Nuclear PDN
Although the safety systems at a U.S. nuclear plant probably won’t be linked by a PDN any time soon, designers of the nonsafety systems are already taking full advantage of the technology. The figure shows a typical PDN architecture that segregates a control network, a performance network, and a support network (all examples of physical networks) but integrates all three via “core” switches to allow plantwide sharing of the data and functionality of all connected devices.
The topology shown is known as an “inverted tree,” with the core switches serving as the trunk and the zone switches providing the limbs extending throughout a plant. Core switches are fed by redundant fail-safe power supplies, typically instrumentation-related power sources. Zone switches generally require only one power supply, but they also can also be dual-powered if necessary. Components considered “mission critical” are powered by two independent supplies. Each of the three networks is itself fully redundant, with dual fiberoptic cables running between its core switches and zone switches.
Usually, devices on the control network support the real-time operation of the plant and thus are considered critical. Devices on the other two networks are deemed less important. The performance network typically plays two roles: keeping track of long-term plant performance and handling monitoring of less-critical equipment. The support network provides two important, but less-time-critical, functions: plantwide communications and integration of all the plant’s diagnostic and maintenance-related systems.
The figure illustrates only one possible PDN topology. Other architectures may be better suited to the layout of a specific plant and/or the extent of its wireless networks. They include the STAR, point-to-point, and mesh configurations, each of which has advantages and disadvantages. The PDN representation shown in the figure was included in a recent utility application for a combined construction and operating license (COL) for a new nuclear unit.
Diagnostic capabilities are inherent in modern digital networks because people and machines are increasingly dependent on the data they deliver. As traffic increases, so does the importance of availability and maintaining bandwidth. PDNs that use high-bandwidth fiberoptic cables and modular network switches are the most reliable, as well as the most scalable to meet expanded needs for data and voice (and video) in the future.
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