05000250/FIN-2017008-03
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Finding | |
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| Title | Potential Failure to Implement Adequate Foreign Material Exclusion Control |
| Description | Inspection Scope The team reviewed licensee documents, performed walk downs associated with the safety -related 3A 4kV switchgear located inside room 071, and interviewed licensee personnel to determine the conditions leading up to the internal bus fault event on the morning of March 18, 2017. The documents reviewed included procedures, work orders, drawings of floor plans, one line diagrams, specifications, correspondence, photographs, licensees NRC Inspection Team Briefing document, and Root Cause Charter description AR 02192198. b. Findings and Observations The team initiated the review by performing a walk down of the 3A 4kV switchgear room to establish an understanding of the conditions inside the room that may have affected the 3A 4kV switchgear. The room , which was significantly smaller than the 3B 4kV switchgear room, provided minimally adequate access around the equipment, such as the switchgear , motor control center s (MCC s), a sequencer panel, a sump pump, and floor mounted air handling units. The current limiting reactor (CLR) , or reactor coil, associated with the event was located in section 3AA06 of the 3A 4kV switchgear. The front of this section is across from a room air handler unit, which directs its air towards the ventilation louvers in the CLR section. The team interviewed members of the licensees failure investigation process team and determined their evaluation of the potential causes for the failure of the reactor coil included: Bus fault in reactor coil cubicle 3AA06 Failed insulator in cubicle 3AA06 19 Fault in reactor coil Bus fault external to the 3AA06 cubicle Load fault with failure to isolate Magnetic properties of the reactor coil interacting with erected scaffold. 3AA06 side panels pushed in from outside reducing air gap Foreign material from internal and/or external sources Bolts installed with nuts facing towards grounded surfaces. Large quantities of conductive dust suspended in air from sweeping prior to fault Each of the potential causes were dismissed for lack of any evidence with the exception of those issues that would have contributed to a reduction in the air gap between uninsulated busses and ground surfaces. The installation of the Thermo-Lag was in progress just prior to the bus fault and according to statements from the installing contractor personnel, they had just exited the room to prepare to go to lunch and had been cleaning up the space before leaving. One of the workers had gone back into the room to check on one last item when the bus fault occurred and suffered injuries as a result of the explosion. Based on interviews and photographs provided, it was determined that the mesh, used to make up the joining pieces of insulation, was conductive. That mesh material was also light weight and made out of carbon fiber. The protective relays operated as expected for almost all components , including the 174/TDO relay in the trip circuit that operated the lockout relay, which in turn opened all the breakers in the 3A 4kV switchgear bus. The lockout relay operation prevented the 3A EDG from closing in on the 3A 4kV switchgear bus. The loss of the bus initiated a loss of steam flow on the turbine. The Unit 3 turbine and generator were motoring for approximately 30 seconds with the transmission system experiencing power swings associated with the loss of the main generator. After 30 seconds, the Unit 3 generator 286/G3 lockout tripped followed by the switchyard breakers opening and isolating the generator in 1.8 cycles. The reactor coil separates the high and low sides of the 3A 4kV switchgear bus. The high side, which was upstream of the reactor coil, had a higher withstand capability for short circuits that the low side of the switchgear bus. There is a slight difference between the overcurrent relays for phases A and B compared to phase C. Tracings provided with the details of current and voltage conditions prior to, during , and after the bus fault reveal an increase in the fault current of phase C preceding the increase in phase A. Photographs of the effects of the bus fault indicated an initial arc located next to what appeared to be phase C bus. However, the target flags in the overcurrent relay s failed to indicate a phase C trip. The entire overcurrent protection system worked as expected except for the delay on the phase C components. The team reviewed procedures and methods prescribed by the licensee to control foreign material contamination. A number of the methods indicated included cutting the Thermo-Lag material outside the switchgear room approximately 15ft from the east door to the room. Some of the final cutting and trimming of the carbon fiber mesh was done inside the switchgear room on top of the scaffolding, which had been fitted with Grifflon net to protect from foreign material particles. In addition, a Pearl Weave material was 20 used to protect against falling objects to the space below. The team was able to confirm a number of these methods used by the conditions of the space during the walk down of the room and the interview transcripts provided by the licensee of the Thermo- Lag installation personnel. However, these methods appear to cover larger pieces of material