05000395/LER-2013-005
| Virgil C. Summer Nuclear Station Unit 1 | |
| Event date: | 10-16-2013 |
|---|---|
| Report date: | 12-11-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(C), 50.54(x) TS Deviation 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 49411 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 3952013005R00 - NRC Website | |
1.0 EVENT DESCRIPTION Recent industry operating experience (OE 305419, EN 49411, EN 49419) regarding the impact of unfused Direct Current (DC) ammeter 1111 circuits in the Control Room described an event that could result in excessive current flow (heating) in the ammeter wiring to the point of causing a secondary fire in the raceway system. As a result of the OE, VCSNS performed a review of ammeter circuitry. On October 16, 2013, at 1315 hours0.0152 days <br />0.365 hours <br />0.00217 weeks <br />5.003575e-4 months <br />, VCSNS determined the described condition to be applicable to VCSNS resulting in an unanalyzed condition with respect to 10CFR50 Appendix R analysis requirements. The condition was reported in accordance with 10CFR50.72(b)(3)(ii)(B) as an unanalyzed condition (EN 49444) on October 16, 2013 at 1508 hours0.0175 days <br />0.419 hours <br />0.00249 weeks <br />5.73794e-4 months <br />.
VCSNS current wiring design for the ammeters contains a shunt in the current flow from each DC battery IBTRY] or DC battery charger IBYCJ. The ammeter wiring attached to the shunt does not contain fuses. It is postulated that a fire could cause one of the ammeter wires to hot short to ground. Concurrently, the fire could cause another DC wire from the opposite polarity on the same battery to also short to ground. This would cause a ground loop through the unfused ammeter cable. The potential exists that the cable could heat up causing a secondary fire in the ammeter raceway. The secondary fire could adversely affect safe shutdown equipment and potentially cause the loss of the ability to safely shutdown.
2.0 EVENT ANALYSIS To assess the extent of condition, the DC ammeter circuits were reviewed. The review included the batteries and battery chargers. The station's battery ammeters (XBA1A, XBA1B, and XBA1X) are local indications only and do not provide indications to the Main Control Board (MCB). Therefore, there is no ammeter wiring routed from the battery ammeters to the MCB. The ammeter wiring for the battery chargers (XBCIA, XBC1B, XBCIA-B, XBC I X, and X BC1X-2X) do provide MCB indications to monitor DC output.
During the apparent cause evaluation, the initial cause identified for the event was determined to be that the original design criteria did not specify protection for shunt circuits. The premise for this event to occur requires two concurrent low resistance grounds (one on the positive side of the charger and one on the negative side of the charger). The Battery System [ED] is designed to withstand one hard ground with no adverse effects. Double short to ground faults of opposite polarity were not considered in the design process.
3.0 SAFETY SIGNIFICANCE The mechanism for failure requires two separate hard (low impedance) grounds to occur: one on the positive side (either of two shunt leads) and one on the negative side. Compatible polarity multiple hot shorts on ungrounded alternating current (AC) and direct current (DC) circuits are discussed in NUREG/CR-6850, "Fire PRA Methodology for Nuclear Power Facilities." "Compatible polarity hot shorts for ungrounded AC and DC circuits are evaluated to be a low-likelihood event; however, sufficient data is unavailable to screen out this particular cable failure mode from consideration, based on the thresholds established in Section 2.5.6. Hence, the evaluation of hot shorts should in general consider this particular failure mode." It should be noted, however, that fuses are installed on the negative side, and that most (approximately 95 percent) are less than 15 amperes. If the fuse blows in the negative conductor, the circuit would open, eliminating the concern.
3.0 SAFETY SIGNIFICANCE (continued) Testing done by the NRC and industry and documented in NUREG/CR-7100, "Direct Current Electrical Shorting in Response to Exposure Fire," shows that "short-circuit faulting in the dc-powered cables often led to destructive damage to the cable conductors. That is, it was often observed that the arc formed during faulting was sufficient to sever a conductor.
"The effect observed was analogous to welding operations where the welding rods are consumed in the process." Thus in a fire, any postulated damage to a cable is expected to be in the fire exposed area rather than elsewhere in the cable run.
Based on the testing it is likely that the arc will sever one or the other shorts to ground. With either of these ground paths broken, the current flow will be terminated, and thereby eliminating the concern of overheating of the cables.
The cables in question are constructed of thermoset material. Testing performed for other stations in similar applications have determined that Institute of Electrical and Electronics Engineers (IEEE)-383-1974 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.
Self-ignited cable fires will be assumed to be implausible for thermoset cables. (References: NUREG/CR-6738, VCSNS Final Safety Analysis Report Section 8.3.3, "Fire Protection for Cable Systems," and VCSNS Fire Protection Evaluation Report Section 2.2.1, "Inventory of Combustibles.") 4.0 CORRECTIVE ACTIONS As part of the risk insights resulting from the analysis to transition from Appendix R to National Fire Protection Association (NFPA) 805, roving fire watches were instituted for certain Fire Areas/Zones. Except for Fire Area CB4, all of the Fire Areas/Fire Zones containing the circuit routing for the Battery Chargers were included in the existing fire watch areas. As an interim action, CB-4 was added to the existing fire watch list.
An action has been added to VCSNS corrective action program to implement a permanent solution for this condition as part of transitioning the plant from Appendix R to NFPA 805.
5.0 PREVIOUS OCCURRENCE There have been no previous occurrences in the last three years.