ML20056B822
| ML20056B822 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 03/22/1971 |
| From: | Duncanson R NORTHERN STATES POWER CO. |
| To: | Morris P US ATOMIC ENERGY COMMISSION (AEC) |
| References | |
| NUDOCS 9102110411 | |
| Download: ML20056B822 (12) | |
Text
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NORTHERN STATES POWER COMPANY Minneapolis, Minnesota 55401
uld be contro11ed' by the bypass valves.)
- 4) As a precaution, operators had been dispatched to strategic ' points.
i within the plant in anticipation of actions that might be necessary to recover from the test if plant response to the trip test did not go -
as expected.
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Account of the Occurrenm 025527 Generator output breaker SNS opened manually.
025530 Generator output breaker 8N4 opened manually.
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025530 The plant response to the load rejection test was proceeding to as ex ected.
One alarm UNINTERROPTIBLE MG SET DC MOTOR 025617 RUNNifGwasreceived.khis action apparently resulted because of a slight frequency increase due to an expected increase in turbine generator speed.
025617 Turbine lockout occurred, causing loss of plant auxiliary power.
This properly resulted in the trip of
- 11 recire pump and MG set
- 12 recire pump and LG set
- 11 circulating water pump
- 12 CRD pump
- 11 reactor feedwater pump
- 11 Reactor Protection System LG set i
- 12 Reactor Protection System LG set i
The emergency diesel generators started when power.was lost, but their output breakers did not close because the essential buses were properly re-energized automatically via the 1 AR transformer.
i 025641 Scram occurred as Reactor Protection S stem LG sets' output y
j breakers were automatically tripped by their underfrequency relays. The LG sets had been coasting down for about 24 l
seconds following the loss of power.
Because~ of the loss of power to the protection system, all the scram relays de-energized and primary containment isolation occurred.
I 0257 The RCIC system was operated to inject water into. the vessel.
l The lowest reactor water level recorded was +14" (scram = 6")
at approximately 0257, 20-30 seconds after the scram.
r 0258 IRMs were downranged to follow the decay of reactor power.
0258 Operator verified that the uninterruptible LG. set had switched back to a-c power. #12 RPS LG set was returned to operation.
(The #11 RPS LG set output breaker was. reset app'roximately six minutes after scram.
Operator in the reactor biilding a failed to reset the #11 RPS LG set output breaker when #pparently 12 RPS LG set was properly reset.)
0258 The #11 feedwater pump and #12 CRD pump were restarted. The RCIC system was transferred back to the TEST condition. Water level in the vessel was increasing at the rate of approximately.
2 inches per minute.
0257 An attempt to start both recirculation pumps failed; the #12 to the 0303 # pump was started twice but tripped from a lockout each timelure 11 pump could not be restarted because of the apparent fai
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of a permissive interlock to operate properly. Operators were also unsuccessful in closing the suction valves to isolate s
each pump in an effort to protect the seals of the shutdown purps.
(Due to the known potential damage to the pump seals, the operators had been instructed to isolate a pump within ei minutes i f the pump could not be restarted after a trip.)ght (See Section 4 of the report for more details.)
0310 Drywell pressure had risen from 14.65 to 15.25 psia.
Observation of the recire pump seal pressure indicators confirmed that the seals on the #12 pump.had failed.
0312 Torus temperature had risen slightly as a result of dumping RCIC exhaust steam into the torus.
The #11 RHR pump and #11 RHRSW pump were started to cool the torus. The RHR heat l
exchanger service ' water outlet control valve, CV-1728, failed to open. #11 PHRSW pump was tripped and restarted twice without the valve opening.
Following the third start, the operator switched the val"e controller to manual and the valve i
opened.
After flow was established the controller was returned to AUTO and the valve operated properly.
031502 The reactor feedpump was taken out of service because water level had risen to 50 inches.
Water was also being added to the vessel by the CRD pump.
0318 The CRD flow to the vessel was decreased by shutting off the charging water header.
The RCIC (in the TEST mode) had tripped off due to high water level, but was subsequently restarted and operated in the TEST mode.
0325 Drywell pressure was increasing at the rate of about 2 psi per hour.
Attempts to close the suction valves to the recire pumps were still not successful.
