05000400/LER-2007-001
| Docket Number | |
| Event date: | 03-09-2007 |
|---|---|
| Report date: | 05-08-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4002007001R00 - NRC Website | |
I. DESCRIPTION OF EVENT
At 07:40 a.m. on March 9, 2007, with the reactor at approximately 100 percent power, surveillance test OST-1005 "Control Rod and Rod Position Indicator Exercise Quarterly Interval Mode 1-3," was being performed to satisfy Technical Specification (TS) 4.1.3.1.2 where each operable shutdown and control rod not fully inserted into the core is determined to be operable by movement of at least 10 steps in any one direction at least once per 92 days. Shutdown Bank A had been inserted from the "park" position of 228 steps to 218 steps in accordance with OST-1005, and was being withdrawn to the "park" position when a rod control urgent alarm was received causing Shutdown Bank A to stop at 220 steps (group 1 rods at 220 steps and group 2 at 221 steps). The urgent alarm originated in the rod control logic cabinet and precluded motion of the rods in Shutdown Banks A and B. As required, Operators suspended OST-1005 and entered procedure AOP-001 "Malfunction of Rod Control and Indication System." TS 3.0.3 was entered at this time due to the inability to satisfy TS 3.1.3.5 for the shutdown bank rods not fully withdrawn as specified in the Core Operating Limits Report (COLR).
In addition, due to the inability to move shutdown or control rods, the control rods were verified to be above the Rod Insertion Limits to satisfy TS Surveillance 4.1.1.1.1, and shutdown margin was subsequently verified by completion of OST-1036 "Shutdown Margin Calculation Modes 1-5.
Trouble shooting commenced at approximately 09:15 a.m.. A failed Slave Cycler Logic Card in the Westinghouse Rod Control System [AA] (card model 3359C80G01 s/n 0210) was replaced with a logic card from stock. At approximately 10:26 a.m., Shutdown Bank A was withdrawn to its "park" position of 228 steps.
The plant was in TS 3.0.3 for 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> and 53 minutes. Operators exited TS 3.0.3 at 10:33 a.m. and applied TS 3.1.3.1 Action C for having more than one rod inoperable due to a rod control urgent failure or obvious electrical problem. This was applied pending completion of the Post Maintenance Test for the replacement Slave Cycler Logic Card.
TS 3.1.3.1 Limiting Condition for Operation (LCO) relating to Action C was satisfied at approximately 2:40 pm following completion of withdrawal of Control Bank D to the original pretest condition with the completion of the post maintenance test OST-1005.
There were no other inoperable structures, systems or components that contributed to this event.
Testing of the failed Slave Cycler Logic Card on March 14, 2007 revealed that chip Z4 "locked-up" in an indeterminate state. The most likely mechanism for this failure is component aging of this chip. A planned corrective action is to establish a structured program of card replacements. This action is being tracked through the corrective action program.
This condition is being reported as a condition prohibited by Technical Specifications in accordance with 10 CFR 50.73(a)(2)(i)(B).
Energy Industry Identification System (EIIS) codes are identified in the text within brackets [ ].
II. CAUSE OF EVENT
The cause was a failed Slave Cycler Logic Card in the Westinghouse Rod Control System [AA] (card model 3359C80G01 s/n 0210). This failure prevented the rod control system from operating and required Operators to enter TS 3.0.3 due to the inability to satisfy TS 3.1.3.5 for the shutdown bank rods not fully withdrawn as specified in the COLR.
Testing of the failed Slave Cycler Logic Card revealed that chip Z4 "locked-up" in an indeterminate state. This chip is a high threshold NAND logic gate and can fail either "locked-up" or drift high. The most likely mechanism for this failure is component aging. The date code of the failed chip is 1972.
III. SAFETY SIGNIFICANCE
This event has no actual safety significance.
Actual Safety Consequences:
The rod control malfunction during OST-1005 was a failure originating in the rod control logic cabinet. The malfunction precluded motion of the rods in Shutdown Banks A and B. Shutdown Bank C could have been moved using the "bank select" mode since its motion is not controlled through the same rod control circuits as Shutdown Banks A and B. At the time of the failure, Shutdown Bank A rods were positioned at 220 steps (group 1 rods at 220 steps and group 2 at 221 steps). The bank had been inserted from the "park" position of 228 steps to 218 steps in accordance with OST-1005 and was being restored to the "park" position at the time of the failure. TS LCO 3.1.3.5 normally requires all shutdown bank rods be fully withdrawn; however, a Special Test Exception is provided by the LCO to allow the performance of this rod exercise surveillance test. The actual consequence of the rod control malfunction was the inability to insert or withdraw the rods except for Shutdown Bank C. This condition rendered the affected rods inoperable and required entry into TS 3.1.3.1 Action C, which provided 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> to correct the condition. Additionally, Operations personnel entered TS 3.0.3 since the Special Test Exception was no longer valid and the applicable Action statement for TS 3.1.3.5 allows only one rod to not be fully withdrawn. TS 3.0.3 allows one hour to correct and then six hours to be in Hot Standby.
Although the affected rods were inoperable, throughout this event all rods were capable of manually or automatically tripping into the core to shutdown the reactor.
Potential Safety Consequences:
The minimal insertion of the shutdown Bank A rods did not impact core shutdown margin, neutron flux distribution or any core operating limits or peaking factors that might be assumed in the initial condition of any transient or accident analysis. Axial flux difference remained stable during the entire performance of the surveillance test. The inability to move the control rods has the potential to impact core response should a reactor shutdown become required, or a transient such as a turbine runback been imposed on the plant. Various computer generated reactivity plans projected that a shutdown performed with control rods at the height during this event would result in the indicated Axial Flux Difference (AFD) turning in the positive direction and exceeding the bands prescribed in Technical Specifications. The short term operation beyond TS AFD limits would have no adverse impact on the fuel pellets or rods during either a planned downpower or in subsequent power operations. Throughout this event all rods would have been capable of manually or automatically tripping into the core to shutdown the reactor.
This condition is being reported as a condition prohibited by Technical Specifications in accordance with 10 CFR 50.73(a)(2)(i)(B).
IV. PREVIOUS SIMILAR EVENTS
Cabinet Card failure. Diagnostic tests and visual examinations were completed for the installed Power Cabinet Cards but not for the Slave Cycler Logic Cards. However, this type of test is static in nature and it is unlikely that it would have predicted or detected the type of age related failure associated with the Slave Cycler Logic Card. The planned corrective action below for establishing a structured card replacement program should diminish the frequency of rod control card failures.
V. CORRECTIVE ACTIONS
Completed corrective actions include replacing the failed Slave Cycler Logic Card. A planned corrective action is to establish a structured program of card replacements. This action is being tracked through the corrective action program.
VI. COMMITMENTS
This document contains no new regulatory commitments.