ML20149K387
| ML20149K387 | |
| Person / Time | |
|---|---|
| Site: | Indian Point  |
| Issue date: | 05/28/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1988-020-00 | |
| Download: ML20149K387 (6) | |
Text
B-30 Accident Sequence Precursor Program Event Analysis LER No:
Event
Description:
Date of Event:
Plant:
247/88-020 RWST level indication inoperable December 11, 1988 Indian Point Unit 2 Summnary While operating at 100% power, both level indicators to the refueling water storage tank were inoperable when cold weather caused the sensing lines to freeze.
With both channels of RWST level indication to the control room inoperative, operators would be unable to accurately deter-mine the appropriate time for change over from safety injection phase to recirculation phase in the event of a loss of coolant accident.
This type of event could potentially cavitate the safety injection and/or residual heat removal pumps and render sump recirculation inoperable.
The core damage estimate for this event is 6.9 x 10-6.
The relative significance of this event compared with other potential events at Indian Point 2 is shown below.
1 E-8 1 E-7
\\ LER 247/88-020 IE-6 I E-5 I E-4 1 E-3 1 E-2 I
AFW Unavail (360 hrs)
Trip w/1 AFW Train Unavail Event Description While at 100% power, operators received a series of RWST high alarms between 0633 and 0651 on Dec. 11, 1988.
Due to previous problems with sensor LT 920, operators interpreted the spurious alarm to be caused by line noise and removed the alarm from the control room computer scan at 0651 (computer point L6064), relying on the redundant control panel SB-i indicator light for RWST level indication.
Subsequent analysis by the Indian Point plant staff indicates that the sensing lines to both level
B3-31 transmitter LT 920 and redundant transmitter LT 5751 may have been commencing to freeze at this time due to the subfreezing temperatures and inadequate instrument heating and insulation.
At 1819, the computer point for LT 920 was returned to service and immediately started to alarm high.
Because the accuracy of LT 920 was in question, the high alarm state was not immediately investigated, and the computer point L6064 was again removed from the computer scan.
At 2330, the strip chart recorder for LT 5751 was discovered to be inoperable.
A few minutes later on December 12 at 0045, a control room operator noticed the high RWST indication from the control panel SB-i for LT 920 and confirmed that LT 5751 was inoperable.
Recognizing a problem with all RWST control room annunciators, an operator was dis-patched to locally check the RWST level using level, indicator LI 921.
At 0105, the operator reported to the control that the RWST level was high and that the level transmitter sensing lines were frozen.
The senior watch supervisor immediately declared a limiting condition for operation (LCO), as both instruments that provide RWST level indication for containment sump switchover in the event of a loss of coolant accident were inoperable.
A power reduction was immediately initiated.
Local heaters were applied to the RWST level instrumentation.
LT 920 was returned to service at 0231, terminating the LCO.
LT 5751 was returned to service on December 13 at 1000.
The event was caused by lack of insulation on an instrument root stop valve, combined with lack of heat trace on the valve.
The following also contributed to the event:
Inadequate design of the existing z-wire chemelex and simplex electric heat trace systems in that they do not indicate when the circuit fails.
- Improper maintenance in that the failed heat trace circuit was recorded as working in October 1988, when in fact it was not.
- Inadequate documentation of verification of freeze protection adequacy prior to the onset of cold weather.
- Lack of procedural guidance on specific operator actions required when an automatic monitoring system is degraded or inoperable.
Event Related Plant Design Information The RWST has two level transmitters (LT 920 and LT 5751) and one level indicator (LI 921).
The level transmitters annunciate via control room panel indications and either may also annunciate to the control room computer when connected.
Either LT 920 or LT 5751 is normally suffi-cient to provide RWST level indication to the control room.
Addi-tionally, the RWST level may be determined locally at the tank using level indicator LI 921.
B-3 2 ASP Modeling Assumptions and Approach This event has been modeled as a loss of high-pressure recirculation.
In the event of unavailability of RWST level indication, failure of the low-pressure injection pumps and piggy-backed safety injection pumps would occur if sump switchover was effected too early (since inadequate sump level would exist for recirculation) or too late (since the RWST could be completely drained).
