ML20056A043

Evaluation of Safety Equipment Outages for Significance at Zion (Revised)

ML20056A043
Person / Time
Site:	Zion  File:ZionSolutions icon.png
Issue date:	07/31/1990
From:	Manning F NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD)
To:
Shared Package
ML20056A041	List: ML20056A040 ML20056A043
References
TASK-AE, TASK-T925 AEOD-T925, AEOD-T925A, NUDOCS 9008030121
Download: ML20056A043 (5)

Text

_

AEOD TECHNICAL REVIEW REPORT UNITS:

Zion Units 1 & 2 TR REPORT NO.: AEODfl925A I

DOCKET NO:

5000295 & 304 DATE:

July 5,1990 (Rev Date)

LICENSEE:

Commonwealth Edison Co.

EVALUATOR / CONTACT:

NSSS/AE:

Westinghouse /Sargent F. Manning and Lundy

SUBJECT:

EVALUATION OF SAFETY EQUIPMENT OUTAGES FOR SIGNIFICANCE AT ZION (REVISED)

SUMMARY

This report provides a revised evaluation'of four Zion event notifications (ens) reporting i

equipment outages on November 22-23,1989. An event notification is an immediate telephone notification to the Nuclear Regulatory Commission (NRC) of signif' cant nuclear plant operating events. The event notification requirements are specified in 10 CFR 50.72, the Code of Federal Regulations.

The four ens are evaluated to estimate their combined effects on safety. The prblems report by these ens are related and this requires that they be evaluated together. The ens involve outages or degradations of plant safety systems that are necessary to cope with a loss of offsite l

power (LOOP) event.

L The ens are evaluated in the context of a postulated loss of offsite power (LOOP) during the period the equipment problems and outages could have coexisted. If a LOOP had occurred the diesel generators and corresponding emergency service water (ESW) pumps would have been commanded to start, if either the diesel generators or corresponding emergency service water pumps were to fail, and offsite power not be recovered within some critical time period, core damage could occur. The critical time period depends on the likelihood of reactor coolant pump seal failure (s) or station battery depletion in the interval before recovery of offsite power. The occurrence of seal failure (a small LOCA) would require AC power to provide makeup coolant injection of the reactor coolant system, while the station battery depletion would cause loss of ability to control and monitor the auxiliary feedwater heat removal function. Prevention'of or mitigating the effects of either of these problems requires recovery of AC power.-

The conditional core damage probability for the combined EN events is estimated to be about 5.6E-7. It is judged that these events are of low safety significance.

[

9008030121 900731 gDR ADOCK0500pg3

.,.v.

e IN'IPODUCTION This repcct evaluates Zion ens 17174,17175,17183, and 17181. The ens concern equipment failure or ouages which could affect the plant response to a loss-of offsite power event, ne plant informction for this evaluation is taken mainly from the four ens which provide basic information about the events. Additional verbal information was obtained in a conference telephone call on November 28,1989 with the Zion NRC resident inspector and senior resident inspector, and the NRC project manager for ZION. A correction to the assumptions >f the original analysis htve been provided by Zion plant personnel (see Reference 3) and these corrections of plant configuration have been included in this revised analysis.

The ens variously refer to D/Gs (&sel generators) or EDGs (emergency diesel generators).

This equipment will be referred to as diesel generators (DGs) in this report consistent with the Zion final safety evaluation report nomenclature.

Discussion Eytot Descrintion. The ens evaluated in this report are quoted below. Only the first three events listed are felt to have a measurable impact. The ens are:

Event 17174, occurred 11/22/89 at 07:15 CST. "The plant entered a LCO to be in hot S/D within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> due to both service water pumps being declared ino.wrable. This occurred during the performance of a safeguards test on unit 2 "0" D/d when it had a first out annunciator alarm showing bearing temperature high. During this type of test the D/G does not get a trip signal, but it was secured and declared inoperable. Since this D/G is the emergency power supply for the "2A" emergency service water pump, this pump was also declared inoperable. Also the "2B" emergency service water pump was out for maintenance and it too was inoperable. Therefore both ESW pumps were inoperable and the plant entered TS 3.8.7.C to be in hot S/D in 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. They are investigating cause of the annunciator alarm. The RI was informed."

Event 17175, occurred 11/22/89 at 08:115 CST, "The plant has started reducing power at the rate of 0.5%/ Min in order to be in hot S/D in 4 hrs. They are also working on the "2B" ESW pump to get it returned to service. The RI was informed."

Update at 1227. "The "2B" ESW pump was started at 1008 CST to begin a 4 hour4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> run prior to being declared operable. The power decrease was terminated at 1035 CST per a discretionary enforcement agreement with Region 3. The unit will continue to opemte at its current power level. 23% until the test of the ESW pump is completed. Licensee informed the RI."

Update at 1534. "The Licensee terminated the unusual event at 1430 CST after the "2B" ESW pump was declared operable. The unit remains in 7 day technical 2

. - - -. ~..

specification action statement #3.15.2.C due w the "0" diesel generator being inoperable. Licensee informed the RI."

Event 17183, occurred 11/23/89 at 11:20 CST. "The commoa swing EDO and #1 of 2 unit 2 EDGs inoperable while unit 2 at power. With the #0 swing EDG inoperable, plant operators were starting 1 of 2 unit 2 EDGs (#2A) during an operability test. A local operator noticed an exhaust gas test connection ca one of the cylinder heads on #2A EDG vibrating open. The local operator stopped the EDG and turned DC control power to the EDO off which rendered #2A EDG incperable. Licensee entered TS LCO A/S 3.0.3 which requires either #0 or #2A EDC to be returned to operable status within I hour or to be in at least hot S/D mode with'.n the next 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. At 1200 (CST), Licensee completed repairs to #2A EDG, declare (. it operable, and exited TS LCO A/S 3.0.3.

