ML20135G742
| ML20135G742 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 05/14/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1991-003-00 | |
| Download: ML20135G742 (5) | |
Text
B-288 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LERNo.:
Event
Description:
Date of Event:
Plant:
Summary 323/91-003 Containment sump isolation valves and containment spray pumps deenergized during hot shutdown September 1, 1991 Diablo Canyon 2 Both Diablo Canyon 2 residual heat removal (RHR) containment sump isolation valves were depowered for 6 h in mode 4 by locally opening the valve breakers. In this mode, power should have been interrupted by opening series contactors in the control room, which would have allowed rapid restoration of power to the valves if their operation was required following a loss-of-coolant accident (LOCA). The conditional core damage probability estimated for the event is 2.1 x lQ-6. The relative significance of this event compared to other postulated events at Diablo Canyon 2 is shown below.
LER 323/91-003 lE-7 IE-6 I IE-5 IE-4 IE-3 IE-2 I
I
- I I
I I
I LOOP+l J L1.00p L360h EP Trip MfRAFW L precursor cutoff 360hAFW Event Description While Diablo Canyon 2 was in hot shutdown, a walkdown of the control room boards revealed that power had been removed from both RHR containment recirculation sump suction valves, 8982A and B. The cause of power loss was that the 480-V breakers serving the RHR valves had been opened locally in preparation for entering cold shutdown. A procedure had been revised in 1988 in violation of Technical Specification (Tech Spec) requirements, to require local opening of the breakers associated with the valves. The Tech Specs require control room operation of the valves while the plant is in hot shutdown - the procedure should have specified use of the contactor located in the control room for removing power from the valves. The valve breakers were open for
B-289 approximately 6 h.
The walkdown also revealed that, due to personnel error, the control power to both containment spray (CS) pumps had been deenergized. The CS pumps were deenergized for approximately 1.5 h before power restoration.
Additional Event-Related Information, The RHR system consists of two trains. During high-pressure recirculation (HPR), each RL{R pump takes suction from the containment sump via separate containment isolation valves. After the sump water is cooled by the RHR heat exchangers, it is supplied to the suctions of the safety injection (SI) and charging pumps. RHR pump 1 provides flow to SI pump 1 and both charging pumps; RHR pump 2 provides flow to SI pump 2.
The CS system is also a two-train system that initially provides water fromn the reactor water storage tank (RWST) to two spray ring headers for containment pressure suppression following a LOCA. After" the RWST is empty, CS flow is provided from the RHR system.
ASP Modeling Assumptions and Approach The event has been modeled as a 6-h unavailability of HPR. Local recovery of HPR at the recirculation valve breakers was assumed to be possible (the utility estimated this would take -15 min). Since the Accident Sequence Precursor (ASP) models only address core damage, the unavailability of the CS pumps was not considered.
Analysis Results The core damage probability estimated for the event is 2.1 x 10-6. The dominant core damage sequence, highlighted on the following event tree, involves a postulated small-break LOCA with successful high-pressure injection and failure of HPR. This estimate is believed to be conservative, since the actual event occurred in mode 4 and the ASP model success criteria and timing assume operation at power.
B-290 LOCA IRT IAFW MFW HPI HPR PORVJ I J OPEN SEQ END NO STATE 71 72 OK CD CD OK 73 CD 74 CD OK 75 CD (1) 76 CD 77 CD 78
.ATWS (1) OK for Class D Dominant core damage sequence for LER 323/91-003
B-291 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier:
323/91-003 Event
Description:
Containment sump isolation valves deenergized Event Date:
09/01/91 Plant:
Diablo Canyon 2 UNAVAILABILITY, DURATION-6 NON-RECOVERABLE INITIATINMG EVENT PROBABILITIES TRANS 1.5E-03 LOOP 6.8E-05 LOCA 6.2E-06 SEQUENCE CONDITIONAL PROBABILITY SUNS End State/Initiator Probability CD TRANS 1.0E-08 LOOP
- 0. 0E+00 SEQUENCE COND ITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
71 loca -rt
-afw -hpi HPR/-HPI CD 2.lE-06 1.5E-01 non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
71 loca -rt
-afw -hpi IIPR/-HPI CD 2.1E-06 l.5E-01 non-recovery credit for edited case Note:
For unavailabilities, conditional probability values are differential values which reflect the added risk due to failures associated with an event.
Parenthetical values Indicate a reduction in risk compared to a similar period without the existing failures.
SEQUENCE MODEL:
c:\\asp\\1999\\pwrbseal.czrp BRANCH MODEL:
c:\\asp\\1999\\diablo2.s11 PROBAB3ILITY FILE:
c:\\asp\\1989\\pwr~bsll.pro No Recovery Limit Event Identifier: 323/91-003
B-292 BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail trans loop loca rt rt/ loop emerg.power a fw afw/emerg.power mfw porv.or.srv.chall porv.or.srv.reseat porv.or. srv. reseat/eslerg.power seal. loca ep. rec (si) ep. rec hpi hpl (f/b)
HPR/-HPI Branch Model:
l.OF.2+opr Train 1 Cond Prob:
Train 2 Cond Prob:
porv.open Sbranch model file
- forced Minarick 05-26-1992 10!46t03 2.5E-04 2.OE-05 2.4E-06 2.8E-04 0.OE+00 5.4 E-04 3.8E-04 5.OE-02 1.0E+00 4.OE-02 3.OE-02 3.OE-02 3.2E-01 6.5E-01 1.1lE-01 1.OE-03 l.OE-03 1.5E-04 > 1.OE+00 l.OE-02 > Unavailable 1.5E-02 > Unavailable l.OE-02 1.0E4-00 5.8E-01 4.3Z-01 1.2E-01
- 1. 0E+00 8.0E-01 2.6E-01 3.4E-01 7.05-02 1.OE+00 1.1E-02 1.0E+00 1.OE+00 1.05-400 l.OE+00 8.4E-01 8.4E-01 1.05+00 > 3.4E-01 1.0E+00 l.OE-02 1.05-03 4.OE-04 Event Identifier: 323/91-003