ML15113A570
| ML15113A570 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 11/17/1998 |
| From: | Essig T NRC (Affiliation Not Assigned) |
| To: | Foster W BABCOCK & WILCOX CO. |
| References | |
| PROJECT-693 TAC-M99020, NUDOCS 9811250109 | |
| Download: ML15113A570 (22) | |
Text
UNITED STATES 0
NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 November 17, 1998 Mr. W. W. Foster, Chairman B&WOG Steering Committee Director of Safety Assurance Oconee Nuclear Station PO Box 1439 Seneca, SC 29679
SUBJECT:
STAFF ASSESSMENT OF THE BABCOCK & WILCOX OWNERS GROUP SUBMITTAL RELATING TO CRACKING IN UNISOLABLE SECTIONS OF MAKE UP/HIGH PRESSURE INJECTION LINES AT OCONEE UNITS 1, 2 AND 3 AND OTHER BABCOCK AND WILCOX PLANTS (TAC NO. M99020)
Reference:
Letter from W. W. Foster, DPC, to the Document Control Desk with enclosed B&WOG Report "Interim B&W Owners Group Report on HPI/MU Nozzle Cracking" (FTI Document No. 51-5000239-00), dated June 5, 1997
Dear Mr. Foster:
On April 22, 1997, Oconee Unit 2 was shut down because of unidentified reactor coolant system (RCS) leakage exceeding the technical specification limit by as much as 12 gpm. The licensee, Duke Power Company (DPC) subsequently identified an unisolable leak in the Make Up/High Pressure Injection (MU/HPI) line 2A1, resulting from a through-wall crack in the weld connecting the MU/HPI pipe and the safe-end of the 2A1 reactor coolant loop (RCL) nozzle.
The thermal sleeve in this nozzle was also found to be loose and cracked. DPC also performed inspections of the MU,!,PI lines in Oconee Units 1 and 3. No flaws were found in Unit 1 nozzles and sleeves. However, partial-wall flaws were found at the same location in nozzle 3A1 in Unit 3. Likewise, the thermal sleeve in this nozzle was also loose and cracked.
As a result of the findings at Oconee Units 2 and 3, the Babcock and Wilcox Owners Group (B&WOG) performed inspections of the MU/HPI nozzles in the following plants: Crystal River Unit 3, Davis Besse Unit 1, Arkansas Nuclear One Unit 1 and Three Mile Island Unit 1. No cracks were detected in any nozzles in these units.
B&WOG submitted an interim report, referenced above, describing the event at Oconee and the inspections performed at Oconee and the other B&WOG plants. The Mechanical Engineering Branch has completed its review of the interim report and their safety assessment is enclosed.
9811250109 981117 PDR ADOCK 05000269 P
Mr. W.
November 17, 1998 If you have any questions regarding this matter please contact Mr. Joseph Birmingham of my staff by phone, 301/415-2829, or email, jIb4@nrc.gov.
Sincerely, Thomas H. Essig, Acting Chief Generic Issues and Environmental Projects Branch Division of Reactor Program Management Office of Nuclear Reactor Regulation Project No. 693
Enclosure:
As stated cc:
Mr. M. Schoppman, Manager Rockville Licensing Operations Framatome Technologies, Inc.
1700 Rockville Pike, Suite 525 Rockville, MD 20852-1631
Mr. W.
November 17, 1998 If you have any questions regarding this matter please contact Mr. Joseph Birmingham of my staff by phone, 301/415-2829, or email, jlb4@nrc.gov.
Sincerely, Thomas H. Essig, Acting Chief Generic Issues and Environmental Projects Branch Division of Reactor Program Management Office of Nuclear Reactor Regulation Project No. 693
Enclosure:
As stated cc:
Mr. M. Schoppman, Manager Rockville Licensing Operations Framatome Technologies, Inc.
1700 Rockville Pike, Suite 525 Rockville, MD 20852-1631 DISTRIBUTION: Safety Assessment for B&W HPI concern, TAC M99019
,Hard Copy E-Mail vCehtral File G. Lainas T. Essig PUBLIC J. Strosnider F. Akstulewicz PGEB r/f E. Sullivan J. Birmingham SRXB r/f K. Wichman B. Elliot OGC J. Roe D. LaBarge ACRS M. Hartzman DOCUMENT NAME: G:\\JLB\\B&W-HPIS.SA OFFICE PM/PGEB:DRPM SC/P
-DRPM AC/PGEB; PM NAME JBirmingham:sw F
c TEssig
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DATE It /t'/98 W
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STAFF ASSESSMENT BY THE OFFICE OF NUCLEAR REACTOR REGULATION MECHANICAL ENGINEERING BRANCH DIVISION OF ENGINEERING BABCOCK AND WILCOX OWNERS GROUP RESPONSE TO CRACKING OF UNISOLABLE SECTIONS OF MAKE UP/HIGH PRESSURE INJECTION LINES AT OCONEE UNITS 1, 2, AND 3
1.0 BACKGROUND
On April 22, 1997, Oconee Unit 2 was shut down because of unidentified reactor coolant system (RCS) leakage. The leakage was subsequently identified as an unisolable leak in the make up/high pressure injection (MU/HPI) line 2A1, resulting from a through-wall crack in the weld connecting the MU/HPI pipe and the safe-end of the 2A1 reactor coolant loop (RCL) nozzle.
