05000263/LER-2015-004, Regarding Past Inoperability of Turbine Stop Valve Scram Function Exceeded Technical Specification Requirements
| ML15233A477 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 08/21/2015 |
| From: | Gardner P Xcel Energy |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| L-MT-15-067 LER 15-004-00 | |
| Download: ML15233A477 (4) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| LER closed by | |
| IR 05000263/2016004 (13 February 2017) | |
| 2632015004R00 - NRC Website | |
text
Xcel Energy August21, 2015 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Monticello Nuclear Generating Plant Docket 50-263 Renewed Facility Operating License No. DPR-22 Monticello Nuclear Generating Plant 2807 W County Road 75 Monticello, MN 55362 L-MT-15-067 10 CFR 50.73 LER 2015-004-00 "Past lnoperability of Turbine Stop Valve Scram Function Exceeded Technical Specification Requirements" Enclosed is the Monticello Nuclear Generating Plant (MNGP) Licensee Event Report (LER) 2015-004-00 concerning malfunction of the turbine stop valve scram limit switch. This condition is reportable to the NRC in accordance with 10 CFR 50. 73(a)(2)(i)(B), as operations prohibited by Technical Specifications.
Summary of Commitments This letter contains no new commitments and no revisions to existing commitments.
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Peter A. Gardner Site Vice President, Monticello Nuclear Generating Plant Northern States Power Company-Minnesota Enclosure cc:
Regional Administrator, Region Ill, USNRC Project Manager, Monticello Nuclear Generating Plant, USNRC Resident Inspector, Monticello Nuclear Generating Plant, USNRC
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 01/31/2017 (02-2014)
, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
YEAR 2015
- 6. LER NUMBER I
SEQUENTIAL I NUMBER 004 REV NO.
00 2
- 3. PAGE OF 3
On May 20 through May 25, 2015, during the refueling outage, the turbine stop valve (SV) [V]Iimit switches [33] were replaced.
On May 23, turbine stop valve closure calibration checks were performed as post maintenance testing.
This check identified that the closed indication light for SV-4 position indication did not illuminate at any time.
On May 25, investigations were performed. Binding was found in the movement of the actuating arm for the limit switch for SV-4. Adjustments were made to the switch and connecting arms. The stop valves were cycled several times to provide evidence that the switches were performing as designed.
Following completion of the maintenance, the turbine stop valve closure calibration check was successfully performed.
Reactor startup commenced on May 28. As part of normal startup activities, the stop valves were opened in preparation for turbine operation. It was noted that when SV-4 opened, the stop valve closure scram relay did not energize as expected. At this time investigation was performed and found that an adjustment of the stop valve stem follower was required. The adjustment was performed and the valve cycled to verify the limit switch would actuate in the required test band. The turbine stop valve closure calibration check was successfully performed for SV-4.
Reactor power reached 40% during startup on June 1 at approximately 2155 hours0.0249 days <br />0.599 hours <br />0.00356 weeks <br />8.199775e-4 months <br />.
On June 24, 2015 at 1130 hours0.0131 days <br />0.314 hours <br />0.00187 weeks <br />4.29965e-4 months <br />, during performance of the turbine stop valve closure scram test, closure of SV-4 did not actuate the required relays [RL Y]. At 1245 hours0.0144 days <br />0.346 hours <br />0.00206 weeks <br />4.737225e-4 months <br /> the fuses [FU] were removed to meet the Technical Specification required action to place the channel in trip. At the time of discovery the plant was in Mode 1 at 95% power.
EVENT ANALYSIS
The event is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B) Operation or Condition Prohibited by Technical Specifications. A past operability evaluation determined that there was not sufficient evidence to conclude that SV-4 stop valve closure scram function was operable since the limit switch replacement during the refueling outage. Therefore, the TS 3.3.1.1 Limiting Condition for Operation had not been met since the plant exceeded 40% rated thermal power on June 1. This exceeded the TS required action time to place the channel in trip.
This event is not considered a Safety System Functional Failure per NEI 99-02 Revision 7.
SAFETY SIGNIFICANCE
Although the limit switch for SV-4 would not have provided input to its associated scram logic, the stop valve closure scram would have occurred as designed since the limit switches associated with the other three stop valves were verified to be functioning as designed. Since any three stop valves can cause a stop valve closure scram, the scram would still have occurred. By design, all four stop valves close at the same time on a turbine generator trip signal. Other than manual test stroking (one valve at a time) there is no operating scenario where only some of the stop valves close.
The turbine stop valve closure scram initiates a scram earlier than either the nuclear instrumentation system or primary system high pressure. The nuclear system high pressure scram in conjunction with the automatic pressure relief system is adequate to preclude over pressurizing the primary system. The turbine stop valve closure scram that provides addition margin to the primary system pressure limit.
CAUSE
The cause of the failure is unknown at this time but has been isolated to the limit switches through troubleshooting efforts. The limit switches are located in a high radiation area that precludes investigation. The cause will be supplemented upon completion of investigation.
CORRECTIVE ACTION
The fuses for the logic associated with SV-4 have been removed to meet Technical Specification requirements. This causes half subchannel trip in place on channels A2 and 82. With the subchannels in trip, the likelihood of a scram is increased. A troubleshooting plan is in place to perform investigation of the limit switches at the next available opportunity.
PREVIOUS SIMILAR EVENTS
There were no similar Licensee Event Reports during the past three years.
ADDITIONAL INORMATION The Institute of Electrical and Electronics Engineer codes for equipment are denoted by [XX].