ML20147A536
| ML20147A536 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 05/26/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1983-045-00, LER 1983-046-00, LER 1983-052-00 | |
| Download: ML20147A536 (1) | |
Text
LERs 237/83-045, -046, -052 Event
Description:
Core Spray A, LPCI A, and SOC A Inoperable, Scram Date of Event:
June 8, 1983 Plant:
Dresden 2 Summary On June 8, 1983, Dresden Unit 2 was operating at approximately 100% power when the circuit breaker for core spray (CS) injection valve M02-1402-25B was found tripped after the valve had been exercised. The breaker trip setting was found to have been adjusted incorrectly some time before. On June 15, 1983, Dresden was operating at approximately 70% power when low-pressure coolant injection (LPCI) train A was aligned to pump down the suppression pool. As the system was being secured, it was discovered that the bolts attaching the train A minimum flow bypass valve motor to the valve (M02-1501-13A) had broken, allowing the motor to separate from the valve. On June 21, 1983, operators were attempting to align shutdown cooling (SDC) following a scram, when the A train SDC return valve failed to open. An investigation revealed that a packing leak on another valve allowed water to enter the SOC return valve motor, causing a fault and burning out the motor. Dresden 2 was returned to service after an extended outage on April 25, 1983. Two scrams occurred during the time that the CS, LPCI, and SDC systems were inoperable, one on June 11 and the other on June 20. The unit was returned to service immediately after the June 11 scram but was shut down after the June 20 scram.
This event is complicated by multiple overlapping equipment unavailabilities. The CS train B failure identified on June 8 was assumed to have existed for half of a one-month surveillance interval. The LPCI A minimum flow valve failure identified on June 15 was assumed to have resulted in failure of the A LPCI train and to have existed for half of a one-month surveillance interval. The SDC A return valve failure identified when the system was demanded on June 21 was assumed to have existed for half of the interval since the last known demand on the system, at the end of the outage on April 25. The potential impact of these events was therefore evaluated as the sum of the effects of the following: a scram with SDC A return inoperable (June 20), a scram with LPCI A and SDC A inoperable (June 11), 7 days of unavailability of LPCI A, CS B, and SDC A; 7 days of unavailability of LPCI A and SDC A, 8 days of unavailability of CS Band SDC A; and 6 days of unavailability of SDC.
Calculated conditional core damage probabilities for the events involving unavailabilities were small relative to those for the scrams and were neglected. The conditional core damage probability estimated for the combined scram events is 3.3x10-6. The dominant core damage sequence involves the observed transient, failure to scram, and failure of the recirculation pump trip breakers to operate.