ENS 43146

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v t e ENS Event
02:43 Dec 9, 2006
Title	Invalid Efw Actuations Due to an Issue in Sg Dryout Protection Circuit
Event Description	This report is being made per paragraphs 50.73(a)(1) and 50.73(a)(2)(iv)(A) to address two actuations of the Emergency Feedwater (EFW) System on Oconee Unit 1 on 12/8/06 by the Steam Generator (SG) Dry-out Protection Circuit. EFW is a system named in 50.72(b)(3)(iv)(B). By design, the SG Dry-out Protection Circuit actuates whenever two Extended Start-up level indications (two of two actuation logic) in either SG are lower than the setpoint (i.e. below 21 inches) for more than 30 seconds. This start circuit is not credited in Oconee safety analyses as performing a required safety function, therefore this signal is considered an INVALID signal with respect to 50.73 (a)(2)(iv)(A). Upon actuation, the circuit starts both Motor Driven Emergency Feedwater Pumps (MD EFWPs) on the affected unit. It does not send a start signal to the Turbine Driven Emergency Feedwater Pump. Per the startup procedure, Condensate Booster Pumps (CBP) feed the SGs until a main Feedwater pump (FDWP) is placed in service. Operators begin startup of a FDWP when the Reactor Coolant System (RCS) temperature is between ~395 degrees to 475 degrees. The Unit is then maintained at ~ 475 degrees RCS temperature and 500 - 550 psig SG pressure until a FDWP is in service. On 12/8/06, a Unit 1 startup from a refueling outage was in progress. RCS temperature was approximately 475 deg. (Mode 3) when the RCS heat-up was suspended due to issues with the 1A FDWP Turbine which delayed placing it in service. Steam pressure was being maintained approximately 530 psig (in the procedural band of 500-550 psig) per the unit startup procedure. In addition, a 'SG Hot Soak' was in progress. For this evolution, SG level was raised to 75% (Operating Range) then Operators took manual control to individually blow down 1A and the 1B SGs to low levels (25 inches) while attempting to maintain level above 21 inches. During this evolution, 1B SG levels decreased below minimum level. Operators initiated actions to recover level but at 2143 the 1A and the 1B MD EFW Pumps auto-started on dry-out protection. From their observation of the level displays, the Operators thought that the actuation logic had not been met (i.e. level had not been less that 21 inches for 30 seconds). Therefore they considered the actuation to be spurious. Based on this understanding and concerns about possible overfeeding and RCS overcooling (due to the relatively low decay heat load starting up from a refueling outage), EFW was secured and reset at 2144. SG Blowdown activities continued per procedure. At 2207, in response to lowering 1A SG levels, the dry-out protection feature again initiated an auto start of the 1 A and 1 B MD EFW Pumps. Based on his review of SG levels, the CR SRO believed that the second EFW actuation was invalid and directed that EFW be secured. EFW was secured and reset at 2208. At 2241 hrs SG outlet pressure was lowered and maintained within a band of 490-520 psig. Subsequent review of computer data indicated that a dry-out condition (i.e. less than 21 inches SG level for 30 seconds) did exist for both actuations. Therefore both actuations were valid in that actual plant conditions satisfied the initiation criteria, but, as stated above, the SG Dry-out Protection signal is considered an INVALID signal with respect to 50.73 (a)(2)(iv)(A). Additional review by Mechanical-Civil Engineering personnel determined that the Unit 1 CBPs operate at approximately 15 psig lower discharge pressure than the Unit 2 and 3 CBPs, apparently due to lower discharge pressures on the Unit 1 Hotwell pumps. That being the case, the Unit 2 and 3 CBPs, apparently due to lower discharge pressures on the Unit 1 Hotwell pumps. That being the case, the Unit 1 CBPs in service could not develop sufficient head pressure to maintain adequate flow to the SGs over the full operating operating band of 500 - 550 psig allowed by current operating procedure guidance. As steam pressures approached or exceeded 535 psig, the ability to feed the SGs using the CBPs was reduced, ultimately leading to the dry-out condition. The issue of these pump discharge pressures being slightly lower than expected had not been recognized previous to this event. The event investigation noted that during past startups, a main FDWP was normally placed into service promptly, which may have avoided using the CBPs to feed the SG above 535 psig. In this case, alignment of a main FDWP was delayed, which led to the event. Specific Information Required per NUREG 1022: (a) The specific train(s) and system(s) that were actuated. The specific train(s) and system(s) that were actuated were the 1 A and 1 B Motor Driven (MD) Emergency Feedwater (EFW) Pumps and the associated A and B trains of EFW. The Turbine Driven EFW pump does not receive this signal and was not challenged by the event. (b) Whether each train actuation was complete or partial. Both actuations were considered complete (i.e. all necessary components responded to the start signal and associated control signals to provide EFW to the SG(s)). (c) Whether or not the system started and functioned successfully. In both cases, the system started and operated successfully until secured by Operations personnel. Initial Safety Significance: None, there was no significant transient. Corrective Action(s): As stated above, EFDW was secured and reset at 2208. At 2241 hrs SG outlet pressure was lowered and maintained within a band of 490-520 psig. Unit 1 continued startup and subsequently reached Mode 1. This event has been entered into the site corrective action program for resolution. The licensee notified the NRC Resident Inspector.

ENS 43146 +/-
Where
Oconee South Carolina (NRC Region 2)
Reporting

10 CFR 50.73(a)(1), Submit an LER, Invalid Actuation
Time - Person (Reporting Time:+1424.22 h59.343 days <br />8.477 weeks <br />1.951 months <br />)
Opened:	Randy Todd 10:56 Feb 6, 2007
NRC Officer:	Pete Snyder
Last Updated:	Feb 6, 2007
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v • d • e

v t e Emergency Notification System
Oconee with 10 CFR 50.73(a)(1), Submit an LER
WEEKMONTHYEAR2012-04-23T00:58:00 [Table view]
[ENS list for Oconee]

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