05000237/LER-2007-001
| Docket Numbersequential Revmonth Day Year Year Month Day Year N/A N/Anumber No. | |
| Event date: | 01-18-2007 |
|---|---|
| Report date: | 03-20-2008 |
| 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 |
| 2372007001R01 - 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 Conditions Prior to Event:
Unit: 02� Event Date: 1-18-2007 �Reactor Mode: 1� Mode Name: Power Operation Power Level: 100 percent Reactor Coolant System Pressure: 1000 psig
B. Description of Event:
On January 18, 2007, at 2110 hours0.0244 days <br />0.586 hours <br />0.00349 weeks <br />8.02855e-4 months <br /> (CST), with Unit 2 at approximately 100 percent power, DNPS control room personnel were notified of a through wall linear crack at the Unit 2 Standby Liquid Control System (SLC) [BR] Tank [TK] temperature switch well. The temperature switch well contains SLC was declared inoperable and Technical Specification (TS) 3.1.7, "Standby Liquid Control (SLC) System," was entered. A review of the possible repair options determined that a repair to the tank could not be completed within the allowed Completion Time of TS 3.1.7.
On January 19, 2007, DNPS verbally requested a Notice of Enforcement Discretion (NOED) associated with TS 3.1.7. The NOED requested a temporary 72-hour extension of the Completion Time of Required Action B.1 for TS 3.1.7. The intent of the NOED was to avoid a plant shutdown as a result of compliance with TS 3.1.7, Required Action C.1, which required Unit 2 to be placed in Mode 3 operation (i.e., hot shutdown) on or before 1710 hours0.0198 days <br />0.475 hours <br />0.00283 weeks <br />6.50655e-4 months <br /> on January 19, 2007. The NRC verbally granted the NOED at 0503 hours0.00582 days <br />0.14 hours <br />8.316799e-4 weeks <br />1.913915e-4 months <br /> (CST) on January 19, 2007. The system was restored to operable status by encapsulating the cracked component on January 20, 2007 at 0015 hours1.736111e-4 days <br />0.00417 hours <br />2.480159e-5 weeks <br />5.7075e-6 months <br /> (CST) within the time allowed by the NOED.
The written NOED request, "Request for Enforcement Discretion, Technical Specification 3.1.7, Standby Liquid Control (SLC) System," was submitted to the NRC on January 22, 2007, consistent with the guidelines provided in Regulatory Issue Summary 2005-01, "Changes to Notice of Enforcement Discretion (NOED) Process and Staff Guidance," and NRC Inspection Manual Part 9900, "Operations — Notices of Enforcement Discretion.
This event is being reported in accordance with 10 CFR 50.73(a)(2)(i)(B), "Any operation or condition which was prohibited by the plant's Technical Specifications," and 10 CFR 50.73(a)(2)(vii)(D), "Any event where a single cause or condition caused at least one independent train or channel to become inoperable in multiple systems or two independent trains or channels to become inoperable in a single system designed to mitigate the consequences of an accident." The SLC Tank was inoperable for a period of time that exceeded TS 3.1.7 allowed Completion Time, and the degraded component rendered both trains of SLC injection inoperable until the system was restored to operable status within the time allowed by the granted NOED.
� C.� Cause of Event:
The cause of the event was the result of transgranular stress corrosion cracking (TGSCC) that initiated from the threaded inner diameter of the Type 304 stainless steel 1 inch to 3/4 inch Hex- Reducing bushing.
During a maintenance outage in November 2006, DNPS Unit 2 drained the SLC Tank to replace the Boric Acid. During this evolution, an existing leak at the threaded connection to the temperature switch well was repaired. The components associated with the temperature switch well include a 1 inch Half-Coupling welded to the tank wall, a 1 inch to 3/4 inch Hex-Reducing bushing threaded and welded to the Half-Coupling and a 3/4 inch Fenwall Thermowell threaded into the Hex-Reducing bushing. The leak at the threaded connection between the Fenwall Thermowell and the Hex- Reducing bushing did not jeopardize the operability of the SLC Tank. The repair required increased seal welding of the Hex-Reducing bushing to the Half-Coupling and replacement of the threaded connection between the Fenwall Thermowell and the Hex-Reducing bushing with seal welding. The leak was successfully repaired.