Subsequent to this event, the NRC staff requested that the MU/HPI nozzles in all B&WOG plants be assessed or examined for similar cracks. In Reference 1, B&WOG submitted a report describing the inspection results, inspection histories and details of the thermal sleeves in the B&WOG plants with the exception of the Oconee plants. However, the Oconee licensee, Duke Power Company (DPC), submitted separately a Licensee Event Report (Reference 2) describing in detail the circumstances of the failure at Oconee Unit 2.
2.0 EVENT DESCRIPTION 2.1 Oconee Unit 2 On April 22, 1997, at 12:50 p.m., Oconee Unit 2 was shut down because of unidentified RCS leakage exceeding the technical specification limit of 1 gpm. From the time of initial leak indications on April 21 1997, at approximately 10:45 p.m., until reactor pressure was sufficiently reduced, the leakage rate rose from approximately 2 gpm to a maximum leakage rate of approximately 12 gpm. The leakage was subsequently identified as an unisolable leak in the MU/HPI line 2A1, resulting from a through-wall crack in the weld connecting the MU/HPI pipe and the safe-end of the 2A1 RCL nozzle.
The Oconee 2A1 MU/HPI nozzle assembly consists of the MU/HPI 2%"-diameter pipe/safe-end/thermal sleeve (see Figure 1 - Original Design). The sleeve is attached by contact rolling to the inner surface of the safe-end. A 1 "-diameter "warming" line taps into the bottom of the MU/HPI pipe immediately upstream of the pipe/safe-end weld.
In Reference 3, DPC submitted the results of a metallurgical examination of the weld. DPC determined that the crack consisted of a 3600 inside surface flaw. The flaw depth increased gradually from about 30 percent into the wall until it became through-wall over a 770 arc length (see Figure 2). The examination also found a gap in the contact area between the thermal sleeve and the safe end, indicative of loss of contact that caused the thermal sleeve in this line Enclosure
to loosen (see Figure 1). The thermal sleeve was found to be cracked, with portions missing from the end that extends into the RCS flow path. Significant wear damage was observed at both the upstream (the rolled end) and the downstream end. Small cracks were also found in the pipe in the vicinity of the "warming" line nozzle.
DPC also performed video examinations of the other thermal sleeves of the Unit 2 HPI system.
No evidence of damage was found. Ultrasonic Testing (UT) and Radiographic Testing (RT) of the welds and the thermal sleeves in the other HPI nozzles showed no indications of cracking or loosening, or other signs of degradation.
Although the cause of the cracking is not well understood, DPC has identified a number of thermal/mechanical conditions that may have contributed to the through-wall cracking of the 2A1 pipe to safe-end weld. The precise contribution to cracking of each of these conditions is not presently known. However, DPC has hypothesized that, in addition to the thermal cycling experienced at the nozzle during heat up/cool down and other plant transients, a likely contributor to the fatigue may have been the alternate heating and cooling of the weld by intermittent mixing of the hot reactor coolant turbulent penetration, the water flowing through the warming line, and the cooler normal makeup water flowing through the associated MU/HPI line.
This had the effect of loosening the sleeve sufficiently to develop a gap in the contact area between the thermal sleeve and the safe end. Once the gap developed, additional reactor coolant in-leakage is thought to have occurred through the gap. The growth of the gap may also have been influenced by flow-induced vibration of the sleeve due to vortex shedding in the reactor coolant flow. Although the precise contribution of the gap to the thermal cycling is unclear, it is.believed that it accelerated the crack propagation rate in the piping.
This phenomenon was identified as the probable cause for similar safe-end cracking observed at Crystal River Unit 3 and other B&W plants (including Oconee) in the early 1980's. This issue was previously addressed in Information Notice (IN) 82-09 (Reference 4) and Generic Letter (GL) 85-20 (Reference 5).
