05000251/LER-2010-005
| Docket Number Sequential Revmonth Day Year Year Month Day Yearnumber No. | |
| Event date: | 9-9-2010 |
|---|---|
| Report date: | 10-12-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 46246 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 2512010005R00 - NRC Website | |
DOCKETFACILITY NAME (1) LER NUMBER (6) PAGE (3)NUMBER (2) �YEAR NUMBER NUMBER
DESCRIPTION OF THE EVENT
On September 9, 2010 at approximately 16:34 hours, the unit entered Technical Specification (TS) 3.1.3.3 Action as a result of the Control Bank C Group 1 Step Counter [AA, CTR] failing to increment. The reactor was subcritical in Mode 3 progressing to reactor startup. The reactor trip breakers were opened as required by the Action of TS 3.1.3.3. The TS requires that the reactor trip breakers be opened if the Bank demand counters (group 1 and group 2) are not within +/- 2 steps of each other. All rods fully inserted. The unit remained in Mode 3. This was a manual actuation of the Reactor Protection System. Therefore, an 8-hour report (EN 46246) was made in accordance with 10 CFR 50.72(b)(3)(iv) to the NRC Operations Center.
The apparent cause of the event was failure of a Step Counter Battery. The Group Step Counters are driven by lithium ion batteries. The failure of the battery was unpredictable, with minimal consequences. There was no reason to expect early failure. The failed battery was replaced, along with the remainder of the Step Counter Batteries. A five year search of the plant, INPO, and NRC databases was performed and did not reveal any previous similar occurrences. Therefore, no further actions have been recommended.
CAUSE OF THE EVENT
The apparent cause of the event was the failure of the Step Counter to increment as required because of an early failure of the installed battery. The battery failed in such a way as to not alert operators. When the battery failed, it did not have enough energy to light the failed battery warning indicator, yet it provided enough energy to drive the LCD display by displaying a Zero (it was even able to increment to 1) but not enough energy to supply the driver circuits for the counters. Since these batteries were still early in their useful life, and no work had been done to the rod control cabinets, no post maintenance test (PMT) was necessary/performed. With an expected life of 8 to 10 years, a battery replacement cycle of 36 months, and a good service history to date of replacement batteries, there was no reason to suspect the impending battery failure.
ANALYSIS OF THE EVENT
During operation of the Rod Control System, the Logic Cabinet generates and sends pulses to the Power Cabinets. When the Power Cabinet receives this pulse it initiates a Rod Step. When the Rod Step cycle is complete a reply pulse is sent back to the Logic Cabinet. Once this reply pulse is received at the Logic Cabinet, the Logic Cabinet then triggers a relay which in turn sends out a 100 Vdc pulse to the associated Group (demand) Step Counter on the Control Room Operator's Console. When this pulse is received at the Group (demand) Step counter the step counter increments in the required direction (+1 if rods out and -1 if rods in). These Group (demand) Step Counters are driven by lithium ion batteries which are projected to last 8-10 years based on the manufacturer's literature. The current Preventive Maintenance (PM) cycle has the site replacing these batteries on a 36 month basis, which is well within the 8-10 year expected life. These batteries are replaced during refueling outages after which the Group Step Counters receive PMT by operating the rod control system during cold rod stepping, which is the only true PMT of the group step counters. During the recent Unit 4 forced outage, a battery failed in the Group 1, Bank C, Group Step Counter, and went unnoticed FACILITY NAME (1) DOCKET _ NUMBER (2) LER NUMBER (6) PAGE (3) Turkey Point Unit 4 05000251 until the rods were withdrawn and the counter did not increment. With Unit 4 in Mode 3 at the time, the TS required the reactor trip breakers to be opened. These batteries were installed during Unit 4, Cycle 24 refueling outage (circa May, 2008) and were thoroughly tested prior to installation; therefore there was no reason to suspect early failure. The Group Step Counters all appeared to be functional when the reactor reset startup pushbutton was pushed and counters reset and displayed Zero. A review of the design schematics of the Group Step Counters revealed that the display may indeed work while the counting circuits may not due to low battery supply voltage.
REPORTABILITY
The event is considered to be reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) due to failure of the step counters to be within +/- 2 steps of each other, requiring the reactor trip breakers to be opened in accordance with the Action of TS 3.1.3.3 (a manual actuation of the Reactor Protection System.)
ANALYSIS OF SAFETY SIGNIFICANCE
At the time of the event, the reactor was in Mode 3 (sub-critical) and the plant remained in mode 3. The reactor trip breakers were opened and all systems functioned as required. There was no impact on safety.
CORRECTIVE ACTIONS
Corrective actions were taken to replace all of the Unit 4 step counter batteries. Each unit contains these same batteries. The current PM replaces the batteries on a 36 month basis on batteries that are expected to last eight to ten years. The PM basis was reviewed and determined to be correct. No further adjustments were recommended at this time.
ADDITIONAL INFORMATION
component function identifier (if appropriate)]. Condition Report 2010-00578725 was initiated due to this event.
FAILED COMPONENTS IDENTIFIED: Step Counter battery.
PREVIOUS SIMILAR EVENTS: None in the last five years.