05000237/LER-2013-009

LER-2013-009, HPCI Failed to Achieve Rated Flow Due to Failure of Stop Valve to Open

Dresden Nuclear Power Station, Unit 2
Event date:	11-30-2013
Report date:	01-29-2014
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident
Initial Reporting
2372013009R00 - NRC Website

12. LICENSEE CONTACT FOR THIS LER

FACILITY NAME TELEPHONE NUMBER (Include Area Code) Glen Morrow - Regulatory Assurance Manager 815-416-2800

13. COMPLETE ONE UNE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT

CAUSE SYSTEM COMPONENT MANU- FACTURER

TO EPIX CAUSE SYSTEM COMPONENT

MANU-

TO EPIX

X BJ TRB G084 Y

14. SUPPLEMENTAL REPORT EXPECTED

III YES (If yes, complete 15. EXPECTED SUBMISSION DATE) P.14 NO

MONTH DAY YEAR

ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines) On November 30, 2013, Operations was performing a fast start of the Unit 2 High Pressure Coolant Injection (HPCI) system. When the initiation signal was given, the HPCI Turbine Stop Valve failed to go full open. The apparent cause of the U2 HPCI system failing to reach rated flow was failure of a spare cover plate within the control valve manifold.

A failure analysis was completed which attributed the failure to a 360 degree circumferential fracture due to thermal fatigue cracking as demonstrated by a relatively brittle appearance and multiple crack origins around the spare cover plate circumference. The fracture occurred on a grooved lip that was designed to allow for thermal expansion during system warm up. As a result of the failure, the spare cover plate was replaced and additional future inspections are being planned.

This event is being reported in accordance with 10 CFR 50.73(a)(2)(v)(D), "any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident" and 10 CFR 50.73(a)(2)(i)(B), "Any operation or condition prohibited by the plants Technical Specifications.

NRC FORM 388 (01.2014) Event Time: 1553 hours0.018 days <br />0.431 hours <br />0.00257 weeks <br />5.909165e-4 months <br /> CST Unit: 02 Event Date: 11-30-2013 APPROVED BY OMB: NO. 3150-0104 EXPIRES: 01/31/21 Reported lessons learned are incorporated Into the licensing process and fed back to industry.

Send comments regarding burden estimate to the FOIA, Privacy and Intonation Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by Internet e-mail to Infocollects.Resourceenrc.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 collactlon 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

PLANT AND SYSTEM IDENTIFICATION

Dresden Nuclear Power Station (DNPS) Unit 2 is General Electric Company Boiling Water Reactors with a licensed maximum power level of 2957 megawatts thermal. The Energy Industry Identification System codes used in the text are identified as [XX].

A. Plant Conditions Prior to Event:

Reactor Mode: 1 Mode Name: Power Operation Power Level: 015 percent

B. Description of Event:

On November 30, 2013, Operations was performing a fast start of the Unit 2 High Pressure Coolant Injection (HPCI) system [B,.1]. When the initiation signal was given, the HPCI Turbine Stop Valve failed to fully open which prevented the HPCI Main Pump from achieving its required flow.

Prior to the failure of the fast start of the Unit 2 HPCI System, the HPCI pump satisfactorily performed the low pressure test in accordance with Surveillance Requirement (SR) 3.5.1.7. Reactor pressure is allowed to be increased to normal operating pressure after the low pressure test has been satisfactorily completed because there is no indication or reason to believe that HPCI is inoperable.

However, failure of SR 3.5.1.6 (i.e., HPCI fast start) represents a failure to meet Technical Specification 3.0.4 which is subsequently a condition prohibited by Technical Specifications.

The HPCI subsystem is designed to pump water into the reactor vessel under Loss of Coolant Accident (LOCA) conditions which do not result in rapid depressurization of the reactor pressure vessel with a mission time of 10 minutes. The loss-of-coolant might be due to a loss of reactor feed- water or to a small line break which does not cause immediate depressurization of the reactor vessel.

The HPCI subsystem includes a steam turbine driving a two-stage high pressure pump and a gear- driven, single-stage booster pump, valves, high pressure piping, water sources, and instrumentation.

The HPCI subsystem is designed to pump 5600 gpm into the reactor vessel within a reactor pressure range of 1135 psia to 165 psia. The Westinghouse LOCA analysis uses 5000 gpm HPCI flow between 1120 psi and 150 psi vessel overpressure. The turbine is driven with steam from the reactor vessel. As reactor pressure decreases, the control valves throttle to pass the required steam flow to maintain the set pump flow rate.

The stop valve is a "poppet" style valve, consisting of a pilot (i.e., inner) valve and an outer valve. A hydraulic piston opens the pilot poppet valve against steam inlet pressure, but lacks sufficient force to open the main (i.e., outer) valve until the steam flow through the pilot valve raises the pressure beneath the main valve. Steam pressure beneath the main valve must be at least 80 percent of the steam inlet pressure before the hydraulic piston can lift the main valve from its seat. The main valve cannot open unless the control valves are closed because there will not be a buildup of pressure beneath the main valve.

