05000313/LER-2008-001

LER-2008-001,

Arkansas Nuclear One
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(IV)(A)
3132008001R00 - NRC Website

MONTH DAY YEAR YEAR SEQUENTIAL REV NUMBER NO MONTH DAY YEAR FACILITY NAME DOCKET NUMBER

05000 � 05000 /2 Limn. euvrArr re. riffs LER Arkansas Nuclear One APPROVED BY OMB NO. 3150-0104 � EXPIRES 8/31/2010 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />. Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the Records Management Branch (T-6 E6), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by intemet e- mail to bjsl@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202 (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose 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.

4. TITLE Two Manual Reactor Trips From Power in Response to Abnormal Control Rod Movement Caused by Control Rod Drive Control System Component Degradation.

SYSTEM COMPONENT MANU- REPORTABLE CAUSE � SYSTEM � COMPONENT MANU- � REPORTABLE FACTURER � TO EPIX � FACTURER TO EPIX BalnigEMMINIM Mitiinalikan

CAUSE

A. Plant Status At the time of each occurrence of this event, Arkansas Nuclear One, Unit 1 (ANO-1) plant conditions were as follows:

Occurrence 1: At 0855 CST, on December 12, 2008, ANO-1 was in Mode 1 following startup from Refueling Outage 21 (1R21) with the reactor at approximately 32% power and in preparation for performance of a Nuclear Instrumentation (NI) [IG] calibration.

Occurrence 2: At 1212 CST, on December 20, 2008, ANO-1 was in Mode 1 at approximately 100% power.

B. Event Description

Occurrence 1: At 0855 CST, on December 12, 2008, following startup from 1R21, ANO-1 was at approximately 32% power with rod control in automatic, in preparation for a NI calibration when a malfunction in the Control Rod Drive Control System (CRDCS) [AA] caused the group 7 control rods to be released. Group 7 control rods were almost fully inserted before the CRDCS re-latched and caught the control rods prior to full insertion. Once re-latched, CRDCS began driving the rods back out, as the abnormality had apparently cleared. Operators received an asymmetric rod alarm and noted an abnormal rod pattern on Group 7 with reactor power lowering. At that time, Operations manually tripped the reactor in accordance with plant procedure. Post trip responses were normal with all plant systems functioning as expected and with no safety system actuations.

Troubleshooting efforts concluded that the most likely cause of the condition was the programmer assembly for the Group 7 control rods which was replaced with a spare unit. Testing was conducted and ANO-1 returned to power on December 13, 2008, at 0630 CST.

Occurrence 2: At 1212 CST, on December 20, 2008, while at approximately 100% power, Operators again received an asymmetric rod alarm and noted an abnormal rod pattern on Group 7. Operators manually tripped the reactor in accordance with plant procedures. Post trip responses were normal with all plant systems functioning as expected and with no safety system actuations. Secondary safeties lifted for approximately 1 minute.

Troubleshooting efforts and a more detailed cause investigation was initiated. The programmer assembly and two ABT relays were replaced. Testing of the system was performed and ANO-1 was returned to power operation on December 23, 2008, at 1714 CST.

C. Root Cause

The eight control rods of group 7 are typically positioned under automatic control based on demand signals from the Integrated Control System (ICS) to the CRDCS to maintain reactor power at the required level. The ICS signal is provided to the programmer via relays to move rods depending on the command initiated. Firing of silicone controlled rectifiers (SCR) is supervised by a programmer dedicated to group 7. The programmer is a microcontroller based component which responds to commands from ICS (or alternately, manual control) by sequencing the firing of the six phases of power as needed to provide rod motion. If no movement requests are initiated the microcontroller will maintain two phases continuously energized to hold the rods in a fixed position. If the programmer does not provide any SCR firing demand outputs, the rods will be released.

The programmer control power is derived from two redundant AC power sources. The primary source is provided from B631, the secondary source is provided from A501. A control power Automatic Bus Transfer (ABT-3) scheme provides power to the programmer from the designated primary AC source and rapidly switches to the secondary source if a loss of primary voltage is sensed. This is accomplished via the switching of relays K1 and K2 located in the ABT-3.

The most probable root cause of the dropping of the group 7 rods is the intermittent failure of the K1 and/or K2 ABT relays. The K1 and K2 relays were original equipment, and were found to be degraded.

A possible root cause is a failure associated with the programmer assembly. The programmer assembly presents a single point failure vulnerability. The failure of the 15 V power supply (internal to the programmer assembly) would directly cause the failure of the Programmer Micro Controller Unit.

D. Corrective Actions

The degraded K1 and K2 relays and the programmer assembly were replaced and tested.

Additional online monitoring requirements were implemented to obtain additional diagnostic information.

Additional inspection requirements were implemented for the CRDCS programmers.

Preventive Maintenance Program strategy improvement was initiated for the CRDCS.

E. Safety Significance

The safety significance associated with these occurrences is considered to be minimal.

Occurrence 1: Although Group 7 Control Rods re-latched and began to withdraw, prompt operator action to perform a manual reactor trip prevented any automatic protective setpoints from being approached. At the time of the reactor trip, the nuclear overpower trip setpoint was set lower than normal, 40% versus 104.9%, while performing physics testing following startup from 1R21 refueling outage. Post trip plant response was normal with no complications and the plant was stabilized in Mode 3, Hot Standby, without incident. All safety systems performed as designed with no automatic safety features actuating.

Occurrence 2: No outward rod motion occurred as a result of this event due to the fact that Group 7 rods re-latched at different heights. This caused an out inhibit associated with the CRD system to prevent outward rod motion. Post trip transient response for this event was normal with no complications and the plant was stabilized in Mode 3, Hot Standby, without incident. All safety systems performed as designed with no automatic safety features actuating. Main Steam safeties lifted for approximately 1 minute as a result of this reactor trip. This is considered to be a normal response due to plant power at 100% at the onset of the event. All main steam safeties reseated properly as designed.

Nuclear safety, Industrial Safety, Radiological Safety and General safety of the public were not challenged at any time during these events.

F. Basis for Reportability The first occurrence of the event was reported on December 12, 2008, at 1048 CST, as a 4-hour Non- Emergency 10CFR50.72(b)(2)(iv)(B) RPS Actuation (scram).

The second occurrence of the event was reported on December 20, 2008, at 1323 CST, as a 4-hour Non-Emergency 10CFR50.72(b)(2)(iv)(B) RPS Actuation (scram).

A manual reactor trip in response to actual plant conditions is reportable pursuant to 10CFR50.73(a)(2)(IV)(A).

G. Additional Information

There are no previous similar events reported by ANO.

Energy Industry Identification System (EllS) codes are identified in the test as [XX].