05000220/LER-2014-002

LER-2014-002, Unanalyzed Condition Due to Unfused Motor Operated Valve Control Circuit

Nine Mile Point Unit 1
Event date:	5-8-2014
Report date:	7-8-2014
Reporting criterion:	10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition
Initial Reporting
ENS 50098	10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition
2202014002R00 - NRC Website

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A. PRE-EVENT PLANT CONDITIONS:

Prior to the event, Nine Mile Point Nuclear Station Unit 1 (NMP1) was operating at rated thermal power.

B. EVENT:

On May 8, 2014 at 1645, NMP1 discovered that an un-fused control circuit associated with the Reactor Water Cleanup Isolation Valve 12 (IV-33-04) could short circuit due to a fire in the circuit cable routing. The short circuit could cause the cable to self-heat and cause secondary fires along the associated cable route.

The Reactor Water Cleanup Isolation Valve is powered from DC Valve Board 12 with a 50-ampere (50A) fuse to protect the motor circuit. The existing design protects the motor circuitry but the control circuitry would not be protected by the 50A fuse during some fault conditions. Should a fire occur in a fire area where the control circuits are located, the control circuit conductors could be susceptible to a newly discovered failure mode being called a DC ground equivalent hot short, which occurs when both the positive and negative control power wires short to a common ground allowing DC current to flow through the ground path. With a certain amount of resistance in the connection, the conductors would be damaged by excess current which the 50A fuse would not clear because it's inappropriately sized. The potential for cable heating and ignition outside of the fire area of concern is introduced if the fault condition is not interrupted before the breakdown of cable insulation. This condition could adversely affect other safe shutdown equipment and potentially cause the loss of the ability to conduct a safe shutdown in accordance with 10 CFR 50 Appendix R.

The determination regarding this unanalyzed condition resulted from an extent of condition review for the event described in LER 2013-002 which was expanded to included direct current (DC) motors. An event notification (EN # 50098) was made in accordance with 10 CFR 50.72 (b)(3)(ii)(B).

Nine Mile Point Unit 2 (NMP2) was reviewed and is unaffected by the condition of the un- fused control circuit for the Reactor Water Cleanup Isolation Valve at NMP1.

This event is documented in the plant's corrective action program as CR 2014-004630.

TO THE EVENT:

There were no inoperable systems, structures, or components that contributed to the event.

D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES:

1969 - Original Construction and Operation The original plant wiring design and configuration for the containment isolation valve did not include separate overcurrent protection for motive power and control wiring.

2012 - Issuance of NUREG 7100, Direct Current Electrical Shorting in Response to Exposure Fire (DESIREE-Fire): Test Results The DESIREE-FIRE test data revealed a newly observed failure mode in which multiple shorts to ground (from ungrounded systems) cause spurious operation in a circuit. This failure mode was identified as "ground fault equivalent hot short".

2013 - Issuance of NUREG 2128, Electrical Cable and Test Results Analyses During Fire Exposure (Electra-Fire) This industry guidance consolidated the results of three major test programs exploring electrical functionality of cables under fire conditions.

2014 - Industry Operating Experience (OE) Review/Consultation with Exelon Corporate Fire Protection Personnel An expanded extent of condition review was completed for the event reported in LER 2013-002. This effort included review of industry findings. The review was expanded from DC ammeter circuitry to now include DC motors (pump motors and MOV motors). The review identified valve IV-33-04, a reactor cleanup isolation valve powered from DC Valve Board 12 with a large fuse (50A) to protect the motor circuit.

E. OTHER SYSTEMS OR SECONDARY FUNCTIONS AFFECTED:

No systems or secondary functions were affected by this event. A postulated MOV control circuit fire could potentially damage safe shutdown equipment.

F. METHOD OF DISCOVERY:

This event was identified during a review of industry OE and consultation with corporate Exelon fire protection personnel regarding the potential for secondary fires being caused by DC Motor control circuit wiring.

G. MAJOR OPERATOR ACTION:

As a compensatory action, operations has initiated a fire inspection each shift to monitor the associated Fire Areas (1 and 10) until fuses are installed within the control circuitry of the MOV.

