05000364/LER-2001-002

LER-2001-002,

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3642001002R00 - NRC Website

Westinghouse -- Pressurized Water Reactor Energy Industry Identification Codes are identified in the text as [XX]

Description of Event

On June 23, during unit startup activities, the turbine latch pushbutton [JJ] was noticed to have its light bulbs burnt out. The operator removed the latch pushbutton to change the bulbs, and found the button had failed apparently due to heat induced embrittlement. Since the turbine can be latched locally, i.e., without the use of this button, it was not replaced at that time and the unit startup continued. The normal function of typical lighted type pushbuttons on this panel is that the lights will be lit if the function has already been completed.

On June 26, with the reactor at 100% power, a replacement pushbutton assembly was obtained. The reactor operator consulted with Digital Electrohydraulic Control System [JJ] support personnel and other operators about the possible effects of inserting the button into the panel. He reasoned that since the turbine was already latched, the function of the pushbutton had already been completed and no action would occur. Based on this belief he concluded that depressing the pushbutton did not represent a risk to the unit. He inserted it into place at 0646. This action requires depressing the pushbutton to lock it into place. Depressing the pushbutton caused the latching mechanism [JJ], a pneumatically operated cylinder, to reposition the turbine trip-latch lever to the latch position, per design. When the latch pushbutton was released, the air cylinder blowdown rate, in combination with the spring tension and inertia of the mechanism, allowed the mechanism to travel from the latch (reset) position past the neutral position to the trip position, directly tripping the turbine. The turbine trip caused the reactor trip.

All safety systems functioned as designed following the trip.

DOCKET (2)

Cause of Event

This event was caused by an existing equipment problem in that the combination of mechanical inertia, spring tension, and pneumatic cylinder blowdown rate allowed the mechanism to overtravel to the trip position when released from the latch position. This equipment problem was not fully understood nor recognized as a trip threat. Due to this lack of understanding no procedural guidance existed to prevent latch pushbutton lamp maintenance at power.

A contributing cause was that operators incorrectly concluded that depressing the pushbutton would not initiate additional actions since the turbine was latched, and therefore did not constitute a risk to continued operation.

Safety Assessment All safety systems functioned as designed following the trip. There was no abnormal release of radioactive material during this event; therefore the health and safety of the public were unaffected by this event.

Since all safety systems functioned as designed, this event does not represent a Safety System Functional Failure.

Corrective Action Administrative controls have been placed on the latch pushbutton on both units to preclude actuation when the turbine is already latched.

The latch mechanism on both units will be repaired or modified, or the remote latching feature disabled, at the next outage of sufficient duration.

Training material for operator training on turbine control has been revised and the revised material will be covered in Licensed Operator Retraining by December 14, 2001.

Additional Information

The following LER has been submitted in the past 2 years on a reactor trip due to unexpected system response:

Control System Response