ML20084Q057
| ML20084Q057 | |
| Person / Time | |
|---|---|
| Site: | Quad Cities  |
| Issue date: | 03/12/1974 |
| From: | Stephenson B COMMONWEALTH EDISON CO. |
| To: | Oleary J US ATOMIC ENERGY COMMISSION (AEC) |
| Shared Package | |
| ML20084Q060 | List: |
| References | |
| BBS-74-53, NUDOCS 8306130106 | |
| Download: ML20084Q057 (4) | |
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Post O'f.cc Box 216 Cordova. Illinois 61242
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({,{:j March 12, 1974 9
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'%fu John F. O ' Leary, Director Directorate of Licensing Regulation U. S. Atcmic Energy Cormission Washington, D. C. 20545
Reference:
Quad-Cities tiucinar Power Stat on, Unit 1 i
Docket flo. 50-254. License DPR-29 Appendix A, Section 1.0.A 3 and 1.0.A.5
Dear Mr. O ' Leary:
The purpose of this letter is to inform you of an explosion which occurred in the Unit 1 off ges hold up volume en Merch 2, 1974 ct about 8: 10 p.ra.
The pressuri:ation of the off gas pipe broke the rupture disc in the 1A Air Ejector Room and resulted in an unplanned release of radioactivity through the turbine building ventilation systen. The release was determined to be well within the Technical Specification limits for gaseous and particulate effluents. This abnormal occurrence was reported to your of fice and Region 111-Directorate of Regulatory Operation on March 3,1974 by telegram.
EVENT 5 PRIOR TO OCCURRENCE Unit I had been taken off the line on March 1,1974 for ninor main-tenance and a rod pattern exchange. Hot functional testing of the recodiner train portion of the augmented off gas system was con-ducted during the ensuing startup on March 2nd. At about 7 a.m. on that date the unit was on the line with the recombiner in operation.
Test data was being taken and the systcn operated without incident.
At 10 a.m. the unit scremmed from 200 MWe due to a turbine trip and isolation which was unrelated to operation of the recombiner. At j
this time the recombiner was purged end isolated. The unit was then brought back on the line in the afternoon. Power was increased at 100 MWe/hr to 400 tide. At 7.:30 p.m. the unit reached 400 tide at which time the rate of increase was reduced to 50 MWe/hr.
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Mr. J. F. O ' Leary March 12, 1974 D_ESCRIPTION OF OCCURRENCE At about 8:10 p.m. on March 2,1974 the control room operator had three alarms come up almost simultaneously. They were:
(1) off-gas filter high differential pressure; (2) off-gas high flow; and (3) off-gas to stack high pressure.
At about the same time the operator in radwaste heard a loud noise.
The unit operator notified the shift engineer and the alarms cicared almost irrediately. The operator then received high radiation alarms f rom three of the Area Radiation monitors in the vicinity of the air ej ector rooms. The of f gas flow recorder also shewed a reduction in l
off gas ficw. This info.mation lead the shift personnel to suspect I
the rupture disc had blown on the 1A air ejector which was in service, in accordance with the station's abnormal procedure for off gas ex-l plosions, the shift engineer dispatched the shift foreman and a l
radman in full radiation protective clothing to the 1A air ejector room to investigate the problem. He also ordered a reduction in power to 350 We to reduce the dose rate. At about 8:30 p.m. the initial entry was made into the 1A air ejector room.
In less than 30 seconds the men made a survey, noticed the rupture disc bicwn and left the room. Operators were then dispatched, again in full radiation protective clothing, to take the lA air ejector out of service and put the ID ejector in service. This operation was completed at about 9:15 p.m.
The chimney release rate of gross radioactivity had increased from i
about 23,000 uC/sec before the explosion to about 180,000 uC/sec.
l From this level i.t decreased steadily and appeared to stabilize at about 60,000 uC/sec. At about 9:30 p.m. the station health physicist had arrived end he conducted another survey of the lA SJAE room with a radman. The results of this survey also indicated that the radi-ation levels had not decreased greatly as expected. The reason for this was soon discovered to be that the discharge valve from the 1A SJAE, valve no. 5411A, had not been closed when the 16 SJAE was placed in service. Thus for a period of time the discharge from the IB SJAE was entering the vent system via the broken rupture disc in the 1A room. When an operator closed the 5411A valve at about 10:30 p.m., the chimney release rate rapidly decreased to the pre-explosion level since the off gas flow was again being discharged entirely into the hold up pipe.
