05000499/LER-2011-003
| South Texas | |
| Event date: | 30-1-2011 |
|---|---|
| Report date: | 01-30-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii)(D), Common Cause Inoperability 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 4992011003R00 - NRC Website | |
generated turbine trip signals were defeated by a maintenance work activity that installed a jumper in both channels (Train R and S) of non-class relays to the turbine trip circuit. Specifically, a total of four relays K947R & S and K948R & S were defeated by the jumpers. The functions affected included the turbine trip from reactor trip breakers open (P-4), turbine trip from a reactor trip signal (P-16), and the turbine trip from Steam Generator HI-HI (P-14). With the reactor trip breakers open, a P-4 signal is generated that locks in a turbine trip signal, which closes the throttle valves (TV) and governor valves (GV). These signals are addressed in TS 3.3.2 Item 5a and 5b and are required in Modes 1, 2, and 3.
This maintenance work activity is normally done during Modes 5, 6, or defueled. The jumpers were installed per Electro Hydraulic Control System Calibration Preventive Maintenance Instruction (PMI) to support maintenance activities which included stroking the turbine throttle and governor valves.
The PMI allows some sections to be performed in Modes other than 5, 6, and defueled as long as the Main Steam Header is not pressurized. However, Section 5.4 of the PMI, which installs the jumpers, was not included in this exception. Closure of this work package and removal of the jumper were not identified as a Mode 3 restraint.
On November 20, 2011 at 0546 hours0.00632 days <br />0.152 hours <br />9.027778e-4 weeks <br />2.07753e-4 months <br />, Unit 2 transitioned from Mode 4 to Mode 3.
11 FORM 366 (10-2010) Reactor Trip breaker indication on Control Panel 4 indicated open and the Main Turbine Electro Hydraulic Control Console on Control Panel 6 indicated the Main Turbine latched. An investigation began to determine if this condition was appropriate for the current plant conditions. Subsequently, the Shift Manager directed maintenance to remove the electrical jumpers. The Main Turbine was manually tripped and the electrical jumpers were removed at approximately 0930 AM on 11/20/2011.
During further review of the condition by Control Room Staff, it was recognized that the plant Mode change that occurred earlier in the day with the electrical jumpers installed.was likely a violation of Technical Specifications.
After an investigation of the facts surrounding this event, this condition was declared reportable on 11/30/2011 pursuant to:
a 10CFR 50.73(a)(2Mi)(B'):
Unit 2 changed Modes from 4 to 3 with TS 3.3.2 Item 5a and 5b and the associated LCO Actions 20 and 25 for not met for Mode 3. This is a condition prohibited by TS 3.0.4.
a 1 OCFR 50.73(a)(2)(v)(C) and (D): Since both channels of the turbine trip were defeated, and this function is assumed in the safety analysis, this condition Could have prevented the fulfillment of the safety functions to mitigate the consequences of an accident and control the release of radiation.
On 11/30/2011 at 1513, a notification pursuant to IOCFR 50.72(b)(3)(v)(C) and 10CFR50.72(b)(3)(v)(D) was made to the NRC Operations Center.
0 10CFR 50.73(a)(2)(vii)(D): The single cause or condition of having defeated both channels of turbine trip signals due to performance of the associated work activity defeated both channels of the turbine trip which are required to mitigate the consequences of an accident.
A review of the performance of this activity in previous outages was conducted. It was identified that during 2RE14 in April of 2010, a work package for this activity was not closed until after Mode 3 and that that a similar event had occurred.
E. METHOD OF DISCOVERY
The Technical Specification non-compliance was discovered during the observation of Control Room outage activities by a Quality Inspector.
I1. EVENT-DRIVEN INFORMATION
A. SAFETY SYSTEMS THAT RESPONDED
N/A FORM 366 (10-2010) On 11/20/2011 at 0546, Unit 2 met the plant~conditions and entered MODE 3 Hot Standby with the electrical jumpers in place preventing automatic tripping the Main Turbine. The Main Turbine was manually tripped and the electrical jumpers were removed at approximately 0930 AM on 11/20/2011. Therefore, the duration of safety system inoperability was approximately 3 hrs and 45 minutes.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
The purpose of the turbine trip on rector trip is to prevent overcooling the RCS after a reactor trip.
Without the turbine trip on a reactor trip, the turbine would continue to draw steam from the steam generators until the main steam isolation valves close due to a compensated steam line pressure signal. This could result in a significant decrease in RCS coolant temperature which could result in a return to criticality and the fuel experiencing Departure from Nucleate Boiling (DNB). However, this event is bounded by the double-ended steam line break analysis which shows that DNB does not occur. A failure of the turbine trip on reactor trip also could result in the uncovery of the steam generator tubes, which could in an increase in calculated dose due to a reduction in the partition factor assumed in the safety analysis. However, the radionuclides in the steam would steam would plate out in the condenser which would more than offset the decrease in partition factor assumed in the safety analysis. Therefore, the failure of the turbine to trip on a reactor trip does not significantly degrade plant safety.
Although Unit 2 was in Mode 3, with the reactor trip breakers open, and turbine throttle valves closed while the jumpers were installed, this condition is conservatively considered to be a safety system functional failure. There was no work process prevention to assure the jumpers would be properly removed; consequently, this condition COULD have prevented the fulfillment of the control of the release radioactive materials or mitigating the consequences of an event.
A risk analysis was performed to determine the risk significance of having P-16 bypassed or defeated while changing plant Mode for Mode 4 to Mode 3. Since a P-1 6 actuation would result in a signal to close the main turbine governor and throttle valves and it has been shown that the throttle valves were in the closed position for the entire time period P-16 was bypassed, the evaluation concluded that there was no increase in the risk of core damage or large early release.
Consequently, there is no adverse effect on the health and safety of the public.
Ill. CAUSE OF THE EVENT The MODE requirement prerequisites in the Preventive Maintenance Instruction were revised without full consideration of the Operational restrictions associated with changing plant conditions during procedure performance.
FORM 366 (10-2010) A. To ensure the jumpers are carried in the procedure that is easily tracked for mode restraints:
a. Remove mode restrictive steps of PMI-IC-EH-0003 (section for turbine trip defeat jumper installation) and, b. Add mode restrictive steps in the 7300 Protection System Channel Trip Function Bypass procedure (PMP08-BS-0002).
B. Revise procedure 0PAP01-ZA-0102 (Plant Procedures) to include process for revision of PMI's.
C. Develop and distribute lessons learned to all procedure writer certified personnel on the importance of close evaluation of re-performing License Compliance Reviews when Review and Comments cycle result in intent change to a procedure step.
V. PREVIOUS SIMILAR EVENTS
A review of work history for the last 3 years indicated the associated maintenance activity had been performed five times. There was only one other instance when this work package was not closed out until after Mode 3. This condition occurred during Unit 2 Refueling Outage in April 2010.
VI. ADDITIONAL INFORMATION
N/A r FORM 366 (10-2010)