Event Number: 47871 Kewaunee
BOTH TRAINS OF RESIDUAL HEAT REMOVAL DECLARED INOPERABLE
“At 1742 EDT on 04/27/2012, while in Mode 5, both trains of RHR (Residual Heat Removal) were declared inoperable due to a through wall leak on a socket welded connection of ASME code class piping. Currently, with both trains of RHR in service for decay heat removal, the leakage impacts redundant equipment required to fulfill a safety function. In the current condition, both trains are required to be operable to meet Technical Specification [TS] 3.4.7, ‘RCS Loops – MODE 5, Loops Filled’.
Any loss of Shutdown Cooling is serious. This is especially so when both trains of Residual Heat Removal are affected. Had the RHR system become incapable of Decay Heat Removal, then extraordinary measures would have to be taken to prevent fuel damage.
This event involved a leaking Socket Weld fitting. These leaks are typically within the ASME bounds for acceptable leakage. Socket Welds, however, have been found to fail due to exposure to vibration over time. Depending on the location of this Socket Weld, and the potential for other similar fittings to fail, the severity of this event is unknown.
This event has to be evaluated under a Loss of Offsite Power and Station Black Out conditions to determine the Core Damage Risk. Also, it has to be evaluated for Worst-Case scenario when the Reactor was freshly shutdown and the Decay Heat Load is high. The evaluation should include other conditions in Mode 5 (cold shutdown), such as Reduced Inventory.
The illustration provided above in the lower left hand corner depicts where RHR taps off Reactor Coolant Loop “A” through Loop Isolation Motor Operated Valves 1A/B and 2A/B. Because both RHR Loops were deemed Inoperable (but Available), my guess is that the leak is on the Common Line where Relief Valve RHR-33 and 33-1 are installed or perhaps at the Temperature Element TE 630.
The illustration provided below shows more detail.
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