IR 05000249/1976007

Letter Referring to Inspection at Dresden Unit 3 - 05000249/1976007 Regarding an Off-Gas System Explosion on 04/29/1976

ML17252B146
Person / Time
Site:	Dresden
Issue date:	05/19/1976
From:	Fiorelli G NRC/RGN-III
To:	Brian Lee Commonwealth Edison Co
References
IR 1976007
Download: ML17252B146 (12)

Text

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UNITED STATES NUCLEAR REGULATORY COMMISSION REGION Ill 799 ROOSEVELT ROAD GLEN ELLYN, ILLINOIS 601 Commonwealth F.dison Company Docket No. 50-249 ATrN:

Mr. Byron Lee, J Vice President P.O. Box 767 Chicago; Illinois 60690 Gentl.emeii a

.Thia refers to the inspection conducted by :nessrs. T. Essig, F. A. Maura and M. C. Schumacher of this office on April 29

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and May:3, 1976, of activities at Dresden Nuclear Power Station, Unit 3 authorized by *NRC Operating License No. DPR-25. and to the discussion of our findings with Mr. Stephenson and others of your staff at the conclusion of the inspectio The enclosed copy of our inspection report identifies areas eXamined during the inspection *. Within these areas, the inspection consisted of a selective examination of procedures and representative records, observations, and interviews with personne *

No items of noncompliance with NRC requirements vere iden-tified within the.scope of this inspection. However, we have 4uestioned certain aspects of plant operation following the off-gas eJ..t>losion on April 29,. 1976. These matters are*

emphasized in the "Management Interview" section of the enclosed report. It is_our understanding based on* discussions with representatives of your staff that1 1~

• The GeLi system for stack charcoal cartridge activity detection will be calibrated as discussed in item A of the management intervie.

Procedure DOA 5400-1, Revision 0 1 governing actions following an off-gas explosion will be revised to giv more specific guidance to operators as discussed in item C of the lilSUagement 1ntel'Vie Please provide your response as to when these corrective actions will be completed within twenty days of receipt of this lette '*

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Commonwe,a.lth Edison

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MAY In accordance with Section 2.790 of the NRC's "Rules of Practice," Part 2, Title 10, Code of Federal-Regulations, a copy of this letter and. the enclosed iDspection report will be placed in the NRC' s Public Document Room*. except as follow If this report contains information that* you or your contractors believe to be proprietary, you must ~pply in writ~ng to this office~ within.. twenty:day~ of y'Our receipt of tliis,.letter, to withhold such information from public diselosur The application must include a full statement of the reasons for which the information:. is considered proprietary, and should.. be prepared so that proprietary inf on:iation identified in the application is contained in an enclosure to the applicatio We will gladly discuss any questions you have concerning this*

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Enclosure:

IE Inspectio~ Report No. 050-249/76-07

REGION III==

Report of Operation Inspection IE lnspection Report No. 050-249/76-07 Licensee:

Commonwealth Edison Company P~ O. Box 767 Chicago, Illinqis * 60690 Dresden Nucl~ar. Power Station Unit 3 Morris, Illinois Type of Licensee:

BWR (GE) 810 MWe Type of Inspection:

Special, Announced Dates of Inspection:

April 29 and May 3, 1976 Principal Inspector:

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~F. A. Maura*_

Accompanying Personnel:

.T. Essig f

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--;zy '/p"l't.IL,L M. C. Schumacher (April 29, 1976)

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• **4LJ, Cress-i.,ell

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Other Accompanying Personnel:

None-d/c?(~rg----

Reviewed By:

W. S. Litt~e, Chief Nuclear Support Section

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License No. *DPR-25.*

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• . SUMMARY OF FINDINGS Inspec~fon Summary An inspection was conducted on April 29 and May 3, (Unit 3, 76-07)

regarding-an off-gas system explosion which. occurred on April 29, 197 Enforcement Ite *Non ***Licensee Action on Previously Identified Enforcement Items

.Not inspected~

Other Signific~nt Items Systems and Component An explosion occurred in the off-gas system while the recombiner and charcoal filters were not in us The cause has not been determined.* It appears the explosion occurred* somewhere in the 30 minute h6ld up pipe~

+/-he licensee has decided not to blind flange the r.upture. discs as* previously recommended by the General Electric Compan Facility items (Plans and Procedures)