A reexamination of radiographs made in April 1996 of the Oconee 2A1 nozzle revealed that DPC had failed to identify the gap which had developed in the safe-end/thermal sleeve contact area. DPC also had failed to follow all the original recommendations in NRC GL 85-20 for augmented ultrasonic testing (UT). DPC had performed the recommended UT of the safe ends of the MU/HPI lines; however, they did not inspect the adjacent piping as recommended. In addition, DPC failed to UT the weld between the safe-end and adjacent pipe, a discontinuity where cracking could be expected to form.
2.2 Oconee Unit 3 DPC also reviewed the 1996 radiographs of the safe-ends in Oconee Unit 3. The 3A1 MU/HPI line was found to have a gap in the safe-end/thermal sleeve contact area. As a result of the gap in the 3A1 safe-end, Oconee Unit 3 was shut down on May 2, 1997. UT examinations identified apparent cracking in the 3A1 safe-end. This safe-end was removed and metallurgically examined. A visual examination also revealed cracks in the thermal sleeve.
Minor gaps in the other safe-end/thermal sleeve contact areas were determined not to have grown, the rolled area of the thermal sleeve was acceptable, and UT examinations of the other Oconee Unit 3 HPI nozzle assemblies revealed no cracking.
2.3 Oconee Unit 1 The Oconee Unit 1 nozzles have a double thermal sleeve design (Figure 3). Radiographic inspection in the period from 1983 to 1989 indicated that no gap existed in three of the four thermal sleeves. The thermal sleeve in the 1B2 (HPI) line had a gap, which had not grown during the inspection period. Advantages of the double thermal sleeve as stated by DPC include (1) greater stiffness, (2) greater thermal resistance, and (3) reduced flow area, with corresponding increased flow velocity.
2.4 Compensatory Actions As corrective action, DPC replaced the cracked safe-ends and the safe-end/pipe welds in Oconee Units 2 and 3. The thermal sleeves were also replaced with an improved sleeve design, also shown in Figure 1. In addition, DPC also assembled a Failure Investigation Process (FIP) team to determine the root causes of the event in Unit 2. The team concluded that DPC had failed to implement an effective HPI nozzle inspection program based on available industry recommendations, and had failed to effectively evaluate known problems (industry and in-house experience) and implement appropriate corrective actions (from 1982 through the present). The FIP team published its findings in Reference 6. The staff has examined this report and finds it thorough and complete.
DPC also submitted a revision of the Oconee Units 1, 2, and 3 third interval ten-year Inservice Inspection (151) Program. This revision commits to meet or exceed the requirements in GL 85-20, previously committed to, regarding examination of HPI system components. The staff reviewed the submittal and determined that the proposed 151 program is acceptable, with the condition that all future ultrasonic examinations should be performed based on the 1992 Edition, with the 1993 Addenda, of the ASME Code Section XI, Appendix VIII requirements, or be based on calibration blocks with cracks that simulate the type of crack observed in Oconee Unit 2. DPC also agreed to submit the fourth interval ten-year ISI program for all three Oconee units six months prior to the start of this interval. The staff reviewed the proposed program and found it acceptable, subject to the condition stated above (Reference 7).
Other measures taken by DPC include the temporary installation of thermocouples in Oconee Units 2 and 3, to measure temperature time histories in the affected zones, and increasing the minimum make-up flow in these units. The make-up flow in Unit 1 was already higher than in the other two units.
2.5 Other Babcock and Wilcox Owners Group (B&WOG) Plants Subsequent to the event at Oconee Unit 2, other B&WOG plants also performed evaluations and inspections of the MU/HPI nozzles and attached piping. These were Davis-Besse Unit 1, Crystal River Unit 3, Arkansas Nuclear One Unit 1 and Three Mile Island Unit 1.
In response to a staff request for information dated May 27, 1997 (Reference 8), B&WOG submitted an interim report (Reference 1) on the cracking found in Oconee Unit 2, and the results of the inspections of the other B&WOG plants. No indications were reported in any nozzles of these systems. The staff has reviewed this report and finds it generally acceptable.
The interim report did not attempt to determine the root cause of the cracking. As part of the generic investigation of this event, B&WOG committed, through Framatome, to conduct a more detailed evaluation of the root cause of the cracking at Oconee Units 2 and 3. This evaluation will be based on analytical studies using computational fluid dynamics methodology. A final report describing the results of these btudies is scheduled to be submitted sometime in 2000.
3.0 STAFF ACTIVITIES On June 6, 1997, the staff of Region II completed a special inspection of Oconee, which focused on the integrated efforts by DPC to investigate the causes of the event and the corrective actions undertaken. The staff concluded that there had been a failure by DPC to adequately implement augmented inspections to detect cracks in HPI lines, and to correct known problems on a timely basis. The staff published its findings in a special inspection report dated June 27, 1997 (Reference 9).
