Nuclear Regulatory Authority Experimental Program to Characterize and Understand High Energy Arcing Fault (HEAF) Phenomena: Basic Arc Test Experimental Data (NUREG/IA-0470, Volume 2)

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Publication Information

Manuscript Completed: June 2021
Date Published: October 2021

Prepared by:
S/NRA/R: H. Kabashima, F. Kasahara, H. Eguchi
NRC: S. Mehta, D. Stroup, N. Melly, S. Turner (Consultant)

Regulatory Standard and Research Department
Secretariat of Nuclear Regulatory Authority (S/NRA/R)
Tokyo, Japan 106-8450

S/NRA/R Project Managers
H. Kabashima, H. Eguchi

NRC Project Manager
M. H. Salley

Division of Risk Analysis
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

Prepared as part of:
The Agreement between NRC and NRA in the Area of Fire-Related Research

Published by:
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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Abstract

A High Energy Arcing Fault (HEAF) occurred in a high-voltage (6.9 kV) switchgear (SWGR) in Unit 1 of the Onagawa Nuclear Power Plant of the Tohoku Electric Power Company on March 11, 2011 during the Great East Earthquake in Japan. While HEAF events are not common, they have occurred in nuclear power plants (NPP) worldwide. The operating experience seen from the Onagawa event illustrate that HEAFs can present a potential threat to the safe operation of NPPs. As a result, the nuclear power industry has placed a new emphasis on understanding and developing evaluation methods for these events.

To investigate the HEAF event sequence and to understand the phenomena, the Regulatory Standard and Research Department, Secretariat of the Nuclear Regulation Authority (S/NRA/R) (Japan) conducted HEAF tests by simulating the design and operating conditions of the SWGR HEAF at Onagawa NPP in addition to simulating the HEAF energy effects using a "Rocket Fuel Arc Simulator" (RFAS). Tests of 480 V Motor Control Center (MCC) and Distribution Panel (DP) cabinets were also conducted to understand HEAF characteristics. Previous tests performed in 2013 through 2015 described in Volume 1 of this publication used realistic SWGR, MCC, and DP cabinet configurations. The SWGR tests showed damage similar to the damage in the  earthquake but less severe. Data for temperatures, heat flux, heat release rate, arc energy, and pressure were  collected and analyzed. The results were generally as expected but provided a new appreciation for and  recognition of the high thermal energy from the oxidation of aluminum bus bars used in the SWGR tests.

The objective of the tests performed in 2016 and described in this report was to study basic arc electrical properties and thermal effects using one arc event with arc energies from 0.6 MJ to 137.3 MJ. The energy was set by changing the arc voltage, current, and arc duration. The supply open circuit voltages (OCV) were 7.1 kV for the SWGR and 484 V and 600 V for the DP. No MCCs were tested. The test items had similar geometry to the previous tests but internal walls and obstructions were removed and the front and rear exterior panels were removed to allow direct observations of the arc. Metal targets with temperature detection labels and samples of cables and plastics were also placed at various distances from the arc to assess ignition and damage at various distances from the arc. Bundles of cables were placed near the arc to investigate cable ignition and damage. The key observations were:

• The DP would not sustain an arc at the same conditions as the January 2013 tests, so some tests had very short duration and low energy. The OCV was increased from 484 to 600 volts in the final two tests and the arc sustained for the planned duration (up to 4 seconds).
• It was very difficult to get heat flux measurements because the external flames and plasma quickly escaped through the open panels and contacted the slug calorimeters and the data was not valid.
• Except for cases where cable samples were very close to the arc (<50 cm) the cable samples did not ignite. For targets that had minor to heavy damage, post-test cable resistance measurements showed only one failure (short circuit between the cable conductors) in more than 300 samples.
• The cable bundles in the tests did not have sustained fires probably because the cabinets were open and the heat escaped.

The results and data in this report in combination with operating experience and other HEAF test data, will be used by international teams to develop consensus conclusions on HEAF behavior, understand the potential for HEAF damage including ensuing fires, establish HEAF evaluation criteria to support Fire Hazard Analyses (FHA), and recommend protection measures.