05000483/LER-2013-003
| Callaway Plant Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4832013003R00 - NRC Website | |
1. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The Control Room Emergency Ventilation System (CREVS) provides a protected environment from which operators can control the unit following an uncontrolled release of radioactivity. The CREVS consists of two independent, redundant trains that pressurize, recirculate, and filter the control room air.
Each CREVS train consists of a filtration system train and a pressurization system train. Each filtration system train consists of a fan, a prefilter, a high efficiency particulate air (HEPA) filter, an activated charcoal adsorber section for removal of gaseous activity (principally iodines), and a second HEPA filter that follows the adsorber section to collect carbon fines. Each pressurization system train consists of a fan, a moisture separator, an electric heater, a HEPA filter, and an activated charcoal adsorber followed by a second HEPA filter.
Ductwork, valves or dampers, and instrumentation also form part of the CREVS system. The CREVS is an emergency system which may also operate during normal unit operations. Actuation of the CREVS by a Control Room Ventilation Isolation Signal (CRVIS) places the system in the emergency mode of operation. Actuation of the system to the emergency mode of operation closes the unfiltered outside air intake and unfiltered exhaust dampers, and aligns the system for recirculation of the air within the Control Room envelope (CRE) through the redundant trains of HEPA and charcoal filters. The emergency (CRVIS) mode also initiates pressurization and filtered ventilation of the air supply to the CRE.
The control room pressurization system draws in outside air, processing it through a particulate filter charcoal adsorber train for cleanup. This outside air is diluted with air drawn from the cable spreading rooms and the electrical equipment floor levels within the control building and distributed back into those spaces for further dilution. The control room filtration units take a portion of air from the exhaust side of the system for dilution with portions of the exhaust air from the control room air-conditioning system, and then process it through the control room filtration system adsorption train for additional cleanup. This air is then further diluted with the remaining control room air-conditioning system return air, cooled, and supplied to the CRE.
2. INITIAL PLANT CONDITIONS:
On 3/23/2013, the plant was in MODE 1 at 96-percent rated thermal power (due to the commencement of coastdown for the Refuel-19 outage). Apart from the CREVS train that was identified to be inoperable per the condition described in this LER, no other significant equipment was concurrently inoperable.
3. EVENT DESCRIPTION:
On 3/23/2013, during restoration from surveillance testing (Containment Isolation Dampers Operational Test), status indication for the CREVS 'B' train Control Room filtration fan was lost in the Control Room, and the associated dampers closed. This occurred approximately 22 minutes after the fan had been started. An Operations Technician (OT) was sent to investigate the fan and associated feeder breaker. He reported that the breaker indicated Closed/Not Tripped and the fan had no abnormal indications. In the Control Room the fan had no status indication, and the associated dampers indicated closed. After review of electrical schematics, the Control Room staff determined that the thermal overload protection devices on the feeder breaker were likely tripped based on indication present on the dampers and the loss of status indication in the Control Room for the fan.
During subsequent troubleshooting, the CREVS 'B' train Control Room filtration fan was started and operated in the recirculation mode (without the 'B' train Control Room pressurization fan operating) for approximately 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and 22 minutes on 3/24/2013. After the CREVS 'B' train Control Room filtration fan was intentionally secured, the thermal overload heaters were replaced to ensure the overloads had not originally caused a nuisance trip. After replacement of the thermal overload heaters on 3/25/2013, the fan was started twice to allow for in-rush EMAX data and running EMAX data to be collected. The CREVS 'B' train Control Room filtration fan was operated successfully for approximately 6-7 minutes both times (once again without the 'B' train Control Room pressurization fan operating). About an hour later, the fan was restarted and operated in the CRVIS lineup (i.e. the 'B' train Control Room pressurization fan was also operating). Again, after 21 minutes of operation, the thermal overloads for the filtration fan tripped.
Later on 3/25/2013, the CREVS 'B' train Control Room filtration fan was again started for troubleshooting and testing. During this run, thermography was performed and revealed a localized hot spot with a temperature reaching as high as 240 degrees Celsius (464 degrees F) on the 'C' phase of the fan's starter.
This localized overheating occurred on the starter's 'C' phase line-side stationary contact termination. A recorder monitoring all three-phase currents during the testing indicated that approximately 18 minutes after the fan started, a condition occurred in which the 'C' phase current went to approximately zero amps while the other two phases experienced an increase in phase current. This condition lasted approximately 42 seconds before all three phase currents returned to their previous running current values. The fan was secured approximately 27 minutes after it had been started based on the discovery of the hot spot on the 'C' phase starter contact and the noted condition that occurred during the testing.
While the fan did not trip during this test, the Motor Control Center (MCC) cubicle door was open during this entire test, allowing some of the heat generated by the hot connection to dissipate to the surrounding environment instead of being strictly confined within the feeder breaker cubicle.
From subsequent inspection of the 'C' phase line-side stationary contact termination, it appeared that a loose screw was responsible for the local hot spot. There was evidence of arcing at the affected connection.
4. ASSESSMENT OF SAFETY CONSEQUENCES:
The CREVS filtration fans are important for maintaining post-accident Control Room habitability.
