ML030860084
| ML030860084 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 03/26/2003 |
| From: | NRC/NRR/DLPM/LPD1 |
| To: | |
| References | |
| EA-03-009, TAC MB7609 | |
| Download: ML030860084 (29) | |
Text
Calvert Cliffs Unit 2 Reactor Vessel Head Penetration Inspections
"* Inspections planned in accordancewith the guidance provided by NRC Bulletin 2002-02 as informed by additionalguidance provided in NRC Bulletins 2002-01 and 2001-01, inspection experience at other US PWRs, inspection vendor experience and inspection production and performance at similar design PWR heads.
"* Qualified Bare metal visual exam showed no evidence of leakage.
"* No leaks = no through wall cracking
"* No leaks = safe operation for an additionalcycle
- Takes time for a through wall crack to facilitate creation of a circumferential crack
- Takes time for a circumferential crack to grow to a size that would threaten structural integrity margin
- No leak = many years of operation prior to hypothetical structural integrity challenge for nozzle
"* Boric acid corrosionalthough aggressive does not threaten structural integrity when identified promptly
- Of all RVHP leaks, all but one resulted in negligible boric acid corrosion
- Single Instance of severe boric acid corrosion occurred after at least four, probably ten, years of active leakage
- No leak = many years of operation prior to hypothetical structural Integrity challenge for nozzle Bottom line: inspections to date demonstrate CCNPP U2 is safe to operate for an additional cycle.
Attachment
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Head Vent 0J /-@\ @ @Jil,065CEDMs Stud Holes (54) 2
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Calvert Cliffs Unit 2 Reactor Vessel Head Penetration UT Inspections
"* CEDM nozzles have thermal sleeves that prevent access with a rotatingprobe
"* Blade probe technology used in Europe modified for CCNPP configuration
"* Time of flight diffraction forward scatter
"* Demonstratedin EPRI demonstration program 5
Calvert Cliffs Unit 2 Reactor Vessel Head ICIs and Vent Line UT Inspections
- Inspection Complete (8 ICIs & 1 Vent Line
- No indicationsin ICIs or vent line e ICI and RV vent line used Rotating probes.
3600 data from bottom of nozzle to 2 inches above nozzle plus leak path assessment for ICls. The vent line does not have interference fit; hence no UT leak path assessment 6
Calvert Cliffs Unit 2 Reactor Vessel Head CEDM UT Inspections PWSCC driven by stress. Maximum possible stress approximately equals yield strength - anything beyond yield is relieved by plastic deformation
"* Lower yield strength results in lower driving force for cracking
"* PWSCC also driven by time at temperature.
- Entire CCNPP U2 head has same susceptibility with respect to temperature
- The entire head consists of two heats. They have similar susceptibility with identicalyield strength, 37.5 ksi. Differences in microstructureare unknown, requiring treatment of the two heats separately.
- We will perform 28 inspections of heat NX7926 and 37 inspections of heat NX9737 to determine neither heat has cracks.
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Calvert Cliffs Unit 2 Reactor Vessel Head CEDM UT Inspections Blade probe has issues with delivery and liftoff
- Nozzle distortion is a product of low yield strength and weld shrinkage
- Distortion causes problems with probe coupling to nozzle preventing local UT analysis
- Distortion prevents access to azimuthal portions of some nozzles
- In some nozzles blade access prevents ability to perform leak path assessment in some azimuths and at some elevations
- Probe has a thickness larger than the gap between the nozzle and the thermal sleeves for azimuthal portions of some nozzles at certain elevations along the nozzle preventing gathering of data beyond approximately 1.5" above top of high hillside 8
Illustration of distortion effects 0
Gap becomes too small. In some cases nozzle and thermal sleeve touch.
10 Calvert Cliffs Unit 2 Reactor Vessel Head Penetration Inspections NRC Order
- Need clarification as to what can be addressed in 60-day report or need relaxation
- Have a second inspection window
- Changing probe designs in hopes of improving coverage
- Based on ICI results, removal of thermal sleeves would improve coverage, although complete coverage would still not be guaranteed.
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Calvert Cliffs Unit 2 Reactor Vessel Head Penetration Inspections Order issued three days before outage start. Need for relaxationidentified for requirementto gather data 2 inches above j-groove weld. Alternative was to remove thermal sleeves. Insufficient time to plan for thermal sleeve removal. Extreme hardship to make change without adequate time to plan for dose reduction. Thermal sleeve removal with manual re-weld takes 130+ Man-rems.
