ML022750053
| ML022750053 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 08/29/2002 |
| From: | Sheron B Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| Download: ML022750053 (11) | |
Text
DAVIS-BESSE RPV HEAD CORROSION ACTIVITIES:
STATUS AND FUTURE PLANS ON VHP NOZZLE AND RPV HEAD INSPECTIONS Brian Sheron US Nuclear Regulatory Commission Office of Nuclear Reactor Regulation NRC/INPO Conference August 29, 2002 BACKGROUND ON VHP NOZZLE CRACKING
& RPV HEAD DEGRADATION HISTORY
First cracking of CRDM nozzles identified in France in 1991 - axial cracking
Industry analyses - axial cracking not a safety concern; circ. cracking unlikely
NRC issued Generic Letter 97-01
Spring 2001 Outages -- Circumferential flaws detected (boric acid deposits)
Oconee Units 2 & 3 - 2 nozzles 165 through-wall cracks
Chronology of circumferential cracks
Axial cracks in J-groove welds or HAZ allow leakage into annular region
Leakage to vessel head OD may be restricted by interference fit of nozzles
Circumferential cracks initiate on OD and grow in aggressive environment
NRC issued Bulletin 2001-01 (August 2001) - inspections for circumferential cracks
Spring 2002 Outages - vessel head degradation identified at Davis-Besse
NRC issued Bulletin 2002-01 (March 2002) - no head degradation at other plants
NRC issued Bulletin 2002-02 (August 9, 2002) - inspections to prevent leakage Typical Reactor Vessel Head - Oconee Unit 1 (Babcock & Wilcox)
Schematic View of B&W Design CRDM Nozzle Area FUTURE INSPECTIONS
Management by leakage detection not considered sufficient
Visual examination inspectability
Insufficient Davis-Besse root cause report
Technical specification limit of no pressure boundary leakage
Need further information to support visual examinations alone
On-going MRP activities
Support not sufficiently mature at this time
Lack of understanding about wastage (corrosion) rates
Lack of predictive models for crack growth in welds
Cracking is becoming more prevalent as plants age BULLETIN 2002-02 EXAMPLE INSPECTIONS
High Susceptibility Plants ( > 12 EDY) - ~ 33 plants
UT of nozzle base metal every RFO
Surface examination (eddy current or PT) every RFO
Bare metal visual every RFO
Moderate Susceptibility Plants ( > 8 EDY & <12 EDY) - ~ 15 plants
UT of nozzle base metal at RFO after next RFO and then every other RFO
Surface examination (eddy current or PT) at RFO after next RFO and then every other RFO
Bare metal visual at next RFO and then every other RFO
Low Susceptibility Plants ( < 8 EDY) - ~ 21 plants
UT of nozzle base metal within 5 years and then at least once every 5 years
Surface examination (eddy current or PT) within 5 years and then at least once every 5 years
Bare metal visual within 3 years and then at least once every 5 years PLANNED SUPPLEMENT TO BULLETIN 2002-01
Bulletin 2002-01 issued March 18, 2002
Within 60-days: Describe boric acid corrosion prevention program for ensuring integrity of reactor coolant pressure boundary
Licensee responses inadequate due to:
Lack of specificity regarding BAC prevention programs
Inspection techniques
Scope and frequency of inspections
Degree of insulation removal
Evaluation criteria
Corrective actions
Staff could not make a reasonable assurance finding that the boric programs have been implemented effectively.
OUTLOOK FOR ADDRESSING VHP NOZZLE AND RPV HEAD INSPECTIONS
Review Bulletin 2002-02 responses - status of inspection plans
Pursue regulatory actions for plants with inadequate inspections
Develop technical basis to support inspection programs
Pursue changes to ASME Section XI
Section XI Task Group on Alloy 600/182/82 Issues
Request for formation of task group on changes to inspection requirements
Insulation removal
Inaccessible components
Local corrosion areas
Revise 10 CFR 50.55a as appropriate INDUSTRY NEEDS TO TAKE MORE PROACTIVE ACTIONS TO ADDRESS ISSUES
Industry needs to provide adequate technical justification to reduce necessary reliance on supplemental non-visual examinations
New heads with Inconel 690 tubes will have to follow the same inspection criteria as head with Inconel 600 until industry provides technical basis for changes NRC WEB-SITE INFORMATION Alloy 600 Cracking (including Circumferential Cracking of CRDM Nozzles) http://www.nrc.gov/reactors/operating/ops-experience/alloy600.html RPV Head Degradation http://www.nrc.gov/reactors/operating/ops-experience/vessel-head-degradation.html