05000287/LER-2016-001

LER-2016-001, 1 of 3

Oconee Nuclear Station, Unit 3
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
2872016001R00 - NRC Website

LER 16-001-00 for Oconee, Unit 3, Regarding Reactor Building Cooling Unit Inoperability Exceeds Technical Specification Completion Time

ML16250A009
Person / Time
Site:	Oconee
Issue date:	08/26/2016
From:	Batson S L Duke Energy Corp
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
ONS-2016-077 LER 16-001-00
Download: ML16250A009 (6)

Background

TS 3.6.5, "Reactor Building Spray and Cooling Systems" credits a combination of Reactor Building Cooling Units (RBCUs) and Reactor Building Spray (RBS) to ensure the reactor building remains below its design pressure and temperature in post-accident conditions. This event is reportable because the discovered rolled leads provides firm evidence that the 3C RBCU was inoperable for a duration that exceeded the TS allowed Completion Time. The inoperability also existed during plant start-up such that the plant entered the TS 3.6.5 modes of applicability without meeting the Limiting Condition for Operation (LCO). These conditions constitute operation prohibited by TS and are reportable per 10 CFR 50.73(a)(2)(i)(B).

The RBCU and RBS systems [BK] are Engineered Safeguards systems. They are designed to ensure that post-accident heat removal capability can be attained to limit and maintain the post-accident conditions to less than the containment design values.

The RBS system consists of two separate trains of equal capacity, each capable of meeting the design basis.

The RBCU system consists of three RBCU trains. Each cooling train is equipped with cooling coils, and a fan driven by a two-speed electric motor. During normal unit operation, typically two reactor building cooling trains operating at low or high speed cool the containment atmosphere. The third train is usually on standby. Upon receipt of an emergency signal, any operating trains will automatically trip, then all trains will start in low speed after a 3 minute delay. Low speed operation during post-accident conditions prevents motor overload from the higher density atmosphere. Analysis results of RBCU train performance for post-accident conditions supports that any combination of two trains can provide 100% of the required cooling capacity during the post-accident conditions.

EVENT DESCRIPTION

On June 28, 2016, a Unit 3 Reactor Building entry was made to investigate anomalies in the RBCU inlet temperature readings. Specifically, the inlet temperature to the 3C RBCU were reading low.

Maintenance and Engineering determined that the 3C RBCU fan was running in the reverse direction in low speed. Based on this discovery, the 3C RBCU was declared inoperable and Condition B of TS 3.6.5 was entered (a 7 day Completion Time).

An onsite investigation team determined that, during the preceding outage, the RBCU motor control center (MCC) contactor leads were inadvertently "rolled" during routine maintenance. The rolled leads created an incorrect order of the electrical phase connections to the motor, which changed the rotational direction of the motor. The terminations were corrected and the fan was observed and tested to verify proper operation. TS 3.6.5 was exited on June 29, 2016.

The determination that the error (rolled leads) occurred during the outage constitutes firm evidence that the RBCU was inoperable while the plant was operating in Mode 2 or higher for approximately 45 days before the condition was corrected. The maximum Completion Time allowed by TS 3.6.5 for one inoperable RBCU is 7 days, after which Condition D would be entered. Condition D allows an additional 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> to be in Mode 3.

Also, because TS 3.0.4 is applicable to TS 3.6.5 for Unit 3, entry into a Mode of applicability for TS 3.6.5 without meeting the LCO is prohibited.

Therefore, entering a Mode of applicability (Modes 2 and 1) with the 3C RBCU inoperable is prohibited by TS 3.0.4, and the span of a 45 day inoperability is prohibited by TS 3.6.5.

During the 45 day span, the other two RBCU trains remained operable. Also both RBS trains remained operable, except for two occasions when a RBS train was declared inoperable to facilitate pump and valve testing. The duration of each RBS inoperability was less than the 24-hr Completion Time allowed by TS 3.6.5, Condition C which applies to the inoperability of one RBS train and one RBCU train.

CAUSAL FACTORS

The cause is attributed to human error in the technician's improper application of the configuration control process.

A contributing factor was that a recent change to the maintenance procedure for this activity did not integrate the maintenance standard for configuration control.

CORRECTIVE ACTIONS

Immediate The 3C RBCU terminations were restored and returned to service.

Result of Cause Analysis 1. The technicians involved received remedial training/counseling from supervision.

2. A procedure change has been initiated to enhance configuration control for the lifted lead aspects in the maintenance procedure for this activity.

SAFETY ANALYSIS

A probabilistic risk assessment (PRA) evaluation was conducted for the period of RBCU inoperability to determine the significance of this event. The Oconee PRA model credits the 3A, 3B and 3C RCBUs to operate on low speed for containment heat removal. The analysis considered internal events, high winds, flood and fire risk. Due to the multiple failures of other equipment that must occur in order for an 3C RBCU failure to be of any consequence, the PRA analysis determined that the increase in core damage and large early release frequencies due to a failure of 3C RBCU are negligible. Therefore, the risk impact associated with this event report is well below any risk significance threshold.

ADDITIONAL INFORMATION

Similar events for the preceding three year period:

the improper positioning of an electrical sliding link. No other LERs were found to be caused by similar personnel errors.

Energy Industry Identification System (EIIS) codes are identified in the text as [XX].