- Additional limitationsin coverage identified during the examination.
Unanticipatedhead configurationissues and blade probe design issues present coverage problems. Full coverage would require change in NDE approach that would take months to appropriatelydevelop and deliver.
Thermal sleeve removal alternativeis dose prohibitive. We have determined, however, that the currently obtainedscope and coverage is sufficient to make an assessment of safety. The additionaldata that would be obtained via dose intensive thermal sleeve removal would not enhance the conclusion of safety.
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Calvert Cliffs Unit 2 Reactor Vessel Head Penetration Inspections Specific deviations from Order
- With thermal sleeve in place it is not possible to achieve coverage for full two inches above top of j-groove weld on high hillside.
Have submitted relaxation request
- Example: All CEDM nozzles have a dimensional change that prevents access for the blade probe at the high hillside portions of the weld.
- With the blade probes it has not always been possible to achieve UT coverage in the nozzle material adjacent to the J-groove weld and below the j-groove weld on every penetration
- Example: For some nozzles no data could be acquired below the weld.
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Clarification of Intent of the Order 3600 Coverage With the blade probe it has not always been possible to achieve 360-degree coverage at all elevations on every nozzle
- Example: For peripheral nozzles the thermal sleeve prevents blade probe access on the low hillside portion of the weld. For most nozzles there is some data missing at some elevations.
UT data collected on every nozzle 14
Clarification of Intent of the Order Assessment of Leakage into Interference Fit Zone
"* The interference fit is not uniform around the circumference of the nozzles. For certain azimuths on certain penetrations the shrink fit is not detectable by UT. It is not possible to assess leakage into the shrink fit region where the shrink fit region does not exist.
"* With the blade probes it is currently not possible to perform UT assessment of leakage into interference fit region for the high hillside azimuths of peripheralRVHPs.
- Example: For peripheral nozzles the thermal sleeve prevents blade probe access on the high hillside portion of the weld. It will not be possible to perform the leakage assessment by UT for these azimuths.
Most of every nozzle gets Successful qualified bare metal leak path assessment by UT visual (BMV) exam provides an Note: Vent Line does not have alternative leakage assessment:
interference fit BMV completed; no leakage Assessment by BMV only I l*tdot,*, 15 UcLt;L;Lclu
Calvert Cliffs Unit 2 Reactor Vessel Head Penetration Inspections
- Specific nozzle discussions
- 28 nozzles of Heat NX7926
- Nozzle 7,8,4,6,10,25,5 inspected so far 16
Coverage @ weld root is defined by boundary between weld region and region above j-groove weld Region above j-groove Weld region Below the weld region 17
Nozzle 7
- Discussion
- Removal of thermal sleeve to permit access with rotating probe would provide no additional safety or quality for this RVHP
- Low susceptibility material
- Much other UT information available for this heat on this head that indicates cracking is not occurring
- Bare metal visual exam indicates a leak does not exist
- Hypothetical nearly through wall crack would not lead to nozzle ejection or severe head corrosion for at least two additional years of operation
- Hypothetical through wall circumferential cracks in nozzle adjacent to weld not a structural integrity concern due to remaining intact nozzle and weld above the hypothetical crack
- For material below j-groove weld the issue is loose parts. Hypothetical "missed" crack would not grow to size over following cycle to create loose parts concern.
Conclusion:
No additionalsafety to be achieved by removal of thermal sleeve 18
Nozzle 8 Discussion is same for nozzle 7 with added discussion of the assessment to determine whether leakage occurredinto the interference fit zone. For a portion of the circumference of this nozzle at the elevation of the interference fit the interference was not sufficiently "tight" to detect. With no detectable interference fit it is not possible to assess whether the interference fit has been degraded.
- With no interference fit leakage would be visible on the vessel head
- Most of the circumference could be evaluated. No leakage
- No indications in RVHP Successful qualified bare metal visual (BMV) exam provides an alternative leakage assessment:
Nozzle 4 BMV completed; no leakage detected
- Discussion is exactly as for nozzle 8 19
Nozzle 6 Discussion same as for nozzles 4 and 8 except
- Weld root coverage was lower. The weld root has no volume, so is not considered as part of the structural integrity assessment. The weld root coverage assessment is used to provide information on the extent of coverage in the area above and adjacent to the weld. A low number for this region indicates some missing data at the extreme lower side of the region above or at the extreme upper side of the region below. Actual cracks have dimensions larger that the dimension of the weld root, so missing data at the weld root does not interfere with the ability to detect cracks in adjacent material.