The plant vulnerability is conservatively assigned at 19.7 h corresponding to 0651 on December 11 when the instru-mentation indication may have become unreliable, to 0231 on December 12 when LI 920 was returned to service.
The likelihood of not recovering RWST level indication depends on the initiating event.
For large-break LOCAs, little likelihood of recovery would exist, since the RWST is drained in -0.5 h. For a small-break LOCA or if high-pressure injection is being used for bleed and feed, the RWST is drained more slowly, and detection of the faulted instrumentation and local recovery is consid-ered possible [p(nonrecovery) =
0.34 was assumed in the analysis].
For the unavailability period, a large-break LOCA does not signifi-cantly contribute to the risk differential associated with this event
[p(cdllarge-break LOCA) < 19.7/(8760 x0.7) x 1.0 x 10-41, and hence only the impact on transient and small-break LOCA sequences has been addressed.
Analysis Results The dominant sequence for this event involves a postulated loss of coolant accident during the 19.7-h RWST level indication failure vulnerability period (p =
2.0 x 10-5) with failure of the operator to initiate high-pressure recirculation.
This sequence is highlighted on the following event tree.
B-33 LOCA IRT IAFW IMFW HPI HPR PORV I
I I___
OPEN SEQ END NO STATE OK 71 CD 72 CD OK 73 ED 74 ED OK 75 CD 76 ED 77 ED 78 ATWS Dominant Core Damage Sequence for LER 247/88-020
B-34 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier:
247/88-020 Event
Description:
RWST level indication inoperable Event Date:
12/11/88 Plant:
Indian Point 2 UNAVAILABILITY, DURATION= 19.7 NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS LOOP LOCA SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator
- 9. 1E-03 l.0E-04
- 6. 9E-06
- 6. 9E-06
- 0. OE-400
- 0. OE+00
- 0. QEs-O
Sequence 71 loca -rt
-afw -hpi NPR/-BPI End State CD
-- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State 71 loca -rt
-afw -hpi NPR/-NPI CD non-recovery credit for edited case Note:
For unavailabilities, conditional probability values are differential values added risk due to failures associated with an event.
Parenthetical values indicate compared to a similar period without the existing failures.
SEQUENCE MODEL:
c:\\asp\\sealmod\\pwrbseal.cmp BRANCH MODEL:
c:\\asp\\sealmod\\indpoint.sll PROBABILITY FILE:
c:\\asp\\sealmod\\pwr bsll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov trans 4.6E-04 1.OE+00 loop 3.1E-05 1.7E-01 loca 2.4E-06 4.3E-01 rt 2.8E-04 1.2E-01 rt/loop G.0E+GG 1.0E+00 Prob N Rec t t 6.9E-06 4.3E-01 Prob N Rec**
6.9E-06 4.3E-01 which reflect the a reduction in risk Opr Fail Event Identifier: 247/88-020
B-35 emerg.power a Ow afw/emerq.power mfw porv.o r.srv.cha 11 porv.or.srv. reseat porv.or.srv.reseat/emerq.power seal.loca ep.rec(sl) ep. rec hpi hpi (f/b)
HPR/-HPI Branch Model:
1.OP.2+opr Train 1 Cond Prob:
Train 2 Cond Prob:
porv.open 5.4E0-04 3.8E-04 5.00-02 2.00-01 4.00E-02 2.0OE-02 2.0 0-02
- 2. 1E-01 6.0OE-01
- 5. 6E-02 3.00-04 3.00-04 1.5E-04 > 1.5E-04 1.00-02 1.5E-02 1.00-02 8.00-01 2.6E-01 3.4E0-01 3.4E-01 1.00E+00 1.10E-02
- 1. 00+00 1.0 0+00 1.00+00 1.00+00
- 8. 4 -01
- 8. 4 -01 1.00+00 1.00+00 1.OE-02 1.00-03 > 3.40-01 4.OE-04
- branch model file
- forced Minarick 11-0 9-198 9 11:52:36 Event Identifier: 247/88-020