On July 9, 1997, the staff published NRC IN 97-46, "Cracking in High-Pressure Injection Piping,"
to alert the industry of the circumstances of this event (Reference 10).
During the review of the inspections performed by DPC, the staff became aware that ASME Section XI requires no volumetric (ultrasonic) examination of Class 1 piping in the range of 1" to 4" nominal pipe size (NPS). This is inconsistent with the requirement for Class 2 piping, which does specify volumetric examination for piping in this size range. To eliminate this inconsistency in the Code, the staff has requested that the ASME include volumetric examination of Class 1 piping in this size range. However, because publication of revised Code requirements cannot be expected in the near future, and considering the safety significance of the HPI system, the staff has included provisions for this examination in currently proposed rule-making on limitations to the NRC endorsement of the ASME Code under 10 CFR 50.55(a). The rulemaking package went out for public comment 1998.
4.0 CONCLUSION
DPC has replaced the cracked safe-end welds in Oconee Units 2 and 3, and has also installed improved thermal sleeves. DPC has also submitted an ISI plan, which the staff finds acceptable. DPC has also adopted other compensatory measures, such as increasing the minimum make-up flow and installing thermocouples for monitoring the temperature. The staff finds these measures acceptable.
Based on its evaluation of the B&WOG and DPC responses to various staff requests for information, and meetings between the staff and DPC, the staff concludes that B&WOG has satisfactorily and reasonably addressed potential cracking of the safe-ends in the HPI/MU nozzles in all B&WOG plants.
Attachments:
- 1. Figure 1 - Units 2 and 3 Thermal Sleeves
- 2. Figure 2 - Warming Line Flow and Crack Orientation
- 3. Figure 3 - Unit 1 Thermal Sleeve
5.0 REFERENCES
- 1.
Letter from W. W. Foster, DPC, to the Document Control Desk with enclosed B&WOG Report "Interim B&W Owners Group Report on HPI/MU Nozzle Cracking" (FTI Document No. 51-5000239-00), dated June 5, 1997
- 2.
Oconee Nuclear Station, Unit Two, Licensee Event Report dated May, 21, 1997
- 3.
Metallurgical Analysis Report #2181, "ONS 2 & 3 HPI/MU Nozzle Components," Duke Power Company, June 30, 1997
- 4.
NRC Information Notice 82-09, "Cracking in Piping of Makeup Coolant Lines at B&W Plants," dated March 31, 1982
- 5.
NRC Generic Letter 85-20, "Resolution of Generic Issue 69: High Pressure Injection/Make Up Nozzle Cracking in Babcock and Wilcox Plants," dated November 11, 1985
- 6.
Oconee Nuclear Station, Unit 2, Failure Investigation Process Final Report, "Pipe Weld Failure, 4/21/97, PIP 2-097-1324," dated July 24, 1997
- 7.
Letter of October 23, 1997, from D. E. LaBarge, NRC, to W. R. McCollum, DPC, "High Pressure Injection System Augmented Inservice Inspection Program - Oconee Nuclear Station Units 1, 2 and 3"
- 8.
Letter of May 27, 1997, from J. L. Birmingham, NRC, to W. W. Foster, Chairman, B&WOG Steering Committee
- 9.
Letter of June 27, 1997, from J. P. Jaudons, NRC, to J. W. Hampton, DPC, with enclosed NRC Special Inspection Report
- 10.
NRC Information Notice 97-46, "Unisolable Crack in High Pressure Injection Piping,"
dated July 9, 1997 SAFE END TO NOZZLE WELD MAIN (81-METALIC)NOZZLE(C.S.)
COOLANT PIPE CLADDING PP SAFE END TO SAFE END PIPE WELD (S.S.)
z
-- LOCK VALVE
- U)
L
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" 6000aS. S. SW HALF COUPLI NG oC PIPEAL CD l[S.S.WARMING(COOLING) LINE PIPE FIGURE 1 THERMAL SLEEVE 2A1, 2B1, 3A1, 381, 3B2
OD crack tip @ 3440 (100%TW)
OD crack 330' 30 ti M R IO (100% TW) 3000."6 270' Metalography (30% TW)
Metallography (26.5% TW) 240*
120*
210*
10 Metaliography10 (24% TW)
Nato View Is looking In direction of M/U flow Warming Line Flow FIGURE 2
SAFE END TO NOZZL WELD MAIN BI-ME TTAL IC)
NOZZLE(C.S.)
