ML20235C857
| ML20235C857 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 09/17/1987 |
| From: | Musolf D NORTHERN STATES POWER CO. |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| GL-87-12, NUDOCS 8709250037 | |
| Download: ML20235C857 (13) | |
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Northem States Power Company 414 Nicollet Mall Minneapolis, Minnesota 55401 Telephone (612) 330-5500 September 17, 1987 Generic Letter 87-12 Director of Nuclear Reactor Regulation Attn: Document Control Desk U S Nuclear Regulatory Commission Washington, DC 20555 PRAIRIE ISLAND NUCLEAR GENERATING PLANT DOCKET NOS.
50-282 LICENSE NOS.
DPR-42 50-306 DPR-60 Response to NRC Generic Letter 87-12, Potential for Loss of Residual Heat Removal While the Reactor Coolant System is Partially Filled Attached is our response to NRC Generic Letter 87-12, Loss of Residual lleat Removal (RHR) While the Reactor Coolant System (RCS) is Partially Filled.
We believe that the procedures currently in place, and the additional procedures, training, and modifications described in this response provide essurance that this problem will not occur at the Prairie Island Nuclear Generating Plant.
Please contact us if you have any questions related to the information we have provided.
DM kw David Musolf Manager - Nuclear Support Services c:
Sr Project Manager, NRC Sr Resident Inspector, NRC Regional Administrator, Region III, NRC G Charnoff Attachment 8709250037 870917 DR ADOCK O 22 ggl tn
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UNITED STATES NUCLEAR REGULATORY COMMISSION l.
NORTHERN STATES POWER COMPANY PRAIRIE ISLAND NUCLEAR GENERATING PLANT Docket No. 50-282 50-306 RESPONSE TO NRC GENERIC LETTER 87-12 Northern States Power Company, a Minnesota corporation, by this letter dated September 17, 1987 hereby submits information required by NRC Generic Letter 87-12 for the Prairie Island Nuclear Generating Plant.
This letter contains no restricted or other defense information.
NORTHERN STATES POWER COMPANY By DJ k Aw David Musolf 4
Manager - Nuclear Support Services On this
/ M _' day of
/4[7 before me a notary public in and for said County, personally / appeared David'Musolf, Manager, Nuclear Support Services, and being first duly sworn acknowledged that he is authorized to execute this document on behalf of Northern States Power Company, that he knows the contents thereof and that to the best of his knowledge, information and belief, the state-ments made in it are true and that it is not interposed for delay.
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" " "**v: ::::::::::::::- :::::::_e JUDY L KLAPPERICM NOTARY PUBilC-MINNESOTA ANOKA COUNTY My Commission Empires Sept 29,1991 i
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' Director of NRR.
Northern States Power Company September 17,.1987 i
Attachment Page 1 PRAIRIE ISLAND RESPONSE TO GENERIC LETTER 87-12
-(l) Draindown of the reactor coolant system'(RCS) to a midplane condition is. required-to vent and drain the steam generator tubes to allow primary manway removal.for performirg steam generator maintenance. Rarely does work other than steam generator maintenance require midplane operation.
The need for additional controls with the reactor coolant system at midplane:1evel is understood and the appropriate precautions are exercised.-
a)
The evolution from full power operation to a partially filled condition typically occurs in approximately 65 hours7.523148e-4 days <br />0.0181 hours <br />1.074735e-4 weeks <br />2.47325e-5 months <br />. This time-is utilized to cooldown the RCS, including the steam generator tubes and the pressurizer steam space.
Sixty five hours also provides time for fission product decay in the fuel.
b).
Steam generator temperature is required to be less than 200*F prior to draining the RCS to midplane. Typically, during the draining process the secondary' side of the steam generators are filled and drained several times to cool the steam generator tubes and to control steam generator chemistry.
c)
Regardless of the status of the RCS, all maintenance and testing activities are coordinated through the Shift Supervisor. The work control process directives stipulate that work packages are not be released until immediately prior to actual commencement. Outage
' activities that could alter RCS level or affect residual heat removal pump operability are planned to avoid RCS perturbations. Operations that could mask a RCS level change, such as RCS purification, are stopped prior to starting the draindown, d)
The reactor coolant pressure boundary is designed with sufficient margin to insure that when stressed under operating, maintenance, testing and postulated accident conditions, the boundary behaves in a nonbrittle manner and the probability of rapidly propagating fracture is minimized.