( At this tine, it was found that the drywell sample isolation valves would not open.)
0334 The Standby Diesel Generators were stopped.
0340 The drywell pressure continued to increase and indicated 15.4 psia.
It was decided to vent the drywell through the Standby Gas Treciment System.
The SGTS had automatically star +ed when power was lost and was drawing its suction from the reactor building.
The 2" vents from the drywell and torus were opened to the SGTS for about two minutes; however, this was apparently not adequate and the drywell pressure indicated 15.6 psia. The 18 inch vent, line was opened for approximately one minute and the pressure decreased rapidly to 14.1 psia in the drywell and 13.9 psia in the torus. Stack gas activity during venting was approximately 15 ue/sec. No additional venting of the drywell through the SGTS system occurred until approximately 0920 after drywell air sample were analyzed.
(Lower level drywell temperatures had
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j increased apprgximately twenty to twenty five degrees to a maximum of 101 F.
Midlevel drywg11 temperature had increased about thirty five degrees to 106 F and the top levg1 drywell 1
temperature had increased about ten degrees to 120. )
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1 0355 The plant chief electrician was called to assist in getting
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the recirculation pump suction valves closed.
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i 0355 Reactor pressure remained steady at 920 psig.
Water was to being added to the vesseliwith the CRD pump.
Water was being 4
0415 extracted through the cleanup system and RCIC steam wasabeing extracted from the vessel with the RCIC in TEST.
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0410 The suction valve to the #11 recirculation pump closed while i
an operator was continuing his attempts to close the valm j
from the control room.
(Apparently a limit switch problem i
j-cleared to allow this action) s 1
i 0410 Observation of the pump seal pressure indicated that the inner j
seal on the F11 pump might have failed.
l 0410 in preparation for re-establishing condenser vacuum, the outboard MSIV's were opened, and it-was attempted to pressurize between the
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inboard MSIV's and the turbine stop valves by using the main i
steamline drain line.
However, it was found that valve MD-2374, j
the inner drain line outboard isolation valve, would not open.
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0415 The chief electrician arrived on site.
After a quick briefing j
of the situation, the #12 recire pump suction valve was closed j
by manually holding in the closing contactor on the valve motor.
0420 NSP Plant Superintendent was notified. He proceeded to the site.
4 0425 NSP Plant Results Engineer.was notified.
He proceeded'io-the site.
1 0425 The chief electrician assisted in opening the main steam line i
to-drain isolation valves. This was accomplished by manually j
0430 holding in the opening contactors at the motor' control centers.
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0430 NSP Radiation Protection Engineer was notified.
He proceeded' to j
the site.
0433 Turbine sealing steam was established.
The condenser vacuum pump was started.
Reactor depressurization was begun.
1 0445 G.E. Operations Manager arrived on site.
0500 The NSP engineer began recording the reactor pressure readings every three minutes to determine the reactor saturation temperatures.
This data was being taken as part of a cooldown rate study; however, an additional purpose was accomplished i
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0610 The final scram trip from 16w condenser vacuum was cleared (at approximately 570 psig reector pressure) and the scram was reset. The scram reset cleared the drywell sampling system isolation signal.
0700 Reactor temperature was 433 F.
Drp/ml. ell sample showed total activity (particulate) of 3 26 x 0745 uct No iodine was detected.
I 0830 The cooldown rate study was terminated at approximately 365 F.
2 0920 Dyrwell venting through the SGTS was initiated.
1135 Shutdown cooling vias begun with #11 RHR pump.
-11 1200 Drywell sample showed total activity of 3.61 x 10 uci/ml.
1430 Entry was made into the drywell by radiation protection man and j
two others for general survey of the drywell.
No unusual I
conditions were found.
Drywell temperature and humidity were normal and no moisture or moisture damage was evident.
Air j
sample taken near the #12 recire pump showed 3.39 x 10 10 u ci/ml.
j 1600 Reactor vessel pressure was 15 psig and the reactor temperature was 250 F.
1622 SGTS taken out of service (Subseauent DOP tests of the HEPA filters showed greater than 99.9% efficiency.)
1628 Containment was purged with normal ventilation flow discharged i
through the reactor building ventilation stack after the reactor water temperature had decreased below 212 F.
j 1800 Radiation protection personnel entered the drywell and completed a contamination level survey.
