05000298/LER-2003-004, Re Manual Reactor Scram Due to Main Turbine High Vibration
| ML032110361 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 07/22/2003 |
| From: | Jason Christensen Nebraska Public Power District (NPPD) |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| NLS2003076 LER 03-004-00 | |
| Download: ML032110361 (6) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2982003004R00 - NRC Website | |
text
Nebraska Public Power District Always there when you need us NLS2003076 July 22, 2003 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555-0001
Subject:
Licensee Event Report No. 2003-004 Cooper Nuclear Station, NRC Docket 50-298, DPR-46 The subject Licensee Event Report is forwarded as an enclosure to this letter.
Sincerely, John Christensen Plant Manager
/dwv Enclosure cc: Regional Administrator USNRC - Region IV Senior Project Manager USNRC - NRR Project Directorate IV-I Senior Resident Inspector USNRC NPG Distribution INPO Records Center Records COOPER NUCLEAR STAON P.O. Box 98 / Brownville, NE 683210098 Telephone: (402) 825-3811 / Fax: (402) 825-5211 www.nppd.com
Abstract
On May 26, 2003, at 1321 hours0.0153 days <br />0.367 hours <br />0.00218 weeks <br />5.026405e-4 months <br />, there was a step change in main turbine vibration indication from less than 4 mils to 10.2 mils. A manual reactor scram was initiated from 89% power. Subsequent to the scram, reactor vessel water level dropped to approximately 30 inches below instrument zero, resulting in Primary Containment Isolation System Group 2, 3, and 6 isolations, start of High Pressure Coolant Injection and Reactor Core Isolation Cooling systems, and trip of the Reactor Recirculation pumps. An evaluation of plant response determined all control rods fully inserted and systems controlling reactor pressure and level responded as designed.
The root cause for the turbine blade failure in the low pressure turbine is currently unknown. An engineering assessment has concluded that the most probable causal factor for blade failure is high cycle fatigue. CNS will perform further analyses to determine the root cause(s) or most probable cause(s) for high cycle fatigue blade failures. As an interim action, the inspection interval for the two rotors installed for the remainder of Fuel Cycle 22.
is being reduced from four fuel cycles to one fuel cycle.
Immediate actions taken were to manually scram the reactor, trip the main turbine, and place the unit in a cold shutdown condition. An initial visual exam of the turbine blade root specimen was obtained, which concluded the crack is consistent with high cycle fatigue. Magnetic particle and eddy current testing were performed on both faces of the last four rows on both LP1 and LP2 spare rotors. Blades with crack indications on the spare rotors were replaced. The in-service LP1 and LP2 rotors were replaced with the spare rotors.
NRCFORM366(7-2001)
(ff more space is required, use additional copies of (f more space is required, use additional copies of NRC Forn 366A)
The HPCI and RCIC actuations, and Reactor Recirculation pump trips occur at the Level 2 Technical Specifications setpoint of 42 inches below instrument zero. A review of the setpoint calculation determined that the actual trip point of the related instruments could be lowered while still maintaining adequate margin to the Technical Specifications setpoint based on the approved setpoint methodology for CNS. Lowering the setpoint reduces the probability of a loss of forced circulation in the reactor vessel and challenges to safety systems if a similar event were to occur in the future. Prior to plant startup from the forced outage, the settings of the instruments related to the Level 2 setpoint were adjusted from 25.59 inches below instrument zero to 33.43 inches below instrument zero.
BASIS FOR REPORT This event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) as "any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B) of this section." The following systems from paragraph (a)(2)(iv)(B) actuated during this event: Reactor Protection System, HPCI, RCIC, and Containment Isolation System Groups 2 and 6.
CAUSE
The root cause for the turbine blade failure in the low pressure turbine is currently unknown. An engineering assessment has concluded that the most probable causal factor for blade failure is high cycle fatigue. CNS will perform further analyses to determine the root cause(s) or most probable cause(s) for high cycle fatigue blade failures. As an interim action, the inspection interval for the two rotors installed for the remainder of Fuel Cycle 22, is being reduced from four fuel cycles to one fuel cycle.
SAFETY SIGNIFICANCE
The May 26, 2003, scram and the associated plant and operator responses fall within the bounds of CNS probabilistic risk assessment transient initiator T3A. The T3A transient scenario contains the following sequence of events:
This transient occurs when the reactor scrams due to various trips such as manual scram, turbine-generator trip or other automatic trip signals without a loss of offsite power. This transient does not result in an immediate loss of the condenser as a heat sink but can cause trip of the feedwater system. The feedwater system can be restarted once the trip signal is removed.
The risk significance of this event does not significantly affect the CNS risk as described by the probabilistic risk assessment and established by the baseline reliability of equipment or systems. The use of the condenser as a heat sink during shutdown was not affected by the event and the actual damage was limited to the main turbine components. The risk is considered to be much less than the 1 E-06 threshold for risk significant changes in core damage frequency. The condition does not challenge a fuel, reactor coolant pressure, primary containment, or secondary containment boundary. The condition does not impact the plant's ability to safely shutdown or maintain the reactor in a safe shutdown condition. The plant was not placed in an unanalyzed condition nor was there any impact on compliance to plant license or design requirements for safety functions or important to safety component functions. Consequently, the safety significance of this event is very low.
(f more space is required, use additional copies of NRC Form 366A)
CORRECTIVE ACTIONS
Immediate Actions:
- 1.
Manually scrammed the reactor, tripped the main turbine, and placed the unit in a cold shutdown condition.
- 2.
Obtained initial visual exam of the blade root specimen, which concluded the crack is consistent with high cycle fatigue.
- 3.
Performed magnetic particle and eddy current testing on both faces of the last four rows on both LP1 and LP2 spare rotors.
- 4.
Replaced blades with crack indications on the spare rotors.
- 5.
Replaced the in-service LP1 and LP2 low pressure rotors with the spare rotors.
Long Term Actions:
CNS will perform further analyses to determine the root cause(s) or most probable cause(s) for high cycle fatigue blade failures. As an interim action, the inspection interval for the two rotors installed for the remainder of Fuel Cycle 22, is being reduced from four fuel cycles to one fuel cycle.
PREVIOUS EVENTS There have been no recent reportable events related to turbine vibration at CNS. LER 2003-003 documents exceeding the Technical Specifications heat-up rate limit. LER 94-015 documents excessive heat-up and cooldown rates during stratification events.
ATTACHMENT 3 LIST OF REGULATORY COMMITMENTS© I
Correspondence Number:
NLS2003076 The following table identifies those actions committed to by Nebraska Public Power District (NPPD) in this document. Any other actions discussed in the submittal represent intended or planned actions by NPPD. They are described for information only and are not regulatory commitments. Please notify the Licensing & Regulatory Affairs Manager at Cooper Nuclear Station of any questions regarding this document or any associated regulatory commitments.
COMMITTED DATE
COMMITMENT
OR OUTAGE Conduct analyses to determine the cause(s) for the high cycle 3/19/2004 fatigue failure on the low pressure turbine blade As an interim action, reduce the inspection interval for the RE22 two rotors installed for the remainder of Fuel Cycle 22 from four cycles to one cycle PROCEDURE 0.42 REVISION 13 PAGE 14 OF 16