05000278/LER-2009-001
| Docket Number Sequential Revmonth Day Year Year Month Day Year 05000Number No. | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 2782009001R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 3 was in Mode 2 with power descending in preparation for a planned outage to replace the 3C Main Transformer when this event occurred on 1/21/09. The unit was being shutdown using control rods (i.e., soft shutdown). There were no other structures, systems or components out of service that contributed to this event.
Description of the Event
As a result of control rod interference monitoring testing performed by Operations personnel on 1/28/09, it was determined that three control rods (EIIS: JC) were inoperable during the unit shutdown that was performed on 1/21/09. This testing determined that Control Rods 14-55, 18 55 and 42-55 could be inoperable for operational conditions involving time periods when reactor pressure is below 850 psig (Mode 2 operations). This testing, however, determined that above 850 psig reactor pressure, the control rods were OPERABLE. In accordance with Technical Specification (TS) Limiting Condition for Operation (LCO) 3.1.3, Control Rod OPERABILITY, each Control Rod must be OPERABLE with the unit in Mode 1 or Mode 2.
Unit 3 had entered Mode 2 at approximately 0513 hours0.00594 days <br />0.143 hours <br />8.482143e-4 weeks <br />1.951965e-4 months <br /> on 1/21/09. Reactor pressure was reduced to below 850 psig at approximately 0530 hours0.00613 days <br />0.147 hours <br />8.763227e-4 weeks <br />2.01665e-4 months <br />. Operations personnel observed that Control Rods 14-55 and 18-55 did not properly settle, after being fully inserted, at approximately 0618 hours0.00715 days <br />0.172 hours <br />0.00102 weeks <br />2.35149e-4 months <br /> and approximately 0927 hours0.0107 days <br />0.258 hours <br />0.00153 weeks <br />3.527235e-4 months <br />, respectively. At 0618 hours0.00715 days <br />0.172 hours <br />0.00102 weeks <br />2.35149e-4 months <br />, reactor pressure was approximately 642 psig and at 0927 hours0.0107 days <br />0.258 hours <br />0.00153 weeks <br />3.527235e-4 months <br />, reactor pressure was approximately 408 psig. Mode 3 was entered at approximately 1018 hours0.0118 days <br />0.283 hours <br />0.00168 weeks <br />3.87349e-4 months <br />, thereby exiting the Mode of applicability for Control Rod OPERABILITY.
Additional extent-of-condition control rod testing was performed on 1/23/09 while the unit was shutdown and it was discovered that Control Rods 42-55 and 46-55 also did not properly settle.
Prior to unit startup on 1/26/09 (i.e., entering Mode 2), Control Rods 14-55, 18-55, 42-55 and entered Mode 2 on 1/26/09 at approximately 0900 hours0.0104 days <br />0.25 hours <br />0.00149 weeks <br />3.4245e-4 months <br />. The control rods were considered inoperable, pending control rod interference monitoring testing performed on 1/28/09.
Based on the testing performed on 1/28/09, Control Rods 14-55, 18-55, and 42-55 are assumed to have become inoperable at 0530 hours0.00613 days <br />0.147 hours <br />8.763227e-4 weeks <br />2.01665e-4 months <br /> on 1/21/09 when reactor pressure was reduced to below 850 psig. Mode 3 was entered at approximatley 1018 hours0.0118 days <br />0.283 hours <br />0.00168 weeks <br />3.87349e-4 months <br />, thereby terminating the event.
� Description of the Event, continued This report is being submitted pursuant to:
10CFR 50.73(a)(2)(vii) — Common Cause Inoperability of Independent Trains — This occurrence is reportable under this criterion since three control rods were inoperable for approximately 4.75 hours8.680556e-4 days <br />0.0208 hours <br />1.240079e-4 weeks <br />2.85375e-5 months <br /> on 1/21/09 during Mode 2 unit operations for a common cause (i.e., control rod interferences).
Control Rods 14-55, 18-55 and 42-55 were restored to an OPERABLE status on 1/28/09 at approximately 0630 hours0.00729 days <br />0.175 hours <br />0.00104 weeks <br />2.39715e-4 months <br /> subsequent to control rod interference monitoring testing. The three control rods are only considered OPERABLE for time periods when reactor pressure is at or above 850 psig.
