ML021290524
| ML021290524 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 05/07/2002 |
| From: | Sands S NRC/NRR/DLPM/LPD3 |
| To: | |
| References | |
| ML021150460, TAC MB4881 | |
| Download: ML021290524 (46) | |
Text
NRC FORM 658 U.S. NUCLEAR REGULATORY COMMISSION (9-1999)
TRANSMITTAL OF MEETING HANDOUT MATERIALS FOR IMMEDIATE PLACEMENT IN THE PUBLIC DOMAIN This form is to be filled out (typed or hand-printed)by the person who announced the meeting (i.e., the person who issued the meeting notice). The completed form, and the attachedcopy of meeting handout materials, will be sent to the Document Control Desk on the same day of the meeting; under no circumstanceswill this be done later than the working day after the meeting.
Do not include proprietarymaterials.
DATE OF MEETING The attached document(s), which was/were handed out in this meeting, is/are to be placed 05/07/2002 in the public domain as soon as possible. The minutes of the meeting will be issued in the near future. Following are administrative details regarding this meeting:
Docket Number(s) 50-346 Plant/Facility Name DAVIS-BESSE NUCLEAR POWER STATION TAC Number(s) (ifavailable) MB-4881 Reference Meeting Notice ML021150460 Purpose of Meeting (copy from meeting notice) FORTHCOMING MEETING WITH FIRSTENERGY NUCLEAR OPERATING COMPANY ON ROOT CAUSE ANALYSIS REPORT NAME OF PERSON WHO ISSUED MEETING NOTICE TITLE STEPHEN SANDS PROJECT MANAGER OFFICE NRR DIVISION DLPM BRANCH PD 111-2 Distribution of this form and attachments:
Docket File/Central File PUBLIC NRC FORM 658 (9-1999) PRINTED ON RECYCLED PAPER This form was designed using InForms
Root Cause Analysis of the Reactor Pressure Vessel Head Degradationat the Davis-Besse Nuclear Power Station May 7, 2002 1
Agenda Introduction John Wood
- Discovery of RPV Head Degradation Mark McLaughlin
- Root Cause Investigation Steve Loehlein i I:
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- Concluding Remarks John Wood FENOC 2
Root Cause Summary Inadequate inspection of the RPV closure head prevented early detection of nozzle leakage, resulting in prolonged boric acid corrosion and significant degradation.
Discovery of RPV Head Degradation Mark McLaughlin Field ActivPities Team Leader FENOC
RPV Head Configuration
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RPV Head Configuration RV HEAD INSULATION -
SERVICE STRUCTURE -` - RnD4 NOZZLES
%.,I \ Lý IV SUPPORT STEEL 18 ACCESS OPENINGS 2" MIN GAP BETWEEN "MOUSE-HOLES" AT INSULATION AND TOP DAVIS BESSE OF RV HEAD FENOC 6
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C'ontrolRod Drive Nozzle Bolts Typical Contirol Rod Drive Nozzle (Babcock & Wi lcox)
Alloy 600 Nozzle Low-Alloy Steel Reactor Vessel Head FENOC 8
Discovery Steps
- February 16 13 RFO (refueling outage) starts
"*February 24 Visual examination starts
- Restraint on plant restart due to boron
" February 26 on head Ultrasonic (UT) examinations started
- February 27 Flaw found on nozzle 3
- Restraint on plant restart due to flaw on noZzle"I
" March 5 UT examinations completed
- Nozzle 2 & 3 confirmed leak paths and backwall anomaly
" March 8 Nozzle 3 cavity confirmed and reported to NRC FENOC - Initial Root Cause Team formed 9
UT Examination Results Nozzle # Summary of Results 1* 9 Axial Flaws, 2 through-wall (TW) 2* 8 Axial Flaws, 1 Circumferential Flaw, 6 TW 3* 4 Axial Flaws, 2 TW 5* 1 Axial Flaw 46 No Flaw Indication 47 1 Axial Flaw 58 No Recordable Indications
- Heat number M3935 material FENOC 10
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Root Cause Investigation Steve Loehlein Root Cause Investigation Team Leader FENOC 14
Root Cause Investigation Team
- Team included FENOC staff
- Steve Loehlein, (Beaver, Valley),Team Lead - BS, PE
- Chuck Ackerman, (Davis-Besse) - BS
- Ted Lang, (Davis-Besse) - MS, PE
- Todd Pleune, (Davis,-Bese) - PhD
- Neil Morrison, (Beaver Valley) - BS FENOC 15
Root Cause Investigation Team l Team augmented by industry experts from FirstEnergy, Framatome ANP, Dominion Engineering, and EPRI
- Mark Bridavsky, FirstEnergy, Beta Labs Failure Analysis Expert -,PhD
- Stephen Hunt, Dominion Engineering, Corrosion Expert - PE
- Steve Fyfitch, Framatom'i ANP, Metallurgical Expert - MS
- Christine King, EPRI, Material Reliability Program Manager FENOC 16
Key Questions
- Was there a new mechanism that caused this degradation?