Procedure DOA 5400-1, Rev. 0, will be revised to include guidance on how to determine the plant has returned to normal following an off-gas explosion and what criteria will be used if the plant is not to be shutdow Managerial Items Non Noncompliance Items Identified and Corrected by Licensee None, Deviations Non Status of Previously Reported Unresolved Items Not reviewe......... I'-.,...... __ *-*--

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Management Interview A-management interview was -c-on-ducted*-wrtlf-Messrs ~--stepnensan*; Able~ ***

Ragan, Watts, Adam and Willaford at the conclusion of the inspection on May 3, 197 The inspector discussed the licensee's following actions relative to effluent monitoring, sampling, and sample analyse The inspector indicated that there appeared to be a real need to c.alibrate* the GeLi system for stack charcoal.cartridge Had such a calibration been performed, the licensee might have been able to determine the quantity of radioiodine on the charcoal much.earlier than he was able to do with NaI spectrometry, in vi~w of the interference from noble gases and daughters present on the charco~l; A licensee representative acknowledged the need for such a calibration, and indicated

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that plans were underway to perform such a calibratio (Paragraph 2, Report Details)

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Th~ inspeotor indicated that. the lice.nsee took a rather long.

time to recognize that the stack monitor was still indicating

.a release of radioactivity after the off-gas monitor was indicating that little or *no releases of radioactivity was occurrin The inspector pointed out that if the stack---- --

monitor had been followed more closely and that if th licensee had recognized that radioactive materials other than noble gases (e.g. radioiodine and particulates) might have been released following the off-gas explosion, the licensee might have collected and analyzed the stack sample (filter plus charcoal) in a more timely manne (The stack sample was collected at 0930, approximately 4,5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> after the explosion.)

The inspector indicated that although the after-the-fact sample analysis showed that the radioiodine and particula~e releases were about 40% of the release rate limits contained in the Technical Specification, the actual release c~uld have easily exceeded the.release limit In the latter situation, promp action to terminate the release would certainly have been desirable but probably would not have been made, given the same followup actio The licensee acknowledged the comment (Paragraphs 2 and 3, Report Details) The inspector noted that procedure DOA 5400-1, Rev. O, dated May 1975 requires the unit be shutdown, the rate of shutdown to be determined by the shift engineer, but that no option to remain at power is give In addition the procedure gives no

• guidance to help the operating crew determine whether the

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switchover of air ejectors. were effective or not. It took the licensee an excessive amount of time to determine that gases were still escaping to the turbine building after the changeover of air ejector From this expe-rie.nce it is obvious the -operat-ing crew*,. and man-agemeiff personnel, failed to use all available information in the control room to determine the g~ses were siill escapin~ ~hru the rupture disc The licensee felt the procedure gave an option to remain at power, but agreed that it was too* brief and would be improved*.

They will also include the parameters and. criteria that should be used to determine the syste~ has returned to normal following the transfer of air ejector The licensee also indicated the desirability to program this event in the simulator so that the operating crew could improve thir response*performanc (Paragraphs 2 and 4, Report Details)

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The inspector inquired about the licensee's plans regarding the rupture discs modification (Ml2-3-75-8) and was informed that it would be cancelle (Paragraph S, Report Details)

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. The inspector expressed his disappointment regarding the diligence with which the licensee is studying the event in an effort to determine its cause and possible corrective actio As of Hay 3 no one in the licensee's staff was prepared to discuss the recordings obtained pr.ior to,* during, and after the event with the inspecto (Paragraphs 4 and 6, Report Details)

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. --REPORT-DE'fAI-LS

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~ Persons.Contacted B. Stephenson, Station Superintendent A~ Roberts, Assistant Station Superintendent R. Ragan, Operating Engineer, Unit 3 *

D. Adam, Radiation Chemistry Supervisor C~ Sargent, Unit 3 Lead Engineer G. Bromba, Engineer G. Bergan, Chemist R. Schumacher, Radiation-Protection Foreman V. Chaney, Radiation Protection Foreman D. O'Keefe, Radiation Protection Foreman Sequence of. Events The unit was operating at approximately 600 MWe with the "A" SJAE, the "B off-gas filter, and the modified off-gas system in service; an off-gas release rate of....-V260 x 103 pCi/sec, after a 2 minute delay downstream of the.SJAE; and a chimney release rate of.N 70 pCi/se Maintenance on the mechanical vacuum pump was scheduled and in order to reduce the dose rate in the area of the mechanical vacuum pump the modified off-gas system was being bypasse The following sequence of events was obtained through interviews of plant management personnel, and reviews of logs and recorder chart /29/76 0000