2
- 11 recire pump (seal area) - 131,415 dpdem2 2 -
- 12 recire pump (seal area) - 566,15g dpdem Lower level from 46 to 9449 dpd100cm Upper level from 2 to 1565 dpv100cm2 1900 Reactor pressure was 0 psig.
2140 Reactor temperature at 100 F.
March 11 0828 Drywell air sample taken near the #
recirculation pump seal showed total activity of 5.16 x 10-uci/ml.
No iodine was detected.
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0900 Work was begun in the drywell on the #11 recire pump seal after the pump seal area was decontaminated.
l 3.
Re nonse in the Oenurrence
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Three aspects of personnel response to the occurrence nerit comment.
i The general response of supervisors and operators to the occurrence a.
j was excellent.
Priorities of actron co,oerning the reactor conditions, j
restoration of plant auxiliary power, recirculation pump considerations, j
turbine generator considerations and general plant conditions were proper and correct. The initial i,mproper resetting of the sh2 RPS j
W set output breaker and the relatively late callout of the electrician j
to analyze and complete the work ; required to close the recirculation I
aump suction valves are the only apparent actions which could or should i-1 ave been completed more promptly. - -
b.
The decision to vent the drywell through the Jiandby Gas Treatment System, without an air sample analysis, is considered as proper action consistent with the procedural responsibilities afforded to senior licensed personnel as described in Volume A of the Operations Manual.
Momentary venting of the drywell approximately 40 minutes after the shutdown l-was decided with the knowledge and understanding of existing plant
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conditions.
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The Reactor Log entries (late entries) describing operator actions in response to the occurrence understandably cannot provide a detailed timely account of all operator actions.
Recognizing this, consideration h
is being given to furnish the control room with a tape recorder to allow P
the operators to maintain an account of their act.ons during extremely busy tines and provide the means for more accurate and complete late entries in the Reactor Log.
p 4.
Corr'ective Actions Reouired i
j The following aspects of the occurrence required investigative or corrective t
actions.
l a.
Cause of Turbine Lockout j
The cause of the turbine trip cannot be absolutely established but the most probable cause was a high water level trip from a moisture 3
j separator tank.
Ubisture separator drain tank high' levels are alarned; however, the moisture separator high. level trips which operate the e
turbine lockout relay directly are not alarmed.
No other possible l
initiating conditions are known to have existed at the time of the i
turbine lockout.
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The four moisture separator drain tank drain valves and controls were i
checked out and one drain valve was found to operate improperly.
i The valve was repaired.
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The moisture separator tank high level switches have been connected j
to the computer and will initiate a sequence of vents log
- entry for any future moisture separator tank high level condition.
l b.
Failure of the Recirco1= tion Pomns to Restart i
When the turbine lockout occurred, both recirculation pumps are tripped
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directly from the turbine lockout relays. The turbine lockout bypass j
switches were placed in the " bypass" position; however, the pug l
drive motor breakers would not close.
Further investigation revealed i
that the loss of power to the RPS hG sets causes primary isolation i
which also trips the recirculation pumps.
Following the return of the RPS IG sets to service, the No.12-recirculation pug was started but tripped from a lockout approximately 5 seconds following the generator j
field application. A second attempt to start the No.12 pump resulted i
in a similar lockout trip.
Attempts to restart the No. 11 recirculation j
pump were unsuccessful because of the failure of a valve position permissive interlock to operate properly.
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The failure of the No.12 pump to start has' been traced to an incorrect i
setting on the pump differential pressure time delay relay installed i
in the auto start sequencing logic and a zero shift in the pump
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differential pressure transmitter at 1000 psig. -The differential j
pressure transmitters for both pumps have been calibrated to eliminate the zero shift problem and the entire auto start sequencing logic for j
j both pumps has been checked for proper time delay settings.
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Recirculation pumps are started with the suction valve open, discharge i
valve closed and the discharge bypass valve open. These valve positions j
are interlocked to the pump start circuit and if they are not properly l
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set, the pump motor breaker is prevented from closing.
All recirculation l
J system valve interlocks have been checked and readjusted where necessary to assure proper interlock operation.
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-Proper operation of the recirculation pumps and valves has been.
verified by testing in the cold condition.
