05000410/LER-2001-004
| Event date: | 10-15-2001 |
|---|---|
| Report date: | 12-14-2001 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4102001004R00 - NRC Website | |
I. Description of Event
On October 15, 2001, at approximately 0959 hours0.0111 days <br />0.266 hours <br />0.00159 weeks <br />3.648995e-4 months <br /> Nine Mile Point Unit 2 (NMP2) scrammed from approximately 100 percent power when all Main Steam Isolation Valves (MSIVs) closed. The MSIV closure occurred as technicians were restoring steam flow transmitter 2MSS*FT12A to service following calibration. All control rods fully inserted on the scram.
During the event the maximum reactor system pressure was approximately 1100 pounds per square inch gage (psig) resulting from the MSIV closure. The Safety Relief Valves (SRVs) were used for pressure control until the MSIVs were reopened, which occurred at approximately 1600 hours0.0185 days <br />0.444 hours <br />0.00265 weeks <br />6.088e-4 months <br />.
Post scram both the "A" and "B" feedwater pumps were running. Because of known leakage past the level control valves (LCVs), the reactor operator took manual control of the LCVs, verified them shut and closed the feedwater pump isolation valves in order to prevent a high water level in the reactor vessel. However, at approximately 1003 hours0.0116 days <br />0.279 hours <br />0.00166 weeks <br />3.816415e-4 months <br /> when an SRV was opened for pressure control, reactor water level swelled above the level 8 setpoint causing both feedwater pumps to trip as designed. At 1009 hours0.0117 days <br />0.28 hours <br />0.00167 weeks <br />3.839245e-4 months <br /> the "A" feedwater pump was re-started. At 1027 hours0.0119 days <br />0.285 hours <br />0.0017 weeks <br />3.907735e-4 months <br /> an SRV was opened for pressure control and reactor water level swelled above the level 8 setpoint causing the "A" feedwater pump to trip.
After the second level 8 condition, the operating crew decided to lower reactor pressure within a range of 550 psig to 650 psig so that the condensate booster pumps could be used for inventory control. At 1043 hours0.0121 days <br />0.29 hours <br />0.00172 weeks <br />3.968615e-4 months <br />, the SRV that had been opened for pressure control was closed, which caused the reactor water level to shrink to the level 2 setpoint. At the level 2 setpoint Reactor Core Isolation Cooling (RCIC) and High Pressure Core Spray (HPCS) actuated as designed along with the Division III diesel generator (HPCS power supply). Primary Containment Isolation Valve groups 2,3,6,7,8 and 9 also isolated, as designed, on the level 2 signal. Primary Containment Isolation Valve groups 4 and 5 had received an isolation signal at level 3. RCIC and HPCS were used to restore reactor water level at which point the HPCS injection valve was closed. RCIC remained in service until reactor pressure was reduced such that the condensate booster pumps could provide inventory control. HPCS was returned to standby at approximately 1105 and RCIC was returned to standby at approximately 1113 hours0.0129 days <br />0.309 hours <br />0.00184 weeks <br />4.234965e-4 months <br />. Reactor pressure was lowered to approximately 150 psig to allow re-opening of the MSIVs. At approximately 1603 the MSIVs were re-opened and the Turbine Bypass Valves were used for pressure control.
A post scram review concluded that restoration of steam flow transmitter 2MSS*FT12A caused the MSIV closure. When the "High" side of 2MSS*FT12A was being opened, a pressure pulse was generated that was sensed as a high flow on transmitter 2MSS*FT13A. These flow transmitters share a common reference leg but input to separate MSIV isolation logic channels. The pressure pulse resulted in a high flow signal from transmitter 2MSS*FT13A to the logic circuit. As a result of the calibration procedure, a high flow signal was already present from transmitter 2MSS*FT12A in the MSIV isolation circuitry. High flow signals from both of the flow transmitters resulted in the closure of the MSIVs.
II. Cause of Event
The cause of the reactor scram was closure of the MSIVs. The cause of the MSIV closure was that the testing procedure was not sufficiently tolerant of the variables involved such as valving the transmitters in and out of service by different personnel. Contributing causes were ineffective change management, in that the risks associated with moving the surveillance from being performed while shutdown to being performed while at power were not adequately assessed.
