05000410/LER-2013-003

LER-2013-003, Unanalyzed Condition Caused by Unfused Control Room DC Ammeters

Nine Mile Point Unit 2
Event date:	10-22-2013
Report date:	12-20-2013
Reporting criterion:	10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition
Initial Reporting
ENS 49464	10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition
4102013003R00 - NRC Website

A. PRE-EVENT PLANT CONDITIONS:

Prior to the event, Nine Mile Point Unit 2 (NMP2) was operating at rated reactor power.

B. EVENT:

On October 22, 2013, it was discovered that six unfused ammeter indication circuits associated with the NMP2 Safety related Direct Current (DC) 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.

Fire patrols were established as a compensatory measure, and fire suppression and detection systems were confirmed functional in the affected areas. Once the six affected circuits were deenergized by lifting of leads at the DC bus, the fire patrols were suspended. Deenergizing the circuits at the DC bus ensures that if a fire in one of the cable routing areas were to occur, secondary fires would not occur due the unfused circuits.

The six unfused safety related ammeter circuits at NMP2 do not affect Nine Mile Point Unit I.

An event notification was made in accordance with 10 CFR 50.72(b)(3)(ii)(B) for the unfused DC ammeter circuits in the control room resulting in an unanalyzed condition on October 22, 2013 at 1928 (EN# 49464).

EVENT:

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

D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES:

Original Contruction and Operation Original construction standards did not recommend installation of fuses in the associated DC ammeter circuits. IEEE 384, "Standard Criteria for Separation of Class 1E Equipment and Circuits," which was the industry standard at the time, did not require an "isolation device" on the ammeter wiring, as long as the circuits were considered "associated" with the safety train. Although the ammeters were considered "instruments," they were bolted to a power conductor leading to the conclusion that they were part of the safety train.

2013 Issuance of NUREG-2128, Electrical Cable Test Results and Analysis During Fire Exposure (ELECTRA-FIRE) This NUREG consolidated the results of three major test programs exploring electrical functionality of electrical cables under fire conditions. Included was the Nuclear Regulatory Commission (NRC) Direct Current Electrical Shorting in Response to Exposure Fire (DESIREE-FIRE) program. 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 Davis Besse Operating Experience (OE) 305419 This OE identified that, during a fire, DC ammeter circuits without protective fusing could short to ground and with the proper polarity it was possible to cause high current in these unfused legs of small diameter cabling with the potential to cause secondary fires. These secondary fires could then result in damage to adjacent cabling.

E. OTHER SYSTEMS OR SECONDARY FUNCTIONS AFFECTED:

A postulated control room fire could lead to the shorting event occurring in the ammeter circuits, which subsequently could lead to additional fires in both Division 1 and 2 cable chases and the divisional DC Switchgear rooms challenging systems required to support safe plant shutdown.

F. METHOD OF DISCOVERY:

On October 22, 2013, review of industry OE regarding the impact of unfused DC ammeter circuits in the control room determined that the described condition was applicable to NMP2.

G. MAJOR OPERATOR ACTION:

Fire patrols were established as a compensatory measure in affected areas of the unit.

H. SAFETY SYSTEM RESPONSES:

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

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, ammeter 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, 'ELECTRA-FIRE' was published in 2013.

This event has been entered into the Nine Mile Point Nuclear Station corrective action program as condition report number CR-2013-008749.

III. ANALYSIS OF THE EVENT:

This event is reportable in accordance with 10CFR50.73(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.

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

The configuration of the DC ammeter circuits has remained unchanged from original construction at NMP2 and as such is a legacy issue. The original design did not include any protection (i.e. fuses) for the ammeter circuit. The original guidance (Generic Letter 81-12, "Fire Protection Rule," and later Generic Letter 86-10 "Implementation of Fire Protection Requirements") for completing a post-fire Safe Shutdown analysis did not identify the need for the evaluation of DC ammeters as they were not relied upon to bring the unit to a Cold Shutdown state. Hence they were not analyzed for the original Appendix R Safe Shutdown analysis.

Guidance used as part of the transition to Multiple Spurous Operation (RG 1.189, Revision 2) at NMP2, mainly NEI-00-01, "Guidance for Post-Fire Safe Shutdown Analysis" and RIS-2005-30, "Clarification of Post-Fire Safe-Shutdown Regulatory Requirements" does not identify the need for the evaluation of the ammeter circuits. The methodology described in these documents requires the evaluation of circuits which are part of the required Safe Shutdown train of equipment, as well as any circuits which could cause spurious actuations of equipment — equipment not part of the required train of Safe Shutdown equipment which could affect the units ability to safely shutdown. As these ammeter circuits are indicating circuits only and could not cause spurious actuation of any equipment, they were not evaluated under this guidance and were not evaluated for the upgrade of the Safe Shutdown analysis for the transition the to Multiple Spurious Operations.

Currently, the safety related DC ammeter circuits have been deenergized at the DC buses preventing this postulated scenario from being able to occur.

Based on the above discussion, it is concluded that the safety significance of this event is low and the event did 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:

1. The six unfused safety related DC ammeter circuits have been deenergized at the DC buses preventing this postulated scenario from being able to occur.

B. ACTION TAKEN OR PLANNED TO PREVENT RECURRENCE:

1. A modification will be developed, issued and installed to provide adequate fuse circuit protection to preclude the postulated event from occurring at NMP2.

A. FAILED COMPONENTS:

There were no other failed components that contributed to this event.

B. PREVIOUS LERs ON SIMILAR EVENTS:

Plant submitted LER 2013-004 (Revision 1) on November 13, 2013 for an unanalyzed condition postulated with a fire and resulting hot short that has a similar cause. There have been no previous similar LERs for NMP2.

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

COMPONENT

Ammeter

D. SPECIAL COMMENTS:

None IEEE 803 FUNCTION IEEE 805 SYSTEM