ML20056B750
| ML20056B750 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 07/23/1971 |
| From: | Duncanson R NORTHERN STATES POWER CO. |
| To: | Morris P US ATOMIC ENERGY COMMISSION (AEC) |
| References | |
| NUDOCS 9102080492 | |
| Download: ML20056B750 (8) | |
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Regulat:ry File Cy NSP NORTHERN STATES POWER CCMPANY Minneapolis, Minnesota 55401 I
July 23,1971 1
Dr. Peter A. Morris, Director Division of Reactor Licensing United States Atomic Energy Commission Washington, D.C.
20545
Dear Dr. Morris:
MONTICELLO NUCLEAR GENERATING PLANT Docket No. 50-263 License No. DPR-22 Reactor isolation from 90% Power and Subsequent Events A condition occurred at the Monticello Nuclear Generating Plant on July 14, 1971, which we are reporting in accordance with the reporting requirunents of Section 6.6.B3 of Appendix A, Technical Specifications, of the Provisional Operating License DPR-22.
The Region t il Compliance Office has been notified in accordance with the reporting requirements of 6.6. A of the Technical Specifications.
The attached report, " Reactor Isolation from 90% Power and Subsequent Events" describes the details of the occurrence and the corrective actions taken as a result of the occurrence.
Yours very truly, hb
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4 uttx.ewwows R. O. Duncanson, Jr., P.E.
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UONT ICELLO NUCLE AR GE!.ERAllW PLANT 7
July 23,1971 S ubjec t: Reactor isolation from 90% Power and Subsequent Events 1.
Su marv Descriet inn of Occurrence At approximately 1340, Wednesday, July 14th, a reactor isolation occurred while operating at approximately 90% of rated power. The isolation was initiated by noise spiking on the main steam line flow sensors while completing scheduled surveillance testing.
After clearing the isolation condition, condenser vacuum was re-established. Shortly thereafter, the gaseous activity in the reactor building vent stack and the off-gas stack increased and airborne contamination was detected in the turbine building.
Maximum activity Icvels of approximately 35,000 uc/sec at the off-gas siack and 20,000 uc/sec at the reactor building vent stack existed for approximately l
30 minutes.
Concurrent with the reactor building veni stack release, an increase in the discharge canal activig was measured. The measured liquid activity of approximately 2.5 x 10 oc/cc has been determined to be the result of cooling tower scrubbin2 of the reactor building vent stack discharge which was blown from ihe plant ioward ihe iowers.
2.
Detailed Descriotion of tho Occ ur r e nc e a.
Summarv of Condiiions At the time of the occurrence the reactor was operating at a steady power level of approximately 90%.
b.
Account and Analvsis of ihe Occurrenco
( L'oi e:
Where times are listed to the second, they wcre taken from the computer alarm prinitr; all other times are according to the control room log, correlat ion with recording charts, or from personnel observations).
134E01 A reac t or sc ram f r u.
90% power occurred due to a high flow isolation of the Main Steam leolation blves.
The isolation was caused by a spike introduced i n t he high st e am flow of fferential pressure switches while valving one of the switches into service following surveillance testing.
134F04 The normal low reactor water level transient which followed the scram ini tiated a group 2 and 3 isolation and started the Standby Gas Treatment System.
Iniiiation of the Standby Gas Tre.atment System isolated the reactor building ventilation system.
134S The reactor pressure was controlled automatically by the relief valves.
to The relief valves opened 9 times during this period mainiaining the reactor 1411 pressure below 1100 psig.
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134935 The reactor water level increased to the high level setpoint causing a 3
turbine lockout. The iurbine lockout initiated an automatic closed transfer s
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. of auxiliary power from the #11 main auxiliary transformer to the IR reserve auxiliary transfer er and < tarted both diesel ponerators.
Tre Turbine Lockout also tripped both reactor recirculation pumps. All of these interlocks operated properly.
134843 No.12 Reactor Feedwat er Pump was manually tripped.
1350 An attempt was ma+ to re-establish steam flow to the condenser after it had been determined that the isolation was a result of the surveillance j
test.
