ML20006C154
| ML20006C154 | |
| Person / Time | |
|---|---|
| Site: | Zion File:ZionSolutions icon.png |
| Issue date: | 01/31/1990 |
| From: | Novak T NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| To: | Joyce T COMMONWEALTH EDISON CO. |
| References | |
| AEOD-T925, NUDOCS 9002060399 | |
| Download: ML20006C154 (2) | |
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'JAN 3 21990 -
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- Mr. T. P. Joyce Staticn Manager Zion Generating Station Comonwealth Edison company 101 Shiloh Blvd Zion, Illinois 60099
Dear Mr. Joyce:
We have performed an analysis on the safety significance of an operating situa-tion which occurred at the Zion station during November, 1989. The analysis is representative of the analyses we undertake for significant plant operation experiences. The Zion plant situation analyzed involved degradations or outages of the diesel generators and the emergency service water systems. The conditions were pertinent to both plant units, and affected the ability of the station to cope with a loss of offsite power initiating event, which could have randomly occurred during the degradation or outage interval. A copy of our report describing the analysis performed is enclosed for your information and use.
erformed The analysis of the effects of these plant conditions on safety was p(ASP) using techniques developed by the NRC's Accident Sequence Precursor program. The ASP program methods allow estimating the quantitative signifi-cance of combinations of actual and postulated operational events and plant system responses. This method allows mapping the degradations, failures, and pertinent plant conditions onto an event tree to show the most likely postu-lated scenarios and, when the tree is quantified, provide a perspective of the significance of the event.
This operating situation and the associated concurrent plant' conditions may be pertinent to the assumptions and analysis in your probabilistic risk assessment for Zion. 'If you would like to discuss the attached analysis, we would be happy to do so. Also, if you have new information on the equipment operability t
which could change some of the assumptions of the enclosed analysis, we would
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welcome the information. Although the risk significance of these events were moderate, the analysis indicates the need to examine concurrent degradations or failures of systems along with other factors of plant operations. A copy of the 1987 ASP annual report is also enclosed to help put this enalysis in perspective in terms of other operating events. This information is provided for your use.
No action is required or requested by this letter.
Orig nal Signed t?%
& Wy-h) homas M. Novbk, Director Division of Safety Programs Office for Analysis and Evaluation of Operational Data
Enclosures:
as stated cc: w/ enclosures:
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AEOD TECHNICAL REVIEW REPORT j
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' Zion Units <1.& 2 TR REPORT.NO.:~ AEOD/T925-l l#,
LUNITS:J
,DOCKETcNO: 5000295 &=304 DATE:- December 18, 1989' LICENSEE:
Commonwealth Edison'Co.
EVALUATOR / CONTACT: F. MANNING l
NSSS/AE:
Westinghouse /Sargent
'. e and Lundy-EVALUATION OF SAFETY' EQUIPMENT OUTAGES FOR
~
SU BJECT:
SIGNIFICANCE AT ZION
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SUMMARY
This ' report: provides an evaluation of four Zion event
. notifications (ens) reporting-equipment outages on November 22-23, 1989.
An event notification'is an immediate telephone notification to the'NRC of significant nuclear plant operating events.
The event notification requirements are specified in
~10CFR50.72, the code of federal regulations.
The-four ens are evaluated to estimate their combined effects on safety.
The problems reported by'these ens are related and this 4
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 power (LOOP) event.
'The ens are -evaluated in the context of a postulated loss of
, of fsite power (LOOP)Eduring the period the equipment problems and outages:could have coexisted.
If-a LOOP had occurred and emergency service water (ESW) heat removal had not been successfully provided1for the operating diesel generators- (DGs),
the.D3s may have failed within about 5 minutes.
Were that to occur, 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) i sg; 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 to the reactor coolant system,.while the station battery depletion would cause loss of ability to control and monitor the auxiliary y
feedwater heat removal function.
Prevention of or mitigating the
. ef fects 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 SE-5.
It is judged that these events are of moderate safety significance.
M i-l N 1[ E U
V INTRODUCTION
'This report' evaluates Zion ens 17174, 17175, 17183, and 17181.
- The_ ens concern equipment failure or outages which could affect
.the plant response to a loss-of-offsite power event.
=The plant information 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.
The ens variously refer to D/Gs (diesel 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 Eyent Description.
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 inoperable.
This occurred during the performance of a' safeguards test ~ on unit 2 "O" D/G 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 r
L 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 hrs.
They are investigating cause of the annunciator alarm.
The RI was informed."
Event 17175, occurred 11/22/89 at 08:15 CST.
