05000317/LER-2011-003
| Calvert Cliffs Nuclear Power Plant, Unit 1 | |
| Event date: | 10-21-2011 |
|---|---|
| Report date: | 12-16-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 3172011003R00 - NRC Website | |
A. PRE-EVENT PLANT CONDITIONS:
Unit 1 was operating at 95.5 percent power and Unit 2 was operating at 99.5 percent power on October 21, 2011.
B. EVENT:
At 0000 on August 28, 2011, numerous alarms were received for the 1A Emergency Diesel Generator (EDG) (train A, one of two safety-related EDGs dedicated to Unit 1). Water was intruding down the diesel generator intake piping resulting in a short circuit on the 1A2 engine speed switch circuit. Based on these indications the 1A EDG was declared inoperable and appropriate Technical Specifications were implemented. During the event, there was no demand for the 1A EDG.
On October 21, 2011, it was determined that the condition that caused the 1A EDG inoperability may have existed since construction of the 1A EDG Building in 1996.
C. INOPERABLE STRUCTURES, COMPONENTS, OR SYSTEMS THAT CONTRIBUTED
TO THE EVENT:
There were no additional inoperable structures, components, or systems that contributed to the event.
D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES:
On August 27, 2011, operators made preparations for the impending Hurricane Irene.
At 1800, the Shift Manager briefed operators regarding the need to look for water intrusion.
Frequency for operator rounds was increased to every two hours.
At 2248, Unit 1 reactor tripped. At this time, the Shift Manager made the decision for no one to go outside for personnel safety reasons and suspended the rounds to the 1A Diesel Generator Building. This prevented the next rounds scheduled for 2340.
On August 28, 2011, at 0000, the 1A EDG was declared inoperable due to receiving several alarms. Both of Unit 2 EDGs and the 1B EDG was verified operable and the OC Diesel Generator was verified available.
At 0040, operators observed water leaking from the 1A2 exhaust stack onto some of the engine relays.
At 0400, craft discovered that the 1A EDG control circuitry was attempting to establish a generator field. The associated circuits were de-energized to prevent damage to the circuit.
On October 21, 2011, it was determined that the condition that caused the 1A EDG inoperability may have existed since construction of the 1A EDG Building in 1996.
E. OTHER SYSTEMS OR SECONDARY FUNCTIONS AFFECTED:
The Technical Specifications require one EDG from the other unit to be capable of supplying power to the Control Room Emergency Ventilation System and the Control Room Emergency Temperature System. The 1A EDG provides this Technical Specification function for Unit 2.
No other systems or secondary functions were affected.
F. METHOD OF DISCOVERY:
The failure of 1A EDG on August 28, 2011, was self-revealing. On October 21, 2011, it was determined that the condition that caused the 1A EDG inoperability may have existed since construction of the 1A EDG Building in 1996.
G. MAJOR OPERATOR ACTION:
Operators entered Technical Specification 3.8.1.B and 3.8.1.E, "AC Sources — Operating" and performed verification of power distribution system operability of off-site power and equipment in train B in accordance with station procedures. When it was discovered that the 1A EDG was continuing to attempt to establish a generator field, craft de-energized the associated circuits to prevent damage to the circuit.
H. SAFETY SYSTEM RESPONSES:
There were no demands for safety system actuations during this event.
II. CAUSE OF EVENT:
The inoperable 1A EDG is documented in station condition report number CR-2011-008708.
The penetration around the 1A2 combustion air intake pipe on the 80 foot level of the 1A Diesel Generator Building was unable to perform its design function of being leak tight. Dripping water penetrated the 1A EDG 1A2 engine speed switch connector causing the speed switch circuit to short. This shorted speed switch caused the field flash circuit to attempt to flash the 1A EDG.
Since the generator was not turning, a field was not established and the field flash circuit protective features were enabled. Because these protective features were enabled, the field flash circuit would not have been available for subsequent EDG starts. An evaluation has determined that the most likely root cause(s) are limited to improper installation, manufacturing defect, degradation, or inadequate original design considerations of the penetration around the 1A2 combustion air intake pipe.
generator. The three other EDGs are not susceptible to this condition as this is the only location in the plant for this type of penetration with this specific configuration of boot seals that is open to atmospheric conditions.
III. ANALYSIS OF EVENT:
While performing a root cause analysis for the inoperability of the 1A EDG, it was determined that the 1A EDG may have been inoperable from original construction (1996).
This event is reportable in accordance with the following:
- 10 CFR 50.73(a)(2)(i)(B); "Any operation or condition which was prohibited by the plant's Technical Specifications.
Since the 1A EDG may have been inoperable from original construction, the following occurrences are examples of situations when the site unknowingly exceeded the Technical Specification allowed completion times.