COOLANT PIPE CLADDING SAFE ENETO SAFE END PIPE WED (S.S.)7
--BLOCK VALVE THERMAL SLEEVE FLOW THERMAL SLEEVE WELD BUTTONS 2
S. S.
1"6000*S.S.SW HALF COUPLING PIPEI
- "--S.S.WARMING(COOLING)
LINE FIGURE 3 -
UNIT 1 THERMAL SLEEVE
o UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 November 17, 1998 Mr. W. W. Foster, Chairman B&WOG Steering Committee Director of Safety Assurance Oconee Nuclear Station PO Box 1439 Seneca, SC 29679
SUBJECT:
STAFF ASSESSMENT OF THE BABCOCK & WILCOX OWNERS GROUP SUBMITTAL RELATING TO CRACKING IN UNISOLABLE SECTIONS OF MAKE UP/HIGH PRESSURE INJECTION LINES AT OCONEE UNITS 1, 2 AND 3 AND OTHER BABCOCK AND WILCOX PLANTS (TAC NO. M99020)
Reference:
Letter from W. W. Foster, DPC, to the Document Control Desk with enclosed B&WOG Report "Interim B&W Owners Group Report on HPI/MU Nozzle Cracking" (FTI Document No. 51-5000239-00), dated June 5, 1997
Dear Mr. Foster:
On April 22, 1997, Oconee Unit 2 was shut down because of unidentified reactor coolant system (RCS) leakage exceeding the technical specification limit by as much as 12 gpm. The licensee, Duke Power Company (DPC) subsequently identified an unisolable leak in the Make Up/High Pressure Injection (MU/HPI) line 2A1, resulting from a through-wall crack in the weld connecting the MU/HPI pipe and the safe-end of the 2A1 reactor coolant loop (RCL) nozzle.
The thermal sleeve in this nozzle was also found to be loose and cracked. DPC also performed inspections of the MU/HPI lines in Oconee Units 1 and 3. No flaws were found in Unit 1 nozzles and sleeves. However, partial-wall flaws were found at the same location in nozzle 3A1 in Unit 3. Likewise, the thermal sleeve in this nozzle was also loose and cracked.
As a result of the findings at Oconee Units 2 and 3, the Babcock and Wilcox Owners Group (B&WOG) performed inspections of the MU/HPI nozzles in the following plants: Crystal River Unit 3, Davis Besse Unit 1, Arkansas Nuclear One Unit 1 and Three Mile Island Unit 1. No cracks were detected in any nozzles in these units.
B&WOG submitted an interim report, referenced above, describing the event at Oconee and the inspections performed at Oconee and the other B&WOG plants. The Mechanical Engineering Branch has completed its review of the interim report and their safety assessment is enclosed.
Mr. W.
November 17, 1998 If you have any questions regarding this matter please contact Mr. Joseph Birmingham of my staff by phone, 301/415-2829, or email, jlb4@nrc.gov.
Sincerely, Thomas H. Essig, Acting Chief Generic Issues and Environmental Projects Branch Division of Reactor Program Management Office of Nuclear Reactor Regulation Project No. 693
Enclosure:
As stated cc:
Mr. M. Schoppman, Manager Rockville Licensing Operations Framatome Technologies, Inc.
1700 Rockville Pike, Suite 525 Rockville, MD 20852-1631
Mr. W.
November 17, 1998 If you have any questions regarding this matter please contact Mr. Joseph Birmingham of my staff by phone, 301/415-2829, or email, jlb4@nrc.gov.
Sincerely, Thomas H. Essig, Acting Chief Generic Issues and Environmental Projects Branch Division of Reactor Program Management Office of Nuclear Reactor Regulation Project No. 693
Enclosure:
As stated cc:
Mr. M. Schoppman, Manager Rockville Licensing Operations Framatome Technologies, Inc.
1700 Rockville Pike,. Suite 525 Rockville, MD 20852-1631 DISTRIBUTION: Safety Assessment for B&W HPI concern, TAC M99019 Hard Copy E-Mail
/Central File G. Lainas T. Essig v PUBLIC J. Strosnider F. Akstulewicz PGEB r/f E. Sullivan J. Birmingham SRXB r/f K. Wichman B. Elliot OGC J. Roe D. LaBarge ACRS M. Hartzman DOCUMENT NAME: G:\\JLB\\B&W-HPIS.SA OFFICE PM/PGEB:DRPM SC/Pg fDRPM AC/PGEB:DRPM NAME JBirmingham:sw F
c4' TEssig DATE It /14/98
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1/98 0/\\ 98
STAFF ASSESSMENT BY THE OFFICE OF NUCLEAR REACTOR REGULATION MECHANICAL ENGINEERING BRANCH DIVISION OF ENGINEERING BABCOCK AND WILCOX OWNERS GROUP RESPONSE TO CRACKING OF UNISOLABLE SECTIONS OF MAKE UP/HIGH PRESSURE INJECTION LINES AT OCONEE UNITS 1, 2, AND 3
1.0 BACKGROUND
On April 22, 1997, Oconee Unit 2 was shut down because of unidentified reactor coolant system (RCS) leakage. The leakage was subsequently identified as an unisolable leak in the make up/high pressure injection (MU/HPI) line 2A1, resulting from a through-wall crack in the weld connecting the MU/HPI pipe and the safe-end of the 2A1 reactor coolant loop (RCL) nozzle.