The design of the spare cover plate (Figure 1) is a combination of an interference fit and a seal weld (Figures 2) to hold it into place. The interference fit is the primary mechanism that holds the spare cover plate in place. The seal weld is not a structural weld and its main purpose is to prevent bypass steam flow. When heat is applied to the valve the expansive forces of the spare cover plate and compressive forces of the turbine casing should hold the spare cover plate in place even without the seal weld intact.

The following indications were observed during the failed start:

• The Stop Valve showed closed and the green light (LS-9) remained on. Video from within the room also indicated minimal valve travel.

• The OPEN, red light (LS-1) did not illuminate.

• Flow of -3200 gpm from the HPCI Main Pump was achieved (>5000 gpm required; -5600 gpm expected).

• The HPCI Turbine reached a speed of -2000 rpm (-3500-3600 rpm expected).

• The HPCI Turbine Trip Light reset (as expected).

• The Motor Speed Changer (MSC) remained on the low speed stop (verified through video) and the Main Control Room (MCR) indication for the low speed stop remained lit.

• HPCI Turbine Exhaust Pressure reached 24 psig (-30-40 psig expected at rated conditions).

• HPCI Achieved a discharge pressure of 367Ibs (Via Computer Data Point) This event is being reported in accordance with 10 CFR 50.73(a)(2)(v)(D), "any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident" and 10 CFR 50.73(a)(2)(i)(B), "Any operation or condition prohibited by the plants Technical Specifications.

C. Cause of Event:

The apparent cause of the Unit 2 HPCI system failing to reach rated flow was a failure of a spare cover plate within the control valve manifold. A failure analysis was provided to Dresden which attributed the failure to a 360 degree circumferential fracture (Figure 3) due to thermal fatigue cracking. The failed control valve spare cover plate was original plant equipment that had been in- service for over 40 years and over 350 system startups.

During each system startup, steam is admitted into the control valve manifold which causes a significant temperature gradient to form across the spare cover plate, and it leads to different rates of thermal expansion between the small spare cover plate and the relatively large turbine casing. These thermal stresses, during system startup, led to fatigue cracking, and allowed for the eventual dislodging of the spare cover plate.

A review of operation and work history shows that there were two potential times the spare cover plate could have been dislodged:

1. Following the securing of steam to the turbine during the last quarterly run, or 2. During outage related activities of D2R23 The run on September 26, 2013, was observed by the system manager and no related abnormalities were noted prior to, during or after the conclusion of the surveillance. Therefore, the most probable time of failure, based upon the available information, is during D2R23 outage activities (i.e., during control valve stroking without steam).

During D2R23, significant maintenance was performed on the HPCI system, including a full overhaul of the front standard which included several post maintenance strokes of the control valve in the absence of steam. Based upon information provided by GE after the event, "Tripping the valves from full-open position could cause damage to the Stellite coated valve seating surfaces, thus this type of testing should be performed a minimal number of times to insure the proper function of the system.

Additionally, GE stated "Stroking the HPCI valves through the entire range of motion without steam applied is a typical post-outage/pre-startup test.

D. Safety Analysis:

The safety significance of this condition is low. The Automatic Depressurization and Low Pressure Emergency Core Cooling Systems [BO] [BM] were available to provide makeup to the reactor vessel inventory in the event of an accident or transient. Additionally, the respective Isolation Condenser [BL] systems and other High Pressure Injection Systems (i.e., Feedwater System [SK] and Control Rod Drive System [AA]) were available for reactor vessel pressure control and injection, if required.

Therefore, health and safety of the public were not compromised as a result of this condition.

E. Corrective Actions:

As a result of this event, the station has completed or is planning to complete the following:

1. The Unit 2 HPCI Failed Control Valve Spare Cover Plate was replaced.

2. The station is planning an appropriate inspection of the Unit 3 control valve spare cover during D3R23.

3. The station has created an action to revise the scope of future control valve inspections to include NDE on the seal weld and associated lip for the spare cover plate.

Previous Occurrences:

A review of DNPS Licensee Event Reports for the last three years revealed the following reportable conditions related to HPCI inoperabilities:

• LER 237/2012-002, "Inlet Steam Drain Pot Drain Line Leaks Result in HPCI lnoperabilities"

• LER 237/2011-002, "Steam Leak Results in HPCI Inoperability"

• LER 249/2010-003, "Steam Leak Results in HPCI Inoperability" The corrective actions from the above Licensee Event Reports would not have prevented the current HPCI event.

Component Failure Data:

Manufacturer Model S/N Type General Electric (GE) 7TDRV231R1 122610 Turbine Figure 1: HPCI Control Valve Manifold with Dislodged Spare Cover Plate