It was discovered that the outboard main steam line drain isolation valve (MO-2374) could not be opened to pressurize the steam line prior to opening the MSIV's.
135140 Due to the failurc of MO-2374, which prevented normal pressurization of the main steam line, the operator attempted to use the test pushbutton to slowly open one of the MSIV's; however, because of the large differ-ential pressure existing across the MSIV's, another high flow isolation occurred immediately afte r the valve started to open.
135514 The reactor water level was approximately 90 inches.
The reactor water cleanup system was placed back in service with dump flow established to reduce the reactor water level.
1356 A second attempt was made to pressurize the main sicam lines by opening a MSIV using the test pushbutton.
A high flow isolation immediately reclesed the ClV.
1400 Both reactor recirculation pumps were restarted.
1401 Reactor Cl(anup System isolated due to high outlet temperature from the non-regenerative heat exchanger.
The high temperature isolation of the cleanup system has occurred on several previous occasions when the dump flow is increased to reduce the reactor water level. The increased dump flow reduces the flow through the regenerative heat exchangers and results in higher iemperatures to the cleanup demineralizers.
When the high temp-erature isolation occurred, an operator was dispatched to the cleanup system area to manually increase the service water flow.
1403 No. 11 Reactor Fcedwater Pump was manually tripped.
1405 Reactor Water Cleanup System was returaed to service and dump flow re-established to decrease reactor water level.
The condenser vacuum was decreasine due io loss of the air cjector and turbine gland seal steam supply.
In order to maintain condenser vacuum, the mechanical vacuum pump was placed in service dischsrging through the 1.75 minute off-gas line.
1407 LO-2374, Ma i n S t eam Dr a i n Isolation Valve, was opened by manually operating the breaker contacts until the valve was fully open and then pulling the breal er.
This established steam supply to the air ejector and the turbine sicam scal system. The steam flow was sufficient to prevent further relief valve openings.
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. (It was subs equently deternined that the flow switch trip relay fuse which was pulled to complete tne high steam flow differential pressure switch surveillance test prevented the opening of MO-2374).
1407 The stack gas radiation monitor began to show an increase as a normal result of the hi h off-gas flow.
B The No.12 air ejector steam supply pressure went to full scale (200 psig) i and remained there for approximately 5 minutes indicating that air ejector steam supply pressure control system was not operating properly.
The air ejector off-gas flow recorder also went to full scale (200 CFM),
and remained there for approximately 30 minutes.
Although this condition is normal while establishing condenser vacuum, the high steam supply pressure to No.12 air ejector further increased the off-gas flow.
1409 The turbine building Continuous Air Monitor (CAM) which monitors the activity of the turbine building equipment drain sump area began to increase. The increase in activity was the result of losing the air ejector off-gas loop seal when the air ejectors were returned to service at 1407.
The loop seal drains condensation from the 30 minute off-gas holdup line to the turbir:e lauilding equipment drain sump. With the water lost from the loop seal, a direct path was established to vent off-gas from the 30 minute holdup line into the sump.
The 1411 The main steam line pressure had increased to approximately 450 psig.
L" lV's were opened and the line pressure immediately increased to 925 psig.
Difficulty was expcrienced controlling the steam packing exhauster suction is postulated that the steam seal regulator did not respond It pressure.
fast enough to the s dden re pressurization of the steam lines and may havc resulted in leakage of steam to the turbine building through the i
turbine shaft scals.
1412 The mechanical vacuum pump, which is located in the same room as the turbine building equipment drain sump, was shutdown.
As the operator was leaving the sep room area, a portable radiation detector located outside the sump room area alarmed.
A CAM located in the same general area also alarmed.
The Radiation Protection Engineer and assistant who were also in the area and the operator proceeded to the accesu control area for checkout. The three personnel required washing of exposed areas.
The Radiation Protection Engineer informed the Shift Supervisor of the airborne activity in the turbine building. Subsequent entries into the turbine building and the reactor building were made wearing full face masks and protective clothing.
