ML20195D391
| ML20195D391 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 10/27/1988 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20195D389 | List: |
| References | |
| TAC-61491, NUDOCS 8811040345 | |
| Download: ML20195D391 (4) | |
Text
. -.
{" "8%k UNITED STATES NUCLEAR REGULATORY COMMISSION g
5 WASHINGTON, D. C. 20566
's.,...../
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATINGTOLERNO.50-267L86-017 PUBLIC SERVICE COMPANY OF COLORADO FORT ST. VRAIN NUCLEAR GENERATING STATION 90CKETNO.50-267
1.0 BACKGROUND
April 3,1986 event at Fort St. Vrain (FSV) was a grid fluctuation which caused the trip of an undervoltage relay in the helium circulator bearing water pursp control circuit and caused loss of forced circulation. The electrical transient also caused the isolation of the helium purification system.
This was reported in the Licensee Event Report (LER)86-017 and evaluated by the staff in Feference 1.
Cur Safety Evaluation (SE) identified various concerns regarding the licensee's ar.alysis of the event, proposed corrective actions, and reconriended further evaluation and update of the corrective actions.
Public Service Company cf Colorado (PSC) submitted their response to the staff's concerns including a status report on the proposed corrective actions in Referer.ce 2.
In response to the staff's request for additional information, PSC submitted it. References 3 and 4, the design details of the circulator bearir.g water system and details of the proposed corrective actions to be implen,ented during or before the fourth refueling outage.
2.0 EVALUAT1,0?(:
The licensee's proposed corrective actions to rainimize and control grid disturbance and elimir. ate adverse effects of the grid transients on the Heliun Circulator 6 earing Water System and the Helium Purification Systen are as follows.
A.
Corrective actions associated with the circulator bearing water system.
+
1.
Theexistingundervoltagerelays(Westinghouse-AR440)in 3ao the helium circulator bearing water pump control circuits 7dj@
will be replaced by high speed, adjustable setpoint relays oo (ITE2711). The AR440 relays are fixed time and difficult to N
calibrate, whereas the ITE27H relays hve adjustable setpoints, "O
can be easily calibrated, and will reliably operate at the y
selected setpoint.
og CQ 2.
The replacement undervoltage relay functions will be bypassed o
when the reactor is at less than 30% power.
mc 3.
The replacement relays will be functionally tested at each scheduled shutdown and calibrated at least once per 18 months in accordance with the FSV revised Technical Specification to be implen.:.nted before the beginning of fuel cycle 5.
2 4.
Operating procedures will be evaluated and revised as required to address operator actions associated with the grid perturbation.
B.
Corrective actions associated with the Helium Purification Sy~ stem
.V.a lv_e Cont.r.o.l.'.t.'i.rh3il
~~~
1.
Special test T-342 was performed to demonstrate the valve closure actuation signals and the valves functioned as designed. The test results demonstrated the valves functioned acceptably.
2.
The susceptibility of current loops in the high temperature and high differential pressure circuits to electric noise is being addressed by the grounding of abandoned and spare cables and the installation of arc suppressors across selected relay coils. This is an ongoing effort to reduce the susceptibility of essential circuits to noise by eliminating putential noise sources as thty are uncovered.
C.
Corrective actions associated with the in-dep'th review of the e l e c t r i ci..l..n.e.t.w.o.r.l!- ~ ~ ~ ~ ~ ' ~ ~ ~ ' ' " ' " ' ' "'
1.
Correction of a s0ftw'ere deficiency in PSC's grid control
- cnpoter program.
2.
Installation of coordinated overvoltage tripping device on capacitor banks in the PSC's 230kV switchyard.
The helium circulator bearing water system at FSV consists of 3 motor driven pun.ps in each coolitig loop with two pumps normally operating and cr.e in a standby mode.
The centrol schene for each punp includes an undervoltage trip relay which provides an anticipatory signal to the control schemes indicating loss of norma < bearing water and the need for an irriediate response from the back-up bearing watcr system.
