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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

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Outline

Background

Regulatory Actions

Current Status

Conclusion and Remarks

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 .

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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

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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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EAS Bolt and Original bolt-UNIT 2

EAS Bolt Original bolt

Tapping

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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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Arrangement of stress monitoring

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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.

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Thank youThank you