05000263/LER-2016-003

LER-2016-003, HPCI Declared Inoperable Due to Excessive Water Level in Turbine

Monticello Nuclear Generating Plant
Event date:	11-27-2016
Report date:	01-25-2017
Reporting criterion:	10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident
Initial Reporting
ENS 52396	10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident
2632016003R00 - NRC Website

LER 16-003-01 for Monticello Regarding HPCI Declared Inoperable Due to Excessive Water Level in Turbine

ML17146A192
Person / Time
Site:	Monticello
Issue date:	05/25/2017
From:	Gardner P A Northern States Power Co, Xcel Energy
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
L-MT-17-036 LER 16-003-01
Download: ML17146A192 (6)

comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (-1-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by intemet 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.

UNIT CONDITION PRIOR TO THE EVENT

On November 27, 2016, Monticello Nuclear Generating Plant (MNGP) was at 100% power, Mode 1.

EVENT DESCRIPTION

On November 17, 2016, a steam leak was identified originating from the packing area of the High Pressure Coolant Injection System (HPCI) turbine steam supply valve. However, the valve seat was not leaking by (no condensate forming in HPCI turbine).

On November 21, 2016, the HPCI turbine steam supply valve packing was tightened and post- maintenance testing completed at approximately 1248 hours0.0144 days <br />0.347 hours <br />0.00206 weeks <br />4.74864e-4 months <br />. The post-maintenance testing cycled the valve cold (without steam flow). This resulted in seat leakage and subsequent accumulation of condensate in the HPCI turbine. With the HPCI turbine exhaust drain pot level switch [LS] not functioning properly to drain the condensate, the exhaust piping partially filled with water.

On November 26, 2016, at approximately 1625 hours0.0188 days <br />0.451 hours <br />0.00269 weeks <br />6.183125e-4 months <br />, water was noted to be dripping from a temperature element located on the HPCI steam exhaust pipe approximately one foot in elevation above the HPCI turbine exhaust drain pot high level bypass switch actuation set point. Control room operators did not receive the drain pot high level alarm that would notify them of excessive water accumulation thus resulting in sufficient water to make HPCI inoperable upon discovery of the water.

On November 27, 2016, while troubleshooting was in progress, it was discovered that the HPCI turbine exhaust drain pot level switch was not functioning per design to alert the control room and drain condensate from the exhaust piping. Subsequently, HPCI was declared inoperable at 1447 hours0.0167 days <br />0.402 hours <br />0.00239 weeks <br />5.505835e-4 months <br />. The HPCI turbine was drained of condensate manually and was declared available at 1800 hours0.0208 days <br />0.5 hours <br />0.00298 weeks <br />6.849e-4 months <br />.

On November 28, 2016, troubleshooting determined the level switch electrical rocker assembly fell off its pivot-point due to insufficient mechanical strength (missing spot welds) during normal HPCI operating transients. Once off the pivot-point, the rocker assembly stuck in the non-trip (e.g. representative of no condensate) position which is how the switch was found.

On December 1, 2016, a temporary modification replaced the level switch with a different style switch with alarm function only due to the current model being obsolete. HPCI was restored to operable status at 1726 hours0.02 days <br />0.479 hours <br />0.00285 weeks <br />6.56743e-4 months <br />. In addition, station procedures were temporarily revised to allow operator control of the exhaust header drain to gland seal condenser valve. Control room hand switch is positioned to OPEN or AUTO allowing condensate to drain until final corrective actions are implemented to replace the level switch and eliminate seat leakage on the HPCI turbine steam supply valve.

EVENT ANALYSIS

The condition is reportable to the NRC in accordance with 10 CFR 50.73(a)(2)(v)(D) as an event or comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (-1-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by intemet 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.

condition that could have prevented the fulfillment of the safety function of HPCI to mitigate the consequences of an accident. This event is considered a Safety System Functional Failure per NEI 99-02, Revision 7. The condition was reported to the NRC on November 27, 2016, in event notification 52396 in accordance with 10 CFR 50.72(b)(3)(v)(D).

SAFETY SIGNIFICANCE

The function of HPCI is to provide a source of coolant to the reactor core under loss-of-coolant conditions which do NOT result in a rapid depressurization of the pressure vessel, such as a small break in the primary system or a loss of the normal feedwater supply. In addition, the system is relied upon to provide a source of coolant to the reactor core under station blackout conditions. The specific station events when HPCI is necessary include Station Black Out (SBO), Small Break Loss of Coolant Accidents (SBL) and Loss of Feedwater (LOFW). The SBO function of HPCI is not a Technical Specification (TS) required function; however, SBO is a licensing condition in which HPCI is the credited injection source.

