ML20211P517
| ML20211P517 | |
| Person / Time | |
|---|---|
| Site: | Surry  |
| Issue date: | 12/22/1986 |
| From: | Shao L NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| To: | Vollmer R Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20211P447 | List: |
| References | |
| FOIA-87-20 NUDOCS 8703020380 | |
| Download: ML20211P517 (11) | |
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/ 'o UNITED STATES
!", ~ ,i NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555
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...* DEC 2 21986 MEMORANDUM FOR: R. Vollmer, Deputy Director Office of Nuclear Reactor Regulation FROM: L. Shao, Deputy Director, Division of Engineering Safety Office of Nuclear Regulatory Research
SUBJECT:
VISIT TO SURRY UNIT 2 NUCLEAR PLANT ON DECEMBER 12, 1986 TO REVIEW PIPE FAILURE On December 9, 1986, at 2:20 p.m., an 18 inch suction line to a main feedwater
, pump of the Surry Unit 2 ruptured, releasing high pressure high temperature water into the turbine building. The resultant hot steam injured eight maintenance workers, six of which were hospitalized with severe burns.
Four of the six victims have subsequently died of complications from the burns. A group of NRC staff and consultants associated with the NPC Piping Review Comittee visited the plant to review the incident and acertain any [
information that might be applicable to NRC implementations of the Piping i Review Committee report, and, in particular to the proposed draft leak-before-break procedures.
The NRC group that visited Surry Unit 2 are listed in Enclosure 1. This group met with representatives of VEPC0 and Mssrs. V. Panciera and B. R. Crowley of the Region II response team who were on site directing the NRC review of the incident. Region II issued an executive sumary of the pipe rupture event on December 17, 1986 and will issue a more detailed report as soon as a more complete understanding of the facts becomes available.
Surry Unit 2 is a 3-loop Westinghouse plant whose Architect-Engineer was Stone and Webster. The plant was in operation since March 1973, but was down for 2 years during that period to replace a steam generator.
Plant Conditions at Time of Failure Both Surry Units were operating at full power just prior to the pipe failure event. At 2:20 p.m. the "C" Main Steam Trip Valve (MSTV) inadvertantly closed, followed shortly by closure of the other two MSTVs. The resulting signal of low level in the steam generator trippad the reactor and initiated auxiliary feedwater flow. Following the reactor trip, the Main Feedwater (MFW) regulating valve automatically closed. Sixty seconds later a small steam release noise was heard in the Turbine building followed by a very loud noise. The loud noise stopped when the secondary pumps were turned off. An unusual event was declared at 2:30 p.m. and the plant was placed in the cold shutdown condition at 7:03 a.m. on December 10, 1986.
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2 DEC 2 2198S The steam release from the ruptured pipe injured eight employees of two VEPC0 contractors; the Daniel Construction Company of Greenville, South Carolina, and Insulation Services, Inc. of Hopewell, Virginia. These poeple were working on instrument line relocation and pipe insulation in the general area, but not on the affected pipe itself. Six of the injured people were hospitalized for treatment of severe burns. Four of the hospitalized individuals have subsequently died of implications from the burns. The remair,ing two are in serious or critical condition.
One of the injured men working on a scaffold near the break location reported a " pop" a few seconds before a big bang when the pipe failed catastrophically. He was able to jump off of the scaffold and reach the exit door just as the steam caught him.
Pipe Failure Description The pipe failure occurred in the 18 inch diameter suction line to the "A" main feedwater pump (see Enclosure 2) in a 90 degree elbow at a point about I foot from where the pipe is attached to the 24-inch diameter condensate supply header pipe by a short horizontal riser. The riser was installed to the header with a saddle reinforced Tee joint (see Enclosure 3). Reported pipe conditions were 550 psi and 375'F containing solid water. The rupture was a 360 degree circumferential (guillotine) break. In addition, a fragment of 18-inch diameter suction pipe approximately 3 feet x 2 feet in size was ripped off of the end of the broken pipe and flung approximately 20 feet away from the original location into a cable tray. Following the rupture, the broken end of the suction pipe was displaced approximately 10 feet from its original location due to pipe whipping (see Enclosure 4). This occurred through a rotating and twisting of the pipe causing gross distortion and severe buckling at a downstream elbow near the condensate pump (see Enclosure 5). The broken pipe was eventually restrained by an adjacent pipe to which no obvious damage was done.
Materials Failure Description The failure occurred in a forged seamless elbow made to ASTM-A234 fitting specifications for material with a tensile strength of 60,000 psi attached to an ASTM A106 header pipe. The elbow was classified as extra strong which would indicate a nominal wall thickness of .500 inches. VEPC0 metallurgist, i
Joe McAvoy, reported that the primary cause of the failure was erosion /
corrosion which caused gross thinning of the wall thickness over a large area. The pipe thickness had decreased from a nominal .5 inch to an average of .25 inch. The pipe wall had a smooth transition to a minimum thickness of
.06 inch. No evidence of water hamer was observed. However, at this l
l thickness, a minor pressure surge of as little as 200 psi could have fractured the pipe. The failure initiator almost certainly was an overpressure generated l from the reactor trip. This would increase line pressure about 100 psi; however, there are other possibilities that could lead to higher delta pressures. Mr. McAvoy suggested that erosion was caused by turbulance of single phase, high pressure water similar to that reported previously by Westinghouse on a steam generator "J" tube failure. The geometry of the pipe at this location with its sharp bends and short sections may have been a major contributer to such turbulance. The wall thinning may have been l
DEC 2 21986 l caused by erosion / cavitation which is cavitation involving the formation and collapse of bubbles within the liquid. Sharp discontinuities in the flow path can produce cavitation and consequent erosion. However, no evidence of steam erosion (striping) was observed. Some very small laps were found at the crack edge tnat may have acted as crack starters. The crack may have initiated close to a weld but did not follow along the weld. The fracture appearance showed a full thickness plastic zone indicating a ductile fracture. The inside surface had a smooth granular appearance with superficial oxidation. Areas away from the elbow had black scales. The 3 foot x 2 foot fragment was flattened and battered. Many of the wall thickness measurements came from this section. VEPCO plans additional physical and metallurgical tests on these materials and further thickness testing on the system and on Surry Unit 1. The safety systems will be evaluated for similar geometries.