H. SAFETY SYSTEM RESPONSES:

No operational conditions requiring the response of safety systems occurred as a result of this condition.

II. CAUSE OF EVENT:

The cause of this event is that the equipment design issue was not recognized as an unacceptable configuration. The design issue is associated with an evolving industry understanding of the Appendix R common enclosure scope. Specifically, control circuits were not required to be fused in the original design of the plant nor included within the scope of Appendix R evaluation of common enclosure type circuits because the failure mode of "ground fault equivalent hot shorts" was not known to exist until NUREG-2128 was published in 2013.

III. ANALYSIS OF THE EVENT:

This event is reportable under 10 CFR 50.72 (b)(3)(ii)(B) and 10 CFR 50.73 (a)(2)(ii)(B) as any event or condition that results in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety.

There were no actual nuclear safety consequences associated with this event. The event was caused by equipment that did not include fusing of DC control circuits due to not understanding or anticipating a failure mode that had not been known to exist until NUREG- 2128, was published in 2013. There were no actual fires impacting any of the DC control circuits and as a result no actual challenge to the ability of the station to safely shutdown to cold shutdown conditions as required by 10CFR50, Appendix R.

The original design contained 50A fuses for protection of the motor circuit. However, they are inappropriately sized for control circuit cable protection (No. 12 American Wire Gauge (AWG) wire) under some fault conditions. Given the present wiring configuration, should a fire occur in a fire area where the control circuits are located, the control circuit conductors could be susceptible to a newly discovered failure mode being called a DC ground equivalent hot short, which occurs when both the positive and negative control power wires short to a common ground, allowing DC current to flow through the ground path. With the correct amount of resistance in a connection, conductors would be damaged by excess current and the 50A fuse would not open since it is too large. If the fault condition is not interrupted before the breakdown of cable insulation, the potential for self-heating and damage to other cables in associated trays is possible. Based on the extent of condition, for a postulated fire event, the control circuit of IV-33-04 had the correct fusing on the motor control circuitry but lacked adequate fusing on the MOV control circuit.

Currently, a fire inspection each shift has been implemented as a preventative measure to address a scenario involving a fire event. This measure will remain in affect until fuses are installed.

Based on the above discussion, it is concluded that the safety significance of this event is low and the event does not pose a threat to the health and safety of the public or plant personnel.

This event does not affect the NRC Regulatory Oversight Process (ROP) Index items.

IV. CORRECTIVE ACTIONS:

A. ACTION TAKEN TO RETURN AFFECTED SYSTEMS TO PRE-EVENT NORMAL

STATUS:

Operations has initiated a fire inspection each shift to monitor the associated Fire Areas (1 and 10) until fuses are installed within the control circuitry of the MOV.

B. ACTION TAKEN OR PLANNED TO PREVENT RECURRENCE:

Install fuses for IV-33-04 Control Circuit.

A. FAILED COMPONENTS:

There are no failed components associated with this event.

B. PREVIOUS LERs ON SIMILAR EVENTS:

On October 22, 2013, NMP1 identified that un-fused ammeter indication circuits associated with safety related direct current buses could short circuit due to a fire in the circuit cable routing. This ground fault equivalent hot short could cause the cable to self-heat and lead to secondary fires. The unanalyzed secondary fires could adversely affect safe shutdown equipment and potentially cause the loss of the ability to safely shutdown as required by 10 CFR 50, Appendix R. The condition was reportable in accordance with 10 CFR 50.73 (a)(2)(ii)(B) as a condition that resulted in the nuclear plant being in an unanalyzed condition that significantly degraded plant safety, and 10 CFR 50.73(a)(2)(ix)(A) as a condition that as a result of a single cause could have prevented the fulfillment of a safety function for two or more trains or channels in different systems.

C. THE ENERGY INDUSTRY IDENTIFICATION SYSTEM (EllS) COMPONENT FUNCTION

COMPONENT

Reactor Water Cleanup System Isolation Valve IEEE 803 FUNCTION IEEE 805 SYSTEM

ISV CE

Fuse FU CE

D. SPECIAL COMMENTS:

None