The HPCI turbine exhaust drain pot level switch function is designed to alert control room operators and provide an automatic open signal for the exhaust header drain to gland seal condenser valve when condensate accumulates in the HPCI turbine exhaust piping during standby conditions. The HPCI turbine exhaust drain pot level switch is not a safety related component, nor does a single failure affect the safety related function of HPCI. However, failure of the HPCI turbine exhaust drain pot level switch to detect high water level and open the exhaust header drain to gland seal condenser valve could affect HPCI's ability to perform its safety function due to excessive condensate in the HPCI exhaust piping. Operating experience (OE) suggests, if HPCI is started with excessive condensate in the exhaust piping, the exhaust piping rupture discs could rupture, rendering HPCI no longer able to perform its safety function.

From 1248 hours0.0144 days <br />0.347 hours <br />0.00206 weeks <br />4.74864e-4 months <br /> on November 21, 2016, when the turbine steam supply valve was cycled, to 1726 hours0.02 days <br />0.479 hours <br />0.00285 weeks <br />6.56743e-4 months <br /> on December 1, 2016, when HPCI was returned to service, HPCI was inoperable and would not have been able to perform its safety function. This duration was less than the prescribed TS Limiting Condition of Operation (LCO) required action completion time of 14 days per TS 3.5.1, "ECCS — Operating", Condition I, to restore HPCI System to operable status.

During the period of HPCI inoperability, Core Spray was inoperable for quarterly testing for approximately two hours on November 22, 2016. However, Core Spray was available to restore and maintain the reactor vessel coolant during a loss of coolant accident. Therefore, the safety significance of this evolution was low. This instance of simultaneous inoperability was within the TS LCO required action completion time of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> per TS 3.5.1, "ECCS —Operating", Condition J, to restore either system to operable status.

In addition, during the period of HPCI inoperability, both Low Pressure Coolant Injection System (LPCI) subsystems were declared inoperable for Torus Cooling to support HPCI testing for approximately 2.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> on November 30, 2016. MNGP declares both subsystems of LPCI inoperable during operation when Residual Heat Removal is in Torus Cooling mode; however, LPCI remains available to restore and maintain the coolant inventory in the reactor vessel to prevent fuel clad melting following a loss-of-coolant accident. Therefore, the safety significance of this evolution was low. This instance of simultaneous comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (-1-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by intemet 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.

inoperability was within the TS LCO required action completion time of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> per TS 3.5.1, "ECCS — Operating", Condition J, to restore either system to operable status and less than the TS LCO required action completion time of 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> per TS 3.5.1, "ECCS —Operating", Condition L, to be in Mode 3.

CAUSE

Two failures were needed to prevent HPCI from performing its safety function including, (1) HPCI turbine exhaust drain pot level switch failure was the primary cause and (2) HPCI turbine steam supply valve seat leakage that slowly admitted steam into the HPCI exhaust piping was the contributing cause.

The failure of the level switch to perform its function was due to a manufacturer defect (missing spot weld) on the electrical rocker assembly. The level switch electrical rocker assembly fell off its pivot point due to insufficient mechanical strength during normal HPCI operating transients. Once off the pivot point, the electrical rocker assembly was stuck in the non-trip (e.g. indicative of no condensate) position, as found during troubleshooting. With the inability to pivot, the level switch became non-functional and thus failed to alert the control room and provide the automatic function to drain condensate from the HPCI exhaust piping.

After the HPCI turbine steam supply valve packing was tightened, post-maintenance testing cycled the valve cold (without steam flow). This resulted in seat leakage and subsequent condensation accumulating in the HPCI exhaust pipe. Internal OE indicates that once the HPCI turbine steam supply valve begins leaking, it may steam cut the seat and thus internals may need to be replaced to fully eliminate seat leakage.

CORRECTIVE ACTION TAKEN

The HPCI turbine was drained of condensate and a temporary modification replaced the level switch with a different style switch with alarm function only due to the current model of level switch being obsolete.

In addition, station procedures were temporarily revised to allow operator control of the exhaust header drain to gland seal condenser valve. Control room hand switch is positioned to OPEN or AUTO allowing condensate to drain until final corrective actions are implemented to replace the level switch and eliminate seat leakage on the HPCI turbine steam supply valve.

CORRECTIVE ACTION PLANNED

The temporary modification will be removed with a permanent modification that installs a level switch to restore automatic drain function of the standby HPCI turbine and remove operator control.

HPCI turbine steam supply valve will be repaired to eliminate seat leakage.

PREVIOUS SIMILAR OCCURRENCES

comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (-1-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by intemet 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.

There are no previous similar licensee event reports in the past three years.