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Electrical Insulators in a Reactor Accident Environment (NUREG/CR-1682)
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Publication Information
Otmar M. Stuetzer
Date Published: January 1981
Sandia National Laboratories Albuquerque, New Mexico 87185
operated by Sandia Corporation
for the
U. S. Department of Energy
Prepared for
Plant Instrumentation, Control, and Power Systems Branch
Office of Water Reactor Safety Research
U. S. Nuclear Regulatory Commission
Washington, DC 20555
Under Interagency Agreement DOE 40-550-75
NRC FIN No. A-1051-0
Availability
Notice
Abstract
Wire connections in a reactor containment generally are made by means
of many hundreds of insulating blocks ("terminal blocks") which are
protected by metal boxes. A broad investigation was conducted to
determine what effect the heat, steam, and contamination resulting
from a nuclear reactor accident would have on-these terminal blocks.
A comprehensive experimental program was performed at temperatures,
pressures, and time constants characteristic of the Three Mile Island
accident. A model was developed which predicts, within an error
factor of 2, the probability of an electrical breakdown for a wide
range of temperatures, contamination, and protective measures.
"Normally dirty" terminals in a tightly closed protective box with a
6 mm "weephole" in a 480 V circuit had about one chance in 100 of
suffering complete breakdown at Three Mile Island.* If flow retarders
or "breathers" narrowed the effective size of the weephole, breakdown
was less likely by about a factor of 3. For a large scale steam
breakout with a temperature of 170 ° C, however, the breakdown
probability for an unprotected terminal would be about 30% at the
same voltage.
Terminal blocks are probably the weakest links in a reactor's
electrical system, and concern about their presence in safety-related
circuits is fully justified. Some remedial measures and some
improvements for future installations are proposed.
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