ML17252A921

Letter Informing of 02/20/1974 Explosion in the 3B Off Gas Filter - Dresden Unit 3

ML17252A921
Person / Time
Site:	Dresden
Issue date:	03/01/1974
From:	Worden W Commonwealth Edison Co
To:	O'Leary J US Atomic Energy Commission (AEC)
References
WPW Ltr. #147-74
Download: ML17252A921 (4)

Text

Commonw.h Edison R~tatwj: 6t ~

One First National Plaza, Chicago, liliiiois

Address Reply to: Post Office Box 767 &0-249 Chicago, Illinois 60690 WPW Ltr.#147:.74 rlt uL, v to

• -MAR 4

• 1974 a.

Mr. J. F. O'Leary, Director Directorate of Licensing U. s. Atomic Energy Commission Washington, D. C. 20545

References:

1) Notification of Region III of AEC Regulatory Operations Telephone: Mr. F. Maura, 1300 hours0.015 days <br />0.361 hours <br />0.00215 weeks <br />4.9465e-4 months <br /> on February 20, 1974.

Telegram: Mr. J. G. Keppler, 1040 hours0.012 days <br />0.289 hours <br />0.00172 weeks <br />3.9572e-4 months <br /> on February 21, 1974.

2) Dwgs: Sargent & Lundy P&ID M~371 *.

Dear Mr. O'Leary:

This letter is to report a condition relating to the operation of the unit at. about 1125 hours0.013 days <br />0.313 hours <br />0.00186 weeks <br />4.280625e-4 months <br /> on February 20,- 1974 *. At this time, an explosion took place in the "3B" off gas filter. The resultant pressure increase ruptured the 3A" Steam Jet Air Ejector (SJAE) rupture disc and released off gas to the "A" SJAE :.t':oom. Thi\ cpndition is contrary to section l.A.5 of the Technical Specifications ~hich defines an abnormal occurrence as an abnormal degradation of a boundary designed to contain the r~~ioactive material resulting from the fission p~cess.

PROBLEM At the time of the occurrence, the unit was in steady state operation at .a power level of 760 MWe (2232 MWt). The "3A" Steam Jet Air Ejector and the "3B" off gas filter were in service.

The first indication of a malfunction was the complete loss o~

all SJAE flow indication. Simultaneously, alarms annunciated for Off Gas System "3A" Hi-Lo Flow, Off Gas Filter High AP, and* SJAE Liquid Drain Hi Level. The l.atter two alarms cleared immediately. The Off Gas Line High Pressure alarm did not annunciate at the time of the explosion.

Mr. J. F. O'Leary March l, 1974 It was postulated, at this time; that one of three malfunctions had occurred. Either there had been an explosion, the SJAE rupture disc had failed, or there was a malfunction in the SJAE flow instrumentation.

It was not known exactly what had happened, because there had been no change in condenser vacuum and the area radiation monitor (ARM) units in the SJAE area bad indicated no change in gamma radioactivity.

Station personnel were dispatched to the SJAE room to investigate.

They observed dust in the air and heard a blowing sound in the SJAE room, which indicated a possible loss of integrity of some component of the air ejector system. Personnel working in the area at the time of the occurrence reported hearing a loud noise. This information was relayed to the control room.

This information, and the radioactivity indicated by the ARM units in the SJAE area which had started to increase, prompted station management to expedite the shutdown from power at 1159 hours0.0134 days <br />0.322 hours <br />0.00192 weeks <br />4.409995e-4 months <br />. A controlled load reduction had been initiated earlier in the sequence of events, but it was felt that conditions now dictated a more rapid power reduction. The load reduction was accomplished by reducing to minimum recirculation flow immediately, inserting control rods, and manually scramming the reactor from 340 MWe. The time of the scram was 1208 hours0.014 days <br />0.336 hours <br />0.002 weeks <br />4.59644e-4 months <br />. Following the scram, the operator broke condenser vacuum and isolated the reactor. All transients were normal and the cooldown was accomplished using normal station operating procedures.

The evacuation siren was sounded at 1157 hours0.0134 days <br />0.321 hours <br />0.00191 weeks <br />4.402385e-4 months <br /> and all personnel on site were promptly assembled. Generating Stat.ions Emergency Plan (Condition ABLE) was initiated at 1216 hours0.0141 days <br />0.338 hours <br />0.00201 weeks <br />4.62688e-4 months <br />. Following a thorough invest-igation of the condition, GSEP (Condition ABLE) was terminated at 1727 hours0.02 days <br />0.48 hours <br />0.00286 weeks <br />6.571235e-4 months <br />.

Following the assembly of personnel, Radiation Protection established a controlled perimeter and began a station contamination survey.

