05000483/FIN-2015002-03
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Finding | |
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| Title | Inverter NN11 Inadvertently Transferred to its Alternate AC Source |
| Description | On June 9, 2014, the Callaway Plant was in Mode 1 operating at 100 percent rated thermal power when, during a maintenance activity, inverter NN11 unexpectedly transferred from its normal direct current (dc) source to its bypass alternating current (ac) source. This inverter provides power to the NN01 bus which is one of four vital 120 Vac instrument buses at the Callaway Plant. The transfer of inverter NN11 to its bypass source was caused by a momentary loss of power to bus SB038 which supports instrumentation and controls for systems such as the reactor trip system and the engineered safety feature actuation system. This momentary loss of power caused the following plant impacts:
Control rod insertion 612 steps, with an associated pressurizer level and pressur perturbation and subsequent Xenon transien Opening of valve BNLCV0112D, centrifugal charging pump A suction fro refueling water storage tank isolation valve, due to momentary loss of th associated volume control tank level channe Actuation of the steam generator environmental allowance modifier circuit resulting in resetting of the low level setpoint trip from 17 to 21 percent narro range leve Numerous momentary partial trip actuation The NRC inspectors responded to the control room and verified that the plant system responded as designed and that the operators stabilized the plant in accordance wit plant procedures Investigation identified a loose mounting screw that secures disconnect switch NN01-11 to NN01. Maintenance work in the area of the loose termination led to a momentary interruption of power to cabinet SB038, which appeared as a fault condition to the inverter, thus causing the inverter to transfer to its alternate power source. The cabinet, bus, and inverter are seismically qualified and are required to be capable of performing their design basis accident functions following a safe shutdown earthquake. With the degraded electrical termination, which existed for an extended period of time before discovery and repair, the inverter and SB038 loads would not have been capable of performing their design basis function following a safe shutdown earthquake, thus rendering the components inoperable. The direct cause of this event was inadequate thread engagement of the screw securing disconnect switch NN01-11 to the NN01 bus. However, the presence of threads in the switch mounting hole (which is not intended to engage with the bus bar termination screw) introduced the potential for binding during screw installation. The detail of this mounting configuration is not identified on plant drawings of the cabinet or switch provided by the vendor and nothing in the work control process required a detailed comparison of the switch to the work procedures and, as such, it was reasonable that this potential vulnerability was not identified and addressed in the procedure or pre-job walkdown. During the actual installation of the screw, the screw appeared flush and tight with the switch mounting board, meeting the requirements of the work package. The equipment was successfully post-maintenance tested and technical specification surveillance tested for a period of 6 years. There was also no industry or vendor operating experience describing this vulnerability. Based on this information, the inspectors concluded that no performance deficiency existed since it was not reasonable for Callaway Plant personnel to foresee and correct this condition. The licensees root cause analysis determined that the root cause of the event was that work instructions did not include direction to remove the back panel cover of the cabinet to support alignment and thread engagement of the mounting screws during switch installation. Corrective actions taken by the licensee included changes to job planning aids and the maintenance procedures associated with the replacement of this type of switch. The inspectors determined during their review of Licensee Event Report 2014-003-01 that traditional enforcement applies in accordance with Inspection Manual Chapter 0612, Appendix B, Figures 1 and 2, Issue Screening, Inspection Manual Chapter 0612, Section 9, and NRC Enforcement Policy, Section 2.2.4.d, because a violation of NRC requirements existed without an associated Reactor Oversight Process performance deficiency. This issue is considered to be a Severity Level IV violation of 10 CFR Part 50, Appendix B, Criterion V, Instructions, Procedures, and Drawings, based on a conservative bounding evaluation performed using Callaways SPAR model which determined the condition was of very low safety significance (Green) and was similar in significance to NRC Enforcement Policy example 6.1.d.2. This issue was entered into Callaway Plants corrective action program as Callaway Action Request 201403898. Licensee Event Report 2014-003-01 was submitted pursuant to 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by Callaway Technical Specification 3.8.7, Inverters Operating, based on the period of past inoperability of the NN11 inverter and SB038 loads. The inspectors reviewed the licensees submittal and determined that the report included the potential safety consequences and necessary corrective actions, but it did not thoroughly document the event, in that the effects on the plant from the inverter transfer to its alternate ac power source were not described. The licensee entered the licensee event report completeness issue into their corrective action program as Callaway Action Request 201504217 and initiated a corrective action to submit a revision of the licensee event report at a later date. Because it was not reasonable for the licensee to have been able to foresee and correct the condition that caused the switch failure, the NRC determined that no performance deficiency existed. Thus, the NRC is exercising enforcement discretion in accordance with Section 3.5 of the NRC Enforcement Policy and is not issuing enforcement action for the violation (EA-15-152). Further, because the licensees action and/or inaction did not contribute to this violation, it will not be considered in the assessment process or the NRCs reactor oversight process action matrix. This licensee event report is closed. These activities constitute completion of one event follow-up sample, as defined in Inspection Procedure 71153. |
| Site: | Callaway  |
|---|---|
| Report | IR 05000483/2015002 Section 4OA3 |
| Date counted | Jun 30, 2015 (2015Q2) |
| Type: | Violation: Severity level Enforcement Discretion |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71153 |
| Inspectors (proximate) | P Elkmann T Hartman T Pruett M Poston-Brown G Guerra M Brooks M Langelier |
| Violation of: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications Technical Specification - Procedures 10 CFR 50.73 Technical Specification |
| INPO aspect | |
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Finding - Callaway - IR 05000483/2015002 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Callaway) @ 2015Q2
Self-Identified List (Callaway)
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