05000461/LER-2011-007
| Clinton Power Station, Unit 1 | |
| Event date: | 12-08-2011 |
|---|---|
| Report date: | 01-27-2012 |
| Initial Reporting | |
| ENS 47374 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 4612011007R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric — Boiling Water Reactor, 3473 Megawatts Thermal Rated Core Power Energy Industry Identification System (El IS) codes are identified in the text as [XX].
EVENT IDENTIFICATION
Unfused DC Ammeter Circuits Result in Unanalyzed Condition
A. Plant Operating Conditions Before the Event
Unit: 1 Event Date: 12/8/11 Event Time: 1230 hours0.0142 days <br />0.342 hours <br />0.00203 weeks <br />4.68015e-4 months <br /> CST Mode: 5 Mode Name: Refueling Reactor Power: 0 percent
B. DESCRIPTION OF EVENT
During Clinton Power Station's (CPS) review of event ENS 47374, Deficiency Identified in Transition to Performance Based Standard for Fire Protection, CPS identified a similar condition. The station's review determined that the original plant wiring design for the station battery [BTRY] ammeter [II] circuits contains a shunt in the current flow from each direct current (DC) battery. Bolted onto the shunt bar are two Institute of Electrical and Electronics Engineers (IEEE) IEEE-383, Standard for Type Test of Class 1 E Electrical Cables, Field Splices, and Connections for Nuclear Power Generating Stations, qualified leads to an ammeter in the main control room (MCR). The small difference in voltage between the two taps on the shunt is enough to deflect the ammeter in the MCR when current flows from the battery through the shunt. The ammeter wiring attached to the shunt does not have fuses, and if one of the ammeter wires shorts to ground at the same time as another DC wire from the opposite polarity on the same battery also shorts to ground, a ground loop through the unfused ammeter cable could occur. With enough current going through the cable, the potential exists that the overloaded ammeter wiring could damage other wiring in direct physical contact with the cable and result in a loss of the associated safe shutdown function/capability.
Review of the CPS design identified a similar condition to the condition identified in the operating experience review, since no protective devices (fuses or breakers) are installed off the shunts to the MCR DC ammeters. The DC distribution system [EJ] is a floating system with no established ground other than the high resistance ground provided through the ground detector. It is designed in this manner to allow the system to withstand one hard ground (zero resistance) with no adverse effect on the operation of any equipment fed from the DC system. If a second ground with low enough resistance is placed on the DC system, the operation of the grounded equipment could be adversely affected. The two ground paths could short around components to prevent them from operating or provide a current path to spuriously operate a piece of equipment.
The condition identified is contrary to the CPS Updated Safety Analysis Report which states that there are no associated non safe shutdown cables that are not electrically protected and share a common enclosure with safe shutdown cables.
Compensatory fire watch measures have been implemented for the affected areas in the plant and will remain in effect until the deficiency is corrected.
C. CAUSE OF EVENT
Review of industry standards for DC Auxiliary Power Systems identified that the design of this system meets all general design criteria contained in Institute of Electrical and Electronics Engineers (IEEE) Standard IEEE-484, Recommended Practice for Installation Design and Installation of Large Lead Storage Batteries for Generating Stations and Substations, and IEEE-946, Recommended Practice for the Design of DC Auxiliary Powered Systems for Generating Stations. These design requirements do not specify protection for shunt fed ammeter circuits. The design standard used at CPS for the DC system does not specify fusing for the ammeter shunt circuit and references IEEE-946. The premise for this event to occur requires two concurrent extremely low resistance (hard) grounds (i.e., one on the positive side of the battery through the ammeter circuit wiring routed to the control room and one on the negative side of the battery through a second ground on the same battery system). The DC distribution system is floating with no established ground other than the high resistance ground provided through the ground detector [GDET]. It is designed in this manner to allow the system to withstand one hard ground (zero resistance) with no adverse affect on the operation of any equipment fed from the DC system. Since this is a very low probability event, it was likely not considered during the development of the related IEEE standards, which are typically developed and refined based on industry experience.
D. SAFETY CONSEQUENCES
This event is reportable in accordance with 10 CFR 50.73 (a)(2)(ii)(B) as a condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety in that the overloaded ammeter wiring could damage other wiring in physical contact with the cable and result in a loss of the associated safe shutdown function/capability.
There were no actual consequences for this event. An extent of condition review determined that this issue applies to the Division 1, 2 and 4 DC ammeter circuit wiring. The Division 3 DC system is not affected because it does not contain a remote MCR ammeter circuit.
Testing performed for other stations in similar applications have determined that IEEE-383 rated wiring sized for this application is not susceptible to self ignition. Industry experience discussed in NUREG/CR-6738, Risk Methods Insights Gained from Fire Incidents, supports that self ignition and fire propagation of IEEE-383 rated wiring is highly unlikely.
E. CORRECTIVE ACTIONS
Compensatory fire watch measures have been implemented for the affected areas in the plant and will remain in effect until the deficiency is corrected.
A modification is planned to correct the design deficiency.
F. PREVIOUS OCCURRENCES
A review was performed and it was determined that no similar conditions have occurred in the past.
G. COMPONENT FAILURE DATA
None