05000374/LER-2003-002
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 3742003002R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor, 3489 Megawatts Thermal Rated Core Power
A. CONDITION PRIOR TO EVENT
Unit(s): 2 � Event Date: 2/25/03 � Event Time: 1005 Reactor Mode(s): 1 � Power Level(s): 100 Mode(s) Name: Run
B. DESCRIPTION OF EVENT
On February 25, 2003, upon return to full power after refueling outage L2R09, all reactor recirculation flow converters were discovered to be out-of-calibration in the non-conservative direction. As a result, the reactor protection system (RPS)[JC] Average Power Range Monitor (APRIO[IG] flow-biased scram was declared inoperable, and the unit entered'a one-hour time clock in accordance with Technical Specification (TS) 3.3.1.1 to restore RPS trip operability of this function. The flow units were re-calibrated within the hour, and the time clock was exited.
To be operable, the RPS flow-biased thermal power trip function requires that the reactor recirculation (RR) [AD] flow converters be calibrated. The procedure for calibrating the flow converters has two methods to determine core flow. The "alternate" method uses a single control room panel value for jet pump flow, and then applies an eight percent negative bias for conservatism. The "normal" method uses a process computer heat balance (OD-3), with input from the Qualified Nuclear Engineer on any required conservatism. The alternate method is normally used just prior to entering the Run mode, because heat balance data is not available at that power.
Just prior to 25 percent core power, the flow converters are re-calibrated, typically using the data from the process computer heat balance. During the startup from L2R09, the heat balance data was not available at that power due to problems with the process computer, resulting in the need to use the alternate method. There was no procedural limitation or guidance preventing this use of the alternate method, even though it proved to become non-conservative as core flow increased.
The flow units were declared inoperable, as TS Surveillance Requirement 3.3.1.1.3 was not met for all four flow units. As required by the Technical Specification Bases, the APRM flow-biased thermal scram was declared inoperable. It was determined that this event was reportable under 10 CFR 50.73(a)(2)(vii) as an event where a single cause or condition caused at least two independent trains or channels to become inoperable in a single system designed to shutdown the reactor and maintain it in a safe shutdown condition.
C. CAUSE OF EVENT
The cause was determined to be an inadequate procedure. The alternate method to determine core flow was used when calibrating the flow converters just prior to 25 percent power, without re-verifying the parameters using the OD-3 heat balance as core flow increased. Using the control room panel value for jet pump flow is not adequate, unless the calibration is performed more frequently as core flow is increased.
Neither LIS-NR-207, "APRM/RBM Flow Converter to Total Core Flow Adjustment" nor LGP-1-1, "Normal Unit Startup" included a restriction on the use of the alternate method. The procedure simply stated that the alternate method is to be used if the normal method is unavailable.
D. C SAFETY ANALYSIS The safety significance of this event was minimal. The APRM flow-biased thermal scram is not a credited scram per the Technical Specification Bases, as it is a backup to the neutron flux scram. Additionally, the flow-biased scram is clamped at 115.596 power, so at the point of discovery the scram would have occurred at the required value because the setpoint is not a function of flow at flow rates greater than approximately 74.56.
E. C CORRECTIVE ACTIONS
Corrective Actions:
1. All four flow converters were calibrated correctly.
2. A Standing Order was issued to provide guidance on the use of the alternate methodology of LIS-NR-107(207).
Corrective Action to Prevent Recurrence:
3. Procedures LIS-NR-107(207)and LGP-1-1 will be revised to provide guidance on the use of the alternate methodology (AT# 146141-19/20).
F. C PREVIOUS OCCURRENCES
LER NUMBER
TITLE
Average Power Range Monitor Reactor Scram and Rod Block Setpoints Found to be Non-Conservative After Calibration Due to Human Performance Error This LER documented an event where the flow converters were set non- conservatively due to a human performance error by a reactor engineer who had incorrectly calculated a value used in the channel calibration. The corrective actions were focused on preventing similar calculation errors, and would not have prevented this event.
G. C COMPONENT FAILURE DATA This section is not applicable, since no component failure occurred.