 1/2
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For the bolts with larger initial cracks, cracks propagated further due to fatigue and then fatigue cracks grew progressively until failure.
For the cracks with stress intensity factor (△K) below the threshold (△Kth ) , cracks growing tends to stop.
Normal operating load
Root Cause Analysis 2/2
Initial Crack(SCC)
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Unit 1, 7 damaged bolts have been replaced with new ones
Unit 2, anchor function of one damaged bolt has been recovered by specially designed Engineered Anchoring System (EAS)
Corrective Actions 1/3
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Preload tests for the remaining bolts were conducted to ensure they are still maintained in designed condition .
UT examination has been performed on all bolts to verify the structural integrity and will be conducted during following refueling outage thereafter
Corrective Actions 2/3
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4 sets of accelerometer (1V+1H)are installed in 4 quadrant of pedestal area to monitor vibration condition.
Corrective Actions 3/30
270
A1D15D14D13D12D11
D10D9
D8D7
D6D5D4D3D2D1
A2A4A5A6A7
A8A9
A10A11
A3
A12A13
A14A15B1B2B3B4B5
B6B7
B8B9B10
B11B12B13B14B15C1 C2 C3 C4 C5 C6
C7C8C9
C10C11
C12C13C14
C15
90
180
270
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Justification for Continued Operation
A Justification for Continued Operation (JCO) for one cycle with 2.5mm crack indication assumption on all bolts was proposed by Taipower and reviewed by AEC for approval. fracture mechanic & fatigue analysis
Result of evaluation demonstrated that the bolt condition with 2.5mm crack assumption would not raise safety concern or introduce operational problems .
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A continuous monitoring program with 4 sets of vibration monitors requested by AEC has been implemented for early detection of possible vibration.
Taipower also has been requested to establish a special procedure to monitor and record the vibration of the reactor pressure vessel/pedestal. Alarm when any signal >0.1g Notify AEC If any signal >0.1g Unit shall shutdown and conduct VT-3 on all
bolts when 2 or more vibration monitors show >0.1g and <50Hz.
Future monitoring program 1/2
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To ensure the structure integrity of anchor bolts, all 120 anchor bolts for both units shall be inspected by UT examination of subsequent refueling outages of both units.
Establishing a stress monitoring programto monitor if any abnormal loading exist. 103 strain gauges (with temperature element)
were mounted on RPV skirt to monitor the possible loading during refueling outages of unit 1 in Dec. 2013
Future monitoring program 2/2
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Survey on other NPPs Request Taipower to verify the
RPV anchor/support condition of Chinshan & Maansan NPPs
For Chinshan NPPs (BWR-4) Different design , RPV skirt anchor
bolts w/o pretension Perform UT of all bolts for both
units, no abnormal indication found .
For Maansan NPPs(PWR-WH) Different design , no anchor bolts. Visually check RPV cold/hot leg
support shoe & pads, no abnormal condition found.
Chinshan
Maansan
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Current Status 1/2
UT performed during refueling outages of unit 2 in March 2013 and unit 1 in December 2013 showed no abnormal indication.
Results of preload check on 7 replaced bolts of unit 1 after one cycle in December 2013 meet the design requirements.
Vibration monitors : no abnormal ~0.02~0.08g when Rx mode switch turn to
“shutdown” ~0.008g during SRV opening test < 0.005g during normal operation
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Current Status 2/2
Stress monitoring system ( with strain gauges) installed on unit 1 RPV skirt was put in service in January 1, 2014 . System totally failed due to signal loss in January 8,
2014. The cause of failure is not clear. Finite element analysis on the data collected during unit
restart before system failed revealed no abnormal loading .
Stress monitoring system will be installed on Unit 2 during refueling outages in Sep. 2014.
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Conclusion and Remarks Based on the results of review on root cause analysis,
corrective actions taken , evaluation on JCO and proposed monitoring program , AEC approved unit restart application for one cycle operation.
The data of vibration monitoring and UT performed during subsequent refueling outage shows no abnormality .
The cause of unit 1 stress monitoring system failure needs further investigation to prolong its availability.
AEC will continue to oversee the implementation of monitoring program on Kuosheng RPV anchor bolts.