A Comparison of Solid State and Relay Reactor Safety Systems

Solid state circuits demonstrating fast speed of response, miniaturization, and low power consumption have been replacing relay circuits in the logic portion of recent nuclear reactor safety systems. Some of the advantages of relay logic, such as circuit isolation, minimum number of parts, and low cost have been lost in the transition. A solid state and a relay safety system are described, and the application and merits of each are developed. Both system designs can meet the principal safety criterion of having the capability to take safety action when required under all operating conditions and with the presence of any one fault eitherwithin or outside the safety system. Secondary criteria used to compare the merits of each system are operability (freedom from false shutdowns), simplicity in testing, ease of maintenance, speed of response and cost. Reliability considerations include mode of component failure, frequency of testing, and the need for developmental testing.