- Was there adequate guidance/knowledge available to have prevented the degradations'to the RPV closure head?
17 FENOC
Key Conclusions The degradation to the RPV closure head was caused by Primary Water Stress Corrosion Cracking (PWSCC) of the Control Rod Drive (CRD) nozzle which'led to leaks that were undetected allowing corrosion to occur The existing guidance/knowledge is adequate for understanding how to prevent RPV closure head degradation from any CRD nozzle leaks FENOC 18
Root Cause Analysis
"* Purpose and Scope
"* Root Cause Investigation
- Data Gathering & Analysis
- Timeline of Key Events
- Crack Initiation, Leakage, and Conclusions
- Corrosion Rates,.,
"* Causes FENOC 19
Purpose and Scope of Investigation
- Determine root and contributing causes for RPV closure head degradation experienced at CRD nozzles 2 and 3
- Perform a prompt investigation to provide the stakeholders with potential impact and insights 20 FENOC
Data Gathering
- Relevant data gathered
- Condition Reports
- System Engineer's System Performance Books
- Photographs of degraded areas
- Inspection results of degraded areas
- Plant procedures and.other station documents
- Personnel interviews.
- Reference Documents (NRC, Vendor, INPO, EPRI)
- Videotapes FENOC 21
Data Analysis
"* Data sorted in chronological order to create a Sequence of Relevant Events matrix
"* Timeline of Key Events developed
"* Events and Causal Factors Chart developed FENOC 22
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PWSCC of Alloy 600 Materials
- Alloy 600 materials known to be susceptible to primary water stress corrosion cracking (PWSCC)
- Both wrought and weld (Alloy 82/182 materials)
- Three main factors:
- Susceptible material (composition, heat treatment)
- High tensile stress, (operational and residual)
- Aggressive environment (primary water at high temperature)
FENOC 24
Davis-Besse Control Rod Drive Nozzles
"*Cracked CRD nozzles are Alloy 600 material with Alloy 82/1821J-groove welds Heat treatment of nozzles met code requirements (1600-1700 'F vs >1850 'F)
"*Nozzles 1 through 5 are from heat M3935
"*Heat M3935 has experienced more leaks in B&W plants than other heats
"*High residual tensile- stress present adjacent to J-groove weld
"*Higher operating temperature (605'F vs 601'F)
"*No counterbore on nozzle penetrations
"*Interference fit between nozzle and vessel by design FENOC 25
Conclusions Regarding Identified Cracking Cracking mechanism is PWSCC
- Flaw characteristics found at Davis-Besse are similar to other plants with confirmed PWSCC
- No factors indicating sulfide-induced intergranular stress corrosion cracking (IGSCC) due to chemistry transients
- No other cracking mechanism deemed credible FENOC 26
Estimated Crack Propagation Timeframe
"*Longest through-wall cracks estimated to have initiated in 1990 (+/- 3 years)
"*Control rod drive nozzle thickness is 0.62 inch
"*Estimated time for flaw to propagate through-wall is 4-6 years
"*Consistent with proposed EPRI Material Reliability Program crack growth rate curve FENOC 27
Leakage From Cracked Nozzles
"* Through-wall cracking in nozzle or J-groove weld leads to leaks into annulus region
"* Leakage rate is a function of crack length above J-groove weld and degree of cracking through the weld
"* Leakage rate increases significantly as crack lengthens above the J-groove welddue to increase in crack width
"* Previous industry observations-indicated very low leakage rates FENOC 28
Davis-Besse Leakage Rate from Cracked Nozzle Davis-Besse axial cracks above weld were longer than reported from other plants (1.1 inches for nozzle 2 and 1.2 inches for nozzle 3)
Analytical leakage predictions yield wide range of results (.025 to >1 gpm) depending on method and assumed geometry used**-j Estimated leak rate based on boric acid deposits and unidentified leakage are in the range of 0.04 to 0.2 gpm FENOC 29