In addition to the above the following supplementary actions were carried out within a few hours after the explosion:
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(1) The IB high efficiency filter which was in service at the time of the explosion was valved out and the 1A filter was placed in service.
(2) The area outside the turbine building in the vicinity of the off gas line and filters was surveyed. No abnornal radiation Icvels or other evidence of damage to the pipe were detected.
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(3)
Maintenance personnel were called out to replace t.he rupture disc in the 1A SJAE room.
4 DETERMINATICH OF CAUSE AND CORRECTIVE ACTION The exact cause of the detonation ha.s not yet been positively identified but the events and system components are being studi.3d for possible causes. The operation of the recombiner is also being reviewed, although there is no evidence at this time which would indicate this to be the source. A previous investigation of a similar event on Unit 2 in 1973 led to the installation of a fic1d modification to the off-gas filter. This modification was to ground the filter pro-perly to the piping to ensure that a static charge buildup and sub-sequent discharge would not cause e detonation. Further investigation has revealed that the filter core had two pieces of ungrounded metal instead of one and thus the initial modification was inadequate. A second modification I-as been issued for all of the high ef ficiency final filters which clininates the possibility of an ungrounded fil-ter being the cause of a detonation. The IB filter has been installed and other filters will be modified and installed as spares become available.
The station procedure for off gas explosions was reviewed af ter this event and found to be adequate. Some minor additions will be made to the supplementary actions such as recalibration of instruments sub-jected to the force of the explosion and inspection and cleaning of.
the filter element retainer.
. EVALUATION AND SAFETY IMPLICATIONS Chimney particulate sample results were reviewed for the'puriod be-fore and after the explosion. Although an increase was detected on March 2nd, the release rate-remained less than 1% of the limit of Technical Specifi cation 3.8.B.2.b.
When the IB filter was removed from the filter vessel on March 5,1974 some of the filter media was observed to have melted and the filter itself was charred. Some debris was also observed on the inside of.
the filter element retainer, but none was observed downstream of the retainer.. The failed element retainer thus appeared to have effectively
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performed its design function and was cleaned prior to' reinstallation.
The off gas. system design incorporates temperature-and pressure switches to detect an explosion in the hold up pipe. These switches-i alarm and automatically clore 'the air ejector suction valves to
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Mr. J. F. O ' Leary March 4, 1974 stop-flow of the combustible gas into the pipe. Following the ex-plosion the operator noticed the high pressure alarm but not the high temperature alarm. He did not observe an isolation of the off gas line; however, this design feature is intended only to pre-vent feeding any combustion taking place in the pipe and therefore does not seal in.
In the course of the investigation all switches were tested by instrument mechanics and found to be functioning
-properly. It is very probable, therefore, that en isolation signal was received; but the initiating pressure pulse from the explosion was of such short duration that the swi tches reset almost immediately and the valves reopened before their intermediate position was ob-i served by the operator. The annunciator for closure of these valves did not come up, however, this is actuated by limit switches-only after tina valves are fully closed.
Contamination in the turbine building as a result of the explosion was minimal since the air ejector roces exhaust' directly to the fans and the chimncy. Cperating personnel involved wi th the air eJcctors were not significantly contaminated and the maximum dose received by any individual was less than 50 mrem.
The gross radioactivity release rate from the main chimney during the occurrence was also evaluated and environmental samples have been collected as an additional precaution although we do not feel that this action _was necessary. Considering the existing wind of
- 20 mph from the south (1750) the dose at the site boundary which could be attributed to the explosion was calculated to be.17 mrem.
The maximum release rate before the affected air ejector was iso-lated was about 180,000 uC/sec which is only 38% of the limit of Specification 3.8.A.2 in summary the of'f gas system is designed to withstand the force of an~ explosion of this nature and our review of this occurrence
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can only' conclude that this design criteria has been met. While it is disturbing that the exact cause of this explosion, and similar events at other BWR's, cannot be -identified positively, the short duration increase in the release rate does not in any way jeopardize the environment or the health and safety of the public.
Very truly yours, COMMONWEALTH EDISON C0lipANY QUAD-CITIES NUCLEAR POWER STATION jI' y
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'B'. D. Stephen on Station Superintendent BBS/lk 4
Regional. Director, Di rectorate of-Regulatory Operations-Region t il
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