This is strictly a bearing water syster.i control function which is bypassed when backup bearing water is not available (i.e., the emergency feedwater header is not charged and the reactor is at less than 30% power). These relays are dif-ferent from those which provide equipment protection froni the degraded voltage offsite power source, initiate the load shedding and start the emergency diesel generators. At its setpoint, the control function undervoltage relay will trip the normally operating bearing water pump, isolate the normal bearing water
. supply and autcmatically restart the normal bearing water pump following restora-tion of rated voltage power supply. However, an operator action is required to restore the normal bearing water supply. Upon loss of the normal bearing water following the trip of the normal bearirig water pump, 6 ltw differential pressure (delta p) initiates an automatic start of the standby bearing water punp.
If normal bearing water is interrupted in a loop, "backup bearing water" is automatically initiated from the feedwater system, and it is routinely available above approx-imately the 30% power level.
If backup bearing water is unavailable in a loop, "emergency bearing water" is supplied using the bearing water accumulator for a sufficient time to saft-ly shutdown the affected circulator.
E As established in reference 1, the setpoints of these undervoltage relays were too high and caused unwarranted trips. The existing relays have nonadjustable setpoints and thus needed to be replaced by an adjustable setpoint relay. We find this modification acceptable. Also, since the normally operating bearing water pump and the associated standby bearing water pump in each loop are supplied by different 480 volt buses (normal pumps are on one of the redundant buses and the standby on the common bus), a low voltage on normal operating purnps bus will not affect the voltage on the standby pump supply bus, unless there is a total loss of offsite power, i.e., loss of voltage on 2-out-of-3 busas.
For a loss of voltage on 2-out-of-3 essential 480 volt buses, the bu; loads are auto-matica11y isolated and both emergency diesel generators are automatically started.
This design feature allows automatic start of the standby bearing water pump when its associated normelly operating betring water punp is tripped by the undervoltage signal.
In FSV design, a source of backup bearing water is norrcally avail.$ble fron the steam generator feedwater system above 30% reactor power.
Below the 307 reactor power level, there may not be any backup bearing water available. if the norraal bearing water supply is isolated and the pump tripped due to undervoltage. The licensee's corrective action to bypass the undervoltese trip of the norrnal operating bearir.g water pumps below 30% of reactor power is necessary due to this design feature of the backup becring water system and also eliminates the potential for damage to the standby bearing watt:r purp from operating without flow. Bypassing of the ccntrol fur etion undervoltage trip of the normally cperating bearing water purips does nut affect the protection of its mutor from a detrinental undervoltage as this protection is provided by the bus isclation (loss of voltage or degraded grid voltage) undervoltage relays. We, therefore, find the bypassing of the control function undervoltage trip of the norr.. ally operating bearing water purps below 3Cf reactor pcwer level acceptable.
3.0 CONCLUS10HS1 Based on our evaluetien, we conclude that the proposed corrective actions will ensure higher availability of the belium circulator bearir,g water and thus re-duce the nunber of circulator trips due to the loss of bearing water during startup or power cperation. The replacement adjustable undervoltage relays and their surveillance on regular intervals (by the plant Technical Specification) will provide correct operation, reliability and the operability of the undervoltage control scheme for the Helium circulator bt aring water pumps. The proposed corrective actions associated with the Helium Purification System valve control circuit and the transmission system grid enhance the reliability of these systems.
The proposed corrective actions have adequately addressed the concerns that were identified in our preliminary safety evaluation and thus are acceptable.
Dated:
Principal Contributor:
I. Ahmed
)._
i 4
,KEFERENCES:
1.
Letter from K. Heitner, NRC, to R. O. Wil.iams, Jr., Public Service Company of Colorado, enclosing Safety Evaluation dated September 26, 1986,
Subject:
"Fort St. Vrain - Licensee Event Report 86-017."
2.
Letter fron J. W. Gahm, Manager, Nuclear Production, Public Service Company of Colorado to H. N. Berkow, NRC, dated Decerber 19, 1986,
Subject:
"Status Report on LER 50-267/86-017."
3.
Letter from R. O. Williams, Jr., Public Service Company of Colorado, to Jose A. Calvo, NRC, dated January 8,1988,
Subject:
"Status Report on LER 86-017.",*
4 Letter from H. L. Brey, Manager, iluclear Licensing & Resource Managetent, Public Service Company of Colorado, to Jose A. Calvo, NRC, dated July 19, 1988,
Subject:
"Supplenental Information on the Bearing Water Systen; and Licensee Event Report 86-017."
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