ML20084R851
| ML20084R851 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 12/12/1974 |
| From: | Stephenson B COMMONWEALTH EDISON CO. |
| To: | James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III) |
| References | |
| 881-74, AO-50-249-74-35, NUDOCS 8306150624 | |
| Download: ML20084R851 (4) | |
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Mr. Jcmos G. Kcppler, Regional Director
}/j M Directorate of Ret;ulatory Operations-Region III U. S. Atomic Encrcy Co.rission 799 Roosevcit Road Glen Ellyn, Illinois 60137
SUBJECT:
FT"AL REFCP.T OP AE'!OIC'JLL OCCURRE!CE PER SECTIO:I 6.6. A 0F THE TECETICAL SH.CIF]CATIO::3 SFCO:.31JtY CDUTitIECT BLO70UT PANEL FAIIJJ?.E Referencess. 1) Regulatory Guide 1.16 Rev. 1, Appendix A
- 2) Notification of Region III of AEC Regulatcry Operations Telephone: Mr. D. Knopf, 1600 hours0.0185 days <br />0.444 hours <br />0.00265 weeks <br />6.088e-4 months <br /> on November 30, 1974 Telegrams Mr. J. Keppler, 1600 hours0.0185 days <br />0.444 hours <br />0.00265 weeks <br />6.088e-4 months <br /> on December 2, 1974
- 3) Dresden Units 2 and 3 FSAR, Vol. 1, Section 5 3-2, Rev. 3-22-68.
- 4) Dresden Unit 3 Preoperational Test Report C-12-9, Section 6.
- 5) CRC Handbook of Applied Ensincering Science, Tabic F23
-6) Dresden Lotter to Region III, Dated Deccaber 9, 1974 Report Number:
50-249/74-35 Report Date: December 12, 1974 Occurrence.Date: Novcmber 30, 1974 Facility: Dresden liucicarTober Station, f.' orris, Illinois i
IDE;TIFICATIO?!.0F OCCU:GriCE Secondary Containment Blowout Panel Failure.
Preliminary report issued on December 9, 1974 (Ref. 6).
CONDITIO"S PRIOR TO OCCURRENCE Unit 2 was in refuel mode, with core alterations in progress, and Unit 3 was in run r:odo, at a thermal power of 1614 :7.it and a load of 517 K.ie, just
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prior to the occurrence.
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r Mr. Ja=cs G. Kepp December 12, 1974 The 2A, 3A, and 33 reactor building vent fans and the 2C, 3A and 33 reactor build 9ag exhaust fans were operating,juct prior to the occurrence.
DESCRIPPIO:! OP OCCU1REbE At about 1140 hours0.0132 days <br />0.317 hours <br />0.00188 weeks <br />4.3377e-4 months <br />, the 3A and 3B reactor building exhaust fans tripped off and would not reset. Gusts of wind of about 30 m.p.h. were noted. At about the same time, it was reported by maintenance perconnel and fuel handling perconnel independently that a ccction of the west wall of the 2/3 reactor building cuperstructure had come looco.
Investigation revealed that the Unit 3 (west) reactor building blowout panel had disconnected from its bottom edge support beam, breaking the bottom edge rostraining bolts, and was standing open approximately ten inches at the bottom. All refueling activitics on Unit 2 were i=.cdiately halted. At 1145 hours0.0133 days <br />0.318 hours <br />0.00189 weeks <br />4.356725e-4 months <br />, the 2C reactor building exhauct fan trippcd off, but was subcequently reset. The 3A and 3B reactor building vent fans were secured.
It was.noted that the reactor building atmospheric pressure was below outsido atmocpheric pressure. At 1153 hours0.0133 days <br />0.32 hours <br />0.00191 weeks <br />4.387165e-4 months <br />, Unit 2 was put into chutdown mode. At 1208 hours0.014 days <br />0.336 hours <br />0.002 weeks <br />4.59644e-4 months <br />, the 20 i
reactor building exhauct fan tripped off and could not be reset. The 2A reactor building vent fan uas subsequently accured, and "A" standby gas treatment was started. Unit 3 began an orderly chutdoim at a rate of 20'Go per hour. At 1300 hours0.015 days <br />0.361 hours <br />0.00215 weeks <br />4.9465e-4 months <br />, the load drop rate on Unit 3 was increased to 50 K.Te per hour. At 2109 hours0.0244 days <br />0.586 hours <br />0.00349 weeks <br />8.024745e-4 months <br />, Unit 3 turbine was tripped off line. At 2207 hours0.0255 days <br />0.613 hours <br />0.00365 weeks <br />8.397635e-4 months <br />, Unit 3 reactor was put into refuel modo, cooling doun at a rate of 50*P por hour. At o505 hours on 12-1-74, 17 hours1.967593e-4 days <br />0.00472 hours <br />2.810847e-5 weeks <br />6.4685e-6 months <br /> and 25 minutes after the occurrence, Unit 3 reactor was put into shutdown mode at a tcmperature of 211*F.
