05000263/LER-2023-002, Reactor Scram and Containment Isolation Due to Valve Responses During Main Turbine Control Valve Testing
| ML23317A359 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 11/13/2023 |
| From: | Brown G Northern States Power Company, Minnesota, Xcel Energy |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| L-MT-23-048 LER 2023-002-00 | |
| Download: ML23317A359 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2632023002R00 - NRC Website | |
text
(l Xcel Energy*
November 13, 2023 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 Monticello Nuclear Generating Plant Docket No. 50-263 Renewed Facility Operating License No. DPR-22 Monticello Nuclear Generating Plant Licensee Event Report 2023-002-00 2807 West County Road 75 Monticello, MN 55362 L-MT-23-048 10 CFR 50.73 Northern States Power Company, a Minnesota corporation, doing business as Xcel Energy (hereafter "NSPM"), hereby submits Licensee Event Report (LER) 50-263/2023-002-00 per 10 CFR 50.73(a)(2)(iv)(A).
If you have any questions about this submittal, please contact Carrie Seipp, Senior Regulatory Engineer, at 612-330-5576.
Summary of Commitments This letter makes no new commitments and no revisions to existing commitments.
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1J, Greg D Brown Plant Manager, Monticello Nuclear Generating Plant Northern States Power Company - Minnesota Enclosure cc:
Administrator, Region 111, USNRC Project Manager, Monticello, USNRC Resident Inspector, Monticello, USNRC State of Minnesota
ENCLOSURE MONTICELLO NUCLEAR GENERATING PLANT LICENSEE EVENT REPORT 50-263/2023-002-00 3 pages follow
Abstract
On September 27, 2023 at 1041 CDT, with Monticello Nuclear Generating Plant in Mode 1 at approximately 75 percent power for the semiannual Turbine Control Valve (CV) test, an unplanned Reactor Scram, Group I Primary Containment Isolation, and Group II Primary Containment Isolation occurred.
The cause of the Reactor Scram was exceeding the Reactor Pressure Vessel (RPV) high pressure scram setpoint during CV testing. CV-1, CV-2 and CV-3 percentage open did not increase enough when CV-4 was closed for testing. The reduction in steam flow caused the RPV pressure to exceed the scram setpoint. Subsequently, an expected low RPV level caused the Group II Primary Containment Isolation. The cause of the Group I Primary Containment Isolation was a transient in the steam line high flow instrumentation sensing lines for the C Main Steam Line.
Corrective actions included inspection and adjustment of the four CVs to within manufacturer's specifications. Channel calibration and functional tests were performed to verify that all C Main Steam Line flow instruments met requirements. Page of
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CAUSE OF THE EVENT
The cause of the RPV high pressure scram setpoint being exceeded was attributed to performance of Control Valve testing with the operating CVs just below the transition point between the large outer piston and the smaller inner piston of the actuator. The transition requires an increased hydraulic pressure to continue moving the CV in the open direction. In the test configuration, this can result in the CV stalling at the transition point. The transition point can occur at a slightly different percentage open for each valve based on control valve actuator settings, linkage tolerances, and valve wear.
During troubleshooting of the event, it was identified that maintenance performed on the CVs in April 2023 changed the transition points for the valves which affected how the valves respond to system demand at test conditions. Troubleshooting identified that only CV-4 was above the transition point prior to the close stroke test of the other CVs. That is, CV-4 did not move through the transition point and performed the largest percentage open adjustments to control reactor pressure during close stroke testing of CV-1, CV-2, and CV-3. However, CV-1, CV-2, and CV-3 were operating just below their transition points during testing. When CV-4 was stroked closed, CV-1, CV-2, and CV-3 were not able to overcome the transition point to increase the percent open enough, and without the increased steam flow, the RPV pressure exceeded the Reactor Scram setpoint. As expected, the RPV water level lowered to below the Group II isolation setpoint, resulting in a Group II Primary Containment Isolation.
The cause of the Group I Primary Containment Isolation was a sensed, rather than actual, high steam line flow on the C Main Steam Line. Plant process computer data showed a transient on the C Main Steam Line flow transmitter. This transient was sensed as high flow by the Group I isolation high steam flow logic. No actual steam leak existed as determined via redundant indications and system walkdown.
CORRECTIVE ACTIONS
CV-1, CV-2, CV-3, and CV-4 were inspected and adjusted to the manufacturer's recommended control valve actuator settings, including adjustment of the operating cylinder closed end overtravel and control valve cracking points. Hysteresis testing confirmed that actuator transition points were within manufacturer's specifications. The valves were retested satisfactorily.
Channel calibration and functional tests were performed to verify that all C Main Steam Line flow instruments met requirements. No adjustments were needed.
PREVIOUS SIMILAR EVENTS
No previous similar events have occurred at MNGP in the prior 3 years.
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Monticello Nuclear Generating Plant 263 2023 002 00