ML121070714
| ML121070714 | |
| Person / Time | |
|---|---|
| Site: | Robinson  |
| Issue date: | 05/21/2012 |
| From: | Nick Melly, Gabe Taylor NRC/RES/DRA/FRB |
| To: | Markhenry Salley NRC/RES/DRA/FRB |
| Taylor Gabriel 251-7576 RES/DRA | |
| Shared Package | |
| ML121070712 | List: |
| References | |
| Download: ML121070714 (5) | |
Text
May 21, 2012 MEMORANDUM TO:
Mark Henry Salley, Chief Fire Research Branch Division of Risk Analysis Office of Nuclear Regulatory Research FROM:
Gabriel J. Taylor, Fire Protection Engineer /RA T. Pennywell for/
Fire Research Branch Division of Risk Analysis Office of Nuclear Regulatory Research Nicholas B. Melly, Fire Protection Engineer /RA/
Fire Research Branch Division of Risk Analysis Office of Nuclear Regulatory Research
SUBJECT:
H.B. ROBINSON HIGH ENERGY ARC FAULT EVENT CABLE DAMAGE EVALUATION Purpose The purpose of this memorandum is to inform you of the results related to the evaluation of electrical cables exposed to the March 2010 high energy arc fault event at H.B. Robinson Steam Electric Plant Unit 2.
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Background===
On March 28, 2010, a fault on a 4160 volt (V) feeder cable between Bus 4 and Bus 5 occurred causing a high energy arc fault event. A failure to trip of the Bus 4 output breaker (52/24) occurred due to the unavailability of breaker control power. The fault caused a decrease in reactor coolant pump speed due to voltage reduction on Bus 4 that ultimately caused an automatic reactor trip. The fault was terminated when the breaker between the UAT and Bus 4 tripped on UAT sudden high pressure. This actuated the generator lockout relays, resulting in a fast transfer of power to Bus 4 from the UAT to the startup transformer (SUT) via breaker 52/19.
CONTACT:
Gabriel J. Taylor, RES/DRA 301-251-7576 Nicholas B. Melly, RES/DRA 301-251-7916
Following the fast transfer, the fault conditions persisted causing a decrease in voltage on Bus 3 and the 480V safety-related Bus E-2. This caused Bus E-2 to isolate from Bus 3 and become powered from the Emergency Diesel Generator (EDG). These fault conditions were terminated when Bus 4 was isolated and de-energized from the SUT.
A second high energy arc fault and fire occurred due to inappropriate recovery actions.
Operators attempted to reset the generator lockout relay per plant procedures. With the UAT sudden pressure trip signal present, another fast transfer to the SUT occurred, reenergizing Bus 4 and causing a HEAF at breaker 52/24. Breaker 52/19 opened on over-current, cleared the fault, and ended the second event.
Both HEAF events caused physical damage to the electrical components and associated cabinets, along with damaging materials in the near vicinity. In the first event, cables located in conduits exiting the top of a cabinet shorted together and damaged the conduit and potentially damaged electrical cables located in cable trays directly above the damaged conduit.
A detailed evaluation of the event in enclosed in the NRC augmented inspection report.
Additionally, the NRC issued Information Notice 2010-09 informing the industry of the importance of understanding circuit breaker control power indications.
Appendix M to NUREG/CR-6850, EPRI/NRC-RES Fire PRA Methodology for Nuclear Power Facilities, presents a model to account for the fire risk of HEAF events. The model identifies zone of influence where components within this ZOI are assumed to be damage from the effects of the HEAF. This model is based off of a single well documented medium voltage switchgear HEAF event and applicability of these methods to other equipment susceptible to HEAF has been questioned.
Discussion In an effort to understand the effects of the HEAF events that occurred at H.B. Robinson and to improve the current fire PRA methods, the NRC Region II office and the Office of Nuclear Regulatory Research (RES) discussed the possibility of obtaining the electrical cables directly above the Robinson cable-conduit failure location. It was believed that a detailed evaluation of the state of functionality of the cables would allow for a better understanding of the applicability of the NUREG/CR-6850 methods to predict component damage.
The following provides a comparison of the guidance presented in Appendix M of NUREG/CR-6850, in relation to cable damage targets, and the evaluation of electrical cables exposed to the March 2010 high energy arc fault event at H.B. Robinson Steam Electric Plant Unit 2.
Appendix M of NUREG/CR-6850 states:
Any unprotected cables that drop into the top of the panel in an open air-drop configuration will ignite.
