05000352/LER-2012-004
| Limerick Generating Station, Unit 1 | |
| Event date: | 07-11-2012 |
|---|---|
| Report date: | 09-10-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 3522012004R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 1 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
On Wednesday, July 11, 2012, Limerick Unit 1 was operating at approximately 100% power. At 1046 hours0.0121 days <br />0.291 hours <br />0.00173 weeks <br />3.98003e-4 months <br />, an operator reported observing a steam plume on the main turbine. An investigation was initiated to identify the source of the leak. At 2345 hours0.0271 days <br />0.651 hours <br />0.00388 weeks <br />8.922725e-4 months <br />, the Turbine Control Valve / Stop Valve Scram Bypassed alarm was received in the main control room. At 0004 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, an Equipment Operator (EO) in the auxiliary equipment room reported that the reactor protection system (RPS) (EllS:JC) 1C and 1D main turbine first stage pressure instruments (EllS:PIS) were indicating zero and two percent respectively with their trip lights off. The 1A and 1B channels were indicating normally between 94 to 96 percent.
At 0035 hours4.050926e-4 days <br />0.00972 hours <br />5.787037e-5 weeks <br />1.33175e-5 months <br />, the control room supervisor (CRS) determined that the A2 and B2 RPS channels for the turbine stop valve (TSV) closure and turbine control valve (TCV) fast closure trip functions were inoperable. TS 3.3.1 RPS Instrumentation action "a" was entered which required verifying within one hour that one TCV channel in each trip system was operable. This action was completed. Action "a" also required verifying within one hour that three TSV channels in each trip system were operable. This action could not be completed. Therefore, at 0121 hours0.0014 days <br />0.0336 hours <br />2.000661e-4 weeks <br />4.60405e-5 months <br />, action "d" was entered which required taking Action 6 in Table 3.3.1-1. Action 6 required initiation of a reduction in power within 15 minutes and a reduction of turbine first stage pressure until the function is automatically bypassed within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />. At 0133 hours0.00154 days <br />0.0369 hours <br />2.199074e-4 weeks <br />5.06065e-5 months <br />, a power reduction was initiated. At 0306 hours0.00354 days <br />0.085 hours <br />5.059524e-4 weeks <br />1.16433e-4 months <br />, power was reduced to less than 29.5 percent which made completion of the Action 6 two hour requirement not applicable. At 0316 hours0.00366 days <br />0.0878 hours <br />5.224868e-4 weeks <br />1.20238e-4 months <br />, the 1A and 1B RPS channels were automatically bypassed at 22 percent power. TS 3.3.1 action "c" was also entered due to having less than the minimum number of channels in both RPS trip systems.
Action "c" required placing the inoperable channels in one trip system in the tripped condition within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. Action "c" was exited when power was reduced to less than 29.5 percent.
The CRS also determined that two TSV closure channels and two TCV fast closure trip function channels of the end-of-cycle recirculation pump trip (EOC-RPT) logic on one trip system were inoperable. TS 3.3.4.2 EOC-RPT System Instrumentation action "c.2" was entered for four channels inoperable on one trip system which required declaring the affected trip system inoperable. Action "d" was entered since one trip system was inoperable which required restoration within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. The TS actions were exited when power was reduced to less than 29.5 percent and the actions were no longer applicable.
An investigation determined that one of two main turbine first stage pressure instrument lines had failed. The line was repaired and the affected instruments were declared operable. On Saturday July 14, 2012, at 0409 hours0.00473 days <br />0.114 hours <br />6.762566e-4 weeks <br />1.556245e-4 months <br />, Unit 1 power was increased above 29.5 percent.
The investigation also determined that the RPS Instrumentation TS action "a" one-hour action should have been entered at 0004 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> when the EO reported that two instruments were failed. The CRS delayed declaring the instruments inoperable until 0035 while an evaluation proceeded to determine the affected RPS trip systems. For purposes of reporting, the initial alarm at 2345 was firm evidence that the affected instruments were inoperable at the time of the alarm.
This event involved the common-cause inoperability of two independent channels in the reactor protection system. Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(vii).
This event also involved a condition prohibited by TS since the RPS Instrumentation one-hour action to place the affected trip system in the tripped condition was not completed. Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(i)(B).
Two channels of RPS remained operable; therefore, the RPS safety function was maintained during the event.
Analysis of the Event
There was no actual safety consequence associated with this event. The potential safety consequences of this event were minimal. The A2 and B2 RPS channels for the turbine stop valve closure and turbine control valve fast closure trip functions were rendered inoperable by the instrument line failure.
The Al and B1 RPS channels remained operable and capable of initiating an automatic actuation of the RPS system during the event. If the Al or B1 channel for the turbine stop valve closure and turbine control valve fast closure trip functions had failed during a main turbine trip, the RPS system would not have automatically actuated on the TSV or TCV closure. However, the reactor high pressure trip function or the Average Power Range Monitor (APRM) neutron flux upscale trip would have automatically initiated a reactor shutdown when operating at power greater than main steam bypass valve capacity.
The failure analysis identified that the % inch instrument pipe failed at the half-coupling connection to the main steam line. Circumferential fatigue cracks were observed along the weld toe most likely due to reverse bending. The failure by fatigue indicates the line was subject to vibration. A modification will be installed to address vibration induced fatigue on the affected instrument piping.
Cause of the Event
The cause of the event was vibration induced fatigue of the affected instrument pipe.
Corrective Action Completed The failed instrument pipe was repaired.
The "Turbine Control Valve/Stop Valve Scram Bypassed" alarm response procedure was revised to direct a power reduction if trip unit status can not be determined within 15 minutes.
Corrective Action Planned A modification will be installed to address vibration induced fatigue on the affected instrument piping.
Previous Similar Occurrences There were no previous similar occurrences of RPS instrument pipe failures in the past 3 years.
Component data:
System:� JC�(Plant Protection System) Component: PIS (Switch, Indicating, Pressure) Manufacturer:
� R369 (Rosemount Nuclear Instruments Inc.) Model:� 710DUOTT