05000259/LER-2023-002, Full Reactor Scram Due to an Oscillation Power Range Monitor (OPRM) Confirmation Density Algorithm (CDA) Trip
| ML23200A238 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 07/17/2023 |
| From: | Sivaraman M Tennessee Valley Authority |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| LER 2023-002-00 | |
| Download: ML23200A238 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2592023002R00 - NRC Website | |
text
Post Office Box 2000, Decatur, Alabama 35609-2000 July 19, 2023 10 CFR 50.73 10 CFR 50.4(a)
ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Browns Ferry Nuclear Plant, Unit 1 Renewed Facility Operating License No. DPR-33 NRC Docket No. 50-259
Subject:
Licensee Event Report 50-259/2023-002 Full Reactor Scram due to an Oscillation Power Range Monitor (OPRM) Confirmation Density Algorithm (CDA) Trip The enclosed Licensee Event Report provides the details of the full reactor scram from 80%
power due to an Oscillation Power Range Monitor (OPRM) Confirmation Density Algorithm (CDA) trip. The Tennessee Valley Authority is submitting this report in accordance with Title 10 of the Code of Federal Regulations (10 CFR) 50.73(a)(2)(iv)(A), as any event or condition that resulted in a manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B).
There are no new regulatory commitments contained in this letter. Should you have any questions concerning this submittal, please contact Michael W. Oliver, Acting Site Licensing Manager, at (256) 729-7874.
Respectfully, Manu Sivaraman BFN Site Vice President Enclosure: Licensee Event Report 50-259/2023-002 Full Reactor Scram due to an Oscillation Power Range Monitor (OPRM) Confirmation Density Algorithm (CDA)
Trip
U.S. Nuclear Regulatory Commission Page 2 July 19, 2023
cc (w/ Enclosure):
NRC Regional Administrator - Region II NRC Senior Resident Inspector - Browns Ferry Nuclear Plant NRC Project Manager - Browns Ferry Nuclear Plant
Abstract
On May 20, 2023, at 0315 Central Daylight Time (CDT), while performing quarterly turbine control valve (TCV) testing at 80 percent reactor thermal power (RTP) and 70 percent core flow, Unit 1 automatically scrammed due to an Oscillation Power Range Monitor (OPRM) Confirmation Density Algorithm (CDA) trip.
The plant responded to the reactor scram as expected and there were no human performance challenges during the turbine control valve testing or subsequent plant shutdown.
The root cause of the event was determined to be that the sensitivity of the CDA trip during off-normal plant conditions was not embedded into processes or procedures, thus preventing development of appropriate risk elimination or mitigation actions.
The Corrective Actions to Prevent Recurrence are to revise Surveillance Requirements for the Turbine Control Valve Fast Closure on Turbine Trip and RPT Initiate Logic to specify the appropriate power level and associated valve position for performing the test to ensure no control valve oscillations and that conditions remain outside of the OPRM trip enabled region when possible; revise Technical Instructions for the Reactivity Control Plan Development and Implementation to increase awareness of entry into the OPRM trip enabled region due to the risk of spurious scrams from the CDA instability detection method; revise Design Criteria for the Turbine Electro Hydraulic Control System (EHC) systems to add details with regards to the interrelationship between EHC and OPRM; and revise Design Criteria for Neutron Monitoring Systems to add details on OPRM Algorithms.
Page 7 of 7 OPRM. Details should include a precaution that incorporates changes to the turbine control EHC system (including testing methodologies) could cause periodic perturbations. These periodic perturbations can be introduced into the thermal-hydraulic behavior of the reactor system (e.g., from control system feedback, System gains including controller settings and mechanical components like the diode function generator or valve flow position, deviation of process monitoring such as pressure or signals between two LVDTs). These changes can impact delay or combined control and mechanical gains to create perturbations that can potentially drive prolonged neutron flux oscillations within a frequency range expected for reactor instability. The presence of these oscillations is recognized by the CDA as reactor instability, independent of the actual stability of the reactor.
Revise Design Criteria BFN-50-7092 - Neutron Monitoring Systems to add details on OPRM Algorithms. Details should include different types of OPRM Trip Algorithms:
a) CDA, b) PBA, c) GRBA, and d) ABA and an explanation about each. It should also include guidance on the sensitivities of CDA trip, GEH information about susceptibility to spurious scrams, and discussion about changes to Turbine Control EHC can impact OPRM Trip Algorithms.
VII. Previous Similar Events at the Same Site
The BFN Unit 1 OPRM scram is not considered to be a Repeat Event or Operating Experience (OE) Preventable. The CDA trip was added to the OPRM system with the implementation of Maximum Extended Load Line Limit Analysis Plus (MELLLA+) in 2018. Since its implementation, this is the first time BFN has received the OPRM CDA trip, which resulted in an automatic reactor scram. A search of LERs from BFN Units 1, 2, and 3 over the last five years identified no similar events.
A review of OE and IRIS reports associated with MELLLA+, CDA, EHC Systems, and LVDTs did not identify any that would have prevented or mitigated the event, as the CDA trip is relatively new to the industry with no known previous SCRAMs.
All plants in the industry that have implemented MELLLA+ (and therefore CDA logic) were benchmarked and determined that none have restrictions on performing either TCV or Scram Time Testing inside the OPRM trip enabled region.
VIII. Additional Information
There is no additional information.
IX.
Commitments
There are no new commitments.