Analytically PredictedLeak Rates
ANSYS Model -Head Material Intact 6 ANSYS Model - Head Material Corroded
- - .-.. Zahoor Analytical Model -Davis-Besse Nozzle N-3 10 0.1 E
0.01 0.01 0.0X01 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 Crack Length Above Weld (inches)
Leak Rate versus Crack Length FENOC 31
Leakage Rate Conclusions Estimated leakage rate from nozzle 3 crack is consistent with analytical predictions FENOC 32
Source of Corrosion
"* Degradation at nozzle 2 and 3 is due to boric acid corrosion
"* Boric acid corrosion is a known mechanism capable of producing such significant degradation
"* There is a history of boric acid corrosion incidents on RPV heads in the industry FENOC 33
DegradationSequence Stage 1 - Crack Initiation Progression Stage 2 - Minor Weepage / Latency Period Stage 3 - Deep Annulus Corrosive Attack Stage 4 - GeneralBoric Acid Corrosion 34 FENOC
Stage 1 Crack Initiation Progression
"* Nozzle 3 cracks resulted from PWSCC
"* Cracks grew at rate consistent with industry data
"* RCS leakage miniscule FENOC 35
Stage 2 Minor Weepage/Latency Period
- Fit allowed capillary flow path
- Latency period could involve several mechanisms (e.g.,
steam cutting, galvanic corrosion, crevice corrosion, and flow accelerated corrosion)
"* Annular gap increased due to localized corrosion resulting in leakage flow (residualand dry steam) reaching surface
"* Leak rate controlled by number of cracks and size of cracks (length and width)
FENOC 36
Stage 3 Deep Annulus Corrosive Attack
"* Oxygen penetration in annulus increased due to decreasing velocity and differential pressure in annulus
"* Preferential corrosion occurred in the vicinity of crack (consistent with EPRI-6 test)
"* Exiting steam mass flow from annulus region not sufficient to wet surrounding area.as
"* Nozzle 2 progressed to this stage FENOC 37
Stage 4 General Boric Acid Corrosion Corrosion progression limited by crack growth rate and leakage through crack Annulus flooded with moist steam Boric acid accumulates on head Increased leakage provides localized cooling of head allowing greater wetted area Affected area governed by thermodynamics and material properties (e.g., viscosity, density, slope)
General corrosion of oxygenated surface FENOC 38
CorrosionRates From Industry Testing EPRI and industry testing (effect of boric acid on low alloy steel) demonstrates corrosion rates of 0.6 to 5.0 inches per year
- Test was performed using deaerated, high temperature water (600'F)
- Orientation, geometry and materials simulated RPV head nozzles
- Flow rates of 0.01 and 0.10 gpm used in test FENOC 39
Davis-Besse Estimated Reactor Vessel Closure Head CorrosionRates
- 4 years of stage 4 corrosion
- Maximum radial progression -7 inches
- Average rate -2 inches per year
- Lateral direction corrosion rate -1/2 that of axial direction
- Consistent with EPRI Boric Acid Corrosion Guidebook FENOC 40
Probable Timeline o 1990 (+/- 3yrs) Nozzle 3 cracking initiated
- 1994-1996 Nozzle 3 cracking propagates through-wall o 1998 and 2000 Nqzzle leak not identified
- 2002 Corrosion discovered at nozzle 3, minor degradation at nozzle 2 FENOC 41
Root Cause Summary Inadequate inspection of the RPV closure head prevented early detection of nozzle leakage, resulting in prolonged boric acid corrosion and significant degradation.
42 FENOC
Root Cause Confirmation
- Phases 1 and 2
- Samples contain iron oxide
- Chemical form of boric acid
- Phase 3
- Rule out IGSCC
- Characterization of nozzle 3 cavity FENOC 43
Root Cause Confirmation
"* Sample Phase 1
- Corrosion products/boric acid deposits from top of head
- Deposits scraped from CRD nozzle 3 below the flange
"* Sample Phase 2
- Corrosion products/boric acid deposits from nozzle 2 removal
"* Sample Phase 3
- Nozzle 3 and nozzle 3 corrosion area
- Nozzle 2 FENOC 44
Concluding Remarks FENOC 45