No contamination attributable to this occurrence was found, with the exception of the SJAE room. The entire site had been surveyed and released by 2240 hours0.0259 days <br />0.622 hours <br />0.0037 weeks <br />8.5232e-4 months <br /> the day of the incident.

Prior to releasing personnel from the assembly areas, individual contamination surveys w~re performed by Radiation Protection personnel.

Eleven individuals bad some localized low level clothing contamination that could be attributed to the :incident. All contaminated clothing was collected to allow time for decay of short lived radionuclides. The majority of the clothing was released following a four hour decay time and a subsequent survey. Only one of the eleven individuals_had low level skin contamination which was promptly decontaminated with soap and water. As a precautionary measure, whole body counts were performed on all individuals identified as being in the vicinity of the "3A" SJAE room during the condition. Results of these whole body counts indicated no internal contamination that could be attributed to the incident.

• l *, *.:;,;.

Mr. J. F. O'Leary * 'fr'*'

March 1, 1974 INVESTIGATION Following the explosion, the "3B" off gas filter was Di-Octyl Phthalate (DOP) tested and found to be 72% efficient. This filter was replaced and inspected by station personnel. This inspection strengthened the suspicion that the explosion had initiated at the "3B" off gas filter and revealed damage to the filter. The irispection of the filter also revealed that the filter had been grounded in accordance with the procedure supplied by the Commonwealth Edison Station Electrical Engineering Department.

Discussions with MSA, the manufacturer of the filter, have revealed that the filters contain two separate foils and that both should be grounded.

The existing procedure called for grounding one of the foils.

The "3A" SJAE room, all welds, flanges, and accessible off gas piping were inspected. The inspection verified that only the "3A" SJAE rupture disc had failed.

There are two pressure switches in the off-gas line. Because the control room did' not receive an off-gas line high pressure alarm at the time of the occurrence, the instrument mechanics checked each of these switches. One of the two switches was found to be failed in a tripped position. However, for th~s system to alarm and to initiate an isolation, both switches have to trip. The second pressure switch and alarm circuitry were found to be operable. It was therefore concluded that the pressure at this point in the off-gas pipe was not sufficient to trip the second switch and to alarm and isolate the off-gas pipe.

Although no significant release of radioactivity to the environs was believed to have occurred, a precautionary off-site sampling program was initiated. samples of airborne particulate and iodine activity,

• vegetation, soil and milk were collected by the environmental contractor.

Preliminary results from these samples indicate no concentrations of radioactive material attributable to this incident. Direct radiation measurements from paired ionization chambers were determined, both upwind and downwind of the station. No ioniz~tion chamber readings above normal were detected in either of these sectors subsequent to the event. Dose determinations are in the process of being calculated by the meteorological contractor. A final report from the environmental contractor has not been p~epared at this time.

During the incident, the maximum release rate of radioactive gases was 100 millicuries per second with a total release of 224 curies for a 57 minute period. The normal release rate prior to the incident was 29 millicuries per second with a total release of 100 c~~~~ for the same period of time. The net release attributable to the occurrence, therefore, was 124 curies.

. ~* .

Ar '* * " ' :

Mr. J. F. O'Leary . March 1, 1974 CORRECTIVE ACTIONS The "2A" off gas filter was removed and used to replace the damaged "3B" off gas filter. This filter was DOP tested after installa-tion in the "3B" off gas line and found to have a removal efficiency of 99.03%. The "3A" SJAE is currently out of service and the "3B" SJAE has been placed in service. A maintenance request has been issued to replace the "3A" SJAE rupture disc. The failed pressure switch on the off gas holdup pipe has been repaired.

In addition, the procedure for grounding off gas filters has been revised to reflect the new information received from the MSA filter representative. At this time, Unit 3 is operating with a filter grounded at one end only. This unit is scheduled to shutdown on March 4, 1974 for a refueling outage. During this outage, the filters on Unit 3 will be grounded at both ends. The filter on Unit 2 has been modified for grounding at both ends pe! the new procedure.

EVALUATIONS This incident did not endanger the safety of the public or of plant personnel. The immediate actions taken, as described above, were appropriate to correct the condition and bring the status into compliance with the Technical Specifications.

• Dresden has not had any previous hydrogen explosions in in-service off gas filters on either Unit 2 or 3. The new off gas filter grounding procedure should reduce the probability that this incident will recur in the future.

The continued operation of Unit 3 was the subject of On-Site Review 74-48, on February 21, 1974. The conclusions of this review stated that there was no known reason to limit continued operation of Dresden Unit 3.

Sincerely, W, .p,_ -W~(>r~

W. P. Worden

.Superintendent WPW:do