Subsequent to this event, the NRC staff requested that the MU/HPI nozzles in all B&WOG plants be assessed or examined for similar cracks. In Reference 1, B&WOG submitted a report describing the inspection results, inspection histories and details of the thermal sleeves in the B&WOG plants with the exception of the Oconee plants. However, the Oconee licensee, Duke Power Company (DPC), submitted separately a Licensee Event Report (Reference 2) describing in detail the circumstances of the failure at Oconee Unit 2.
2.0 EVENT DESCRIPTION 2.1 Oconee Unit 2 On April 22, 1997, at 12:50 p.m., Oconee Unit 2 was shut down because of unidentified RCS leakage exceeding the technical specification limit of 1 gpm. From the time of initial leak indications on April 21, 1997, at approximately 10:45 p.m., until reactor pressure was sufficiently reduced, the leakage rate rose from approximately 2 gpm to a maximum leakage rate of approximately 12 gpm. The leakage was subsequently identified as an unisolable leak in the MU/HPI line 2A1, resulting from a through-wall crack in the weld connecting the MU/HPI pipe and the safe-end of the 2A1 RCL nozzle.
The Oconee 2A1 MU/HPI nozzle assembly consists of the MU/HPI 22"-diameter pipe/safe-end/thermal sleeve (see Figure 1 - Original Design). The sleeve is attached by contact rolling to the inner surface of the safe-end. A 1 "-diameter "warming" line taps into the bottom of the MU/HPI pipe immediately upstream of the pipe/safe-end weld.
In Reference 3, DPC submitted the results of a metallurgical examination of the weld. DPC determined that the crack consisted of a 3600 inside surface flaw. The flaw depth increased gradually from about 30 percent into the wall until it became through-wall over a 770 arc length (see Figure 2). The examination also found a gap in the contact area between the thermal sleeve and the safe end, indicative of loss of contact that caused the thermal sleeve in this line Enclosure
to loosen (see Figure 1). The thermal sleeve was found to be cracked, with portions missing from the end that extends into the RCS flow path. Significant wear damage was observed at both the upstream (the rolled end) and the downstream end. Small cracks were also found in the pipe in the vicinity of the "warming" line nozzle.
DPC also performed video examinations of the other thermal sleeves of the Unit 2 HPI system.
No evidence of damage was found. Ultrasonic Testing (UT) and Radiographic Testing (RT) of the welds and the thermal sleeves in the other HPI nozzles showed no indications of cracking or loosening, or other signs of degradation.
Although the cause of the cracking is not well understood, DPC has identified a number of thermal/mechanical conditions that may have contributed to the through-wall cracking of the 2A1 pipe to safe-end weld. The precise contribution to cracking of each of these conditions is not presently known. However, DPC has hypothesized that, in addition to the thermal cycling experienced at the nozzle during heat up/cool down and other plant transients, a likely contributor to the fatigue may have been the alternate heating and cooling of the weld by intermittent mixing of the hot reactor coolant turbulent penetration, the water flowing through the warming line, and the cooler normal makeup water flowing through the associated MU/HPI line.
This had the effect of loosening the sleeve sufficiently to develop a gap in the contact area between the thermal sleeve and the safe end. Once the gap developed, additional reactor coolant in-leakage is thought to have occurred through the gap. The growth of the gap may also have been influenced by flow-induced vibration of the sleeve due to vortex shedding in the reactor coolant flow. Although the precise contribution of the gap to the thermal cycling is unclear, it is believed that it accelerated the crack propagation rate in the piping.
This phenomenon was identified as the probable cause for similar safe-end cracking observed at Crystal River Unit 3 and other B&W plants (including Oconee) in the early 1980's. This issue was previously addressed in Information Notice (IN) 82-09 (Reference 4) and Generic Letter (GL) 85-20 (Reference 5).
A reexamination of radiographs made in April 1996 of the Oconee 2A1 nozzle revealed that DPC had failed to identify the gap which had developed in the safe-end/thermal sleeve contact area. DPC also had failed to follow all the original recommendations in NRC GL 85-20 for augmented ultrasonic testing (UT). DPC had performed the recommended UT of the safe ends of the MU/HPI lines; however, they did not inspect the adjacent piping as recommended. In addition, DPC failed to UT the weld between the safe-end and adjacent pipe, a discontinuity where cracking could be expected to form.
