05000298/LER-2015-003
| Cooper Nuclear Station | |
| Event date: | 05-30-2015 |
|---|---|
| Report date: | 07-28-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 2982015003R00 - NRC Website | |
Reported lessons learned are incorporated into the licensing process and fed back to industry.
Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555- 0001, or by Internet e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means 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.
NRC
FORM 366 l.F U.S. NUCLEAR REGULATORY COMMISSION (See Page 2 for required number of digits/characters for each block)
NO
Cooper Nuclear Station 05000298
PLANT STATUS
Cooper Nuclear Station (CNS) was at 0 psig reactor pressure, in Mode 4, Cold Shutdown, at the time of discovery.
BACKGROUND
The Reactor Protection System (RPS) provides timely protection against the onset and consequences of conditions that threaten the integrity of the reactor coolant pressure boundary. Excessive temperature threatens to perforate the cladding or melt the uranium dioxide. Excessive pressure threatens to rupture the reactor coolant pressure boundary. The RPS limits the uncontrolled release of radioactive material from the fuel and reactor coolant pressure boundary by terminating excessive temperature and pressure increases through the initiation of an automatic scram.
The Main Steam system conducts steam from the reactor vessel, via four steam lines, through the Primary Containment to the Main Steam Turbine. Each steam line has two, normally open, Main Steam Isolation Valves (MSIVs), one inside and one outside the Primary Containment. The MSIVs close automatically upon receipt of certain isolation signals to prevent damage to the fuel cladding by limiting the loss of reactor water during a steam line break outside Primary Containment; and also to limit the release of radioactive materials in case of a major leak from Primary Containment.
MSIV closure results in loss of the main turbine and the condenser as a heat sink for the nuclear steam supply system and indicates a need to shut down the reactor to reduce heat generation. Therefore, a reactor scram is initiated on a MSIV-closure signal before the MSIVs are completely closed in anticipation of the complete loss of the normal heat sink and subsequent over pressurization transient.
The RPS MSIV closure signals are initiated from position switches located on each of the eight MSIVs.
Each MSIV has two position switches; one inputs to RPS trip system A while the other inputs to RPS trip system B. Each RPS trip system receives an input from four MSIV-closure channels, each consisting of two position switches (one for the inboard MSIV and one for the outboard MSIV in the same steam line) in series with a sensor relay. The logic for the MSIV-closure function is arranged such that either the inboard or outboard valve on three or more of the main steam lines must close in order for a scram to occur. The design permits closure of any two lines without a full scram being initiated.
EVENT DESCRIPTION
During the October 2014 Refueling Outage 28 (RE28), limit switches A, D, and F associated with inboard MSIVs (80A-D) were replaced.
In January 2015, Quarterly Surveillance Testing was performed on the inboard MSIVs (80A-D). At this time, inboard MSIV C failed to actuate its associated RPS relay. Subsequently, the limit switch and Reported lessons learned are incorporated into the licensing process and fed back to industry.
Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555- 0001, or by Internet e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means 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.
NRC
FORM 366 U.S. NUCLEAR REGULATORY COMMISSION (See Page 2 for required number of digits/characters for each block)
NO
Cooper Nuclear Station 05000298 associated RPS relay were declared inoperable and the associated RPS channel was placed in trip per Technical Specifications (TS) LCO 3.3.1.1, Condition A.
In May 2015, Quarterly Surveillance Testing was performed on the MSIVs. At this time limit switches associated with inboard MSIV A and inboard MSIV B also failed to actuate their associated RPS relay. The limit switches and associated RPS relay were declared inoperable and the associated RPS channel was placed in trip per TS LCO 3.3.1.1, Condition A, resulting in a continuous half scram.
With the plant being in, an increased risk of an inadvertent full scram due to placing the RPS channel logic a in a tripped condition, decision was made to shutdown the plant and replace the limit switches. CNS shutdown on May 29, 2015, and the limit switches were replaced. The limit switches that were removed are being evaluated for cause.
The event is currently under investigation. CNS will provide additional event details, the safety significance, cause evaluation, and corrective action(s) to reduce the probability of recurrence in a supplement to this Licensee Event Report.
BASIS FOR REPORT
This event is potentially reportable under 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by Technical Specifications and 10 CFR 50.73(a)(2)(vii), common cause inoperability.
Other criteria, if determined to be applicable, will be provided in the supplemental report.
PREVIOUS EVENTS
There have been no events reported in the last three years related to MSIV limit switches.