05000260/LER-2017-003

From kanterella
Jump to: navigation, search
LER-2017-003, Manual Reactor Scram Initiated During Startup Due to Multiple Rods Inserting
Browns Ferry Nuclear Plant, Unit 2
Event date: 03-29-2017
Report date: 05-30-2017
2602017003R00 - NRC Website
LER 17-003-00 for Browns Ferry Nuclear Plant, Unit 2 Regarding Manual Reactor Scram Initiated During Startup Due to Multiple Rods Inserting
ML17150A024
Person / Time
Site: Browns Ferry Tennessee Valley Authority icon.png
Issue date: 05/30/2017
From: Bono S M
Tennessee Valley Authority
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
Download: ML17150A024 (7)


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.

I. Plant Operating Conditions Before the Event

At the time of discovery, Browns Ferry Nuclear Plant (BFN), Unit 2, was in Mode 2 at zero percent power, after completing a refueling outage.

II. Description of Event

A. Event Summary On March 29, 2017, at 1842 Central Daylight Time (CDT), during Unit 2 start-up, Operations personnel received annunciators for an Intermediate Range Monitor (IRM) Downscale and a Control Rod Withdrawal Block. Operations personnel noticed that IRM `G' was reading downscale and adjusted the range down one position with no immediate reaction. At 1844 CDT, an upscale spike on IRM `G' caused a half scram on Reactor Protection System (RPS) 'A' trip system. After verifying that the IRM `G' High-High trip signal was cleared, Operations personnel reset the half scram on RPS 'A'. An immediate, concurrent trip signal from IRM 'F' was then received on the RPS 'B' trip system, resulting in multiple rods inserting into the core. When Operations personnel identified multiple rods inserting, a manual reactor scram was inserted at 1844 CDT.

B. Status of structures, components, or systems that were inoperable at the start of the event and that contributed to the event There were no structures, systems, or components whose inoperability contributed to this event.

C. Dates and approximate times of occurrences

Dates & Approximate Times Occurrence March 29, 2017, BFN, Unit 2, achieved criticality, as part of startup activities.

at 1651 CDT March 29, 2017, at 1842 CDT March 29, 2017, at 1844 CDT Operations personnel received annunciators for an IRM Downscale and a Control Rod Block for IRM `G' and adjusted the range down one position for IRM `G'.

IRM `G' read upscale when ranged down, resulting in an unexpected half scram on RPS 'A'. The IRM reading stabilized at this higher value. Operations personnel reset the half scram on RPS 'A' and a trip signal from IRM 'F' was received on the RPS 'B' trip system, resulting in multiple rods inserting into the core. When Operations personnel identified multiple rods inserting, a manual reactor scram was inserted.

2017 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.

E. Other systems or secondary functions affected

No other systems or secondary functions were affected by this event.

F. Method of discovery of each component or system failure or procedural error Operations personnel received annunciators, IRM Downscale and Control Rod Withdrawal Block, for IRM `G'. Additionally, when Operations personnel were directed to reset RPS 'A', multiple rods were inserting into the core.

G. The failure mode, mechanism, and effect of each failed component A faulty pre-amplifier [AMP] gave the IRM `G' the appearance of a low signal. When the IRM `G' experienced a spike on this new setting, it caused a half scram.

The faulty high-voltage connector [57] and degraded grounding inside of the IRM 'F' drawer allowed for spurious signals induction from adjacent and/or overlapping cables.

H. Operator actions

After a spike on IRM `G' caused a half scram on the RPS 'A' trip system, Operations personnel verified that no trip condition was present, and manipulated the RPS 'A' scram reset switch. When resetting RPS 'A', a spurious IRM 'F' signal caused a half scram on RPS 'B' system. This caused multiple rods to insert into the core, requiring Operations personnel to manually scram the reactor.

I. Automatically and manually initiated safety system responses

Operations personnel inserted a manual reactor scram upon noticing multiple rods inserting into the reactor.

III. Cause of the event

A. Cause of each component or system failure or personnel error The root cause was determined to be a lack of performing electromagnetic and radio-frequency interference noise testing to detect abnormalities in nuclear instrumentation.