On January 18, 2007, during a DNPS Senior Manager overview of training in Unit 2, it was identified SLC Tank temperature switch well. DNPS determined that the best repair solution was to remove the temperature sensor from the thermowell and encapsulate the cracked component thereby restoring the integrity of the tank's boundary. During the event, the repair method that was utilized did not require the tank to be drained, thus the SLC Tank remained available to support the injection of Boric Acid into the reactor vessel during a postulated accident.
DNPS during a refuel outage in November 2007 drained the SLC Tank to permit the removal of the cracked component. After removal the cracked component was sent to a vendor's lab for evaluation of the failure mechanism.
The vendor failure analysis report was issued on February 2, 2008. The results of the failure analysis identified the material used in the 1 inch to 3/4 inch Hex-Reducing bushing was consistent with Type 304 stainless steel. A dye penetrant examination identified a 0.29-inch long axial crack on the outer diameter of the bushing that extended into the weld and hexagonal portion of the bushing. A metallurgical section of the crack identified branching transgranular features that are typical of TGSCC. The primary direction of the crack branching indicated the crack initiated from the inner diameter of the bushing and extended into the bushing and coupling weld metal. The crack oxides were evaluated using energy dispersive X-Ray analysis techniques and were found to contain iron, chromium, silicon, and oxygen that are consistent with the base metal corrosion products.
Additionally, a small chlorine peak was identified suggesting that chlorides may have contributed to the cracking.
Stress corrosion cracking occurs when a susceptible material such as Type 304 stainless steel is exposed to tensile stress in a corrosive aqueous environment. The susceptibility of Type 304 stainless steel to chloride induced TGSCC is low at temperatures below 150 degrees Fahrenheit. The SLC tank operates at approximately 95 degrees Fahrenheit implying the TGSCC growth rate was most likely slow, which suggests that the cracking initiated many years ago. The exact source of the chlorides that led to the TGSCC could not be identified. Two potential sources of the chlorides that were postulated in the failure analysis report include the sealant that was applied to the threaded connection during fabrication or brazing flux residue from the manufacturing of thermowell tube / connection.
temperature switch well does not use a Hex-Reducing bushing between the Half-Coupling and thermowell.
D. Safety Analysis:
The safety significance of the event is minimal. During this event, DNPS continued to operate within the requirements of the TS and the NRC granted NOED, which temporarily extended the Completion Time. The SLC Tank remained available to support the injection of Boric Acid into the reactor vessel during a postulated accident. Additionally, the leak from the linear crack was extremely small in relation to the Boric Acid volume contained in the tank and at no time did the quantity of available Boric Acid fall below that assumed in any accident analyses. Therefore, the consequences of this event had minimal impact on the health and safety of the public and reactor safety.
E. Corrective Actions:
The SLC Tank was repaired by encapsulating the cracked component. The system was restored to operable status on January 20, 2007 at 0015 hours1.736111e-4 days <br />0.00417 hours <br />2.480159e-5 weeks <br />5.7075e-6 months <br /> (CST) within the time allowed by the NOED.
The written NOED request was submitted to the NRC on January 22, 2007, consistent with the guidelines provided in Regulatory Issue Summary 2005-01 and NRC Inspection Manual Part 9900.
The cracked component was removed from SLC Tank during the Unit 2 refueling outage in November 2007. The cracked component was sent to a vendor's lab for evaluation.
An inspection of the Unit 3 SLC Tank identified no observable leakage from the tank.
F. Previous Occurrences:
A review of DNPS Licensee Event Reports (LERs) for the last three years did not identified any LERs associated with through-wall leakage of the SLC Tank.
G. Component Failure Data:
The manufacturer of the Type 304 stainless steel Hex-Reducing bushing (1 inch to 3/4 inch) is unknown.