United States Nuclear Regulatory Commission - Protecting People and the Environment

Refining And Characterizing Heat Release Rates From Electrical Enclosures During Fire (RACHELLE-FIRE) — Volume 1: Peak Heat Release Rates and Effect of Obstructed Plume, Final Report (NUREG-2178, Volume 1, EPRI 3002005578)

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Publication Information

Date Published: April 2016

EPRI Project Manager
A. Lindeman

Electric Power Research Institute (EPRI)
3420 Hillview Avenue
Palo Alto, CA 94304-1338

U.S. NRC-RES Project Manager
M. H. Salley

U.S. Nuclear Regulatory Commission
Office of Nuclear Regulatory Research (RES)
Washington, DC 20555-0001

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The Refining And Characterizing Heat Release Rates From Electrical Enclosures During Fire (RACHELLE-FIRE) program involves a working group of experienced fire protection and fire probabilistic risk assessment researchers and practitioners focused on enhancing the methodology used to model electrical enclosure fires in nuclear power plants (NPPs). This report documents the results from the working group's efforts to develop technical information in three areas: (1) classification of electrical enclosures in terms of function, size, contents, and ventilation, (2) determination of peak heat release rate (HRR) probability distributions considering specific electrical enclosure characteristics, and (3) development of a method to account for the impact of the enclosure on the vertical thermal zone of influence (ZOI) above the enclosure during fire.

Electrical enclosures have been classified in classification groups based on their electrical function, contents, and size. Power enclosures, such as switchgear, load centers, motor control centers, battery chargers, and power inverters, are grouped based on their function. Remaining electrical enclosures are classified as small, medium, and large based on their volumetric size. This classification is primarily based on the size because it can be easily assessed by visual inspection during walkdowns without opening the electrical enclosure. Distinctions based on insulation type (unqualified thermoplastic versus lower flammability cable types which include thermoset, qualified thermoplastic and SIS wire) and open versus closed door configurations have been retained for certain enclosure groupings. Peak HRR values may be refined based on visual inspection of the interior of the enclosures if fuel type, fuel quantity, and cable bundling arrangement can be expected to limit fire growth.

Peak HRR distributions for the different classification groups have been developed. These distributions are based on the results of different experimental programs intended to measure the HRR associated with fires in electrical enclosures. The working group evaluated the configuration factors in these fire tests and compared them with the actual configuration of electrical enclosures used in commercial NPPs and the available plant fire event experience. The resulting probability distributions are intended to map the configurations of the electrical enclosures in operation at the plant with the experimental factors evaluated during the test programs. As a result, HRR probability distributions are available for a wide range of electrical enclosure types and configurations.

In order to provide a comprehensive characterization of electrical enclosure fires, the working group evaluated the temperature characteristics of fire plumes associated with these events using the Fire Dynamics Simulator (FDS) program. Computer simulations of various enclosure configurations were developed for evaluating the fire burning inside electrical enclosures and the fire plume temperature characteristics that would be generated. Based on this research, new fire plume temperature profiles reflecting the obstructed nature of fire plumes generated from fires inside electrical enclosures are provided.

Finally, examples, consolidating the information described in this report, are provided. The examples have been selected and designed to illustrate how to incorporate the information documented in this report into existing approaches for modeling fires in electrical enclosures.

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