- The 1A EDG may have been inoperable for a period of over 14 years.
- The 1B EDG was removed from service for scheduled maintenance at 0130 on October 18, 2010 and was returned to service at 1010 on October 22, 2010, a period of over 104 hours0.0012 days <br />0.0289 hours <br />1.719577e-4 weeks <br />3.9572e-5 months <br />.
- The 2A EDG was removed from service to investigate an unplanned start at 1827 on November 16, 2008 and was returned to service at 1912 on November 17, 2008, a period of over 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. With both Train A EDGs inoperable, all Train A site battery chargers and both Train A 125 VDC busses were inoperable, which should have resulted in entry into Technical Specification 3.0.3 on both units.
- The Technical Specifications require one EDG from the other unit to be capable of supplying power to the Control Room Emergency Ventilation System and the Control Room Emergency Temperature System. The 1A EDG provides this Technical Specification function for Unit 2. The 2B EDG was removed from service at 0200 on November 8, 2010 for scheduled maintenance and was returned to service at 0100 on November 11, 2011, a period of 71 hours8.217593e-4 days <br />0.0197 hours <br />1.173942e-4 weeks <br />2.70155e-5 months <br />.
The above conditions existed for a time longer than the completion time allowed by their respective Technical Specifications. Therefore, this event is reportable pursuant to the reporting requirements specified in 10 CFR 50.73(a)(2)(i)(B).
- 10 CFR 50.73(a)(2)(ii)(B), "Any event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety.
Chapter 14 of the Calvert Cliffs Updated Final Safety Analysis Report (Safety Analysis) presents analytical evaluations of the Nuclear Steam Supply System response to postulated disturbances in process variables and to postulated malfunctions or failure of equipment.
Some postulated disturbances analyze the most limiting failure of equipment. In some analyses, failure of one EDG is assumed. However, there are no instances where the failure of both EDGs is assumed. Therefore this event is reportable pursuant to the reporting requirements specified in 10 CFR 50.73(a)(2)(ii)(B).
- 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.
There are occurrences following construction of the 1A EDG when both the 1A and 1B EDGs were simultaneously inoperable. No redundant equipment in the same system was operable and available to perform the required safety function. Therefore this event is reportable pursuant to the reporting requirements specified in 10 CFR 50.73(a)(2)(v)(D).
There were no actual nuclear safety consequences incurred from this event. For periods when both the 1A and 1B EDGs were out-of-service, Unit 1 was more susceptible to a Station Blackout (SBO) event. The SBO diesel generator is designed to provide a power source capable of starting and supplying the essential loads necessary to safely shutdown one unit and maintain it in a safe shutdown condition during an SBO event.
The Nuclear Regulatory Commission Performance Indicator for Safety System Functional Failures for Unit 1 rises from 0.0 to 1 as a result of this event and will remain in the green band.
No other Nuclear Regulatory Commission Performance Indicators were impacted.
A Significance Determination Process evaluation determined that the risk incurred by operating Unit 1 with the 1A EDG in this degraded condition is as follows: Integrated Conditional Core Damage Probability and Integrated Conditional Large Early Release Probability are 7E-07 and 7E-08, respectively. The risk incurred by operating Unit 2 with the 1A EDG in this degraded condition is as follows: Integrated Conditional Core Damage Probability and Integrated Conditional Large Early Release Probability are 6E-07 and 4E-08, respectively.
IV. CORRECTIVE ACTIONS:
A. ACTION TO RETURN AFFECTED SYSTEMS TO PRE-EVENT NORMAL STATUS:
1. The 1A2 engine speed switch assembly was cleaned, dried, and inspected.
If information is subsequently developed, that would significantly affect a reader's understanding or perception of this event, a supplemental licensee event report (LER) will be submitted.
V. ADDITIONAL INFORMATION:
A. FAILED COMPONENTS:
1. The 1A EDG Building penetration seals were manufactured by PCI - Promatec.
2. The 1A2 Engine Speed Switch Assembly was manufactured by Moisy-Servat.
3. The 1A EDG was manufactured by Societe Alsacienne De Constructions Mecaniques De Mulhouse.
B. PREVIOUS LERs ON SIMILAR EVENTS:
A review of Calvert Cliffs' events during the past several years was performed. No previous LERs were identified that involved the same underlying concern or reason for this event.
C. THE ENERGY INDUSTRY IDENTIFICATION SYSTEM (EMS) COMPONENT
FUNCTION IDENTIFIER AND SYSTEM NAME OF EACH COMPONENT OR SYSTEM
REFERRED TO IN THIS LER:
COMPONENT EDS FUNCTION SYSTEM ID
1A EDG DG EK 1A2 Engine Speed Switch Assembly SIS EK 1A EDG Building SEAL NB
D. SPECIAL COMMENTS:
None