• 0120 0122 0150 Modified off-gas system charcoal filters bypasse Modified off-gas system recombiner bypasse No. 1 off-gas analyzer alarmed Hi-Hi instrument checked, zero adjusted, alarm cleare No. 1 off-gas analyzer alarmed agai Throughout the night it continued reading higher than No. 2 analyze Started to reduce loa Contacted Operating Engineer, Rad Protection Technician and Nuclear Enginee... *.

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0300 Staried cutting in recombine Steam on booster jet and preheate Isolation valves remained close, As

. ---***--*.off-ga*s-.monitor reading started.drop.ping,. isolated.steam 0335 0445 0450

• 0452 0459 051 to booster jet and preheate Stopped load reduction @ 422MW show decrease in valu Over the load approximately 30 MW Off-gas started to next hour increased Stopped load increase as of f~gas monitor reading started increasing agai Received off-gas hi-pressure, steam jet air ejector *

liquid drain high level and low leve.l, and t1,.1rbine building high radiation alarm Reduced reactor power by reducing recirculation flow to minimu Decided overpressure of off-gas system had occurre Evacuated turbine building. * Started load reduction by rod insertion in accordance with sequence B-l Started switchover to the "B" SJAE. * Notified Operating Engineer, Assistant Superintendent and Radiation Chemistry Superviso Completed SJAE changeove Stopped load reduction at 375 MW First set of air samples collected inturbine building with high volume sampler Initial concentrations were about 2 x 10-7 pCi/cc, with activity later shown to be due to 32-mi Cs-138 and 83 min.Ba-13 (Concentration in these same samples decreased by factor of 100 due to de.cay during a 6 hour6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> period)

Delared on-site alert (first step in station emergency plan).

Operating Engineer and Radiation Chemistry Supervisor arrive at sit Turbine building was posted as an airborne radioactivity are.~Y'\\t.... -i

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. 0700 0800-0830 0815 0930 1037 1200 1800 Switched off-gas filters (B to A).

Rad Chern supervisor contacted corporate personnel to request collection of-erivlronment:al -samples --(off~sifi~ -.nert: creciarea--

as a result).

Made determination that off-gas appeared to be being discharged to the-turbine building (probably via the failed rupture disc and subsequent leakage from the air ejector exhaust and turbine building exhaust ducts and/or via blown loop seals).

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Attempt to obtain off-gas sample at filter building faile Off-gas monitor response had dropped, but was attributed to load reductio Decision made to take unit off lin Chimney effluent sample (Filter plus charcoal) collected,

'but could not be analyzed immediately_due to interferences from noble gas daughters (primarily Cs-:138) present on the filter and charcoa Unit off line. Proceeding to cool down reacto.

Preliminary estimate of I-131 release was Q.063 yci/sec, on GeLi scan a~d its estimated counting efficiency for the charcoal cartridg Turbine building taken off airborne radioactivity statu During the afternoon of April 29 the licensee entered the air ejector *rooms and found both, "A" and "B" SJAE rupture discs broke The isolation valves of the "A" SJAE were verified to be shu The rupture discs were later replaced, The B off-gas filter at the base of the stack was found destroyed and was replace Results of DOP tests performed on the A and the new B off-gas filters gave a particulate filter efficiency of 99.98% and 99.10% respectiv~l.

Release to Atmosphere On April 30, 1976, additional results became available, and the atmospheric releases appeared to be:

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Medium Charcoal Filter*

Nuclide 1-131 I-131 Co-5 Mn-54 Co-60 Cs-134 Ba-140 Cs-137 Total*

Release Rate, µCi/sec D2 Vent D3 Vent D2/3 Chimne __.J( __ _

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These releases, when c*ombined with the 0.08 pCi/ sec iodine and particulate release rate from unit 1, yielded a total release rate for the site of 44% of the technical specification limit for iodine and particulate releases to the atmospher Analyses

