Effects of Smoke on Functional Circuits (NUREG/CR-6543, SAND97-2544)

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Publication Information

Manuscript Completed: April 1997
Date Published: October 1997

Prepared by:
T.J. Tanaka
Sandia National Laboratories
P.O. Box 5800, MS0747
Albuquerque, New Mexico 87185-0747
Operated by Sandia Corporation

C Antonescu, NRC Project Manager

Prepared for:
Division of Systems Technology
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code W6051

Availability Notice

Abstract

Nuclear power plants are converting to digital instrumentation and control systems; however, the effects of abnormal environments such as fire and smoke on such systems are not known. There are no standard tests for smoke, but previous smoke exposure tests at Sandia National Laboratories have shown that digital communications can be temporarily interrupted during a smoke exposure. Another concern is the long-term corrosion of metals exposed to the acidic gases produced by a cable fire. This report documents measurements of basic functional circuits during and up to 1 day after exposure to smoke created by burning cable insulation. Printed wiring boards were exposed to the smoke in an enclosed chamber for 1 hour. For high-resistance circuits, the smoke lowered the resistance of the surface of the board and caused the circuits to short during the exposure. These circuits recovered after the smoke was vented. For low-resistance circuits, the smoke caused their resistance to increase slightly. A polyurethane conformal coating substantially reduced the effects of smoke. A high-speed digital circuit was unaffected. A second experiment on different logic chip technologies showed that the critical shunt resistance that would cause failure was dependent on the chip technology and that the components used in the smoke exposures were some of the most smoke tolerant. The smoke densities in these tests were high enough to cause changes in high impedance (resistance) circuits during exposure, but did not affect most of the other circuits. Conformal coatings and the characteristics of chip technologies should be considered when designing digital circuitry for nuclear power plant safety systems, which must be highly reliable under a variety of operating and accident conditions.