The design reflects the uncertainties in determining the 1 effects of irradiation on material properties. The RCS heatup/cooldown curves show the system capable of withstanding 525 psig at 85"F.
e)
Should equipment hatch replacement be necessary, it is estimated that it would take approximately one hour provided there is no equipment in the passageway.
If equipment interference exists, this time could extend to as much as eight hours.
i Northern States Power Company Page 2 It is our intention to establish appropriate administrative controls during midplane operation which will assure all direct openings to the outside can be closed within a one-hour period. Restrictions will be placed on use of the equipment hatch to assure that it can be replaced during this period.
f)
Reestablishing the integrity of the RCS pressure boundary during the draindown can be performed in a very timely manner since all breached locations have operations personnel stationed there to control the draining.
Once the steam generator manways are removed, reestablishing the RCS pressure boundary rapidly is not a viabic alternative.
Reinstallation of the primary manways is estimated to take twelve hours.
The use of substitute temporary manway covers capable of withstanding low pressures is being evaluated.
(2) The following instrumentation and alarms are provided to the operators for controlling thermal and hydraulic aspects of the nuclear steam supply system j
(NSSS) during operation with the RCS draining and partially f lied (Unit I numbers are given, Unit 2 has similar equipment).
a)
LI-41079 Pressurizer Cold Calibrated Level Indicator Used to monitor RCS/ Pressurizer Level.
Calibrated for use with the RCS in cold shutdown conditions.
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Not used once tygon tube level indicator and refueling water level indicator are valved in.
b)
LI-41018 Refueling Water Level Indicator Used to monitor refueling cavity and reactor vessel level.
Calibrated from the bottom of the hot leg (0%) to the top of the refueling cavity (100%).
Refueling water level indicator not used for any control purposes such as adjusting RCS level.
Considered a backup for the tygon tube level indicator.
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1 c)
Tygon Hose Level Indicators Used to monitor RCS level typically with TV camera to control room.
Connected to RCS Loop A drain line.
Valved in after approximately 14,000 gallons have been drained i
out of the RCS prior to draining below the top of the hot leg.
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'Page 3 Permanent installation with tape measure fastened to the wall i
adjacent to the tube for exact level indication.
Used for fine control of RCS level, d) 47018-0603 RHR System Low Flow Alarm Alarms on low RHR flow conditions.
Low flow alarm setpoint changed to 400 gpm prior to starting draindown.
Warns control room operators on low RHR flow such as could be caused by suction line vortexing, e)
PT-21145 Pressurizer Relief Tank (PRT) Pressure
,Used to monitor RCS pressure with pressurizer PORV's open during
' draindown.
Correction factor on RCS level is accounted for with PRT pressure.
Calibrated over a range of 0-50 psig, f)
FT-23060 RHR System Flow j
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Control Room monitor / recorder of RHR flow.
Used for RHR flow adjustments.
Feeds RHR low flow alarm.
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LT-24068 Reactor Coolant Drain Tank (RCDT) Level RCS drains to RCDT.
RCDT level used to correlate draindown rate of RCS.
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CVCS Holdup Tank / Refueling Water Storage Tank (RWST) Level CVCS Holdup Tank /RWST are final location for water drained from
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RCS.
Level indication used to perform inventory balance on RCS volume to verify RCS level.
1) 15331/2/3/4 RCS loop Wide Range Temperature
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I Page 4 Nodhem States Power Company l<
Recorders in control room used for gross monitoring of RCS l
temperature.
Transducer located at midplane of loops, and therefore not very accurate for monitoring core exit temperature.
j) 15347 RHR Loop Temperature Monitors temperature of water returning to RCS from outlet of RHR heat exchangers.
Not useful on loss of RHR.
Calibrat'ed from 0-400*F.
k) 12149/12150 RHR Heat Exchanger Outlet Temperature Local indicators on the outlet of each RER heat exchanger.
