05000317/LER-2016-003

LER-2016-003, Unit 1 Automatic Trip on Loss of Load due to Spurious Steam Generator High Level Turbine Trip

Calvert Cliffs Nuclear Power Plant, Unit 1
Event date:	05-31-2016
Report date:	07-29-2016
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3172016003R00 - NRC Website

LER 16-003-00 for Calvert Cliffs Nuclear Power Plant, Unit No. 1 Regarding Automatic Trip on Loss of Load due to Spurious Steam Generator High Level Turbine Trip

ML16216A148
Person / Time
Site:	Calvert Cliffs
Issue date:	07/29/2016
From:	Flaherty M D Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER 16-003-00
Download: ML16216A148 (5)

I. DESCRIPTION OF EVENT:

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

A. INITIAL CONDITIONS:

Unit 1 was operating in Mode 1 at 100 percent power on May 31, 2016 prior to the event.

B. EVENT:

On May 31, 2016 at 1626, Calvert Cliffs Unit 1 reactor [RCT] tripped. The cause of the trip was a spurious high level steam generator [SG] signal due to a failed Steam Generator 11 Channel B High Level Turbine Trip Under Voltage Logic Module [IMOD], 12/4BL-XA27. The spurious high level trip signal resulted in a main turbine [TRB] trip followed by an automatic reactor protection system [JD] (RPS) trip on loss of load. Following the reactor trip, all safety systems performed as designed to safely shutdown the unit in an uncomplicated trip. The unit remained in Mode 3 while troubleshooting was conducted.

The failed under voltage logic module was sent off-site to the vendor to conduct a failure analysis. The analysis identified two most probable causes for the failure. The first most probable cause was an intermittent failure of an integrated circuit of the under voltage logic module. The second most probable cause was an inadvertent solder bridge between pin 6 on the U5 chip and board.

Following installation of a refurbished under voltage logic module and subsequent testing of the module, the unit commenced startup activities. Unit 1 was returned to Mode 1 at 2040 on June 1, 2016. The unit returned to full power on June 2, 2016 at 1040.

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:

May 31, 2016 — 1626: Spurious hi level trip signal from 12/4BL-XA27 resulting in a main turbine trip. The main turbine trip resulted in an automatic reactor trip due to loss of load.

entered Mode 2.

June 1, 2016 — 2040: Unit 1 entered Mode 1.

June 1, 2016 — 2355: Paralleled Unit 1 Main Generator to the grid.

June 2, 2016 — 1040: Unit 1 returned to 100% power.

E. FAILURE MODES:

refueling outage. Following the Unit 1 trip, 12/4BL-XA27 reset shortly after the trip without any external input. The under voltage logic module subsequently tripped and reset several more times during the hours following the reactor trip. After initial on-site troubleshooting, the under voltage logic module was sent off-site to the vender. The vendor's evaluation determined two most probable causes for the failure of the under voltage logic module. The first most probable cause was an intermittent failure of an integrated circuit of the under voltage logic module. The second most probable cause was due to an inadvertent solder bridge between pin 6 on the U5 chip and the board. A refurbished logic module was installed and subsequently tested satisfactorily prior to the unit returning to power.

F. METHOD OF DISCOVERY:

The method of discovery was self-revealing upon the failure of 12/4BL-XA27 while in operation.

This event is documented in the site's Corrective Action Program under IR 02676088.

A. SAFETY CONSEQUENCES:

There were no human performance issues related to the reactor trip or the following recovery and the unit shutdown.

The spurious high level trip signal of 12/4BL-XA27 resulted in an automatic RPS actuation due to loss of load following the main turbine trip. The loss of load trip from 100 percent power is an analyzed anticipated operational occurrence event and the plant responded according to the analysis, with no complications.

The steam generator high level trip function is a non-Engineered Safety Features Actuation System [JE] (ESFAS) function. The purpose of this trip is to protect the main turbine against damage from water carryover.

This event satisfies the criteria in NUREG-1022, Revision 3, for RPS actuation. Therefore, this event is reportable pursuant to 10 CFR 50.73(a)(2)(iv)(A). An immediate event notification report (51967) was also made pursuant to 10 CFR 50.72(b)(2)(iv)(B).

B. CORRECTIVE ACTIONS:

The faulty under voltage logic module was replaced with a refurbished under voltage logic module.

The corrective action planned to prevent recurrence is to replace the current ESFAS (which includes the non-safety high level turbine trip) with a system that will eliminate single point vulnerabilities within ESFAS.

A supplemental licensee event report will be submitted if additional information is subsequently developed that would significantly change the corrective actions for this event.

III. PREVIOUS SIMILAR EVENTS:

A review of Calvert Cliffs' licensee event reports in the last four years was performed. This review did not identify an event that involved the same underlying concern or reason for this event.

A. COMPONENT INFORMATION:

COMPONENT IEEE 803 IEEE 805

FUNCTION ID SYSTEM ID

Logic Module, 12/4BL-XA27 IMOD JC 12/4BL-XA27, 11 SG high level turbine trip under voltage logic module was manufactured by Vitro Labs and was recently refurbished by FTI, Inc.