05000263/LER-2001-006
| Docket Number | |
| Event date: | 02-22-2001 |
|---|---|
| Report date: | 04-19-2001 |
| 2632001006R00 - NRC Website | |
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MONTICELLO NUCLEAR YEAR SEQUENTIAL NUMBER REVISION NUMBER
Description During an assessment conducted in December, 2000, by the Nuclear Management Company (NMC) Fire Protection Program Group, it was found that wiring for several components credited for plant shutdown from the Alternate Shutdown System' (ASDS) panel was not completely independent of the Control Room or Cable Spreading room. The ASDS provides alternative safe shutdown capability as required by 10 CFR 50.48 and 10 CFR 50, Appendix R, in the event of a fire in the Control Room or Cable Spreading Room.
An extent of condition review was initiated by the ASDS system engineer to determine if fire damage to the wiring in question could adversely affect the function of the ASDS. On February 22, 2001, during this review, a vulnerability to hot short failures in the ASDS was confirmed. This vulnerability occurs on loss of offsite power when powering Buse 16 from 12 Emergency Diesel Generator3 (EDG). Under these conditions a hot short is postulated to occur in the wiring connected to relay' 97- 31 located in Control Room panels C-08. This hot short has the effect of energizing the load shed relays for 12 EDG, which in turn strips Bus 16 of all large loads. The net effect is the inability to operate the following loads from the ASDS under loss of offsite power conditions:
1. 12 Residual Heat Removal6 (RHR) pump 2. 12 Core Spray' pump 3. 12 RHR Service Water8 pump Following discovery of this condition, the ASDS controls were declared inoperable, a Limiting Condition for Operation was entered in accordance with Technical Specification 3.13.H.2, and a compensatory fire watch was established in the Control Room and Cable Spreading Room as required by the Technical Specifications.
On February 24, 2001, continuing engineering evaluation identified additional postulated hot shorts that, taken one at a time, could reclose breakers on loads that were shed on loss of offsite power and that are not associated with equipment controlled from the ASDS. While some of these failures 1 EIIS System Code: 8 JL 2 EllS Component Code: BU 3EIIS Component Code: DG 4EIIS Component Code: RLY 'EIIS Component Code: PL 6 EllS System Code: 8 BO 7 EllS System Code: 8 BM 8E1 IS System Code: 8 BI 9EIIS Component Code: XFMR FACILITY NAME(1) DOCKET LER NUMBER (6) PAGE (3)
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would be non-consequential, others could cause 12 EDG or the emergency offsite 1AR transformer9 to be overloaded resulting in a lockout or possible damage. Breakers which could result in source overloading were:
1. 14 Bus to 16 Bus 4KV Supply Breaker, 152-408 2. 14 RHR Pump 4KV Supply Breaker, 152-603 3. 12 Control Rod Drive Pump 4KV Supply Breaker, 152-606 4. 14 RHR Service Water 4KV Supply Breaker, 152-607 Monticello Design Change 01Q055 was designed and installed on an expedited basis to resolve all hot short vulnerabilities described above. Installation and testing were completed on March 8, 2001.
Event Analysis
Analysis of Reportability This condition was determined to be reportable in accordance with 10 CFR Part 50, Section 50.73(a)(2)(v)(B). The hot short vulnerabilities that were identified had the potential to cause inoperability of the ASDS in the event of a fire in the Control Room or Cable Spreading Room coincident with a loss of offsite power.
Safety Significance
A risk assessment performed by Monticello Probabilistic Risk Assessment (PRA) Group concluded that the hot short vulnerabilities described above have a low safety significance. This conclusion was based on the following considerations:
1. The frequency of a fire in the Control Room or Cable Spreading Room.
2. The probability that the Control Room or Cable Spreading Room fire is not suppressed (i.e.
control room evacuation is required).
3. The probability of a loss of offsite power requiring 12 EDG to be the source of power to bus 16 (Note: It takes multiple hot shorts for the fire to cause a total loss of offsite power).
4. The probability of the hot short scenarios described above.
5. The probability of failure of an alternate injection system which is not dependent on AC power.
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There are many other alternative considerations which, if credited, would further lower the calculated safety significance of these hot short vulnerabilities.
Cause
The hot short vulnerabilities described in this report have existed since the original design and installation of the ASDS in 1986.
Backfitting of an ASDS to Monticello involved many interfaces with existing plant design features.
The existence of the hot short vulnerabilities described above represent complex system interactions which were not recognized during the original ASDS design.
Corrective Actions
The following wiring changes were made under Monticello Design Change 01Q055 to eliminate the hot short vulnerabilities described in this report:
- A "b" contact on the 12 EDG output breaker, 152-602, was rewired so that breaker closure blocks the postulated hot short in panel C-08 from energizing the load shed relays. This allows the relays to automatically reset and allow operation of their associated ASDS loads.
- A relay contact was inserted in the negative side of the control circuits for breakers 152-408, 152-603, 152-606, and 152-607. This isolates the closing coil and prevents the postulated hot short from closing the associated breaker.
An extensive engineering review was completed to determine if other similar potential hot short vulnerabilities existed beyond those identified in this report. No other credible or consequential hot shorts were identified.
Failed Component Identification Not applicable.
Similar Events No similar Monticello events have been identified.