Licensee informed RI."

Event 17181, occurred 11/22/89 at 14:45 CST "The licensee discovered a service water ventilation fan aircraft crash damper in the open position with its associated fan off and fan breaker and solenoid valve energized. In this condition, the damper should have been closed. The damper is designed to close in the event high temperatures, such as those caused by an aircraft crash, are sensed at the ventilation intake. The problem has been identified as the solenoid valve which was not allowing air through to close the damper.

The licensee has initiated activities to replace the solanoid valve. Technical specifications require the damper to be closed if the aircraft crash protection system is inoperable but the damper cannot be closed until the solenoid valve is replaced. The damper fails in the open position to allow cooling of the service water pump motors during conditions other than those involving a fire outside the intake structure. The licensee will inform the RI."

Plant and Equipment Status Plant Statu'::

Unit I had finished refueling and was ready for startup except for DG maintenance and repair.

Unit 2 was operating at 99% power when potential DG and ESW problems were discovered.

Because of these problems, the plant could not comply with technical specification requirements regarding ability to respond to a LOOP and the plant entered a limiting condition for operation.

(LCO) requiring shutdown of the plant in 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. The status of the DGs and ESW were as follows:

DG Status:

Unit 1 DG 1 A -

In maintenance for rod bearing problems - not recoverable Unit 1 DG IB -

In maintenance for testing and switchboard out - possibly recoverable in about 30 minutes. (The DG is assumed failed in this analysis.)

Unit 2 DG 2A -

Had gas test connector leak found during performance test requiring 1/2 hour repair - The resident inspector (RI) thought the DG was still 3

ll'

l j

functional if needed. (The DG is assumed failed in this analysis.)

Unit 2 DG 28 -

Available Swing DG 0 -

Indicated high bearing temperature during the performance of a surveillance test. (The DG is assumed failed in this analysis.)

ESW Status:

The Zion station has a total of six ESW pumps. All ESW pumps were operable except pump 28.

ESW 2B was out of seivice for maintenance at the time of the 4 hour4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> S/D LCO. The maintenance was halted and the pump restarted and was run for a required proving 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />.

With ESW 2B proven operable, the LCO was exited about 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br /> after entry (the plant was still in a 7 day LCO to restore 2nd DG & ESW train.)

The ESW systems were cross connected at the time of the event with common parallel ESW feed l

to both units. However, given a LOOP, unit I would supply no ESW since both DGs were in maintenance. Since only DG 2B was available, and since only one ESW pump can be powered l

l from any 4160 volt emergency AC bus, only one ESW pump is assumed available during LOOP.

ESW 2C is powered by DG 2B during LOOP.

The Zion FSAR indicates one ESW pump per unit is required to achieve mitigation and shutdown given accident conditions. (FSAR page 9.6-2, 2nd paragraph and page 9.6-3,1st paragraph.)

Event Slenificance and Postulated Scenario:

l The core damage scenario postulated hetein is for a randomly occurring LOOP to occur during the assumed seven day vulnerability period. Diesel generator 2B would be commanded to start but could fail either the start attempt or fail if ESW pump 2C were to fail. The other diesels are assumed not recoverable. Therefore core damage would occur if the diesel generator 2B or ESW 2C failed and offsite power for the station were not recovered before station battery depletion or reactor coolant pump seal LOCA occurs.

A LOOP occurrence frequency of 4E-2/RV is assumed. A probability of failure to recover -

offsite power within 1/2 hour after LOOP start of.39 is assumed. A probability of failure to recover offsite power before battery depletion or seal LOCA of.17 is assumed. These values are consistent with values from the Accident Sequence Precursor program (Ref 1) and a study-on station blackout (ref 2). A fault duration estimate for DG 0 of seven days is assumed for this analysis. For the 7 day exposure time to a LOOP, the conditional core damage probability (CDPr), given the conditions and assumptions stated above is estimated to be:

4

c-CDPr =

- (exposure time fraction) (LOOP frequency / year) (Fall to recover LOOP short term) (Fail to recover LOOP long term or seal LOCA) (DG 2B +

ESW 2C fail)

I (7/364) (4E-2) (.39) (.17) (1.1E-2) = approximately 5.6E-7

=

EN 17181 was also evaluated with the above three ens. This event involved Unit 1. The Unit l

1 Service Water aircraft crash damper failure (failed open) has no measurable effect on the above postulated scenario. Even if it had failed closed (the opposite of what it did and is designed to do), it would have caused no problem by the NRC resident inspector's assessment because of the large area (room volume) the ESW pumps operate in, the extensive amounts of concrete for heat absorption, and their relatively low ambient air temperatures due to physically being near Lake Michigan water.

CONCLilSIONS The four operating events reported for Zion on November 22 23,1989 were evaluated in.the context of a postulated LOOP during the period the EN described problems could have coexisted.

The core damage probability associated with these events is estimated to be about 5.6E-7 It is judged that these events are of low safety significance. The corrective action taken by the licensee appears to have been adequate. Therefore, no further action by AEOD other than monitoring this type of event is suggested.

REFERENCES:

1.

NUREG/CR-4674, Volume 7. " Precursors to Potential Severe Core Damage Accidents" 1987 A Status Report".

2.

NUREG 1032, " Evaluation of Station Blackout Accidents at Nuclear Power Plants".

3.

Letter, Mr. R. A. Chrzanowski, Nuclear Licensing Administrator, Zion Station to Dr.

T. E. Murley, Director, NRR, dated May 7,1990, subject, " Zion Station Units 1 & 2 l

Evaluation of Safety Equipment Outages for Significance at Zion NRC Docket Nos. 50-295/304".

l l

l 5

l

~ ' "