The Advanced Digital Fieldbus Option for Nuclear Plants

Jack Y. Zhao, U.S. Nuclear Regulatory Commission

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Digital fieldbus technologies, including Foundation fieldbus and Profibus, are increasingly being used with success in the nuclear and fossil fuel power industries. This article compares a conventional control system with a Foundation fieldbus – based digital control system used in a typical circulating water system in a nuclear power plant. As shown in this example, using digital fieldbus technologies can result in significant savings in terms of installation and hardware costs.

Advanced digital fieldbus technology is an all-digital, multidrop, bidirectional serial communication bus that connects microprocessor-based control and field devices. In addition, this technology serves as a local area network for the instrumentation and control systems used within power plants and other industrial facilities that have the built-in capability to monitor and distribute control applications across a network. This technology therefore offers an open architecture for control and information integration. Advanced digital fieldbus technology has already gained wide acceptance in a number of industries, including the oil and gas, petrochemical, and pulp and paper industries. Its popularity is growing in the fossil fuel power industry, but it is being adopted much more slowly in the nuclear power sector.

The benefits achieved with this technology include lower installation costs, easier troubleshooting, improved maintenance, and extensive diagnostics. Some advanced digital fieldbus technology applications have been implemented for nonsafety-related systems to retrofit the analog-based control systems in existing nuclear power plants. For example, Oconee Nuclear Station, located in Seneca, S.C., installed a Foundation fieldbus (FF) network and related field devices to replace obsolete pneumatic and electrical control and monitoring loops for its balance-of-plant systems. The application of advanced digital fieldbus technology has also been proposed for nonsafety-related systems in new reactor systems currently under U.S. Nuclear Regulatory Commission (NRC) review. The technology has also been proposed for use in safety-related systems in the U.S. nuclear power industry. Moreover, digital fieldbus technology has been tested internationally for use in nuclear plant safety applications.

For years, the industry has investigated ways to use digital fieldbus communications with field devices without the need for a separate cable for each. Many types of digital fieldbus technologies exist; however, two digital fieldbus platforms have emerged as leaders for process control systems: Foundation fieldbus and Profibus (process fieldbus). See the sidebar for a discussion of the differences between them.

Foundation Fieldbus vs. Profibus

The digital Foundation fieldbus (FF) – and Profibus-based control systems have much in common. Both technologies can provide the following features:

Fully digital, serial, two-way, and multidrop communication.
Remote access to status and diagnostic information from smart field devices.
A method to communicate to field instruments over fieldbus.
Bus power and intrinsic safety features.
Faster verification and validation of control loop configurations.

However, for FF technology, which is a single communication protocol, field equipment (such as sensors, actuators, controllers, and I/O devices) is first interconnected with a single cable to form a segment known as an H1. Then various H1 subsystems and other devices such as high-speed controllers, data servers, and workstations are connected to form a high-speed Ethernet (HSE) network, which can run at 100 megabits per second. The FF H1, which is usually used for process control with limited discrete applications, is a deterministic network running at a speed of 31.25 kilobits per second (kbps). This communication speed is appropriate for the majority of process conditions in nuclear power plants, but it is not fast enough for some safety-critical discrete applications. The FF HSE can provide peer-to-peer communication capability.

The HSE technology is designed to support fault-tolerant networks and devices in safety-critical monitoring and control applications. All or part of the HSE network and devices can be made redundant to achieve the level of fault tolerance required for a particular application. Because FF devices can communicate with each other directly without having to go through a central computer, advanced control strategies that involve variables throughout the plant can be implemented without the risk of a central computer failure, thus further reducing risk. In addition, control and I/O functions can be distributed to field FF devices, which can further improve the operating safety and reliability of systems.

In contrast, Profibus is not a single-communication protocol; it has a family of protocols. The two main Profibus protocols are Profibus-DP and Profibus-PA. The digital Profibus-DP running at 500 kbps is a better candidate for discrete and safety-critical control, whereas Profibus-PA has a speed of 31.25 kbps (similar to that of the FF) and is more suited for process control. Unlike Profibus-DP, Profibus-PA can be fieldbus-powered. In Profibus-based control systems, the I/O functions are moved to the field instruments; however, no control functions are moved to the field instruments. All control in the Profibus system is still done by the central controller.

FF and Profibus technologies share many similar design features, but each technology has its own specific functions and limitations. In nuclear power plants, there are areas with different levels of automation and control system hierarchy that have different communication needs. It may be necessary to use a mix of different fieldbus technologies to take advantage of each individual digital fieldbus characteristic to achieve more benefits, higher reliability, and adequate safety.

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