05000285/LER-2014-002

LER-2014-002, Reactor Manual Trip due to Control Rod Misalignment

Fort Calhoun Station
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
2852014002R00 - NRC Website

BACKGROUND

Fort Calhoun Station (FCS) is a two-loop reactor coolant system of Combustion Engineering (CE) design.

The Control Element Drive Mechanism (CEDM) functions to raise or lower its respective Control Element Assembly (CEA) in response to signals from the Control Room. The CEDM includes the upper components that drive the movement such as the drive package containing the motor/brake. It also includes lower components such as the drive shaft, bevel gears, and rack and pinion which are contained in the support tube assembly.

EVENT DESCRIPTION

After reaching criticality on January 12, 2014, the control room attempted to reduce power ascension rate, while at zero percent power, by inserting Group 4 CEAs. All Group 4 CEAs inserted with the exception of control rod RC-10-41, which failed to move. Group 4 was inserted further until a 10" deviation existed between RC-10-41 and the remaining Group 4 CEAs. Power continued to rise slowly and the Control Room conservatively decided to manually trip the reactor. On January 12, 2014, at 0323 the reactor was manually tripped due to reactor power rising with RC-10-41 deviation.

The control room entered EOP-00, Standard Post Trip Actions, and exited AOP-2. All CEAs inserted into the reactor as expected following the trip. At 0644 Central Standard Time (CST) NRC Headquarter Operations Office (H00) was notified of the reactor trip per 10 CFR 50.72(b)(2)(iv)(B) and (b)(3)(iv). This event is reportable per 50.73(a)(2)(iv)(A). This issue was entered in to the corrective action system as CR2014-00485.

CONCLUSION

Troubleshooting determined that the rectifier for CEDM RC-10-41 had failed. Additional review of the rectifier failure is being performed and any significant findings will be addressed in a supplement to this LER if needed.

CORRECTIVE ACTIONS

Immediate Corrective Actions

The failed CEDM rectifier and associated fuses for RC-10-41 were replaced.

SAFETY SIGNIFICANCE

Although the failure of the rectifier and associated fuse would not allow the CEDM to be manually repositioned the failure mechanism has no impact on the ability of the reactor trip (redundant rapid shutdown) circuitry to allow tripping this rod or tripping the reactor.

SAFETY SYSTEM FUNCTIONAL FAILURE

This does not represent a safety system functional failure in accordance with NEI 99-02. Revision 7.