that would be appropriately captured by the Pearl Weave or the Grifflon but not the smaller pieces of carbon fiber mesh that could become airborne and migrate around the room. The only apparent control provided for airborne particulate would be the air filter in the air handling unit. This would require the material to be at an elevation low enough to get sucked in by the air return at the bottom of the air handler. Any material suspended in air would be blown out from the air handler and potentially be blown through the louvers in the reactor coil cabinet. Overall, the team concluded that the licensee was taking appropriate actions to evaluate the potential causes for the failure of the 3A 4kV bus. The most likely potential causes of the event involve the introduction of foreign material into the switchgear as well as the configuration and design of the switchgear. Additional review of information related to these potential causes will be required following the conclusion of the licensees root cause evaluation, which had not yet been completed at the time of the inspection. Therefore the team opened two URI s as documented below. i. URI 05000250, 251/2017008- 03, Potential Failure to Implement Adequate Foreign Material Exclusion Controls Introduction: The team identified an URI associated with the licensees potential failure to properly control the spread of airborne particulates generated from the installation of the Thermo-Lag insulation material on cable trays and conduits inside the 3A switchgear room. Description : The documentation provided to install the Thermo-Lag insulation was prescribed in work order 40464284- 03, EC 283459 Install T -Lag of MCC -3B Power Cables in 3A SWGR , dated the 10th of March 2017. This work order refer red to procedure MA -AA- 101- 1000, Foreign Material Exclusion Procedure, for job supervisor to review and approve the foreign material exclusion ( FME ) controls under item 2.3. The supervisor signature was provided on the 17 th of October 2016 for this particular task. However, the signature date was prior to this work order issue date. Section 4.3 of the FME procedure in paragraph 10 stated that , Special precautions need to be taken when work activities (spray painting, sand blasting, grinding, cutting, welding, insulating, chemical cleaning etc.) may generate airborne dust, debris or chemical fumes that could be introduced into operating plant equipment such as motors, switchgear, control panels and electrical cabinets . In addition, section 4.5.1 , Electrical Cabinets , paragraph 1 , directed personnel to visually inspect the surrounding area, particularly overhead, for potential sources of foreign material and to note any nearby ventilation system that may introduce foreign material into the cabinet. In paragraph 2, it indicated that , Where practical, covers should be installed on open electrical enclosures, cabinets, and boxes required to be left open by procedure, plant operations, or maintenance . Section 4.5.2, Switchgear , directed the personnel to follow the measures identified above. In addition, the conductivity of this mesh may have played a significant factor in the resulting bus fault when it migrated into the reactor coil cabinet through the open louvers and formed a low impedance path from the exposed phase C bus to the metal enclosure of the cabinet. Pieces of the black mesh were discovered inside the reactor 21 coil insulated windings, which indicated an absence of screening material or a means to block foreign material migration into the inside of the reactor coil cabinet with its exposed busses. Procedure 0- GMP -102.21, Installation, Modification and Maintenance of Thermo-Lag Fire Barrier Systems , did not contain an engineering evaluation of the carbon fiber mesh used with the system installed inside the 3A 4kV switchgear room. Material safety data sheet (MSDS -0012821) from Cytec Engineered Materials with product name Thornel Pan Based Standard Modulus Carbon Fiber provided a hazard identification of Electrically Conductive Fibers Airborne fibers can short circuit electrical equipment . This URI was initiated to further review the environment created during the installation of the Thermo-Lag in 3A 4kV switchgear room. This environment may have contributed to a degraded isolation of exposed medium voltage bus bars inside the reactor coil cabinet . Following the completion of the licensees root cause evaluation, inspectors will determine whether performance deficiencies exist ed related to the licensees evaluation of the carbon fiber mesh and the foreign material exclusion controls in effect at the time of the event. (URI 05000250, 251/2017008- 03, Potential Failure to Implement Adequate Foreign Material Exclusion Controls) |
| Site: | Turkey Point  |
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| Report | IR 05000250/2017008 Section 4OA5 |
| Date counted | Mar 31, 2017 (2017Q1) |
| Type: | URI: Green |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 2201/004 |
| Inspectors (proximate) | E Stamm G Crespo J Patel J Reyes L Suggs N Mellyj Orrj Reyes N Hobbs R Carrion A Butcavage T Morrissey J Patel A Wilson |
| CCA | H.1, Resources |
| INPO aspect | LA.1 |
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Finding - Turkey Point - IR 05000250/2017008 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Turkey Point) @ 2017Q1
Self-Identified List (Turkey Point)
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