Since 1995, the surveillance had been conducted successfully 64 times with the plant at power, which impacted the crew's sensitivity to the surveillance.
The causes of the low reactor water level condition were training issues relative to transient operation of RCIC and selection of a less than optimum event mitigation strategy. The operators thought that the SRVs could reduce reactor pressure sufficiently to use the condensate booster pumps without reaching level 2. The operators were concerned that using the SRVs for pressure control in conjunction with RCIC would result in reactor water levels that would prevent resetting the reactor scram. The crew understood that SRV operation would result in lower reactor water inventory.
However, the crew did not know the amount of inventory that would be lost with SRV operation. Contributing causes are (1) inadequate communication in that some reactor water level information was not transmitted to the Control Room Supervisor, and (2) the crew did not have adequate simulator experience managing reactor water level and pressure with the MSIVs shut.
Ill. Analysis of Event This event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) having met the following criteria: 10 CFR 50.73(a)(2)(iv)(B)(1) because of the reactor scram; 10 CFR 50.73(a)(2)(iv)(B)(2) because of the level 2 Primary Containment Isolation signals; 10 CFR 50.73(a)(2)(iv)(B)(4) because of the HPCS actuation; 10 CFR 50.73(a)(2)(iv)(B)(5) because of the RCIC actuation; 10 CFR 50.73(a)(2)(iv)(B)(8) because of the Division III diesel generator starting. The closure of the MSIVs is also reported in this Licensee Event Report in lieu of a 60-day telephone report as an invalid actuation of multiple MSIVs in accordance with 10 CFR 50.73(a)(2)(iv)(A).
Post scram, feedwater, RCIC and HPCS functioned as designed to maintain reactor water level.
A Probabilistic Risk Analysis (PRA) of the event concluded that the estimated Conditional Core Damage Probability for this event was 9 E-7 and the change in Core Damage Frequency for this event was 4 E-8. The PRA assessment of this event concluded that it was not risk significant.
Based on the above, the event did not pose a threat to the health and safety of the public or plant personnel.
IV. Corrective Actions
1. The appropriate Instrumentation and Controls technicians were briefed on the event.
2. A post event review was conducted with operating crews.
3. Benchmarked the industry to determine best method to return similar transmitters to service.
4. Reinforced crew team dynamics in the control room with all licensed operators and operations trainers.
5. Modify surveillance procedure, N2-ISP-MSS-R102, based upon benchmarking results. This will be completed by May 30, 2002.
6. Review the remaining tests that are performed at power and other routine activities classified as trip sensitive to ensure the activity does not place the performer in a situation that could lead to an unexpected system actuation.
This will be completed by April 30, 2002.
7. Establish an administrative control to assure that procedures that are normally conducted while shut down are reviewed for barrier adequacy prior to conducting the procedure at power. This will be completed by February 14, 2002.
8. An MSIV isolation scenario will be developed and implemented to train the NMP2 crews. This training scenario will include the effects on reactor pressure control when RCIC is used. The training development and implementation will be completed by February 21, 2002 9. Training will be developed to address the change in reactor water level versus length of time SRVs are open.
Training will be developed by June 12, 2002.
V . Additional Information A. Failed Components: None
B. Previous similar events:
Surveillance Procedure After Issuing an Engineering Change Notice," describes a missed surveillance because a procedure had not been updated. The corrective actions would not have prevented the MSIV closure. LER 97-07, "Failure to Calibrate Hydrogen Recombiner Instruments as Required By Technical Specifications due to Procedure Omission," describes missed surveillance requirements because of an inadequate procedure. The associated corrective actions would not have prevented the MSIV closure.
C. Identification of components referred to in this Licensee Event Report Components IEEE 805 System ID � IEEE 803A Function Main Steam System � SB � N/A Feedwater System � SJ � N/A Turbine Bypass System � JI � N/A Reactor � AC � N/A Control Rod Drive System � AA � N/A Reactor Core Isolation Cooling � BN � N/A High Pressure Core Spray � BG � N/A Condensate System � SD � N/A Pump � BG, BN, SJ, SD � P Valve � SB, SJ, JI, BG � FCV, LCV, ISV, RV, INV Rod � AA � ROD