- The coverage below the weld may be insufficient to facilitate performance of a nozzle specific loose parts analysis. For this nozzle the loose parts assessment will be performed by using information from all the nozzles in this heat. If no cracking is discovered in this heat, then the potential for loose parts due to missing data on this single penetration is small.
Hypothetical weld Nozzle 10 region cracks not structural integrity Discussion concern
- Same as for nozzle 6 except missed a 50 circumferential azimuth above the weld. A fracture mechanics analysis of a hypothetical throughwall circ crack has been performed. A circ crack much larger than 50 would continue to have structural integrity for more than two additional years of operation.
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Nozzle 25
- Discussion
- Same as for nozzle 10 except that the loss of data in the weld root on the RVHP made it difficult to identify how much data was lost at the bottom of the region above the weld. Acceptability of this nozzle relies on examination of the other 26 nozzles from this heat with no cracks or leaks.
Nozzle 5
- Discussion
- Same as for nozzle 25 except a significant circumferential portion of the nozzle could not be inspected. Acceptability of this nozzle relies on the UT information provided by examinations of the other 26 nozzles from this heat.
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Calvert Cliffs Unit 2 Reactor Vessel Head Penetration Inspections e 37 nozzles of Heat NX9737
- Nozzle 2, 3, 9, 12, 1, 33 inspected so far 22
Nozzles 2, 3, 9, and 12 Discussion Nozzle 2 and 3 similar to nozzle 7, 4 and 8 from NX7926.
Even hypothetical through wall cracks in the area of missed coverage would not threaten structural integrity margins Successful qualified bare metal visual (BMV) exam provides an alternative leakage assessment:
BMV completed; no leakage detected 23
Nozzle 1
- Discussion
- The coverage below the weld is insufficient to facilitate performance of a nozzle specific loose parts analysis. For this nozzle the loose parts assessment will be performed by using information from all the nozzles in this heat. If no cracking is discovered in this heat, then the potential for loose parts due to missing data on this single penetration is small
- The loss of data in the weld root made it difficult to identify how much data was lost at the bottom of the region above the weld.
- Acceptability of this nozzle relies on examination of the other 36 nozzles from this heat with no cracks or leaks Nozzle 33
- Discussion
- Same as for nozzle 1 except the shrink fit could not be evaluated due to access problems. Since the BMV indicated no leakage, the available UT data for this nozzle indicates no cracking or leakage; removal of thermal sleeve to perform leak path assessment of this nozzle would provide no additional safety or quality. Acceptability of this nozzle relies on the UT information provided by examinations of the other 36 nozzles from this heat.
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Summary
- Best exams
- 3600 at and above J-groove, good coverage below, leak path assessment
- Analyses can demonstrate structural integrity without additional knowledge about the heat of material
- NX7926 Nozzle 7, 4, 8
- NX9737 Nozzle 2, 3, 9,12
- Other exams
- Coverage at and below j-groove less extensive
- Analyses demonstrating structural integrity need to be supplemented by knowledge that heat is not susceptible. Missed areas covered on other nozzles of same heat.
- NX7926 Nozzle 6,10, 25, and 5
- NX9737 Nozzles 1, and 33 25
Calvert Cliffs Unit 2 Reactor Vessel Head Penetration Inspections
- Plans for next inspection window
- Improve probe to increase coverage Complete examination of remaining nozzles 26
Calvert Cliffs Unit 2 Reactor Vessel Head Penetration Inspections Conclusions
-U2 Head does not have leaks UT information so far indicates no cracking Materials not expected to be susceptible to cracking No safety issue with an additional cycle of operation 27
Calvert Cliffs Unit 2 Reactor Vessel Head Penetration Inspections
- Orderissues
- One relaxation requested
- Issues needing further clarification
- 3600 coverage on each penetration
- Above weld Address these issues in
- Root 60 day report (with
-Weld appropriate safety
- Below weld justifications)
- Leakage assessment
- Leakage assessment each nozzle 28
CCNPP Conclusions with Respect to Order
= Expect remaining exams will show no cracking or degradationand achieve comparableor better coverage
- Compliance assured through combination of inspection, relaxation, and documented clarification
- For issues needing further clarification we propose addressingthem in 60-day report with appropriatesafety justification
- Exams that have been completed are acceptable consistent with above 29