Rough calculations by E. Rodabaugh indicate that pressure at failure for 1 this elbow should be 667 psi for a thickness of .100 inch and that for the estimated pressure of 450 psi the thickness at failure should be .0675 inch.
The reported thinning would also cause changes in stress intensification i factors that would tend to make other loads much higher and possibly con-tribute to the failure.
An unanswered question is whether the elbow was 0.5-inch wall initially or whether it was thinner. In any event there was ample evidence of wall thinning in the short section of riser and in the elbow.
General Observations and Conclusions t
l The Region II team led by Vince Panciera is doing a connendable job of gathering the appropriate information for an expeditious analysis of the problem. The conclusion that the primary cause of rupture was erosion appears reasonable in view of the extreme wall thinning reported. On
- December 13, 1986, we were informed by Region II that the similar suction line elbow on Surry Unit I also had thinning to a similar degree in the same locations tending to confirm this conclusion. We believe it would be prudent to do UT wall thickness measurements planned by VEPCO on the suspect areas of I
piping in other piping as well. In particular, piping should be carefully analyzed for geometric shapes, high flow rates and other conditions that and thickness measures should be performed on
! might lead all areas to similar found suspect erosion,is by th analysis. This evaluation should be done on all safety-related piping regardless of material or design codes used since these variables would not influence the susceptability to erosion. We also conclude that the problem may be generic and that other utilities should be alerted to the problem through an inform tion notic .
wrence C. Shao, Deputy Director Division of Engineering Safety Office of Nuclear Regulatory Research cc: See next page i
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.' 4 DEC 2 2198S cc: J. Sniezek J. Roe H. Denton E. Beckjord J. Taylor N. Grace D. Ross R. Starostecki T. Speis G. Arlotto R. Bosnak J. Richardson I C. Serpan T. Novak L. Rubenstei E. Rossi R. Ballard NRC Visiting Team -
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5 CEC 2 ; :555 ENCLOSUPE 1 PARTICIPANTS IN VISIT TO SURRY NUCLEAR PLANT ON DECEMBER 12, 1986 NRC Visitors L. Shao W. Hazelton M. Mayfield E. Sullivan A. Taboada NRC Consultants S. Bush C. Czajkowski F. Rodabaugh Region II B. Crowley V. Panciera HPg J. McAvoy J. Ogren W. Stewart 4
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Steel piping at the Surry Power Station that ruptured Tuesday afternoon, releasing steam and hot water that injured eight workers. The piping carries water to one of two Unit Two main feedwater pumps, which supply l
water at high pressure to the unit's steam generators. The rupture, which dislodged a 3-foot section of pipe, l
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occurred at the point where the oice distributes water to one of the oumos. No radioactive material was l
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ENCLOSURE 5
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Piping One of two feedwater pumps at Surry Power Station Unit Two that supply the unf t's steam generators. The pumps are that carries pressurized hot water to the pump ruptured Tuesday, injuring eight workers.
Cdpable of pumping 13,800 gallons of water per minute. No radioactive material was released in the incident.
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DATE: b ac, M O u
MEM3RANDUM FOR: Hazel Smith, NRR P-433 '
FROM:
L t Rv66tJmW -
SUBJECT:
FOIA - E1-ao Thisofficehasnodocumentssubjectto'thisrequest.
I have no documents subject to this request.
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n6 A The subjectrequest. enclosed documents listed at Appendix A are responsivew to A
- ther. ' m',A I recoernend release of these documents.
The specific documents enclosed and listed at Appendix B should be withheld document. from public disclosure for reasons discussed for each "PDR" this availability request. (Pfeaseaccession numbers describe: date, l etterlisted at Appendix C relate to to and from, subject.
" accession number)
The following oYfices
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this request but were n(ot notified by)your correspondence to me.o OTHER C0tt4ENTS: '
Ins PD/ftj hie gure
%bc sk PD/ Branch 4hltf Name (printed Or typed)
Date i/~h,
'UONTACT: (X- )
'i APPENDIX A FOIA 87-20
- 1. Memo from R. Vollmer to L. Shao dated December 22, 1986,
Subject:
" Visit to Surry Unit 2 Nuclear Plant on December 12,~ 1986 to Review Pipe Failure."
- 2. Semo to D. Tondi from M. Caruso dated December 22, 1987,
Subject:
"h ip Report: Surry-2 Feedwater Line Rupture / Augmented Inspection."
- 3. Letter from W. L. Stewart (VEPCO) to H. Denton dated December 23, 1986,
Subject:
" Unit 2 Feedwater Pump Suction Line Break Event."
4 Letter from W. L. Stewart (VEPCO) to-H. Denton dated December 23, 1986,
Subject:
" Impact of December 9,1986 Surry 2 Event on 40 Year Operating License Amendment Request."
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