05000321/LER-2013-001
| Edwin 1. Hatch Nuclear Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3212013001R01 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor Energy Industry Identification System codes appear in the text as (EIIS Code XX).
DESCRIPTION OF EVENT
On 2/10/2013 at 0536 EST, while operating at 99.8 percent RTP, a system operator reported that. the "System Inlet High Conductivity" alarm had been received at the local Condensate Polishing System panel. The setpoint for this annunciator is 0.239 micro mhos per cm. At 0540 EST, control room instruments indicated that conductivity levels were increasing for the condenser and the reactor vessel coolant. The conductivity was greater than 0.3 micro mhos per cm in the condensate, while still under 0.1 micro mho per cm at the inlet to the reactor. The shift supervisor entered the abnormal operating procedure for Condenser Tube Leaks/Chemical Intrusion and directed the crew to decrease power to 88 percent RIP, After verifying conductivity levels, the crew continued to reduce power under the direction of shift management to 47 percent RTP at 0647 EST. Inlet conductivity reached 5 micro mhos per cm and the shift supervisor ordered the crew to enter the abnormal operating procedure for a manual SCRAM at 0700 EST.
The unplanned RPS actuation was initiated as a protective action for the reactor vessel pressure boundary, as increasing water chemistry conductivity indicated the possibility of long term degradation of pressure boundary materials, and this condition could have increased the potential for stress corrosion cracking of internal vessel components if it had not been addressed. The Technical Requirements Manual (TRM) 3.4.1 required action statement (RAS) for Condition A directs the shift to return to less than 5.0 micro mhos per cm within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> and to restore reactor coolant system (RCS) chemistry to within the limits of the associated table, T3.4.1-1. The condenser conductivity never exceeded 5 micro mhos per cm prior to the SCRAM or immediately thereafter. Shift management made the conservative decision to enter a manual SCRAM after the power descent, rather than waiting 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. Management believed it to be necessary to ensure that degradation of the reactor coolant pressure boundary was not exacerbated by poor chemistry conditions. The orderly shutdown was completed in accordance with the TRM and was not addressed in HNP-1 Technical Specifications.
CAUSE OF EVENT
Operation of the lA condenser with high hotwell water levels and thermodynamically driven pressure surges taking place under the condenser false bottom were the two phenomena that directly caused the damage within the condenser. Operating the condenser with water levels above its false bottom makes the condenser more susceptible to damage due to not having a vapor space to suppress any surges in hotwell level, and allowing the entrapment of voids underneath the false bottom plates. The leak-by of condensate long cycle cleanup isolation valves reduced the condensate-subcooling margin and introduced voids (steam and heat) under the condenser false bottom. Since the hotwell was water solid, the energy released from the phase transition of water/steam under the false bottom applied stress to the condenser structural members. This overloaded structural components resulting in the fracture of the false bottom and middle divider wall. This failure is unique to the industry. No operating experience was found that identified issues with operating the condenser hotwell at such levels.
1. The as-found design and construction of the lA condenser were not adequate for the current operating parameters. This resulted in unrecognized operation of the lA condenser hotwell in a water solid condition. and increased the hotwell tilt between the Unit 1 condenser hotwells.
2. Preventive maintenance on the hotwell level control loop was not performed.
3. Past corrective actions did not identify the causes of damage found in the condenser to mitigate any future damage.
4. Predictive monitoring plans were not adequately performed to identify energy sources within the condenser and were not updated to include recently identified failure mechanisms.
REPORTABILITY ANALYSIS AND SAFETY ASSESSMENT
The orderly reactor shutdown resulted from conservative actions that are part of an operational strategy laid out in associated abnormal operating procedures that includes the insertion of a manual RPS actuation or SCRAM as a means to limit the potential long term pressure boundary issues that can be created by abnormal reactor coolant chemistry. From this perspective the RPS actuation is an expected culmination of operator actions when the reactor coolant conductivity has a potential to worsen. This event was therefore determined to be reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A): actuation of any of the systems listed in paragraph (a)(2)(iv)(B), which includes any unplanned manual RPS actuation.
This condition occurred at rated power conditions and no monitored safety system parameters were adversely impacted or exceeded. Based on this information this event is considered to have low nuclear safety significance.
CORRECTIVE ACTIONS
Condenser repairs were completed. Instrumentation was installed to measure flow out of the I A condenser and both Unit 1 condensers were equipped with instrumentation to monitor pressure perturbations. Procedure changes were made to maintain the water levels in the 1A and 1B condensers in such a way that pressure perturbations would be minimized, if any should occur.
The main corrective actions that have been performed as a result of this incident include:
1. The north-south divider wall was installed in the IA water box to match the original hotwell design.
2. The 1N61-N003 level transmitter was returned to the I A hotwell to allow monitoring of both the 1A and 1B hotwell level controllers.
3. Necessary changes were made to ensure that normal operating water levels in the Unit 1 hotwells allow for sufficient margin to the false bottom.
In addition, the condensate long cycle cleanup isolation valves will be repaired or replaced as necessary.
Previous Similar Events:
2005: LER 2005-003-001 was written for HNP-2 when the unit was taken off line due to condenser tube leaks. The root cause was a manufacturer defect in tube plugs installed during the previous refueling outage.
The plugs failed on return to power and were replaced with components supplied by a different manufacturer after the cause was determined. This Unit 2 shutdown event due to condenser tube leaks was considerably different than the recent Unit 1 event. The Unit 1 tube leak developed within the latest fuel cycle and was not previously known to exist. The corrective actions from LER 2005-003-001 could not be expected to preclude the 2013 Unit 1 event reported here.