05000461/LER-2017-005

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LER-2017-005, Automatic Reactor Scram During the Performance of Scram Time Testing As a Result of an Invalid Oscillation Power Range Monitor Growth Rate Trip
Clinton Power Station, Unit 1
Event date: 05-30-2017
Report date: 09-28-2017
Reporting criterion: 10 CFR 50.73(a)(2)(iv)(A), System Actuation
4612017005R01 - NRC Website
LER 17-005-01 for Clinton, Unit 1 Regarding Automatic Reactor Scram During the Performance of Scram Time Testing As a Result of an Invalid Oscillation Power Range Monitor Growth Rate Trip
ML17275A173
Person / Time
Site: Clinton Exelon icon.png
Issue date: 09/28/2017
From: Stoner T R
Exelon Generation Co
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
U-604374
Download: ML17275A173 (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 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.

3. LER NUMBER

2017 - 01 005

PLANT AND SYSTEM IDENTIFICATION

General Electric—Boiling Water Reactor, 3473 Megawatts Thermal Rated Core Power Energy Industry Identification System (EIIS) codes are identified in the text as [XX]

EVENT IDENTIFICATION

Automatic Reactor Scram During the Performance of Scram Time Testing As a Result of an Invalid Oscillation Power Range Monitor Growth Rate Trip A. Plant Operating Conditions before the Event Unit: 1 Mode: 1 Event Date: May 30, 2017 Event Time: 2038 Mode Name: Power Operation Reactor Power: 028 percent

B. DESCRIPTION OF EVENT

On May 30, 2017 at

2038 hours
0.0236 days
0.566 hours
0.00337 weeks
7.75459e-4 months

CDT, Clinton Power Station (CPS) experienced an automatic reactor scram from 28 percent thermal power while performing control rod scram time testing (STT) per CPS procedure, CPS 9813.01, Control Rod Scram Time Testing, during startup from refueling outage C1R17. The automatic scram signal was generated by the Oscillation Power Range Monitor (OPRM) Growth Rate Algorithm.

Following the event, General Electric Hitachi (GEH) performed an evaluation of CPS -reactor conditions and determined that the core decay ratio, a measure of reactor core stability, was very low ( data. A review of OPRM data concluded that the event did not occur due to actual core thermal hydraulic instability. It is also concluded that core thermal limits were not challenged by this event.

CPS is licensed with the Detect and Suppress Option III reactor core stability solution which utilizes the Period Based Detection Algorithm. This provides detection of a power instability based on occurrence of a fixed number of consecutive neutron flux period confirmations and a relative power signal exceeding a specific set point. For CPS, the number of period confirmation counts is 14 and relative power set point is 1.12. Furthermore, the Option III stability solution includes a Growth Rate Algorithm (GRA) and an Amplitude Based Algorithm (ABA) that offer defense in depth by providing protection against unexpected power oscillation characteristics.

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.

3. LER NUMBER

Following the automatic trip, plant systems, structures, and components responded as designed and functioned properly. There were no complications from the event.

Interim actions were implemented to increase operating margin to trip setpoints to support reactor startup and completion of STT. They consisted of developing and implementing an operating and monitoring strategy when performing STT in the OPRM enabled region. This included wait times after scram time testing a control rod, utilizing single notch rod withdrawal, pausing after rod withdrawal past a Local Power Range Monitor (LRPM), additional identification of high worth control rods, and review of Average Power Range Monitor (APRM) and LPRM response. In addition, setpoints were raised for the GRA and ABA portion of the OPRMs.

The reactor was restarted on June 2, 2017 and control rod STT was completed without issue on June 4, 2017.

C. CAUSE OF EVENT

The cause of the event is that the OPRM GRA trip function design is unable to distinguish between plant response to system perturbations and onset of thermal-hydraulic instabilities.

D. SAFETY ANALYSIS

There were no safety consequences associated with the automatic scram. This event is reportable under the provisions of 10 CFR 50.73(a)(2)(iv)(A) as an event or condition that resulted in a manual or automatic actuation of the reactor protection system. The condition of the reactor core at the time of the event was stable. Operator actions taken during STT and following the event were in accordance with plant procedures. Systems necessary to maintain the plant per Technical Specification requirements following the automatic plant trip performed as expected and remained available to perform their safety function.

This event report does not identify any safety system functional failures.

2017 - 01 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.

3. LER NUMBER

2017 - 01 005

E. CORRECTIVE ACTIONS

Interim actions were implemented to increase operating margin to trip setpoints to support reactor startup and completion of STT. These actions included:

  • Completed a revision to a plant procedure to establish an operating strategy when performing scram time testing in the OPRM enabled region.
  • Implemented a monitoring strategy to assess the effectiveness of the operating strategy and monitor for expected plant response.
  • Raised the OPRM ABA and GRA set points.

In addition, CPS will work with the industry, as needed, to develop and implement an industry solution for the design of the OPRM GRA to prevent false, spurious trip signals.

F. PREVIOUS SIMILAR OCCURENCES

There are no previous similar occurrences associated with this event. CPS has not experienced a scram in the past related to the OPRM instrumentation.

G. COMPONENT FAILURE DATA

There was no component failure data associated with this event.