Cables in conduit or in a fire wrap are considered protected in this context. In other words, if cables are protected (i.e., not exposed) by conduit or fire wrap, they are assumed damaged, but not ignited, and they do not contribute to the fire load.
Armored cables with an exposed plastic covering are considered unprotected in this context.
The analysis of the electrical cables exposed to the Robinson HEAF event confirmed this guidance. The initiating event was an electrical fault to ground at a cable conduit connection the 4-kV bus 5 that propagated to a second conduit at a 90-degree bend above the bus 5 switchgear in the turbine building. These damaged cables drop into the top of the panel in an open air-drop configuration and were damaged as the result of the HEAF.
Appendix M of NUREG/CR-6850 states:
Any unprotected cables in the first overhead cable tray will be ignited concurrent with the initial arcing fault provided that this first tray is within 1.5 m (5 ) vertical distance of the top of the cabinet. The cable tray fire will propagate to additional trays consistent with the approach provided for the treatment of cable tray fires elsewhere in this document, assuming that the time to ignition of the first tray is zero rather than the normal 5 minutes.
This applies to any cable tray located directly above the panel.
This applies to any cable tray above the aisle way directly in front of, or behind, the faulting cabinet, provided some part of that tray is within 0.3 m (1) horizontally of the cabinets front or rear face panel.
Cables in conduit or in a fire wrap are considered protected in this context.
Armored cables with an exposed plastic covering are considered unprotected in this context.
The electrical cables exposed to the Robinson HEAF event which were sent to Sandia for analysis did not show the type of damage which is proposed in Appendix M of NUREG/CR-6850. When tested for continuity, the cable taken from the first tray did not display insulation degradation between conductors. Appendix M of NUREG/CR-6850 denotes that any tray within 1.5 m (5) vertical distance of the top of the cabinet will be ignited and damaged at time T=0.
The first cable tray at Robinson was located 3 above the top of the cabinet. The model in Appendix M of NUREG/CR-6850 would over estimate the risks associated with damage to the first cable tray located within the ZOI. It should be noted that, based on the post fire analysis plant pictures, the level of damage on the cables exposed directly to the fire appears to be much greater than the sample that was shipped to SNL. This implies that the cable shipped to SNL for analysis were not from the area of greatest heat impact. It is unclear at this point whether the cables at the greatest heat impact area would have shown the same conduit continuity.
Appendix M of NUREG/CR-6850 states:
Exposed cables, or other exposed flammable or combustible materials or transient fuel materials located within this same region (0.9 m (3) horizontally) will be ignited.
Based on the lack of specific location information to accompany the cables shipped to SNL, it is impossible to determine the applicability of this guidance in appendix M of NUREG/CR-6850.
Collaboration between the NRC Region II office, NRC resident inspectors at H.B. Robinson, staff from H.B. Robinson, and RES allowed for several samples of cables exposed to the first HEAF event to be sent to Sandia National Laboratories for additional evaluation. The concept was to evaluate a sample of cables with known locations above the Bus 5 conduit failure to develop a map of fire effects and zone of influence.
SNL received the cable sample and evaluated the physical and electrical state of each sample.
The enclosed SNL report documents this evaluation. The evaluation determined that no cable
sample failed electrically, and the lack of known location resulted in the evaluation be less useful than originally intended. In general, the results of the electrical cable evaluation show that the model in Appendix M of NUREG/CR-6850 would over estimate the risks associated with the HEAF event that occurred at H.B. Robinson Steam Electric Plant in March 2010.
Conclusions Events like the Robinson HEAF provide insights that could be used to advance the state of the art in fire risk assessment. Given the level of effort and resources applied to the evaluation of the Robinson cables, it is unfortunate that more insightful information could not have been obtained. Future research of this format where equipment from utilities is being sent to a laboratory for evaluation can be improved by (1) setting up direct communication between the laboratory, research project manager and utility site representative responsible for the event evaluation and shipment of the component(s), (2) a better understanding on what components will provided the most useful information, and (3) how to ship the component(s) to the laboratory without potentially inducing shipping damage. Addressing these three recommendations will not guarantee success, but will likely improve the research and advance the state-of-knowledge of the specific phenomena being evaluated.
Enclosures:
- 1. SNL letter report, Evaluation of cables from the HEAF fire event at the H.B. Robinson Electric Plant
- 2. H.B. Robinson Steam Electric Plant - Augmented Inspection Team Report 05000261/2010009
ML121070712 OFFICE RES/DRA/FRB RES/DRA/FRB RES/DRA/FRB NAME G. Taylor (T. Pennywell for)
N. Melly M. Salley DATE 5/21/12 5/21/12 5/4/12