Used as backup to control room RHR loop temperature indicator / recorder.
Not useful on loss of RHR.
1) 21160/21161 RHR Pump Discharge Pressure Control Room pressure indicators monitoring the discharge pressure of each RHR pump.
Would show vortexing/ air entrainment via oscillating indicated pressure.
Separated from RCS via check valves and therefore cannot monitor RCS pressure.
m)
Reactor Vessel Incore Thermocouple Located on exit of reactor core in the upper internals support
- assembly, s
Best indicator of core temperature when the RCS is at midplane.
Used to monitor core exit temperature.
(3) The following pumps can be used to control NSSS inventory during midplane operations:
Residual Heat Removal Pumps (2) - both pumps required to be operable by T.S. 3.1.A.3.c.
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Nodhem States Power Company Page 5 one pump required to be Positive Displacement Charging l
Pumps (3) operable (administrative requirement).
Safety injection Pumps (2) - both pumps are capable of operation (unless removed from service for maintenance).
Both pump control switches in pull-lock to meet T.S.
3.1.G requirements.
Technical Specifications applicable to the above pumps have been reviewed.
The existing practices at Prairie Island adhere to these requirements.
(4)
,ontainment closure conditions required for operations while the RCS is partially filled have not previously been specified. Administrative controls are being developed to limit the number and location of leakage paths to those that could be closed within a one-hour period.
(5) The ECS is cooled and drained to midplane using operations procedures ul.3, " Unit Shutdown Procedure"; CIS, " Residual Heat Removal System"; and D2, " Draining the Reactor Coolant System".
Procedure C1.3 provides guidance for taking a unit from 15 percent power to cold shutdown or refueling shutdown conditions with pressurizer level at 30 percent and the RCS depressurized.
Procedure CIS provides guidance for placing the RHR system in service and
?or normal operation of the system. This procedure also addresses RHR system malfunctions for RCS conditions other than mid-plane operation.
Procedure D2 provides detailed steps for draining the RCS from 30 percent level in the pressurizer to mid-plane level.
Included in this procedure are steps to recover RHR should its suction become air bound. The series of operations and basis for each event is given below (Equipment identifica-tions are for Unit 1, Unit 2 has similar equipment):
EVENT Basis 1.
Breakers for RCS Hot Leg The four RCS Hot Leg to RHR motor to RHR motor valves MV-32164 valves auto close on a high pressure and MV-32231 are turned off, signal to prevent overpressurization RHR low flow alarm set to of the RHR system. Turning power 400 gpm.
off to MV-32164 and MV-32231 assures a suction path to RHR is always available should a spurious signal occur while depressurized.
Normal RHR low flow alarm is 1250 gpm.
Reducing the setpoint to 400 gpm keeps the alarm in service when RHR flow is reduced to 1000 gpm.
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%m States Power Company r
EVENT BASIS 2.
RCS purification system Shutting down the purification is shutdown, system eliminates a potential for perturbations of RCS-level
' indication.
3.
Prestart checklist for' Aligns system to drain to CVCS draining completed..
Holdup tank or RWST.
4.
Verify PRT gas space < 4% H,
Verifies non-explosive-mixture.
2 5.
Align N t PRT'and manually Positive pressure raintained on 2
control at approximate.ly 6 psig.
PRT and RCS to aid in draining.
and assures no. level error associated with any potential vacuum within the system.
6.
Drain PRT.
Allows N fl w t system-rem ves 2
water seal.
7.,
Open Pressurizer PORV and Allows N to pressurize the 2
associated isolation valve.
pressurizer and reactor head while Open Pressurizer and Reactor draining. Note: At this point Head vent valves.
level will be reading approximately 2' - 3 " high per pound of N 2 pressure if the tygon tube were valved in.
IB.
Close RCP'#1 seal water return Prevents flow from VCT to RCS valves and seal water bypass.
9.
Operators are stationed at required locations for drain down with communications established.
a)
Control Room Monitors refueling level indicator, assures RHR does not lose suction, monitors PRT pressure to correct l
RCS level indication.
b)
Waste disposal panel Monitors RCDT level to inform operator controlling drain-down if draining rate exceeding RCDT pump rate.
c)
Loop Drain Valve Throttles to control draindown rate.
d)
Continuous watch of level - Notifies man on drain valve of appropriate level.
