05000341/LER-2002-006
| Fermi 2 2 I | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3412002006R00 - NRC Website | |
Initial Plant Conditions:
Mode � 1 Reactor Power � 100 percent
Description of the Event
On December 28, 2002, at 0440 hours0.00509 days <br />0.122 hours <br />7.275132e-4 weeks <br />1.6742e-4 months <br />, a low voltage condition was discovered on Modular Power Unit (MPU) 3, Distribution Cabinet 2. Operations personnel noticed an offset in control room recorders and several control room alarms that were associated with MPU 3. Investigation into this condition concluded that the 120 VAC voltage regulator associated with MPU 3 had degraded such that the output voltage was 93 VAC. MPU 3 is a non-Class lE 120 VAC Instrumentation and Control Power (ICP) System that provides related Balance of Plant (BOP) instrumentation and control loads including several control circuits, indicators and recorders in the control room. In attempting to determine the prudent course of action, a review of the MPU 3 loads was conducted, and it was concluded that loss of the feedwater pumps and the circulating water pumps could occur if operation were allowed to continue, or if on line repair was attempted. Initial plans were made to perform a temporary modification that would bypass the MPU 3 voltage regulator, allowing the incoming 120 VAC to be supplied directly to the loads normally supplied through the voltage regulator. Based upon the risk of performing this work on line, it was decided to shutdown the unit in order to install the temporary modification. A normal reactor shutdown to install the temporary modification was then commenced at 2132 hours0.0247 days <br />0.592 hours <br />0.00353 weeks <br />8.11226e-4 months <br />. Reactor power was reduced to approximately 73 percent using recirculation flow. Further power reduction using control rods was then attempted when it was found that control rods could not be moved using the Reactor Manual Control System (RMCS). The inability to manually move rods during the power reduction was attributed to the degraded voltage on MPU 3. The normal power reduction was stopped and the reactor was manually scrammed on December 29, 2002, at 0010 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br />.
All control rods fully inserted in response to the manual scram signal. Reactor water level decreased as expected. Containment Isolation Group 4, Shutdown Cooling and Head Spray; Group 13, Drywell Sumps; and Group 15, Traversing Incore Probe Systems isolation signals were received as expected. The post scram feedwater logic actuated as designed and the Startup Level Control Valve returned level to the normal operating band. The NRC was notified of this event on December 29, 2002, at 0256 hours0.00296 days <br />0.0711 hours <br />4.232804e-4 weeks <br />9.7408e-5 months <br /> (NRC Event 39476).
This event is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A), as an event that resulted in a manual or automatic actuation of any systems listed in paragraph (a)(2)(iv)(B).
Cause of the Event
The cause of this event was a faulty voltage regulator that resulted in a degraded voltage condition on MPU 3, impacting the unregulated 28 VDC power supply for the rod motion timing circuit in the RMCS. Although this condition did not impact the ability of the control rods to move in response to a scram signal, it did prevent normal rod motion using the RMCS.
Analysis of the Event
The problem with control rod movement after control rod selection was due to the degraded voltage condition of MPU 3 impacting the unregulated 28 VDC power supply for the rod motion timing circuit in the RMCS.
The RMCS allows operational manipulation of the control rods and the surveillance of associated equipment.
This system includes interlocks that inhibit rod movement (rod block) under certain conditions. The RMCS does not include any of the circuitry or devices used to automatically or manually scram the reactor, or the mechanical devices associated with the control rod drive (CRD), or CRD hydraulic system. This system is nonessential for safety. Therefore, this condition did not impact the ability of the control rods to move in response to a scram signal.
The control rods provide the primary means for rapid reactivity control (reactor scram), for maintaining the reactor subcritical and for limiting the potential effects of reactivity insertion events caused by malfunctions in the CRD System. The capability to insert the control rods provides assurance that the assumptions for scram reactivity in the Design Basis Accident (DBA) and transient analyses are not violated. The scram function was not inhibited by the low voltage condition on MPU 3, and all other systems responded as expected. This event did not affect the ability of systems required to maintain safe shutdown conditions, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident. Therefore, it is concluded that there was no adverse impact on safety as a result of this event, nor did it present a threat to the health or safety of the public.
Corrective Actions
Following shutdown, the voltage regulator in MPU 3 was bypassed under a temporary modification to allow an upstream voltage regulator associated with its normal power supply, Bus 72 F, to control the MPU 3 voltage. The MPU 3, Distribution Cabinet 2 regulation is currently being provided by Bus 72F. The upstream electrical supply remains sufficiently regulated to ensure the MPU 3 loads are reliably fed.
Analysis of the MPU voltage regulator problem will be performed once the component has been removed during the upcoming refueling outage currently scheduled for March 2003, and a determination will be made to either repair, modify, or leave as is. Further corrective actions to provide a reliable voltage regulator in the MPU 3 circuitry will be taken in accordance with the Fermi 2 corrective action program. This event is documented in the Fermi 2 corrective action program in CARD 02-16674.
Additional Information
A. Failed Components:
Component: Voltage Regulator Description: 15KVA at 120VAC output Manufacturer: Sola Electric Type: � 33-16-315 B. Previous LERs on Similar Problems None