05000255/LER-2013-002
| Palisades Nuclear Plant | |
| Event date: | 05-05-2013 |
|---|---|
| Report date: | 06-25-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown |
| 2552013002R00 - NRC Website | |
tank [BP;TK] was declared inoperable in accordance with the operational decision-making issue (ODMI) process. Water leakage from the tank had exceeded the pre-established limit of the ODMI process that directed the tank be declared inoperable. Leakage from the tank was quantified at approximately ninety gallons per day. Technical Specification (TS) 3.5.4.6 requires restoration of an inoperable SIRW tank within one hour. If the tank is not returned to an operable status within one hour, TS 3.5.4.0 requires the plant be placed in Mode 3 within six hours and in Mode 5 within the subsequent thirty-six hours.
Due to the inability to repair the leak within the required one hour time frame, a plant shutdown was initiated at approximately 0100 hours0.00116 days <br />0.0278 hours <br />1.653439e-4 weeks <br />3.805e-5 months <br /> on May 5, 2013. The plant entered Mode 3 at 0457 hours0.00529 days <br />0.127 hours <br />7.556217e-4 weeks <br />1.738885e-4 months <br /> on May 5, 2013. At 2358 hours0.0273 days <br />0.655 hours <br />0.0039 weeks <br />8.97219e-4 months <br /> on May 5, 2013, the plant entered Mode 5 to execute repairs. This condition is reportable in accordance with 10 CFR 50.73(a)(2)(i)(A) for the completion of a plant shutdown required by TS. No additional structures, components, or systems were inoperable and contributed to the event at the time of discovery.
SYSTEM DESIGN/FUNCTION
The SIRW tank is original equipment and has been in service since plant start-up. The tank is aluminum constructed, maintained at atmospheric pressure, and has a nominal volume of approximately 285,000 gallons of borated water. The tank is required to contain a minimum of 250,000 gallons of borated water which is sufficient to fill the reactor cavity during refueling operations.
During accident conditions, the SIRW tank is a source of borated water to the suction of the high and low pressure safety injection pumps and the containment spray pumps that provide containment cooling and depressurization, core cooling, replacement inventory, and a source of negative reactivity for reactor shutdown.
CAUSE OF THE EVENT
In June/July 2012, due to previously identified leakage from the SIRW tank floor, several repair actions were implemented to terminate the leakage and prevent the possibility of future leakage.
Suspect welds were ground out and re-welded, some nozzles were replaced, reinforcing plates were installed and sand was injected into identified voids underneath the tank floor.
In May 2013, after water leakage from the tank had exceeded the pre-established limit of the ODMI process that directed the tank be declared inoperable, several examination and testing techniques were implemented to identify the source of leakage. Testing revealed an approximate 3/16-inch through-wall crack in a nozzle reinforcing collar to floor plate weld of the tank. Follow-up analysis determined there was significant lack of fusion in the nozzle reinforcing collar to floor plate weld that resulted in the failure of the weld and subsequent water leakage.
The failed weld had been installed during the repair efforts to the tank in June/July 2012. The welder that fabricated the nozzle reinforcing collar to floor plate weld did not ensure adequate fusion at the weld root. The significant lack of fusion resulted in a weaker than designed weld and as a result, failed. The poorly performed weld was an isolated human performance error by a contract welder utilizing poor welder technique.
ASSESSMENT OF SAFETY CONSEQUENCES
The leakage from the SIRW tank did not challenge nuclear safety. SIRW tank volume was maintained above the required TS limit, and capable of fulfilling its safety function for the maximum mission time, during the period there was leakage from the tank. Therefore, no event or condition existed that could have prevented fulfillment of a safety function.
CORRECTIVE ACTIONS TAKEN
The entire SIRW tank floor was replaced with the exception of an annulus ring around the perimeter. Several initiatives were implemented to preclude potential weld issues during the fabrication of the new tank floor, including welder proficiency training on revised welding techniques and utilization of several types of weld testing methods.
PREVIOUS SIMILAR EVENTS
Palisades has experienced water leakage from the SIRW tank on previous occasions; however the previous events did not involve the same underlying cause.