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Page 7 Northern States Power Cornpany EVENT BASIS 10.
Base line level indication on Water inventory measurement will be the CVCS Hold Up Tank and used to augment level indications, pressurizer are recorded.
11.
RCS draindown started at Overpressure provides approximately 150 gpm from both a known correction A and B RCS loop drains.
PRT factor. RHR throttled overpressure maintained.
RHR to preclude vortexing throttled to approximately 1000 in pump suction.
gpm.
12.
At 20 percent pressurizer level Preparing for operation LI-41018, Refueling Water Level at lower level in RCS.
Indicator, and tygon hose level indicator placed in service.
13.
After 14,000 gallons has been Draining of 14,000 gallons drained from the RCS as corresponds to an RCS level indicated by CVCS Holdup just above the top of the Tank level increase, the RCS primary loop.
(i.e., prior to Loop A drain valve is closed venting and draining S/G tubes),
and the RCS is vented to Loop A drain valve is closed to atmosphere. The tygon hose decrease the draining rate. The level indicator now matches RCS is vented to atmosphere to actual level.
The ECS is vented to atmosphere eliminate the use of a level correction factor for pressure.
The tygon hose level indicator and the Refueling Water Level Indicator are sensed at the Loop A drain valve.
Closing the drain makes these indicators accurate.
14.
Draining from Loop B RCS The tubes are vented and drained continues and the seal on with the volume of air from the the S/G tubes is broken reactor and not from the when the water level in pressurizer. As tubes are the reactor is just below drained, the water level in the the top of the loops, loops will increase as water issues out of the tubes.
15.
After the S/G tubes have Draining is stopped at the drained and RCS level is desired midplane level.
The at midplane (723'-4h") the increase in CVCS Holdup Tank Loop B RCS drain is closed level is expected to be 25,500 -
and final CVCS Holdup Tank 26,500 gallons.
l'his inventory Level is recorded.
change is used as a second verification of RCS level.
Page 8 Nodhem States Power Company EVENT BASIS 16.
The tygon hose level indicator Venting RHR assures no is continuously monitored via TV air buildup. RCS level camera to the control room, the is maintained between RHR system is vented hourly, RCS midplane and 3/4 full.
level is maintained between 723'-4 "
and 723'-11 3/4", and RHR ficw, RHR temperature, RCS level and three RCS thermocouple are recorded hourly.
17.
Purification is returned to service.
Required for RCS cleanup.
Also included in D2 is a section for restoring RHR suction. Assuming a loss of suction due to low level and vortexing/ air entrapment:
EVENT BASIS 1.
Operate a charging pump at Increases level to eliminate maximum speed to restore level, vortexing.
2.
Vent the RHR suction.
Vents off any air entrained in the RHR suction.
3.
In emergency cases open lf S/G manways are off, water the RWST supply to RHR to will issue out of manways.
restore Icvel.
RWST boron concentration may be lower than RCS boron concentration specified for refueling.
j RCS level and RHR flow are administratively controlled at setpoints where it is known that vortexing does not occur. Venting the RHR pump suction hourly gives added assurance that air entrainment is not taking place.
Potential level changes due to air entrainment are understood and will be further reinforced at future training sessions.
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Boiling in the core with and without RCS pressure boundary integrity is mitigated by assuring adequate makeup sources are available to quench the RCS. The urgency of recovering RHR and RCS level is stressed in plant procedures to further preclude boiling in the core.
Calculations have been performed to approximate the amount of time available from loss of RHR to start of core uncovery.
Conservative assumptions were used for heatup of the metal, heatup and boiling of the water, initial water temperature and initial decay heat load.
No credit was taken for metal mass above RCS loop midplane, steam renerator water inventory, or heat losses to I
ambient. Dependent upon initic) decay heat level and time after reactor trip, there is a minimum of one hour to core uncovery. The results of these j
calculations will be reflected in plant procedures to reinforce the urgency j
of recovering RHR during midplane operations.
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l Page 9 Nodhem States Power Company New procedures are being developed to cover situations in which RHR cannot be restored.
