|Monticello Nuclear Generating Plant|
|Reporting criterion:||10 CFR 50.73(a)(2)(iv)(B), System Actuation|
10 CFR 50.73(a)(2)(iv)(A), System Actuation
|2632017005R00 - NRC Website|
|Person / Time|
|From:||Church C R|
Northern States Power Company, Minnesota, Xcel Energy
Document Control Desk, Office of Nuclear Reactor Regulation
|Download: ML17264A321 (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 Infccollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs.
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.
On July 19, 2017, at1149 hours
, it was identified that lube oil temperature for the 12 Emergency Diesel Generator (EDG) [EllS: DG] had made a step change stabilizing at approximately 150°F. The step change indicated that the immersion heater was either not operating properly or was cycling abnormally. On July 20, 2017, at1634 hours
, after troubleshooting, it was identified that the temperature switch controlling the 12 EDG immersion heater had failed, causing it to be continuously on. It was determined that since this condition did not result in temperatures outside the normal operating band, no compensatory measures were required but additional monitoring by Operations was implemented. On July 23, 2017, with the unit in Mode 1, at 100 percent rated thermal power, surveillance procedure 0187-02, "12 Emergency Diesel Generator / 12 ESW [Emergency Service Water System] Quarterly Pump and Valve Test," was performed. After shutting down the 12 EDG in accordance with the surveillance procedure, at1332 hours
, Annunciator 8-C-21 (NO. 12 DIESEL ENG TROUBLE) was logged in the Control Room. An in-plant operator reported that Annunciator C94-A- 13 (HOT ENGINE) was in alarm and could not be reset. This condition resulted in a valid start signal to the 12 EDG Emergency Service Water System [BI] (EDG-ESW) pump (which was already running) and an automatic transfer of the pump control to the Alternate Shutdown System (ASDS) panel.
At the inlet to the lube oil cooler is a manifold containing an engine temperature switch and an immersion heater [EHTR] temperature switch (TS-7195) [TS]. If EDG engine water temperature reaches 190°F the engine temperature switch actuates the hot engine relay, which starts and transfers control of the 12 EDG-ESW pump to the ASDS panel. During EDG engine shutdown, water heated by a 15 KW immersion heater (controlled by TS-7195) circulates through the lube oil cooler by natural circulation to maintain oil temperature between 135°F and 165°F; 85°F is the required minimum temperature to ensure EDG fast start capability.
Investigation identified that the switch contacts for the immersion heater temperature switch (Manufacturer: Square D, Model No. GZW2-5102, Serial No. 40036996) had failed in the closed position, causing the immersion heater to be continuously on when the 12 EDG was shutdown in a standby state. While this condition did not raise temperature out of specification prior to EDG engine operation, it did add enough local heat to bring in the alarm after the engine was shutdown following a run due to the residual heat.
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.
This condition activated the engine temperature switch, in turn actuating the hot engine relay, which resulted in an auto start signal to the already running 12 EDG-ESW pump, and an automatic transfer of pump control to the ASDS panel. The immersion heater temperature switch was replaced on July 25, 2017.
This condition is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A), "Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph 10 CFR 50.73(a)(2)(iv)(B)". Specifically, criteria 10 CFR 50.73(a)(2)(iv)(B)(9) for ESW "systems that do not normally run and that serve as ultimate heat sinks" applies. This event is not classified as a safety system functional failure.
The apparent cause of the 12 EDG hot engine condition was a failed temperature switch for the immersion heater in concert with operating the 12 EDG. Following completion of the surveillance test, the 12 EDG was shutdown and returned to the automatic standby mode. When the EDG is shutdown the immersion heater automatically cycles back on and controls lube oil temperature to between 135°F and 165°F. With the immersion heater failed in the on position along with the residual heat from engine operation a localized hot spot occurred that was sufficient to actuate the transfer of control of the 12 EDG-ESW pump to the ASDS panel.
The immersion heater (and control) is required to maintain EDG engine operability while in the standby state by maintaining EDG lube oil temperature greater than 85°F to ensure fast start capability. With the immersion heater continuously on, the 12 EDG lube oil temperature stabilized at 154°F, well above the minimum and within the required band. Prior to the monthly surveillance test, engine temperatures recorded on operator rounds were within the normal acceptance band.
During the 12 EDG engine run, the coolant temperature was maintained at approximately 175°F, which is within the normal operating range, and below the hot engine alarm setpoint. From the time of problem identification through the 12 EDG surveillance test, lube oil temperature was monitored by operations and never exceeded the normal operating band. This includes when the EDG was in the shutdown ready-to-start configuration and during the EDG monthly surveillance run. Therefore, the 12 EDG remained operable.
Monticello Nuclear Generating Plant 05000-263 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 Infocollects.Resource@nrc.gov, arid to the Desk Officer. Office of Information arid 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.
There was no safety significance to transmittal of the valid start signal to the 12 EDG-ESW pump.
The start of the 12 EDG-ESW pump and transfer of control to the ASDS panel does not prevent the cooling of the 12 EDG or impact the ability of the 12 EDG to perform its safety function. This condition did not impact EDG, EDG-ESW System, or ASDS System operability since the lube oil temperatures never exceeded the normal EDG operating band, and the system logic operated as designed and engine cooling was always available.
The immediate corrective action was to replace the failed immersion heater temperature switch, TS-7195.
The long-term corrective actions are:
1. Evaluate the immersion heater and associated control circuit to determine if the temperature switch design and preventative maintenance frequency are appropriate. Based on the results, take action to address identified deficiencies.
2. Revise plant procedures to include discussion of component(s) not operating correctly or in configurations that are abnormal, so that the impact of the condition is properly understood and controlled.
PREVIOUS SIMILAR EVENTS
On April 29, 2014, it was identified that a similar issue occurred following shutdown from performance of the surveillance. The engine temperature switch and the immersion heater temperature switch setpoint bands were found to be out of adjustment resulting in the 12 EDG engine residual heat actuating the hot engine temperature switch. This resulted in an automatic start signal to the 12 EDG-ESW pump and transfer of pump control to the ASDS panel. Note, the immersion heater temperature switch did not fail during this occurrence. The condition was corrected by installation of new temperature switches with the proper operating parameters.