Hot restarts of_ the pumps and proper operations of the system valves will be completed in non-j junction with 25% power level testing (loss of power test) scheduled i
to occur within approximately one week.
c.
Failure of the Recirculation Pem Suetion Valves to Close and the Main Steam Line Drain Valves to (ken The failure of the recirculation system suction valves to close and the main steam line drain valves to open was caused by improperly 7
set torque limit switches. Torque switch settings on all reciraslation system valves and on the steam line drain valves. have been checked and.
1-properly reset where required.
Testing of the valves has been completed i
under cold system conditions to verify proper operation.
i-The problem encountered with the torque switch settings has prompted a review of the torque switch setting practices and the operational j
' testing of the related valves under their normal operating temperatures.
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- Additional valves have been checked for proper operation and, if necessary, additional valve testing will be conducted at rated temp-erature and pressure prior to further power testing. A program for continual surveillance of torgae switch settings and the operation of their related valves will be established.
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d.
Failure of the Reci rcul at i on Pomn Seals On March 5,1971, the No.11 recirculation pump was inadvertently tripped
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at rated pressure and temperature condi tions but was successfully restarted on the second attempt.
During this pump trip, the seal i
temperature had increased to the point of concern.
Discussions with Bingham pump personnel on March 8th revealed that seal leakoff isolation valves are required in the seal leakoff lines (following the upper seal pressure breakdown coil - see attached figure) to prevent seal damage following a pump trip unless the pump can be restarted within 10 to 15 minutes. Tripping a recirculation pump
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without the isolation valves results in primary, system water leaking 1
past the pump bearing and lower seal resulting in a failure of the lower and upper tungsten carbide seals due to thermal shock conditions. '
m On the basis of the information received from the pump manufacturer, the operators were instructed to isolate a recirculation pump within i
eight minutes after it tripped if the pump could not be restarted.
- Also, the installation of the seal leakoff isolation valves was planned for the next shutdown.
On March 10th, after the plant shutdown occurred, unsuccessful attempts were made to isolate the recirculation pumps after the attempts to restart the pumps were unsuccessful.
Several minutes after the shutdown the inner seal temperatures on both pumps were greater than-3 the 300 t range of the indicator.
The No.12 pump inner seal cavity pressure dropped from a normal operating pressure of approximately 1
.500 psi to approximately 300 psi indicating a failure of the inner and outer seals. The No.11 pump inner seal cavity pressure increased to approximately 750 psi indicating an inner seal failure but no outer seal failure.
Following the shutdown on March 10th, the seals for both recirculation j
pumps were replaced. Tbe 1ower bearing on the No.11 pump was damaged and was also replaced. The seal leakoff isolation valves were installed and the controls checked out. These valves are operated from auxiliary contacts on the drive motor breaker.
When the drive motor breaker trips, the isolation valves close to prevent seal leakage from the upper seal breakdown coil.
By blocking the seal leakoff path, the cooled water i
is trapped in the seals preventing hot primary system water from entering along the pump shaft through the pump bearing and lower seal.
e.
Failure to be Able 1o Onen the Drvwell Samnlino isolatinn Valves from the Control Rnnm
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Following the turbine trip and reactor scram, the primary containment sampling system valves isolated properly; however, after the plant auxiliary power was restored, the sampling system valves could not be
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opened.
No electrical or control system problems could be found. The valves opened when the scram was reset.
Investigation of the situation revealed that the instrunent air supply to the sample valves was from the CRD scram air header which is de-pressurized by the backup scram solenoid valves following a scram.
The air supply to the sampling system valves has been changed and now comes directly from the reactor building air supply header.
f.
Errects or Possible Errect of Drywe11 Vaisture on Nucienr Instromanf ation Following the plant shutdown on March 10th, the nuclear instrunentation responded properly. Subsequent checks of the LPRM channels showed that nine detectors were reading high.
Cable resistance measurements were made on all LPRM cables and-the nine suspect channels were ;found to have low resistance. The cable connectors in the drywell were l
cleaned and the resistance values returned to normal.
Prior to the plant shutdown, one IRM channel had been noisy.
During the shutdown, the detector and preamp were replaced to eliminate the noise problem.
Presently, all nuclear instrumentation is operating properly.
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