Impact of Digital Control Systems on Nuclear Power Plants

Abstract

Instrumentation and control is an integral component in nuclear power plant operation. It provides the operator with the means to understand the status of the plant, and the data needed to make the proper corrective actions when necessary. Instrumentation and control systems can also make automatic changes to the plants parameters when needed, and activate required safety systems to ensure that the plant always stays in a safe state even if the operator makes an error. I&C systems have generally been run on relay-based analog control, and were run as such successfully for several years. In the 1980s, after the emergence of the microprocessor, I&C systems in nuclear plants started to shift toward digital control. Digital control offered benefits over analog control, such as more flexibility, faster computational speed, networking of systems, increased data storage capability, and increased accuracy. However, the transition from analog to digital I&C technology is not simple. The regulations in existence at the time were all meant for analog control, and had to be completely redone for any digital components. More importantly, providing V&V (Verification and Validation) for digital systems was nearly impossible, since it could not be proven that they were indeed reliable. This is because they depend on software, whose reliability could not be predicted, whereas analog systems depended on hardware interlocks whose reliability could be proven more easily. Despite these disadvantages, digital control is shown to be superior to analog control, providing better plant performance and increased plant safety.