"The o
plant has started reducing power at the rate of 0.5%
o
/ 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 se rvice.
The RI was informed."
Uodate 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 be declared i
The power decrease was terminated at 1035 CST per a discretionary enforcement agreement with Region 3.
The unit will continue to operate at its j
l current power level, 23% until the test of the ESW pump l
is completed.
Licensee informed the RI."
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9-b9 s Update at 1534.: '"The Licensee terminated the
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unusual event at-1430 CST _after the "2B" ESW pump was declared' operable.- The unit remains in 7 day technical
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specification action statement #3.15.2.C due to the "O" diesel generator being inoperable.
Licensee informed the RI."
Event'17183, occurred 11/23/89 at 11:20 CST.
"The common swing EDG and. #1 of 2. unit 2 EDGs inoperable -
while unit 2Lat power.
With. the 10 swing EDG-inoperable,= plant operators were starting llof 2 unit 2 i
I EDGs (f 2A) during an operability test.
A local operator noticed an exhaust gas test connection on one of the cylinder heads on #2A EDG vibrating open.
The local operator stopped the EDG and turned DC control power to.the'EDG off which rendered $2A EDG inoperable.
Licensee entered TS LCO A/S 3.0.3 which requires either
- O or #2A EDG to be returned to operable status within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> or to be in at least hot S/D mode within the
.t 1200 (CST), Licensee. completed 1
A next 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />.
repairs _to=#2A EDG, declared 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 solenoid 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 Status:
Unit 1 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 g
l specification requirements regarding ability to respond to a LOOP i
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r 7,
i 1.,
Land 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:
1 L
DG Status:~
Unit -1 DG 1A - In maintenance for rod bearing problems - not recoverable L
Unit 1 DG-1B'- In maintenance for testing &. 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.
E functionable if needed.
(The DG is assumed l
failed in this analysis)
Unit 2 DG 2B - Available Swing DG O - Indicated high bearing temperature during the performance of a surveillance test. [The DG is assumed failed in this analysis).
ESW 6tatust The Zion station has a total of six ESW pumps.
All ESW pumps
-were : operable except pump 2B.
'ESW 2B was out of service 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 hrs.
With ESW 2B proven operable-the LCO was exited about =7 hours 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 to both units.
However, given a LOOP, unit 1 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 from any 4160 volt emergency AC bus, only one ESW pump is assumed available during LOOP.
However, ESW 2B, which is powered by DG 2B during LOOP, was out of service for maintenance therefore effectively no ESW pumps may have been initially available. -Whether ESW pump 2B could be quickly put back in service is open to conjecture.
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 Sionificance and Postulated Scenario:
The core damage scenario postulated herein is for a randomly occurring LOOP to occur during the assumed seven day vulnerability period.
Diesel generator 2B would start but fail within a few minutes from lack of jacket cooling water.
The
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btherdieselsareassumednotrecoverable.
Therefore core damage a
would occur if 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/RY is assumed.
A probability l
of f ailure to recover offsite power within 1/2 hour af ter LOOP start of 39 is assumed.
.A probability of failure to recover of fsite power before battery depletion or scal 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 j
time to a LOOP, the conditional core damage probability (CDPr),
)
given.the conditions and assumptions stated above is estimated to be:
-CDPr =(exposure time fraction)(LOOP freq/yr) (fail recover LOOP short term)(fail to recover LOOP long term or seal LOCA)
= (7/3 64 ) (4 E-2) (. 39) (.17) = approximately SE-5 EN 17181 was also evaluated with the above three ens. This event involved Unit 1.
The Unit 1 Service Water aircraf t 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.
CONCLUSIONS j
The four operating events reported for Zion on November 22-23, 1989 were evaluated in the context of a postulated LOOP during l
the period the EN-described problems could have coexisted.
If a l
LOOP had occurred and ESW heat removal had not been successfully l
provided for the operating DGs, the DGs may have failed within about 5 minutes.
Were that to occur, and offsite power not be recovered before battery depletion, core damage could occur.
The i
core damage probability associated with these events is estimated to be about SE-5.
It is judged that these events are of moderate safety significance.
The corrective action taken by the licensee i
l appears to have been adequate.
Therefore, no further action by AEOD other than monitoring this type of event is suggested, i
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References:
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L1." NUREG/CR-4674, Volume 7. " Precursors to~ Potential Severe Core ~
Damage Accidents: 1987 A Status Report" m
e:: y 2 '. NUREG-1032,." Evaluation of' Station Blackouti Accidents at
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