2017 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.

Browns Ferry Nuclear Plant, Unit 2 05000-260 - 003 - 00 B. Cause(s) and circumstances for each human performance related root cause There were no human performance related root causes associated with this event.

IV. Analysis of the event

The Tennessee Valley Authority (TVA) is submitting this event in accordance with Title 10 of the Code of Federal Regulations 50.73(a)(2)(iv)(A), any event or condition that resulted in a manual or automatic actuation of the RPS. The condition was discovered on March 29, 2017, when IRM '0' appeared to have a downscale signal, from what was later determined to be a result of a pre-amplifier failure. Upscaling the IRM `G' caused an RPS 'A' half scram. Signal noise from the RPS 'A' scram reset switch inadvertently induced a current through a faulty high voltage connector on IRM 'F'. This spurious current spike on IRM 'F' tripped a RPS 'B' half-scram signal, before the 'A' signal had cleared. Upon noticing control rod motion, Operations personnel were procedurally required to insert a manual reactor scram, which constitutes a reportable event.

V. Assessment of Safety Consequences

This event resulted in the manual actuation of a safety system. This event did not result in the inoperability or unavailability of any system to provide their required safety functions. No Emergency Core Cooling Systems [BG] [BJ] [BO] or Reactor Core Isolation Cooling [BN] reactor water level initiation set points were reached. Primary Containment Isolation Systems did not receive an actuation signal and performed as designed. All safety systems remained in standby readiness configuration and were capable of performing their required safety functions. Therefore, this condition had a negligible impact on the health and safety of the public.

A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event Each RPS train had three additional I RMs which remained operable throughout this event.

B. For events that occurred when the reactor was shut down, availability of systems or components needed to shutdown the reactor and maintain safe shutdown conditions, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident Safety system availability was not impacted by this event.

C. For failure that rendered a train of a safety system inoperable, estimate of the elapsed time from discovery of the failure until the train was returned to service Safety system operability was not impacted by this event.

2017 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.

Browns Ferry Nuclear Plant, Unit 2 05000-260 - 003 - 00

VI. Corrective Actions

Corrective Actions are being managed by TVA's corrective action program under Condition Report (CR) 1278595.

A. Immediate Corrective Actions

Troubleshooting was performed prior to re-commencing startup. During that effort, the high voltage connector in the IRM 'F' drawer was replaced, and the IRM `G' pre-amplifier was replaced and its range switch was correlated.

B. Corrective Actions to Prevent Recurrence or to reduce the probability of similar events occurring in the future Corrective Action to Prevent Recurrence is to perform routine pre-outage and outage-related preventive maintenance tasks for noise-induced cable tests to verify the noise has been removed.

VII. Previous Similar Events at the Same Site As described in CR 558437, a similar event occurred on May 24, 2012, at BFN, Unit 3, during startup following a refueling outage. Troubleshooting activities identified that IRM 'A' and Source Range Monitors (SRMs) 'A', 'C' and D' received period alarms due to scram reset switch actuation with a half scram signal. The subsequent investigation revealed that a high voltage cable in the IRM 'A' main control room drawer had a broken insulating shield, and that the scram reset switch induced electronic noise on the control room common ground. The high impedance of the ground, acting through this degraded connector, caused an electronic spike on IRM 'A', leading to a RPS 'A' actuation while Operations personnel reset a previous RPS 'B' half-scram signal, resulting in a half scram.

Corrective maintenance involved adding ferrite beads to the SRM ground wires and to the reset switch ground wire. It was determined that the Unit 3 IRM 'A' and SRMs were only vulnerable to scram reset switch noise when actually resetting a valid half scram. To prevent recurrence, a significant number of connectors were repaired or replaced on all three units. However, these efforts did not focus on finding the noise causes, and were insufficient to prevent recurrence. While nuclear instrumentation is largely standardized, there can be unforeseen problems, especially as components degrade or are replaced over time.

VIII. Additional Information

There is no additional information.

IX. Commitments There are no new commitments.

2017