The Radiation Protection Engineer requested that an air sample in the crea of the turbine building CAM be obtained for analysis._gThe sample, taken uoi/c -
A Spectrum at 1423, showed an airborne concentration of f 4 x 10 13 of the sample indicated the activity to be Rb ' and Cs i
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. Subsequent urinalysis (Eanma and beta) for the three personnel showed no above normal activity.
1414 The stack gas radiaticn monitor reached the high alarm setpoint (8000 uci/sec.).
1415 The activity released in the lower areas of the turbine building was carried into the reactor building exhaust plentrn through the normal ventilation system causing an increase in the indication of the reactor building vent CAM.
1417 The off-gas stack release rate leveled off at 35,000 uci/sec. 141745 The reactor building exhaust plenum high radiation alarm was received. The alarm setpoint of 1.25 mr/hr is equivalent to a reactor building vent stack release of approximately 10,000 uci/sec. 1419 The discharge canal radiation monitor showed an ingease in activity.
The to measured liquid activity of approximately 2.5 x 10 uci/cc has been 1459 de 1rmined to be the result of cooling tower scrubbing of a very small 4
portion of the reactor building vent stack discharge which was blown from the plant toward the towcr by a 10 mph wind.. A gamma spectrum of a sa taken from the canal monitor discharge showed that the activity was Rbgle and Cs135 The analysis confirmed the concentration indicated by the discharge canal monitor.
From approximately 1430 to 1445, the reactor building plenum exhaust fans were shutdown to stop the release of activity from the reactor building vent stack.
Except for this fifteen minute period, the reactor building vent stack release was approximately 20,000 uci/sec. 1430 The charcoal and particulate filters in the reactor building vent sampling system were changed.
Immediate count of the particulate filter indicated a release rate of approximately 1.3f3uci/se 4nd a Comma EPectrum of the filter showed the vity to be Rb and Cs135 An analysis of the charcoal filter showed no i 1430 CAMS located on the reactor building refueling floor and ground level floor to siarted to increase.
1440 A smoke bomb test performed on July 22, 1971, revealed that with the reactor building isolated and the plenum fans shut down, leakage into the reacior building occurs around the reactor building exhaust fan ventilation ducts I
which penetrate the plenum wall.
A small amount of leakage was also detected throuBh a set of double interlocked doors located between the turbine building and the reactor building.
l 1441 The condenser vacuum' had returned to normal with an off-gas flow of approx-l imately 10 cfm. With the reduced flow, the stack gas activity release rate decreased by approximately one decade.
j 1444 An air sample taken from ige reactor building exhaust plenum chowed a y%,
yc i/sec.
A spectrum analysis showed the T
concentrationofgd.6 x 10 l30, activity to be Rb and Cs 4
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t 4 1445 The charcoal and particulate filters in t he off-Eas stack sa pling system were changed.
An i nediate count of it c particulate filter indicated a release rate of 1.99 uci{sec anc ggamma spectrum of the Rbi and Csl A 48 hour5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> analysis filter showed the activity to b(
of the particulate filter showed that a long lived activity release 4 uci/sec occurred.
Analys relcase rate of 4.09 x 10 js of the charcoal filter rate of E.7 x yI uci/sec.
indicated an i Airborne activity was detected in the access control area of the admin-istration building.
An air sample showed the concentration to be 1.
x 3
10-7 uci and Cs "fsec.
A spectrum of the sample showed the activity to be Rb l
With the exhaust plenum fans shutdown 3 a strong draft exists between the turbine building and the administration building. This draft held the access control area door to the turbine building open and carried airborne act ivity into the access control area. The door was inmediately closed.
An air sa,ple at 1510 showed a concentration in the access control area A sample at 1535 showed a concentrat ion of 1.1 x 10-gf 6.7 x 10-9 uci/ce.
oci/cc.
T h( reactor building plenum exhaust fans were restarted.
Normal turbine building ventilation ficw was established.
1500 An air sample was taken on the turbinc building operating floor.
A count o f t he sa pl e s howe d t he ac t i vi t y t o be 3. gAx 10-0 ugi/ce.
A spectrum of t he sample showed the activity to be Rb and Csl5 The reactor building cxhaust plenum monitor indication began to decrease.
