Electrical Insulators in a Reactor Accident Environment (NUREG/CR-1682, SAND80-1957)
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Date Published: January 1981
Otmar M. Stuetzer
Sandia National Laboratories
Albuquerque, NM 87185
Operated by Sandia Corporation for the U.S. Department of Energy
Plant Instrumentation, Control, and Power Systems Branch
Office of Water Reactor Safety Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
Under Interagency Agreement DOE 40-550-75
NRC FIN A-1051-0
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.