05000263/LER-2014-001
| Monticello Nuclear Generating Plant | |
| Event date: | 01-17-2014 |
|---|---|
| Report date: | 03-14-2014 |
| 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)(ii)(A), Seriously Degraded |
| 2632014001R00 - NRC Website | |
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2014 - 001 00
EVENT DESCRIPTION
On January 4, 2014, a step change increase in the Reactor Building Closed Cooling Water (RBCCW) [CC] surge tank level was noted. On January 17, 2014, the source of the inleakage was determined to be an un- isolable Reactor Coolant Pressure Boundary (RCPB) leak. As required by the Monticello Nuclear Generating Plant (MNGP) Technical Specifications (TS), RCPB leakage does not meet the TS 3.4.4 (RCS Operational Leakage) limiting condition for operation (LCO) and a TS required shutdown was performed.
The plant shutdown commenced at 2029 on January 17, 2014, and was completed without incident.
The source of the inleakage into the RBCCW System was determined to be from the Reactor Recirculation System [AD] through a coil failure in the #12 Reactor Recirculation Pump Upper Seal Heat Exchanger [HX].
Following the shutdown the #12 Reactor Recirculation Pump Upper Seal Heat Exchanger leakage rate was measured to be 0.215 gpm.
The cause of the inleakage was subsequently corrected by performing a modification that removed the #12 Reactor Recirculation Pump Upper Seal Heat Exchanger and modifying the system to operate without this heat exchanger by using the excess capacity of the #12 Reactor Recirculation Pump Lower Seal Heat Exchanger.
EVENT ANALYSIS
This event is reportable in accordance with 10 CFR 50.73(a)(2)(ii)(A) as an event or condition that resulted in the condition of the nuclear power plant, including its principal safety barriers, being seriously degraded, since a leak of a RCPB component occurred.
This event is also reportable in accordance with 10 CFR 50.73(a)(2)(i)(A) as the plant completed a shutdown as required by the plant's TS. Finally, this event is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B) as a condition which is prohibited by the plant's TS, since a leak of the RCPB existed for a time period that is prohibited by the MNGP TS.
SAFETY SIGNIFICANCE
The leak of the #12 Reactor Recirculation Pump Upper Seal Heat Exchanger resulted in a small, un-isolable RCPB leak, which required a controlled plant shutdown. There was no radioactive release from the plant.
CAUSE
The apparent cause for the RCPB leak was that there was no established maintenance strategy in place to periodically check the condition of the heat exchanger or replace it. A failure analysis performed indicated that the heat exchanger leaked due to the slow formation of a crack on the external surface of the stainless steel coil that was not detected prior to the leak occurring. RBCCW chemistry constituents (including chlorides) concentrated on the coil surface along with scale due to unrecognized boiling occurring in the lower region of the #12 Reactor Recirculation Pump Upper Seal Heat Exchanger coil. The crack formed in the coil due to the unexpected high concentration of chlorides that formed on the coil surface, combined with a susceptible material and stress, resulting in intergranular stress corrosion cracking (IGSCC).
CORRECTIVE ACTION
The leaking #12 Reactor Recirculation Pump Upper Seal Heat Exchanger was removed and the system was modified to operate without this heat exchanger by using excess capacity in the #12 Reactor Recirculation Pump Lower Pump Seal Heat Exchanger. Based on the extent of condition review, a similar proactive modification to bypass the #11 Reactor Recirculation Pump Upper Seal Heat Exchanger was performed without physical removal of the heat exchanger. In addition, the Recirculation System Sample Cooler was replaced as this heat exchanger was determined to be vulnerable to the conditions that caused the #12 Reactor Recirculation Pump Upper Seal Heat Exchanger to leak.
PREVIOUS SIMILAR EVENTS
There were no previous similar events reported.
ADDITIONAL INFORMATION
Energy industry identification system (EllS) codes are identified in the text within brackets [xx].