ML20135G721
| ML20135G721 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 05/14/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1995-005 | |
| Download: ML20135G721 (13) | |
Text
LER No. 313/95-005 AppendixB voltage regulating circuit for the power supply, resulted in a half-trip of the EFIC system. Train A SG level indication was lost, as was remote control of atmospheric dump valve (ADV) CV-2668 and emergency feedwater valves CV-2646 and CV-2648 (see Fig. B.3.2).
B.3.3 Additional Event-Related Information To adequately remove heat from the reactor core after a scram or a trip, only one of two EFW pump trains must be available to deliver water to at least one of the two OTSGs. The failure of the +5-volt power supply resulted in the loss of EFW flow control valves in the MDEFWP train (CV-2646 and CV-2648) and ADV CV-2668 control in either automatic or manual control (local control of the ADV was still possible).
B.3.4 Modeling Assumptions About I hour after the trip, EFIC Train A failed, resulting in a loss of automatic and manual control of EFW flow control valves CV-2646 and -2648. The licensee event report (LER) for this event is not specific regarding the as-failed position of the MDEFWP flow control valves and the impact of the failure on system performance. If the valves failed closed, then the auxiliary feedwater supply from the MDEFWP would be unavailable. If the valves failed full-open, then they would not be capable of regulating flow. This latter condition could eventually require the operators to trip the MDEFWP to prevent steam generator overfill.
In this case, tripping the MDEFWP would be modeled as a recoverable system failure. Either of the above cases (failed open or failed closed) leads to the unavailability of the MDEFWP; therefore, this event was modeled as a reactor trip with flow from the MDEFWP made unavailable by failure of its EFW flow control valves. Note that failure of the flow control valves in the open position in conjunction with operator failure to control SG level by tripping the MDEFWP could result in failure of the turbine-driven EFW pump (TDEFWP). This potential failure mode was not explored.
Control of EFW flow control valves CV-2646 and CV-2648 was lost when a +5-volt de power supply in EFIC Train A failed. This failure was apparently caused by a random failure of a voltage regulator within the power supply. No information was provided that specifically indicated an increased potential for common-cause failure of the flow control valves in the TDEFWP train, so no increase in common-cause failure probability was modeled.
To implement the assumed failure of the MDEFWP flow control valves, the set of valves associated with the MDEFWP (Basic Event EFW-MOV-CF-DISM) was set to TRUE (i.e., the valves were failed). This setting caused the motor-driven train of the EFW to be failed in the model. The turbine-driven train was still available and was not subject to the common-cause failure (i.e., loss of the +5-volt de power supply in EFIC Train A) that rendered the MDEFWP flow control valves inoperable. Basic event probability changes are noted in Table B.3.1.
NUREG/CR-4674, Vol. 23 B.3-2
LER No. 313/95-005 Appendix B Figure removed during SUNSI review.
Fig. B.3.1 ANO I Emergency Feedwater System.
NUREG/CR-4674, Vol. 23 B.3-4
AppendixB LER No. 313/95-00S Figure removed during SUNSI review.
Fig. B.3.2 ANO 1 Emergency Feedwater System.
B.3-5 NUREG/CR-4674, Vol.13