Significance of analog instrumentation-design philosophy of replacement dump arrest unit at Pickering Station Candu Reactor

This paper discusses the differences of opinion concerning nuclear power plant instrumentation, including safety systems. One popular view point is that modern instrumentation must be microprocessor-based to be acceptable. An alternative view point is that properly designed analog instrumentation is recommended in some applications and has proven to be viable based upon performance and experience. A practical example is discussed in detail, explaining how a combination of discrete analog circuitry, combined with discrete digital circuitry provides a robust solution to a complex instrumentation replacement problem. In this application, a microprocessor-based instrument was designed as a replacement for an obsolete analog instrument. Due to severe licensing difficulties, concerning the software reliability, the instrument was redesigned as a combination of discrete analog and digital circuitry. In the implementation of this circuitry, all complex testing functions of the improved microprocessor-based instrument were accommodated and system accuracy and performance were not compromised over the microprocessor-based instrument. The instrument has met all requirements for reliability and EMI/RFI susceptibility, as well as isolation of analog outputs and the ability to withstand severe transient noise on inputs and outputs without adversely affecting performance.