2.2 Oconee Unit 3 DPC also reviewed the 1996 radiographs of the safe-ends in Oconee Unit 3. The 3A1 MU/HPI line was found to have a gap in the safe-end/thermal sleeve contact area. As a result of the gap in the 3A1 safe-end, Oconee Unit 3 was shut down on May 2, 1997. UT examinations identified apparent cracking in the 3A1 safe-end. This safe-end was removed and
- e 0
metallurgically examined. A visual examination also revealed cracks in the thermal sleeve.
Minor gaps in the other safe-end/thermal sleeve contact areas were determined not to have grown, the rolled area of the thermal sleeve was acceptable, and UT examinations of the other Oconee Unit 3 HPI nozzle assemblies revealed no cracking.
2.3 Oconee Unit 1 The Oconee Unit 1 nozzles have a double thermal sleeve design (Figure 3). Radiographic inspection in the period from 1983 to 1989 indicated that no gap existed in three of the four thermal sleeves. The thermal sleeve in the 1B2 (HPI) line had a gap, which had not grown during the inspection period. Advantages of the double thermal sleeve as stated by DPC include (1) greater stiffness, (2) greater thermal resistance, and (3) reduced flow area, with corresponding increased flow velocity.
2.4 Compensatory Actions As corrective action, DPC replaced the cracked safe-ends and the safe-end/pipe welds in Oconee Units 2 and 3. The thermal sleeves were also replaced with an improved sleeve design, also shown in Figure 1. In addition, DPC also assembled a Failure Investigation Process (FIP) team to determine the root causes of the event in Unit 2. The team concluded that DPC had failed to implement an effective HPI nozzle inspection program based on available industry recommendations, and had failed to effectively evaluate known problems (industry and in-house experience) and implement appropriate corrective actions (from 1982 through the present). The FIP team published its findings in Reference 6. The staff has examined this report and finds it thorough and complete.
DPC also submitted a revision of the Oconee Units 1, 2, and 3 third interval ten-year Inservice Inspection (ISI) Program. This revision commits to meet or exceed the requirements in GL 85-20, previously committed to, regarding examination of HPI system components. The staff reviewed the submittal and determined that the proposed ISI program-is acceptable, with the condition that all future ultrasonic examinations should be performed based on the 1992 Edition, with the 1993 Addenda, of the ASME Code Section XI, Appendix VIII requirements, or be based on calibration blocks with cracks that simulate the type of crack observed in Oconee Unit 2. DPC also agreed to submit the fourth interval ten-year 11 program for all three Oconee units six months prior to the start of this interval. The staff reviewed the proposed program and found it acceptable, subject to the condition stated above (Reference 7).
Other measures taken by DPC include the temporary installation of thermocouples in Oconee Units 2 and 3, to measure temperature time histories in the affected zones, and increasing the minimum make-up flow in these units. The make-up flow in Unit 1 was already higher than in the other two units.
2.5 Other Babcock and Wilcox Owners Group (B&WOG) Plants Subsequent to the event at Oconee Unit 2, other B&WOG plants also performed evaluations and inspections of the MU/HPI nozzles and attached piping. These were Davis-Besse Unit 1, Crystal River Unit 3, Arkansas Nuclear One Unit 1 and Three Mile Island Unit 1.
In response to a staff request for information dated May 27, 1997 (Reference 8), B&WOG submitted an interim report (Reference 1) on the cracking found in Oconee Unit 2, and the results of the inspections of the other B&WOG plants. No indications were reported in any nozzles of these systems. The staff has reviewed this report and finds it generally acceptable.
The interim report did not attempt to determine the root cause of the cracking. As part of the generic investigation of this event, B&WOG committed, through Framatome, to conduct a more detailed evaluation of the root cause of the cracking at Oconee Units 2 and 3. This evaluation will be based on analytical studies using computational fluid dynamics methodology. A final report describing the results of these studies is scheduled to be submitted sometime in 200C.
3.0 STAFF ACTIVITIES On June 6, 1997, the staff of Region II completed a special inspection of Oconee, which focused on the integrated efforts by DPC to investigate the causes of the event and the corrective actions undertaken. The staff concluded that there had been a failure by DPC to adequately implement augmented inspections to detect cracks in HPI lines, and to correct known problems on a timely basis. The staff published its findings in a special inspection report dated June 27, 1997 (Reference 9).
On July 9, 1997, the staff published NRC IN 97-46, "Cracking in High-Pressure Injection Piping,"
to alert the industry of the circumstances of this event (Reference 10).