Contingencies for rapid installation of steam generator manways are being considered to allow alternate makeup to the RCS.
Evaluation of this issue is continuing.
(6) Training provided to the operators related to midplane operation consists of specifics on the RHR system and the reactor coolant level tygon tube indicator. Pressure correction factors for level and RHR pump suction
.vortexing are reviewed.
Concepts associated with level errors induced by air entrainment and de-entrainment and core boiling have not been included in this training, but will be incorporated in future training sessions.
Maintenance personnel training regarding avoidance of perturbing the NSSS and response to loss of decay heat removal while the RCS is partially filled will be implemented.
(7) Two RHR System Engineers from the Operations Engineering group are normally available to pr" vide assistance to the control room operating staff. These two individuals have reviewed the phenomena and instrumentation associated with partially filled RCS operation and have participated in the development of training materials.
(8) Procedures used to draindown and operate with the RCS partially filled have been discussed under item (5) above. These procedures implement additional requirements beyond those needed for normal Mode 5 operation. They include:
a)
RHR flow is reduced to 1000 gpm (proven sufficient to remove decay heat), to minimize the likelihood of vortexing and air injection.
1000 gpm, at RCS midplane, is a known point where vortexing does not
- occur, b)
The RHR pump suction pipe is vented hourly to preclude a long term buildup of air, c)
The RHR low flow alarm setpoint is reduced to 400 gpm rather than its normal 1250 gpm.
This returns the alarm to service rather than keeping it backlit at the reduced flowrate. The 400 gpm setpoint gives the operator immediate indication on loss of RHR.
d)
Prior to draining the RCS, purification is removed from service to preclude perturbation of the RCS.
e)
The draindown of the RCS starts from 30% pressurizer level each time.
The water removed from the RCS is inventoried and compared with the known RCS volume to verify final level, f)
When in the midplane conditions, both trains of RHR are required to be operable.
..e Page 10 Northem States Power Company g)
Prior to draining the RCS, steam. generator temperature is-required to be less than 200*F to assure the stesm' generator does not add heat to the RCS volume, h)
LWhile the RCS is drained down, a roving operator in containment is utilized for quick response to any control room requests and to survey the critical containment locations.
1)
While at midplane, work that could perturb the RCS is limited to that which is absolutely necessary.
j)
Every attempt is made to minimize the time spent with the RCS at midplane.
k)
Use of nozzle dams is evaluated each time steam generator manways are removed.- Time spent at midplane can be reduced by using nozzle dams, however, significant radiation dose will be accumulated.
(9) Many potential changes are being evaluated to strengthen our ability to safely conduct midplane. operations:
a.
Decay heat load versus time to core uncovery curves are being developed.
It is likely they will be placed in plant procedures.
b.
Training of control room operators and maintenance personnel pertaining to midplane operation will be improved, c.
Alternate, readily available, sources of makeup water are being evaluated.
d.
Control of containment openings while at midplane will be implemented.
All direct openings to the outside (i.e. unmonitored and unfiltered release paths) will be capable of being isolated within one hour.
The exact methodology'and details have not yet been defined.
e.
Requirements for steam generator secondary side water inventory will be implemented while the primary manways are on.
These levels may be altered should unusual primary to secondary leakage exist.
f.
Methods to quickly close primary manway openings are being evaluatad.
Again, details have not yet been defined.
g.
Existing RHR system and RCS level system installations will be evaluated to assure good engineering practices have been implemented, h.
Minimum instrumentation requirements for midplane operation will be developed to include incore thermocouple.
1.
Redundant midplane level indication with low level alarms will be installed.
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Page 11 Northern States Power Company Implementation of items (a) thru (h) will be completed prior to the l
scheduled RCS draindown to midplane for the next refueling outages on each unit.
( Tanuary.19P8 - Unit 2, September,1988.- Unit 1).
Implementation of item (i) will be completed prior to the scheduled RCS draindown to midplane for the 1989 refueling outages.
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Should unscheduled outages occur which require RCS draindown to midplane prior to the 1988 refueling outages, many of the above items will be implemented providing reasonable assurance that the operation can be safely conducted, i
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