Analysis of the Event
There were no actual safety consequences associated with this event.
The control rod blade and drive mechanism provides control of reactor power, including the ability to provide a sufficiently rapid insertion of control rods (scram) so that no fuel damage results from any abnormal operating transient and limits fuel damage under accident conditions. The 185 control rods are located uniformly throughout the core on a 12-inch pitch. Four fuel assemblies surround each control rod. Control Rods 14-55 and 18-55 are adjacent control rods. Peach'- Bottom Units 2 and 3 are 'D-lattice' reactor fuel design units. Global Nuclear Fuels (GNF) is the fuel vendor. The fuel is GE14 type fuel. The control rod blades are original equipment supplied by General Electric.
Control Rod Drive (CRD) System Hydraulic Control Unit (EIIS: HCU) scram accumulators are designed with a limited nitrogen pressure and volume, which are sufficient to initiate control rod scram motion. Any force necessary to complete the insertion stroke is supplied by reactor coolant delivered at reactor pressure. As reactor pressure decreases, the amount of pressure available to complete the control rod stroke also decreases, and for pressure down to approximately 550 psig, this means less control rod friction can be overcome. Two low pressure scram events are identified as bounding:
- The Main Steam Isolation Valve (MSIV) isolation pressure scram (Mode 1 operations)
- Scram at lower pressures near 550 psig. The scram at pressures near 550 psig would occur only during startup and soft shutdown conditions (Mode 2 operation).
Concerning the MSIV isolation pressure scram, Control Rods 14-55, 18-55 and 42-55 were considered OPERABLE throughout the event. The unit Primary Containment Isolation System (PCIS) setpoint for low reactor pressure is > 850 psig while the unit is in Mode 1. Decreasing reactor pressure to this isolation setpoint would result in a Group I PCIS isolation, resulting in a unit scram when in Mode 1.
- Analysis of the Event, continued If a low reactor pressure scram was required during the 1/21/09 Mode 2 soft shutdown, it is possible that Control Rods 14-55, 18-55, and 42-55 may not have properly inserted due to the lower reactor pressures at the time the control rods were withdrawn. However, following incomplete insertion during a low-pressure scram due to control rod friction, the control rod could have been manually inserted by re-setting and re-applying the scram. This would have re-established adequate CRD scram accumulator pressure allowing the capability of overcoming channel-control blade friction at lower reactor pressures. In addition, the control rods could also have been manually inserted using normal control rod insertion via the reactor manual control system.
In addition, an event specific analysis was performed that determined that appropriate shutdown margin was maintained, assuming that the affected control rods and the rod of highest reactivity worth in the core did not insert.
Cause of the Event
The cause of the inoperability of Control Rods 14-55, 18-55 and 42-55 is apparently due to fuel channel distortion as discussed in the fuel vendor's safety information communication on this topic (GEH SC08-05, Rev. 1, Updated Surveillance Program for Channel-Control Blade Interference Monitoring). Since control rod slow settle times is the earliest indication of elevated cell friction, the observations made during the events described in this report are consistent with channel distortion.
A root cause investigation is in-progress for this event. Appropriate causes will be determined and corrective actions will be implemented in accordance with the Corrective Action Program.
Corrective Actions
A control rod interference monitoring / testing program has been established. This program is based on the recommendations of the vendor-issued safety information communication concerning channel — control blade interference monitoring.
Control rod interference monitoring / testing was performed on Control Rods 14-55, 18-55, 42 55 and 46-55. Based on the testing results, Control Rods 14-55, 18-55 and 42-55 were determined to be INOPERABLE for reactor pressures below 850 psig.
Other control rods on Unit 3 were tested (stroked and settle time tested) during the planned outage. Other appropriate control rods are being tested in accordance with the methodologies and frequencies of the control rod interference monitoring testing with no additional inoperabilities discovered to-date.
� Corrective Actions, continued Control rod interference monitoring testing was performed for Unit 2 control rods. There were no inoperable control rods detected on Unit 2.
Additional actions, including resolution of the control rod interferences on Unit 3, will be performed in accordance with the Corrective Action Program.
Previous Similar Occurrences There were no previous LERs related to control rod interferences.
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