05000387/LER-2001-003
| Susquehanna Steam Electric Station - Unit 1 | |
| Event date: | 08-07-2001 |
|---|---|
| Report date: | 10-08-2001 |
| 3872001003R00 - NRC Website | |
EVENT DESCRIPTION
On August 7, 2001 with Unit 1 and 2 at 100% power, engineering personnel discovered an error in the reactor heat balance calculation that resulted in a 6 MegaWatt-thermal (MWth) error. Specifically, the main steam (ENS: SB) moisture fraction value was found to be non-conservative. The moisture fraction value used in the calculation was 2.0E-3 (no units). The correct moisture fraction value determined from plant testing is 2.0E-5. The maximum power level of Unit 1 and Unit 2 was administratively reduced by 7 MWth upon discovery of the error.
Subsequent investigation revealed the following summary timeline of events:
- Early 1980's - The moisture fraction value used for initial operation of Susquehanna Unit 1 and 2 was 1.0E-3. This value was the design value for turbine inlet conditions specified by the reactor vendor, General Electric.
- 1996 - The Susquehanna power uprate reviews identified that the reactor heat balance calculation should use actual plant testing results for the moisture fraction instead of the design value. The calculation was revised in an attempt to make the heat balance more accurate. A moisture fraction value of 2.0E-3 was used in error.
- 2001 - The incorrect moisture fraction used in the reactor heat balance calculation is discovered during a review of turbine design specifications for an upcoming turbine replacement project.
CAUSE OF EVENT
The cause of the calculation error is confusion over similar, closely related terms associated with entrained water in steam. Terms such as moisture fraction, percent moisture, carryover and quality represent the same physical parameter and are easily confused. Additionally, the actual plant test value of main steam moisture expressed as a percentage is numerically very close to the design value expressed as a moisture fraction. Procedural control of plant test data was focused on compliance with turbine performance criteria, and was not adequate to ensure the value derived from plant testing was accurately used in the reactor heat balance calculation. As a result, an incorrect but closely related term was used in error.
ANALYSIS / SAFETY SIGNIFICANCE
The calculation error resulted in a non-conservative reactor power indication, but was not significant due to the small magnitude (0.2%) of the error. 10CFR50 Appendix K requires accident analysis from 102% reactor power to allow for instrument inaccuracy. This maximum analyzed power level is 3510 MWth for Susquehanna. Actual instrument accuracy at Susquehanna is better than assumed in the regulations, and this results in a safety margin to the maximum analyzed power level of approximately 10 MWth.
Since the magnitude of the error is less than the available margin, the maximum analytical power level has not been exceeded and the licensing analysis for operating both Susquehanna units remains valid.
Based on consideration of the margin to analyzed maximum power level, there were no adverse consequences to the health and safety of the public as a result of this event.
Due to fluctuations in the actual power level, the average reactor power is typically less than the rated thermal power license limit. An exhaustive historical search for actual shift average power levels was not conducted to quantify the number of occurrences that the license limit was exceeded. However, a review of limited data in 2001 revealed a few instances of shift average reactor power greater than the license limit. This report is submitted per section 2.G of Unit 1 operating license NPF-14 and 2.E of Unit 2 operating license NPF-22 as a violation of license condition 2.C(1), maximum power level. In accordance with guidance in NUREG-1022, Revision 2, the due date for this report is October 8, 2001.
CORRECTIVE ACTIONS
The following describes the corrective action strategy for the reactor heat balance calculation error:
Corrective actions that have been completed are:
- The initial corrective action taken upon discovery of the error was to administratively reduce the maximum reactor power of Unit 1 and Unit 2 by 7 MWth.
- Following the cause determination for the error, administrative and computer-based reactor heat balance calculations were reviewed and corrected to ensure that accurate moisture fraction values are used.
- The basis and accuracy of all parameters in the reactor heat balance calculation has been validated.
Corrective Action to be completed is:
- Procedures and specifications that will be used to obtain and implement plant test data for the upcoming turbine replacement project will be reviewed and revised to ensure that test data is accurately used in reactor heat balance calculations.
ADDITIONAL INFORMATION
Past Similar Events: � LER 50-388/95-003-00, Shift Average Maximum Power Level Exceeded Failed Component: � None