. or air samples and TL dosimeters collected. from the Dresden*

environs (at both up and down-wind locations) indicated that both the airborne concentration of radionuclides and the gamma exposure rate were within the normal range of variatio Review of Recording Charts The inspector reviewed recordings of the following parameters for the period prior to, during, and after the event: off-gas monitors #1-and 2 stack monitor steam jet air ejector flow inlet to holdup pipe flow recombiner temperature generator 1oad The off-gas monitor and stack monitor charts were also compared*

against charts obtained during previous explosions at Dresden and Quad-Citie The overall review showed that:

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_ 9_-c ___________ _. The pressure wave was felt in the "B" SJAE and could have been 'the reason why its rupture disc was found broken. _

Prior to the event the "B" SJAE had been used last in March* 197:6~ ----After the--outage-at-the end-of March the -- --- -

licensee had started the unit with the "A" SJA The flow upstream (SJAE) and downstream (inlet to holdup pipe) of the rupture discs was approximately 15.0 scfm prior to the even Following the detonation the flow to the holdup pipe dropped to 25 SCFH and failed to match the*

SJAE flow after the switchover of air ejectors. It showed that;t/ 83% of the gases were escaping thru the blown rupture disc For some unexplained reason the flow out of the* "A" SJAE increased shortly after it had been isolated, and the flow from the "B" SJAE decrease Although the licensee believes

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• the indication was caused by instrument problems the combined'

flow accounts for the expected flo To dismiss the "A" SJAE flow indication as caused by instrument malfunction raises the question of how the air ejector flow cbuld be cut in half without experiencing a loss of condenser vacuu d, Approximately 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> after the event the off-gas flow to the holdup line increased from 25 to 60 scf For some unknown reason the amount of gases escaping thru the blown discs decreased from.ri/83% to --z/59%. The off-gas monitors showed a sha~p drop at th~ *time of the event, recovered /l/ 60% during the first 4 minutes, and then began a gradual drop during the next half hour to approximately 1/30 of its original valu f, In contrast to the observed behavior of the off-gas monitors the stack monitor showed an instantaneous 50% increase, a 50% decrease, and a 750% increase within one to two minutes after the even Over the next two minutes the reading returned to its original value and then increased a decade over the next 3 minutes and practically remained there until

• the unit was shutdow The final increase of the stack monitor to readings higher than existed prior to the event is typical of that seen during previous explosions and is caused by the reduced holdup time as the gases are being discharged to the stack by the ventilation syste The licensee should have realized by

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0600 hours0.00694 days <br />0.167 hours <br />9.920635e-4 weeks <br />2.283e-4 months <br /> that the changeover of SJAE's had accomplished nothing and that a unit shutdown was require The flow

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The initial peak in the stack monitor differs from February 20, 1974 experience at Dresden and appears to indicated 'that the explosion may not have originated in the final off-gas filter, but somewhere upstream thus sending an initial burst_

of gases up the stac The fact that the second peak was much higher than the first one seems to indicate the explosion was not too far upstream from the filter, __ The licensee's theory that a second explosion (a-second "bang"

_was reported by members of.the operating crew) approximately twe:nty minutes after the first one-had caused the "B" SJAE rupture dist to burst could not-be substantiate '

Rupture Discs Modification Following a series of explosions in the off-gas systems of BWR's -

General Electric recommended in SIL No. 38 dated November 30, 1973 the blanking off of all rupture discs_at the discharge of the steam j.et air ejector SIL No. 38, Supplement 1 dated April 30, 1974 reinforced the earlier recommendatio The licensee's Mechanical and Structural Engineering Department by letter dated June 17, 1974 recommended that the rupture discs not be-blanked off because of the-SJAE 15 psig design pressur On January 22, 1975 the Dresden Station originated a non-safety related modification, M12-3-75-8, to blank off the rupture_ disc Installation was authorized by the Station Superintendent by April 16, 1975, however no action to complete or delete the modification has taken place sinc The inspector was shown a ietter dated June 5, 1975 from Sargent and Ltindy Engineers which also recommends against the blanking of the disc Licensee's Investigation As of the close of.this inspection the licensee had not started an investigation into the cause* of the event and how to prevent a recurrence. - Analysis of the_recording traces had not been performe The inspector was informed that an investigation team had been formed and ~ould start fun~tioning wi~hin a few day It was composed of personnel from Quad-Cities, Dresden, and th~

corporate office,

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