05000318/LER-2015-001

LER-2015-001, Manual Reactor Trip Due to 22 Steam Generator Feed Pump Trip

Calvert Cliffs Nuclear Power Plant, Unit 2
Event date:	12-01-2015
Report date:	03-28-2016
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3182015001R01 - NRC Website

LER 15-001-00 for Calvert Cliffs, Unit 2, Regarding Manual Reactor Trip Due to Steam Generator Feed Pump 22 Trip

ML16029A378
Person / Time
Site:	Calvert Cliffs
Issue date:	01/27/2016
From:	Flaherty M D Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER 15-001-00
Download: ML16029A378 (6)

I. DESCRIPTION OF EVENT:

Energy Industry Identification System (EIIS) codes are identified in the text as [XX].

A. INITIAL CONDITIONS:

Unit 2 was operating at 100 percent power on December 1, 2015 prior to the event.

B. EVENT:

On December 1, 2015 at 1820, Unit 2 turbine driven Steam Generator Feed Pump [SJ] [P] (SGFP) 22 tripped. Operations attempt to reset 22 SGFP, in accordance with the abnormal operating procedure, was unsuccessful. Facing lowering steam generator water level, Operations manually initiated a reactor trip. The unit experienced an uncomplicated trip as all systems operated as designed.

Following the reactor trip, the unit transitioned into an unscheduled outage. Upon inspection, 22 SGFP coupling was found failed. During disassembly, Maintenance discovered that one of the four pump casing hold-down nuts (southeast corner) had backed off from its bolting surface by 1-5/8 inches. Further investigation revealed that the vendor supplied stud tensioning values used in tensioning the hold down studs on both Unit 2 SGFPs during the March 2015 refueling outage were incorrect and resulted in insufficient clamping force being applied to all the studs.

As a result, 22 SGFP pump became misaligned with the turbine to such an extent to exceed the maximum designed angular misalignment of its coupling and subsequently caused the coupling to fail. Although 21 SGFP also had insufficient tensioning applied during the refueling outage, it was found to still be within acceptable alignment. The root cause of the failure was that the Responsible and Reviewing Engineers failed to address the full scope and critical parameters associated with the use of a different tool for installation of studs in lieu of capscrews for the 22 SGFP. The investigation further determined that the southeast stud for 22 SGFP had been pulled out of perpendicular alignment to its base during the 2015 refueling outage due to the stud tensioner not having a flat surface to sit flush upon. Non perpendicularity resulted in stud deflection and binding of the nut during the tensioning process. This combined with the incorrect stud tensioning value used prevented sufficient clamping force to be applied and led to 22 SGFP failure.

The coupling on 22 SGFP was replaced and 22 SGFP was realigned. Corrected tensioning values were then applied to all the hold down studs on 21 and 22 SGFP. To ensure proper clamping force had been applied, a hydraulic power high torqueing tool was subsequently used to verify required clamping force on both 21 and 22 SGFP studs had been achieved. In addition, anti-rotation nuts were installed as a measure to help ensure vibration will not cause the nuts to lose tension resulting in a loss of clamping force.

Unit 2 was returned to Mode 1 operations at 1326 on December 6, 2015 and the unit reached 100 percent power at 0514 on December 7, 2015.

TO THE EVENT:

There were no structures, systems or components inoperable at the start of the event that contributed to the event.

D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES:

December 1, 2015 1820 — Unit 2 manual reactor trip following trip of 22 SGFP.

December 1, 2015 1830 — All EOP -0 (Post Trip Immediate Actions) completed. Entered EOP-1 (Reactor Trip) to perform post-trip recovery actions.

December 1, 2015 1840 — All EOP -1 Safety Functions completed satisfactorily.

December 6, 2015 1326 — Unit 2 returned to Mode 1.

December 7, 2015 0514 — Unit 2 reached 100 percent power.

E. FAILURE MODES:

The cause of 22 SGFP trip was a coupling failure due to excessive pump to turbine misalignment caused by inadequate torque force having been applied to 22 SGFP hold-down nuts. The root cause for the failure was that the Responsible and Reviewing Engineers failed to address the full scope and critical parameters associated with the use of a different tool for installation of studs in lieu of capscrews for the 22 SGFP. This led to a contributing cause where Maintenance used tensioner values that were not formally analyzed by engineering processes.

F. METHOD OF DISCOVERY:

The failure of 22 SGFP was self-revealing. This event is documented in the site's Corrective Action Program under IR 02594406.

