05000440/LER-2013-004
| Perry Nuclear Power Plant, Unit 1 | |
| Event date: | 10-17-2013 |
|---|---|
| Report date: | 12-09-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 4402013004R00 - NRC Website | |
Energy Industry Identification System (EIIS) codes are identified in the text as [XX]
INTRODUCTION
On October 17, 2013, at 1300 hours0.015 days <br />0.361 hours <br />0.00215 weeks <br />4.9465e-4 months <br />, a review of industry operating experience regarding the impact of unfused Direct Current (DC) ammeter [II] circuits in the control room determined the described condition to be applicable to the Perry Nuclear Power Plant (PNPP) resulting in a potentially unanalyzed condition with respect to 10 CFR 50 Appendix R analysis requirements. The original plant wiring design and associated safe shutdown analysis for the Class 1 E batteries [EJ] control room ampere indications do not include overcurrent protection features to limit the fault current.
In the postulated event, a fire in the control room could cause one of the ammeter wires to short to the ground plane. Simultaneously, it is postulated that the fire causes another DC wire from the opposite polarity on the same battery to also short to the ground plane. This could cause a ground loop through the unprotected ammeter wiring, resulting in excessive current flow (i.e., heating) in the ammeter wiring to the point of causing a secondary fire in the raceway system. The secondary fire could adversely affect safe shutdown equipment and potentially cause the loss of the ability to conduct a safe shutdown as required by 10 CFR 50 Appendix R. Interim compensatory measures (i.e., fire watches) were implemented for affected areas of the plant at 1530 hours0.0177 days <br />0.425 hours <br />0.00253 weeks <br />5.82165e-4 months <br /> on October 17, 2013.
The unanalyzed condition was reported to the NRC at 1625 hours0.0188 days <br />0.451 hours <br />0.00269 weeks <br />6.183125e-4 months <br /> on October 17, 2013, in accordance with 10CFR50.72(b)(3)(ii)(B); reference ENF Notification No. 49446. This event report is being submitted in accordance with 10CFR50.73(a)(2)(ii)(B).
At the time of event discovery the PNPP was in Mode 1 at 100% power.
EVENT DESCRIPTION
During PNPP review of industry operating experience regarding unfused DC ammeter [II] circuits that could adversely impact Appendix R safe shut down equipment a similar condition was identified. The review identified DC ammeters associated with Class 1 E batteries [BTRY] that were not fused to provide overcurrent protection.
Five Control Room ammeter circuits, absent of overcurrent protection, were identified with the potential to result in heating and/or a secondary fire along the circuit routing of the ammeter cables which could cause damage to adjacent cables that may be credited for safe shutdown. Three of the ammeter circuits are associated with Unit 1 Divisional 125 volts DC (VDC) Class 1E systems [EJ] and two are associated with the Unit 2 Divisional 125 VDC Class 1 E systems [EJ].
The design for these Control Room DC ammeters for the Class 1 E Divisional batteries is consistent with common industry design for ammeter indications from DC buses. The design contains a shunt in the current flow from each battery. This shunt produces a voltage difference between the two taps of the shunt when current flows from the battery through the shunt. Bolted to the shunt are two wires which are connected to the Control Room ammeters. When current flows through the shunt, the voltage difference produced correlates to amperage indicated on the ammeter.
With no overcurrent protection provided for these ammeter circuits, a postulated fire in a fire area containing these ammeter cables could cause one of the ammeter wires to short to ground. At the same time, the postulated fire causes another DC wire from the opposite polarity on the same battery to short to ground. This could cause a ground loop through the unfused ammeter circuit resulting in current in excess of the cables ampacity rating.
This high current could cause heating and/or arcing effects from the ammeter cable which could result in a secondary fire along the raceway these circuits are routed in outside the original fire area, damaging adjacent cables that may be credited for safe shutdown.
The circuits for the ammeters are routed through fire areas that contain other circuits required for safe shutdown. In some instances, the ammeter circuits are routed in raceways adjacent to other safe shutdown circuits. Lack of fuse protection for these circuits and routing in areas of other safe shutdown circuits is contrary to the requirements of 10 CFR 50, Appendix R Section III.G to ensure circuits required for safe shutdown remain free of fire damage.
CAUSE OF EVENT
The original plant design contained a latent design error related to wiring and isolation that constituted a fire protection program deficiency which could adversely affect the ability to achieve and maintain safe shutdown of the plant in the unlikely event of a control room fire.
EVENT ANALYSIS
This event is being reported in accordance with 10CFR50.73(a)(2)(ii)(B) in that the overcurrent condition in the DC ammeter circuits could cause damage in other wiring in physical contact with the cable and result in the loss of the associated safe shutdown function required by 10 CFR 50 Appendix R.
There were no actual consequences for this event because a short circuit of the affected ammeter circuits has not occurred. The event investigation and extent of condition review determined that the Division 3 DC ammeter circuit does not impact safe shutdown capability as Division 3 is not a credited system for 10 CFR 50 Appendix R. The Division 3 125 VDC system provides power to the High Pressure Core Spray System [BG].
A Probabilistic Risk Assessment (PRA) evaluation was performed to assess the identified deficiency involving unfused remote DC ammeter circuits. An analysis of this condition estimates a delta core damage frequency (CDF) on the order of 6.1E-08/yr, and a delta large early release frequency (LERF) on the order of 1.85E-09/yr. The delta CDF and delta LERF values are well below the acceptable thresholds of 1.0E-06/yr and 1.0E-07/yr, respectively, as discussed in Regulatory Guide 1.174, "An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis". Therefore, the risk of this event is considered small in accordance with the regulatory guidance.
CORRECTIVE ACTIONS
Interim compensatory measures (i.e., fire watches) were implemented for affected areas of the plant at 1530 hours0.0177 days <br />0.425 hours <br />0.00253 weeks <br />5.82165e-4 months <br />, on October 17, 2013.
Lifting of leads to remove the potential ground loop in the affected DC ammeter circuits was completed at 1435 hours0.0166 days <br />0.399 hours <br />0.00237 weeks <br />5.460175e-4 months <br />, on October 18, 2013, under the temporary modification process.
A permanent plant modification will be completed to install fuses in the five affected DC ammeter circuits.
PREVIOUS SIMILAR EVENTS
A review of LERs and the corrective action database for the past three years identified one previous similar event documented in LER 2011-001 submitted August 23, 2011. LER 2011-001 identified the following condition:
On July 5, 2011, during a review for applicability of a 10 CFR 21 notification made by WorleyParsons (formerly Gilbert/Commonwealth), an original plant design wiring deficiency was identified. The deficiency is in the design of two control room ammeter circuits and results in the plant being vulnerable to a hot short in the unlikely event of a postulated control room fire. The hot short could cause a loss of the capability of the Division 1 train to achieve and maintain safe shutdown of the plant (the site Safe Shutdown Analysis credits Division 1 to shutdown the plant in the event of a control room fire).
COMMITMENTS
There are no regulatory commitments contained in this LER. Actions described in this document represent intended or planned actions, are described for the NRC information, and are not regulatory commitments.