During the review of the inspections performed by DPC, the staff became aware that ASME Section XI requires no volumetric (ultrasonic) examination of Class 1 piping in the range of 1" to 4" nominal pipe size (NPS). This is inconsistent with the requirement for Class 2 piping, which does specify volumetric examination for piping in this size range. To eliminate this inconsistency in the Code, the staff has requested that the ASME include volumetric examination of Class 1 piping in this size range. However, because publication of revised Code requirements cannot be expected in the near future, and considering the safety significance of the HPI system, the staff has included provisions for this examination in currently proposed rule-making on limitations to the NRC endorsement of the ASME Code under 10 CFR 50.55(a). The rulemaking package went out for public comment 1998.
4.0 CONCLUSION
DPC has replaced the cracked safe-end welds in Oconee Units 2 and 3, and has also installed improved thermal sleeves. DPC has also submitted an ISI plan, which the staff finds acceptable. DPC has also adopted other compensatory measures, such as increasing the minimum make-up flow and installing thermocouples for monitoring the temperature. The staff finds these measures acceptable.
Based on its evaluation of the B&WOG and DPC responses to various staff requests for information, and meetings between the staff and DPC, the staff concludes that B&WOG has satisfactorily and reasonably addressed potential cracking of the safe-ends in the HPI/MU nozzles in all B&WOG plants.
Attachments:
- 1. Figure 1 - Units 2 and 3 Thermal Sleeves
- 2. Figure 2 - Warming Line Flow and Crack Orientation
- 3. Figure 3 - Unit 1 Thermal Sleeve
5.0 REFERENCES
- 1.
Letter from W. W. Foster, DPG, to the Document Control Desk with enclosed B&WOG Report "Interim B&W Owners Group Report on HPI/MU Nozzle Cracking" (FTI Document No. 51-5000239-00), dated June 5, 1997
- 2.
Oconee Nuclear Station, Unit Two, Licensee Event Report dated May, 21, 1997
- 3.
Metallurgical Analysis Report #2181, "ONS 2 & 3 HPI/MU Nozzle Components," Duke Power Company, June 30, 1997
- 4.
NRC Information Notice 82-09, "Cracking in Piping of Makeup Coolant Lines at B&W Plants," dated March 31, 1982
- 5.
NRC Generic Letter 85-20, "Resolution of Generic Issue 69: High Pressure Injection/Make Up Nozzle Cracking in Babcock and Wilcox Plants," dated November 11, 1985
- 6.
Oconee Nuclear Station, Unit 2, Failure Investigation Process Final Report, "Pipe Weld Failure, 4/21/97, PIP 2-097-1324," dated July 24, 1997
- 7.
Letter of October 23, 1997, from D. E. LaBarge, NRC, to W. R. McCollum, DPC, "High Pressure Injection System Augmented Inservice Inspection Program - Oconee Nuclear Station Units 1, 2 and 3"
- 8.
Letter of May 27, 1997, from J. L. Birmingham, NRC, to W. W. Foster, Chairman, B&WOG Steering Committee
- 9.
Letter of June 27, 1997, from J. P. Jaudons, NRC, to J. W. Hampton, DPC, with enclosed NRC Special Inspection Report
- 10.
NRC Information Notice 97-46, "Unisolable Crack in High Pressure Injection Piping,"
dated July 9, 1997 SAFE END TO NOZZLE WELO (81 -ME TTAL IC)
MAIN NOZZLE(C.S.)
COOLANT PIPE SAFE END TO SAFE END DZ PIPE WELD (S.S.)
C0
~--LOCK VALVE zLu FLOW G
A, PSLEEVE WELD BUTTONS---
)
IP 21".S l6000aS.S.SW HALF COUPLING0 1S. S. WARMING(COOL ING)
LINE FIGURE 1 -
THERMAL SLEEVE 2A1, 2B1, 3A, 381, 3B2
OD crack tip @ 3440 (100%TW) o*
OD crack 3300 300 tiD 0 RIO (100% TW) 300".
270'g Metallography (30% TW)
Metallography (26.5% TW) 240.
120*
210' 150*
Metallography 10 (24% TW)
View Is looking In direction of MA/U flow Warming Line Flow FIGURE 2
SAFE END TO NOZZL WELD MAIN (B I-METTALIC)
NOZZLE(C.S.)
COOLANT PIPE CLADDING
)
SAFE ENETO SAFE END PIPE WED (S.S.)
--BLOCK VALVE SLEEVE
-- \\ THERMAL SLEEVE WELD BUTTONS 24"S. S.
1"6000*S.S.SW HALF COUPLING PIPE
-1"S.SWARMING(COOLING) LINE FIGURE 3 -
UNIT 1 THERMAL SLEEVE e
1-