|River Bend Station - Unit 1|
|Reporting criterion:||10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material|
10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat
|4582017004R00 - NRC Website|
|Person / Time|
|From:||Chase M L|
Document Control Desk, Office of Nuclear Reactor Regulation
|Download: ML17150A303 (5)|
comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the intonation collection.
River Bend Station — Unit 1 05000-458 2017 004 00
On March 23, 2017, at 0028 CDT, with the plant operating at 100 percent power, the high pressure core spray system (HPCS) [BG]was declared inoperable due to a malfunction of a motor-operated valve (MOV) in the system. During a scheduled test, the HPCS pump test return valve (**TV**)to the suppression pool was given a "close" signal after having been opened for the test. The valve position lights indicated that it fully closed, but system flow parameters did not respond as expected. An operator went to the valve and reported that it appeared that the anti-rotation device on the valve actuator had failed, and that the valve was not fully closed. This valve is a primary containment isolation valve and has a design safety function to close automatically upon receipt of a system actuation signal.
This condition is being reported in accordance with 10 CFR 50.73(a)(2)(v)(B) and 10 CFR 50.73(a)(2)(v)(C) as a potential loss of safety function of the HPCS system and the primary containment isolation function.
Operators diagnosing the condition of the valve determined that the anti-rotation device had become dislocated. An examination of the MOV found that a set screw on the actuator had loosened, allowing the anti-rotation device to slip down the valve stem. When the anti-rotation device slipped far enough, the retainer keys fell out, allowing the valve stem to disengage from the anti-rotation device. The valve limit switches were also displaced by this action, resulting in an erroneous indication in the main control room that the valve was fully closed.
In terminating the test, the MOV was given a "close" signal for at least five seconds in accordance with operations administrative guidelines. At this time, system flow parameter returned to normal, and the operator at the MOV reported that it was fully closed. This MOV is a throttle valve, and the extra amount of time under a "close" signal resulted in the valve actually going fully closed.
The maintenance history of the valve was investigated, and it was found that in 1996, the anti-rotation device was loosened during a scheduled maintenance task. A review of the work documentation package found that no torque value was specified for the set screw, whereas the vendor manual requires the set screw to be torqued to 60 ft-lbs. upon installation.
An extent-of-condition was performed, and two other similar valves in the HPCS system test return line are susceptible to the same failure mechanism. During work in 2014, the anti-rotation devices for those valves were loosened. The applicable work packages did not incorporate the vendor recommendation to apply thread lock to the set screw and torque it to 60 ft.-lbs. The work instructions only directed that the set screw should be "snug tight.
comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
05000-458 2017 004
PREVIOUS OCCURRENCE EVALUATION
River Bend has reported no events caused by a similar failure mechanism in the last three years.
CORRECTIVE ACTIONS TO PREVENT RECURRENCE
The test return valve has been repaired, but the anti-rotation device is slightly bent. As a compensatory action, until the anti-rotation device is replaced, operators must observe valve motion locally whenever it is opened.
A maintenance action request has been initiated to incorporate vendor recommendations for installation of the set screw.
Work orders are being planned to check the set screw torque on the other two susceptible valves for proper installation.
An evaluation of the as-found condition has concluded that the HPCS system and primary containment isolation would have been able to perform their design safety function had an actual design basis event occurred during the test. Upon an automatic system actuation signal, the automatic close signal locks in until the MOV torque switches actuate rather than relying on the limit switches. Even though the anti-rotation device was dislodged, the test return valve would have closed satisfactorily due to the stem packing friction. This event was thus of minimal significance with respect to the health and safety of the public.
(NOTE: Energy Industry Identification System component function identifier and system name of each component or system referred to in the LER are annotated as (**XX**) and [XX], respectively.)