1505 The Standby Gas Treatment System was shutdown and the reactor building vcnt ilat ion system was ret urned to normal.
1509 The reactor building cxhaust radiation monitor indication decreased below the 1.25 mr/hr high level trip point.
1530 An air sample was taken on the turbine building operat ing floor.
A count o f t he sampl e showed t he ac t i v i t y t o b e 1.1 x 10-l uci/cc.
T he C A'l locat c d on the reactor building refueling floor peaked at 1.0 x 10-0 uci/cc.
I 1545 The CA'.1 lopated on the ground level of the r(actor building peaked at l.96 x 10-5 uci/cc.
154S An air sampic taken on the ground level of the reactor building showed a concentrat icn of 7.6 x 10-S uci/ce.
1630 The reactor building vent CAM returned to normal indication.
1706 The airborge activity on the turbine operating floor was measured as 1.24 x 10- uci/cc.
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l 1727 The airborne activity in tht turbine building equipment drain sump area i
was measured as 1.24 x 10-' uci/cc.
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1745 The charcoal and particulate filters in the reactor building vent sampling syst em were changed.
A 48 hour5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> analysis of the a long livgd activity release rate of 163 x 10_garticulate filter showed i
uci/sec. and the release
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rate of 1 1I t o be l ess t han 6.1 x 10-" uci/sec.
d 1800 All indications showed that plant airborne activities had returned to 4
I normal.
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Ac" cunt of Additional Aciiviiirs c.
1700 An NSP consultant was contacted to discuss cooling tower scrubbing of i
July airborne activity. Discussions strongly supported our determination 14 that the discharge canal act ivity was the result of cooling tower i
scrubbing of gaseous activity.
1900 The Operations Committee met to review the occurrence and determine to what actions were requircd prior to restarting the plant.
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2300 T he off-gas HEPA fil ter was DOP tested and found to be 99.9 percent July ef fic ient.
The filter was tested because of possible damage due to j
14 high off pas flows which had occurred while re-establishing condenser i
vac u um.
Jul y The Region t il Compliance Office was advised of the occurrence.
The 15 occurrence was reported on the basis of an unplanned release.
AM July it was determintd to ranain shutdown and go into a turbine screen removal 15 o ut age.
The outage was anticipated to last nine days.
Noon July The Vinnesota Depariment of Healt h was rcquested to analyze the on-15 si te and off-site t hermol uminescent dosimet ers.
The TLDs (12), which PU had been placed on location four days carlier, were analyzed and showed no indicatienc above background.
Jul y Grass, mud and soil samples taken near the cooling towers showed no 16 above normal activity.
Jul y Grass, mud and soil samples taken three miles east (downwind) of the plant 21 showcd no above normal activity.
3.
Corr <ctivo Actions To prevent a recurrence of the unplanned gaseous release, the operating
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procedures for the recovery from an isolation scram have been reviewed 7
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. and supplmented with a temporary memo (Volume f) to the Operations Manual.
The detailed supplemental procedure addrrests itself to the re-establishment of normal operation of the air ejector system and the turbine steam seal systun during the recovery from an isolated condition.
In addition to the procedural changes, the following systen control changes have been completed to assure proper recovery from an isolated condition.
A new cont rol switch has bcen provided in the control room which a.
will allow the operators to isolate the off-gas line loop seal during high off gas flow periods.
The isolation will be removed when off Eas flows return to normal.
b.
The control switch, which is used to isolate the off gas flow to the air ejectors, has been interlocked to also isolate the sicam supply to the air ejectors when the switch is in the CLOSED position.
This interlock will provide the operator with isolaiion capability from the cont rol room and facilitate recovery from an isolated condition.
Tcsting will be performed during the next reactor startup to verify proper operation of the control system changes.
Also, the duct pcnetration seals from the reactor building exhaust plenum to the reactor building have been repaired to prevent the possibility of airborne in leakaEc to the reactor building if the plenum exhaust fans are shutdown.
Analysis of the occurrence is continuing and furthcr actions may be initiated to prevent similar occurrences.
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