II. CAUSE OF EVENT

A switch to use stud tensioning technology on the SGFPs was first made during Unit 1 2014 refueling outage, however it was only used on 12 SGFP. During the Unit 2 2015 refueling outage, the same stud tensioning technology was used on both SGFPs. Prior to the original LICENSEE EVENT REPORT (LER) (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555- 0001, or by internet e-mail to Infocollects.Resource@nrc.gov, and to the CONTINUATION SHEET 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 collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, I and a person is not required to respond to, the information collection.

NUMBER

Calvert Cliffs Nuclear Power Plant, Unit 2 05000 318 2015 -- 001 -- 01 use of this technology, Engineering performed an equivalency evaluation that allowed use of studs to hold down the SGFP pump casing to its pedestal in place of previously used capscrews. However because the evaluation did not rigorously follow Engineering standards and applicable processes, the evaluation justified the use of the stud tensioning technology without adequate review and identification of the critical parameters -associated with use of the stud tensioning technology. As a result, an opportunity to identify the vendor's incorrect hydraulic pressure values was missed. Additionally, the Engineering evaluation failed to ensure formal, systematic notification was made to Maintenance concerning the change. This resulted in a missed opportunity to incorporate Electric Power Research Institute bolted joint guidance into the applicable maintenance work practice that would have helped identify critical parameters that must be obtained or followed by Maintenance to ensure proper stud tensioning is applied.

A. SAFETY CONSEQUENCES:

The reactor protection system [JD] was manually initiated in response to 22 SGFP trip. No other safety systems were initiated following the trip.

The safety consequence of 22 SGFP trip was insufficient main feedwater flow to the steam generators. Facing decreasing water levels in the steam generators, Operations manually tripped the reactor prior to water levels reaching the low level setpoint for automatic reactor trip.

The subject condition satisfies the criteria in NUREG-1022, Revision 3, for an event that results in the actuation of the reactor protection system when the reactor is critical. Therefore, this event is reportable pursuant to 10 CFR 50.73(a)(2)(iv)(A). An immediate event notification report (51577) was also made pursuant to 10 CFR 50.72(b)(2)(iv)(B).

This event was reviewed for potential probabilistic risk assessment impact. The probabilistic risk assessment calculated a conditional Core Damage Frequency of less than 1E-7 and a Conditional Large Early Release Frequency of less than 1 E-8. Both these values result in this event as being of very low safety significance (Green) under the Nuclear Regulatory Commission's Significance Determination Process.

B. CORRECTIVE ACTIONS:

Following 22 SGFP failure, its coupling was replaced and 22 SGFP was realigned. Corrected tensioning values were then applied to all the hold down studs on 21 and 22 SGFPs. To ensure proper clamping force had been applied, a hydraulic power high torqueing tool was subsequently used to verify required clamping force on both 21 and 22 SGFP studs had been achieved. Additionally, anti-rotation nuts were installed as a precautionary measure to ensure the nuts do not lose tension during subsequent operation.

Although 12 SGFP used the same stud tensioning technology and tensioning values as used on Unit 2 SGFPs, 12 SGFP has exhibited normal operating parameters since the 2014 refueling outage. A determination was made to continue operation of 12 SGFP until its February 2016 refueling outage at which time correct tensioning values will be applied. A robust adverse condition monitoring plan was developed for 12, 21, and 22 SGFPs to include the monitoring and recording of pump and turbine shaft and casing vibration, and the monitoring of thrust bearing temperatures.

Corrective actions taken include:

• Implementation of Exelon procedure HU-AA-1212, Technical Task Risk/Rigor Assessment, Pre-job Brief, Independent Third Party Review, and Post-Job Review.

• Conducted critical parameters training and rigor training for Engineering, which included expectations for three pass reviews and verification of assumptions.

Corrective actions to be taken include:

• Conduct briefings with all disciplines that might use values from uncontrolled vendor documents to reinforce adherence to requirements contained in CC-AA-103-1003, Owner's Acceptance Review of External Engineering technical Products.

• Ensure proper barriers are in place to ensure tensioner seating surfaces are flat (perpendicular to stud) on all components where stud tensioning technology is used.

III. PREVIOUS SIMILAR EVENTS:

On May 21, 2013, Calvert Cliffs Unit 2 initiated a manual reactor trip following the trip of 22 SGFP. The failure of 22 SGFP was due to the failure of its coupling. However, in this event, the coupling failure was subsequently determined to be due to areas of incomplete weld fusion on the turbine end of the coupling that occurred during initial component manufacture. This event was described in Calvert Cliffs Licensee Event Report 2013-004-00.

COMPONENT

Pump, Steam Generator Feed, (22 SGFP) IEEE 803

FUNCTION ID

IEEE 805

SYSTEM ID

P SJ 22 SGFP Pump is manufactured by Byron Jackson Pump Division — Borg Warner Corp. (Model DVSR).