05000237/LER-2007-003
| Docket Number Sequential Revmonth Day Year Year Month Day Year N/A N/Anumber No. | |
| Event date: | 07-26-2007 |
|---|---|
| Report date: | 09-24-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 2372007003R00 - NRC Website | |
Dresden Nuclear Power Station (DNPS) Unit 2 is a General Electric Company Boiling Water Reactor with a licensed maximum power level of 2957 megawatts thermal. The Energy Industry Identification System codes used in the text are identified as [XX].
A. Plant Conditiolfs-Prior to Event:
Unit: 02� Event Date: 7-26-2007 �Reactor Mode: 1� Mode Name: Power Operation Power Level: 98 percent Reactor Coolant System Pressure: '1000 psig
B. Description of Event:
On July 26, 2007, at 1706 hours0.0197 days <br />0.474 hours <br />0.00282 weeks <br />6.49133e-4 months <br /> (CDT), with Unit 2 at approximately 98 percent power, DNPS control room personnel were notified of a very small through-wall leak on the Unit 2 High Pressure Coolant Injection System (HPCI) [BG] Inlet Drain Pot drain [DRN] line 2-2323-1, 90-degree elbow upstream of valve 2-2301-29. This line is the drain from the Unit 2 HPCI Drain Pot to the main condenser. To repair the leaking location, the piping was isolated which resulted in the isolation of the Unit 2 HPCI.
The Unit 2 HPCI was declared inoperable and Technical Specification (TS) 3.5.1, "ECCS Operating," was entered. The elbow was replaced.
An ENS call was made on July 26, 2007, at 2011 hours0.0233 days <br />0.559 hours <br />0.00333 weeks <br />7.651855e-4 months <br /> (CDT) for the above-described event. The assigned ENS event number was 43526.
The Unit 2 HPCI was declared operable on July 27, 2007, at 1809 hours0.0209 days <br />0.503 hours <br />0.00299 weeks <br />6.883245e-4 months <br /> (CDT).
This event is being reported in accordance with 10 CFR 50.73(a)(2)(v)(D), "Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident." HPCI is a single train system and is credited in mitigating the consequences of an accident.
C. Cause of Event:
The apparent cause of the through-wall leak was liquid impingement erosion of the exterior curve of the 90-degree elbow. DNPS had replaced this elbow during a HPCI line upgrade in 1997 with A335 P11 Chrome Moly alloy steel to reduce the lines susceptibility to flow accelerated corrosion (FAC).
Although chromium alloy steels are immune to FAC, they are still susceptible to wall loss from mechanical thinning mechanisms such as cavitation, liquid impingement, flashing, and solid particle erosion.
The leaking 90-degree elbow was sent to a vendor for analysis. The vendor verified the elbow's chemistry was consistent with A335 P11 Chrome Moly alloy steel. The vendor's evaluation concluded that the elbow leak was the result of internal liquid impingement erosion thinning. Liquid impingement erosion is caused by the impacts of high-velocity water droplets on the component's inner surface. The through wall leak occurred toward the downstream side of the elbow, where the droplet impact angle was the highest. This HPCI line carries a steam/condensate mixture from the approximately 400°F/1000 psig conditions in the drain pot to the vacuum conditions in the condenser.
Due to the pressure drop, the two-phase mixture accelerates as the condenser is approached. Liquid impingement erosion occurs when the impacts from the high velocity water droplets are high enough to damage the pipe surface. Velocity and impaat angleare two factors that can affect the erosion.
DNPS has scheduled non-destructive testing inspections on the piping components in the Unit 2 and 3 HPCI Drain Pot drain lines that have the highest susceptibility to impingement erosion (i.e., elbows, valves, orifices and piping immediately downstream of these components). DNPS will upgrade the A335 P11 Chrome Moly alloy steel elbow to Inconel material and modify the design of the line to reduce susceptibility to liquid impingement erosion or initiate preventive maintenance monitoring.
Additionally, DNPS will evaluate other high-pressure safety piping which is not included in the FAC Program for susceptible areas for impingement erosion.
D. Safety Analysis:
The safety significance of the event is minimal. TS 3.5.1 allows Unit 2 to remain at power for 14 days with an inoperable HPCI if the Isolation Condenser System (IC) is operable. Unit 2 was in compliance with TS 3.5.1 during this event as the IC was operable and HPCI was inoperable for approximately 1 day. Therefore, the consequences of this event had minimal impact on the health and safety of the public and reactor safety.
E. Corrective Actions:
Unit 2 HPCI Inlet Drain Pot drain line elbow with the leak was replaced.
Non-destructive testing inspections will be performed on the piping components in the Unit 2 and 3 HPCI Drain Pot drain lines that have the highest susceptibility to impingement erosion.
DNPS will upgrade the A335 P11 Chrome Moly alloy steel elbow to Inconel material and modify the design of the line to reduce susceptibility to liquid impingement erosion or initiate preventive maintenance monitoring.
DNPS will evaluate other high-pressure safety piping which is not included in the FAC Program for susceptible areas for impingement erosion.
F. Previous Occurrences:
A review of DNPS Licensee Event Reports (LERs) for the last three years did not identified any LERs addressing piping leaks caused by liquid impingement erosion.
G.�Component Failure Data:
NA