05000321/LER-2009-004
| Docket Number Sequential Revmonth Day Year Year Month Day Yearnumber Na 05000 | |
| Event date: | 5-10-2009 |
|---|---|
| Report date: | 06-19-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3212009004R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor Energy Industry Identification System codes appear in the text as (EllS Code XX).
DESCRIPTION OF EVENT
On May 10 at 1041 EDT, when the Unit 1 reactor mode switch was taken to the run mode following a planned maintenance outage a full reactor Scram was initiated. The following alarms were received in conjunction with the Reactor Scram: "IRM BUS A UPSCALE TRIP OR INOP", "IRM BUS B UPSCALE TRIP OR INOP" and "IRM UPSCALE". Investigation revealed that the `1A' IRM (Bus A) (EllS Code IG) and the '1H' IRM (Bus B) had spiked upscale when the mode switch was taken to run, resulting in a reactor scram. The plant had been experiencing increased noise on the IRM's since the morning of May 6, 2009. Multiple corrective actions had taken place to reduce and eliminate noise on the IRM's which had been experienced with plant component manipulations prior to the startup and subsequent scram.
CAUSE OF EVENT
The cause of the scram was an electrical noise spike transmitted into the IRM's. The noise signal was sufficient to cause IRM "A" and "H" to trip on "high-high" neutron flux level.
The scram occurred simultaneously with the movement of reactor mode switch from "Start- Up" to "Run". Post-incident testing revealed that IRM noise spikes were being induced by RPS (El IS Code JC) relay actuations which generated electro-magnetic interference (EMI).
It is concluded that noise induced spikes from the RPS relay actuations caused the scram.
REPORTABILITY ANALYSIS AND SAFETY ASSESSMENT
This event is reportable in accordance with 10 CFR 50.73 (a)(2)(iv)(A), which requires the licensee to report any event or condition that resulted in a manual or automatic actuation of any Engineered Safety Feature (ESF), including the Reactor Protection System (RPS).
The IRM scram function provides for reactor protection during startup, shutdown, and low power operations. Because the IRM "high-high" neutron flux trips were due to electrical noise introduced to the instrumentation, no actual over power event occurred. The safety function of the IRM's was not diminished. There were no safety consequences as a result of this event. All control rods (El IS Code AA) inserted and plant systems operated as expected following the scram. There were no systems or components inoperable during the event that could have contributed to the event. The reactor scram posed no safety consequences to the health and safety of the general public or plant personnel.
safety. This analysis is applicable to all power levels and operating modes in which a LOCA is postulated to occur.
CORRECTIVE ACTIONS
Extensive investigation and repair activities to correct degraded cables and connections and to improve the grounding of the neutron monitoring system improved the noise response of the IRMs.
The plant installed five ferrite beads (Fair-Rite Type 31 material) on each cable entering and/or exiting the preamplifier NEMA enclosures on each of the eight IRM channels. These ferrite beads were positioned in close proximity to the preamplifiers. The ferrite bead works to add common mode impedance to IRM cables, thereby causing EMI to be reflected toward its source. The ferrite bead responds to the magnetic field produced by the common mode current carried on the conductors within the ferrite bead. The ferrite bead does not affect the differential mode signal (current) within the cable on which the ferrite bead was placed.
Post-installation testing prior to restart demonstrated that noise spikes from RPS relay actuations into the IRM instruments were of much lower amplitude.
ADDITIONAL INFORMATION
Other Systems Affected: None Failed Components Information: None Commitment Information: This report does not create any new permanent licensing commitments.
Previous Similar Events:
There are no similar events within the past two years in which a reactor scram was initiated due to electrical noise spike on the nuclear instrumentation.