05000528/LER-2008-003
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | |
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| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 5282008003R00 - NRC Website | |
All times are Mountain Standard Time and approximate unless otherwise indicated.
1. REPORTING REQUIREMENT(S):
This LER is being submitted pursuant to 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by Technical Specifications (TS) for the delayed entry into a Limiting Condition for Operation for an inoperable safety injection tank and 10 CFR 50.73(a)(2)(i)(A) as a shutdown required by TS. Notification of this event was made to the Nuclear Regulatory Commission (NRC) on June 5, 2008, at 02:40 via the Event Notification System (ENS) on ENS 44274.
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
There are four Safety Injection Tanks (SIT) (EIIS: BP) in each unit which contain borated water and are pressurized with nitrogen. They discharge their contents to the Reactor Coolant System (RCS) (EIIS: AB) following depressurization as a result of a Loss-of-Coolant Accident (LOCA). Each tank is piped into a cold leg of the RCS via a safety injection nozzle located on the RCS piping near the reactor vessel inlet. The tank vent nozzle is located on the upper head of the tank which is in contact with the nitrogen blanket. Each tank is approximately 41 ft.
high and 9 ft. in diameter. The operating level is maintained at approximately 32 ft. to 34 ft.
The upper level alarm limit is set at approximately 35 ft. Technical Specifications requires boron concentration be in the range of 2300 parts per million (ppm) to 4400 ppm; and that the nitrogen cover pressure be maintained from 600 pounds per square inch, gauge (psig) to 625 psig. The SIT operates at a temperature range of 60° Fahrenheit (F) to 140° F.
3. INITIAL PLANT CONDITIONS:
On June 5, 2008, Palo Verde Unit 1 was in Mode 1 (power operations), operating at approximately 100 percent power. There were no major structures, systems, or components inoperable at the start of the event that contributed to the event.
4. EVENT DESCRIPTION:
On June 4, 2008, during a scheduled containment entry, SIT 1A was inspected for leaks. A leak was located at the joint between the tank head and the SIT vent line nozzle. Upon exiting from the containment, at 13:30, maintenance personnel communicated the results of the inspection to the day shift Control Room Supervisor (CRS). This information was misinterpreted by the CRS who did not consider a potential impact to the vessel integrity and did not pursue clarification. On June 5, 2008, while reviewing corrective action documentation for the SIT 1A leak inspection, the night shift Shift Manager determined SIT 1A was inoperable and, at 02:25, SIT 1A was declared inoperable and TS LCO 3.5.1, Condition B, was entered requiring SIT 1A to be returned to operable status within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
To repair SIT 1A, Unit 1 control room staff commenced a plant shutdown at 23:56 on June 5, 2008, and manually tripped the reactor at 01:48 on June 6, 2008. Control room staff entered LCO 3.5.1, Condition C, at 02:25 on June 6, 2008, when the allowable time had expired for Condition B. LCO 3.5.1, Condition C, requires the plant to be placed in Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and pressurizer pressure to be reduced to less than 1837 psia within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. Unit 1 exited LCO 3.5.1, Conditions B and C at 05:30 on June 6, 2008, when pressurizer pressure was reduced to less than 1837 psia.
Investigation revealed a small amount of nitrogen leakage through the nozzle weld for the 2 inch nozzle vent line at the top of SIT 1A. Since the original weld for this connection is on the inside of the vessel and repairing the weld at the inside location was not practical, the nozzle repair consisted of making a new weld to attach the nozzle to the outer surface of the vessel.
This repair method complies with American Society of Mechanical Engineers (ASME) Code requirements. Unit 1 returned to power on June 8, 2008. However, since the flaw that resulted in the nitrogen leak could not be characterized and was left in place, an ASME Relief Request was submitted on July 11, 2008.
5.ASSESSMENT OF SAFETY CONSEQUENCES:
The size of the leak did not impact plant operator's ability to maintain SIT 1A within the allowable TS pressure band, therefore, the plant remained within safety limits throughout the event. An analysis was performed that demonstrates that the worst case flaw in the nozzle or attachment weld will not propagate such that the structural integrity of the new weld or the tank boundary is affected.
The condition described in this LER did not result in any challenges to fission product barriers or in any offsite releases. Therefore, there were no actual adverse safety consequences or implications as a result of this event and the event did not adversely affect the safe operation of the plant or health and safety of the public as a result of the weld flaw on SIT 1A.
This condition would not have prevented the fulfillment of any safety function and did not result in a safety system functional failure as defined by 10 CFR 50.73(a)(2)(v).
6.CAUSE OF THE EVENT:
The cause of the leak was the loss of SIT 1A vessel integrity due to a through-wall nitrogen leak. The leak appears to be from a possible flaw in the weld material between the vessel and the vent line nozzle. The vessel base metal was demonstrated to be unaffected.
Ineffective involvement from Operations personnel in the containment entry activities caused the delayed entry into LCO 3.5.1, Condition B, which led to exceeding the allowed outage time A for LCO 3.5.1, Condition B and C. The expectation for the Operations Department to participate in containment entry activities while the unit is at power was unclear. Operations personnel did not attend the pre-entry brief where the entry team had discussed the potential impacts to SIT 1A operability. As a result, when the entry team exited the containment to notify the control room of their findings, the CRS believed the call was primarily to inform the control room staff that the team had exited containment. Although he acknowledged that some details of the leak location were provided, he did not have a clear understanding of the leak nor its location. He also knew that the SIT 1A was not currently exhibiting any indication of abnormal leakage and relied on the corrective action process to ensure that the information was routed for impact and operability review.
7. CORRECTIVE ACTIONS:
The following corrective actions have been implemented:
- A repair consisting of adding a new weld with a reinforcing partial fillet weld to attach the nozzle at the outer surface of the vessel was completed. The design of the new configuration complies with the design and construction Code (ASME Section III, 1974 edition through Winter 74 Addenda).
- An analysis performed on the original flaw and determined that the worst case flaw would not propagate such that the structural integrity of the new weld or the tank boundary would be affected.
- ASME Relief Request 39 was submitted on July 11, 2008, for the original weld on the inner surface that was left in place.
- The vent nozzle connections on the other three SITs in Unit 1 were inspected using the snoop method and no leakage was detected.
- The Containment Entry at Power procedure, 40DP-9ZZ01, will be revised to add expectations for Operations personnel regarding operational impact of negative findings in containment.
Work Orders were initiated to evaluate the condition of the SITs in Units 2 and 3.
Any additional corrective actions taken as a result of this event will be implemented in accordance with the APS corrective action program. If information is subsequently developed that would significantly affect a reader's understanding or perception of this event, a supplement to this LER will be submitted.
8. PREVIOUS SIMILAR EVENTS:
No similar conditions have been reported in the prior three years.
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