Specifically, the CREVS provides airborne radiological protection for the CRE occupants, as demonstrated by the CRE occupant dose analyses for the most limiting design-basis accident fission product release presented in the FSAR (Chapter 15A). By design, the worst-case single active failure of a component of the CREVS, assuming a loss of offsite power, does not impair the ability of the system to perform its design function. With only one CREVS filtration train affected by the identified condition (as explained further in Section 5 below), the CREVS was still able to perform its required function based on operability of the "A" train. Thus, the identified condition/event was not significant with regard to safety consequences.
5. REPORTING REQUIREMENTS:
This LER is submitted pursuant to 10CFR50.73(a)(2)(i)(B) to report a condition prohibited by the Technical Specifications.
The CREVS is subject to the requirements of Callaway Technical Specification (TS) 3.7.10, "Control Room Emergency Ventilation System (CREVS)." Per the Limiting Condition for Operation (LCO) of this Technical Specification, both trains of CREVS must be Operable during Modes 1, 2, 3, and 4 and during movement of irradiated fuel assemblies. With one CREVS train inoperable for reasons other than an inoperable CRE or Control Building envelope (CBE) boundary, Condition A applies and associated Required Action A.1 must be entered, which requires restoring the inoperable CREVS train to Operable status within the specified Completion Time of 7 days. With TS 3.7.10 Required Action A.1 and its associated Completion Time not met in Modes 1 through 4, Condition C applies such that Required Actions C.1 and C.2 must be entered, which direct the plant to be in Mode 3 in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in Mode 5 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />, respectively.
With the CREVS 'B' train being declared inoperable on 3/23/2013 at 2243, Required Action A.1 of TS 3.7.10 was entered with its 7-day Completion Time in effect. As noted previously, however, the motor starter for the CREVS 'B' train Control Room filtration fan was promptly replaced such that the CREVS 'B' train was restored to operable status on 3/26/2013 at 0659, i.e., within 56 hours6.481481e-4 days <br />0.0156 hours <br />9.259259e-5 weeks <br />2.1308e-5 months <br /> and 16 minutes.
In light of the as-found condition discovered on 3/23/2013, i.e., the degraded condition of the 'C' phase line-side stationary contact termination in the starter for the CREVS 'B' train filtration fan, a "past" operability determination was performed. The intent was to determine at what point the CREVS 'B' train became degraded to the extent that it would no longer be able to fulfill its 30-day mission time in response to a demand. Although there was no firm evidence to determine specifically when that occurred, it may be assumed that the overheating condition due to the loose screw at the termination for the fan starter developed over time due to a combination of thermal cycling and mechanical agitation. Prior to the trip on 3/23/2013, which occurred approximately 22 minutes after the Control Room filtration fan was started, the fan had been run on 3/5/2013 and 3/7/2013 for about 18 minutes each day. Although the fan did not trip during either of those two runs, there may have been insufficient time for enough heat to build up to trip the fan on thermal overload. Prior to this, on 2/11/2013, the fan was run for 91 minutes (in CRVIS lineup) without tripping. It is reasonable to assume that a steady-state temperature would have been reached in the cubicle within the 91-minute time duration when the fan was operating. Based on the fan operating for this length of time without its thermal overload devices tripping, there is reasonable assurance that the filtration fan was operable on or before 2/11/2013.
Review of control room logs and maintenance records for the CREVS 'A' train during the period of CREVS `B' train inoperability due to the identified condition, i.e., from 2/11/2013 until 3/26/2013, did not identify any concurrent periods of CREVS 'A' train inoperability. Consequently, the identified condition did not involve a condition that could have prevented fulfillment of a safety function.
However, based on the period of "past" inoperability of the CREVS 'B' train due to the identified condition (from 2/11/2013 to 3/26/2013), the CREVS `B' train was inoperable for a period of time longer than allowed by TS 3.7.10 (based on the Completion Times of Required Actions A.1 and C.1).
Consequently, the condition is required to be reported pursuant to 10 CFR 50.73(a)(2)(i)(B) as a condition or operation prohibited by the Technical Specifications.
6. CAUSE OF THE EVENT:
The most probable cause of failure is tripping on thermal overload, due to the high-resistance connection on the CREVS 'B' train Control Room filtration fan's starter. Specifically, localized overheating was found by thermography on the starter's 'C' phase line-side stationary contact termination. It is reasonable to assume that this connection in the starter loosened over time due to a combination of thermal cycling and mechanical agitation.
7. CORRECTIVE ACTIONS:
Corrective actions include adding instructions to the maintenance procedure for cleaning and inspection of motor control centers to require periodically checking the tightness of termination screws and inspecting for signs of overheating in similar starters.
8. PREVIOUS SIMILAR EVENTS:
A review of internal Operating Experience was performed, and no similar Operating Experience (in which the stationary contact termination screws in ITE, Gould, or Telemecanique starter/contactors were loose or causing overheating) was found. This specific event appears to be a random, isolated failure and thus is not considered to be a common cause issue or a single cause impacting multiple components.