DRSIGNATIO:10F APPARE.'T CAUSE OF CCOURRE!OE (Other)
The Unit 2 and Unit 3 reactor buildings were apparently overpressuriced due to the tripp off of the exhaust fans and failure of the vent fans to trip. The vent fans develop a total pre =sure of less than 9 inches of water (Ref. 4), or less than 47 pounds per square foot. A 30 m.p.h.
wind can exert a vacuum prescure of at most 2.2 pounds per square foot, even accounting for gusting above the average velocity (Ref. 5). This totals 49 2 pounds per square foot.
The design blowout point of the panel is 70.0 pounds per square foot, which is determined by the aluminum restraining bolts that coeure it.
These bo3.tc are certified to plastically deform at a specific tensile stress cc=ensurate with 70 pounds per cquare foot (Ref. 3). The vent fans, even aucmented by the vind, should thuc to incapable of blowing out the panel.
(It chould l
- require a wind velocity of at least 70 m.p.h., with the fans, to blow out the panc1). The conclucicn drawn is that the panel restraint bolts failcd prematurely. Reactor building proccure trancicnts following exhauct fan trips on windy days are not unccamon, and the cuculative straining of those bolts during their lifettne may have centributed to their prc=ature failure.
i Vork request //11346 was issued to check the Units 2 and 3 reactor building l
Vent fan preccure trip cystems. This check completed on 11-30-74, revealed l
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-Mr. Jancs G. Ecpp Decc=ber 12, 1974 that the systems will operate normally, i.e. the vent fans vill trip off when 1f.ic reactor building at?.ocpheric prescuro execeds the outside atmospheric pressure by more than 1 inch of water (0.036 pai). This further lowers the apparent failure point of the blowout panel.
AWJXSIS OF OCCU?.REI!CE Seccndary contain=cnt was breached, resulting in mandatory orderly shutdown of the operating unit. The probability of a reactor incident occurring during the tico that the unit was operating and the cocondary contain=cnt integrity was lost was no greater than under normal operating condition.
The most probable potential hazard was the low level relcaco of atmospheric contaminants, had the reactor building been in a condition of airborne radioactivity at the time of the incident. However, the reactor building airborno radioactivity levels were low at the time of the incident and no airborne radioactivity precautions ucre in effect. A possible secondary hazard van that the panel would dicconnect completely from the building and fall to the ground, possibly injuring plant perconnel. Outsido ucather conditions cado it unlikely that porconnel would have been in the area. In retrospect, no additional hazard to the health and safety of the public resulted.
CORRECTIVE ACTIO!S The imediate corrective actions were to stop refueling of Unit 2 and co=mence an orderly shutdown of Unit 3.
Also, the ground area under the dangling
- panel and the interior area adjacent to the panel vero roped off for safety to plant personnel. A work requent (#11347) uas iscued to rectore secondary containment integrity. This was accompliched by attaching cables to the bottom part of the panel and drauing the panel in with Jacks. The cables and jacks remained in place to secure the panel.
In addition, strong backs were utilized on the upper levels to further strengthen the panel. All edcos joining the panel to the rest of the building were then caulked using a siliconc rubber compound. Following the completion of the tempora-y repairs a check of cecondary integrity was made. The results of this test demonstrated that the secondary integrity critorion was met.
The temporary repair of the blowout panel una resolved by comunication betueen Commonwealth Edicon and its consultant. The findings are sumarized as follows:
- 1) Blowout panels make up approximately 50% of the total. wall.
area.
- 2) The repairs reprocent approximately a.105 reduction of blowout panel area. This taken in light of the design margin will have no effect en the structure above the refueling floor. The roof pancis are capabic of relieving any excess pressure.
Subsequent to completion of the temporary corrective action summariced above and the secondary:containcent integrity test, an on-site review determined I
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Deccaber 12, 1974 Mr. James G. Kepp'[ ) (]
V that the conditions of 10 CFil 50.59 were met and startup was authorized.
A portanent repair pror; ram 13 forthcoting and the repairs will be reported in the coni-annual report following complet, ion.
T[to blowout panel in question is an approximately 20-foot-cquare coction of the rcoctor building cuperatructure west wall, factened along the top and bottom cdcca by special aluminum restraining bolta on 6-inch centers to horicontal steel cupport beams. The remaininc five panels were alco inspected. All were in cood condition with the exception of the couth panel or the Unit 3 cide where como bolta have failed. The condition of this panel has been evaluated.
It has been concluded that its ability to perform its intended function has not been compromised. The couth panel was originally installed, rcmoved to bring in the Unit 3 reactor veccel and then reinstalled.
FAILUPI D!.TA No previcus failures have been noted in this system.
Sincerely, f/
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B[B.S'tsphencoN Superintendent BBS:JGT:cep I
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