Implementation of Reactor Control Rod Position Sensing/Display Using a VLSI Chip

Implementation of Reactor Control Rod Position Sensing/Display Using a VLSI Chip

Imbaby I. Mahmoud (Egyptian Atomic Energy Authority, Egypt)
DOI: 10.4018/978-1-5225-0299-9.ch001


The chapter describes the use of CPLDs and FPGAs devices in Nuclear Power Plant (NPP) Instruments. The design, simulation, implementation and test of a reactor control rod position sensing electronic unit intended as a replacement of the outdated Russian type in old power reactors is presented. The signals are generated from12 ring-shaped pair of inductively coupled coils surrounding the reactor moving rod. The implementation involves both analog and digital design. The designed digital circuit has 12 TTL outputs working in a 1-out-of-12 mode, excluding both double [2-out-of-12] and no-output state. To avoid a flickering display during transition between two neighboring positions, some sort of hysteresis is implemented. One time this hysteresis is implemented through a state machine deriving up/down counter. The state machine is synthesized targeting a Xilinx Spartan XL device. To reduce the possibility of power failure effects, another circuit consists of combinatorial logic and implemented in CPLD is presented. However, energy harvesting methods in NPPs can support counter based design.
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Control rods, which generally include a cluster of elongated rods containing neutron absorbing materials, regulate the core activity within a nuclear reactor. The movable control rods are located within the region of the nuclear fissionable fuel and penetrate the core and fuel to a selected depth which is measured in steps (e.g. between 0 and 231 steps (Beltz et al., 1996; Sexton et al., 2014). When the control rods are inserted into the reactive region they absorb neutrons emitted from the fuel. The number of neutrons in the fuel determines the number of fissions of the fuel atoms that take place, and the number of fissions determines the amount of energy released by the nuclear reactor. Therefore, the number of control rods inserted and the depth of insertion can along with other measures control the amount of energy released by the nuclear reactor. Energy in the form of heat is removed from the reactive region by a coolant which flows through the region and to a heat exchanger. The heat from the reactor coolant is used to generate steam for energy. Thus, reactor fuel considerations make it of the utmost importance to accurately know the position of each of the control rods within a nuclear reactor.

Nowadays there is undergoing work around the world to upgrade old power reactors (Sexton et al., 2014). In particular, electronic part of reactor control system is receiving great attention.

The FPGA based systems are considered as possible replacements for older, analog systems that are commonly used in Nuclear Power Plants (NPPs). Many of these systems are becoming obsolete, and it is difficult to repair and maintain them. FPGAs possess certain advantages over traditional analog circuits, as well as microprocessors, for nuclear I&C applications. The advantages of applying FPGA are to keep the long-life supply of designed units, improving testability (verification), and to reduce the drift which may occur in analog-based system.

In this chapter, we describe a design to replace the outdated Russian type used presently for reactor control rod position sensing/display in old power reactors. In this type, the signals are generated from 12 ring-shaped pair of inductively coupled coils arrangement surrounding the reactor moving rod.

The 12 ring-shaped pair of inductively coupled coils are arranged in line of the path of the rod with an internal iron core attached to the rod moving inside the tube-like structure and used as position sensor (Mahmoud, 2000). It has a length of approximately 1/12th of the full rod displacement. This arrangement is depicted in Figure 1. The primary coils are connected in series with alternating polarity and fed from 220V AC voltage source, while secondary coils give 12 separate voltages relative to their common reference node. Figure 2 shows the voltage waveforms generated from this arrangement.

Figure 1.

Arrangement of 12 ring-shaped pair of coils as position sensor

Figure 2.

Voltage waveforms generated from position sensor

One possible way of upgrading is replacing rod position indicator by neutron flux measurement (Cerny et al., 1997). This approach requires complete subsystem design, implementation and installation.

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