**2.1 Fundamentals of reactor instrumentation and control**

Nuclear reactors initiate and control nuclear fission or fusion reactions. These processes must be monitored and closely controlled to ensure reliable and efficient operation while maintaining the health and safety of the public. The number and type of parameters monitored in a reactor will vary depending on the reactor type and purpose, but both nuclear and non-nuclear instrumentation will likely be used. Nuclear instrumentation includes detectors to monitor neutron and gamma flux for routine reactor monitoring and control as well as reactor safety. Neutron detectors, such as proportional counters and ion chambers, are commonly used to provide source range, intermediate range, and power range monitoring, while gamma detectors are used for post-accident monitoring. These detectors may be out-of-core or in-core, or a combination thereof, depending on the reactor type. Other compact in-core detectors, such as small fission chambers or self-powered neutron detectors, are also commonly used for continuous real-time monitoring of reactor core conditions, including reactor power distributions.

Non-nuclear instrumentation includes sensors used to monitor process parameters, such as temperature, pressure, differential pressure, level, and flow.

### *Cyber-Informed Engineering for Nuclear Reactor Digital Instrumentation and Control DOI: http://dx.doi.org/10.5772/intechopen.101807*

Additionally, non-nuclear instrumentation may be used to monitor other parameters, including control rod position, area radiation, fuel-pin fission gas pressure, vibrations, acoustics, fuel or vessel strain, process fluid chemistry, moisture and gas analysis, and leaks.

Local instrumentation data is transmitted from the sensors to control board indicators, data recorders, applications, and control systems via analog or digital circuits, often through multiplexers or combinatorial logic circuits. Applications are commonly used to auctioneer (e.g., signal selection), aggregate, and/or perform calculations on the data to provide real-time reactor and plant status indications to operators.

While operators will also perform manual actions on a reactor, such as starting and stopping pumps or opening and closing valves, I&C systems are commonly used to automatically control reactor operations and maintain reactor safety. Control systems can be simple, like a single programmable logic controller, or complex, like a reactor control system. Control systems can combine numerous sensors, transmitters, controllers, and actuators to change the physical state of process equipment, such as a valves, pumps, or motors, by using signal feedback loops to monitor and maintain desired conditions. In a nuclear power plant (NPP), non-safety control systems may include feedwater control (or fluid control), turbine control, and reactor control.

Most nuclear reactors will have at least two types of control systems—reactor control systems and reactor safety systems. Depending on a reactor's purpose, there may also be other control systems, such as plant control systems in an NPP or experiment/sample control systems in a research and test reactor. Reactor control systems are used to control the nuclear fission or fusion reaction within specified acceptable fuel design limits by adjusting physical components according to the reactor design. For example, a reactor control system may raise or lower control rods in a light water reactor (LWR), turn control drums in a heat pipe reactor, or start or stop feedwater flow in a research reactor.

In an LWR, reactor control systems are used to maintain desired thermal megawatts by balancing primary and secondary systems. For example, as shown in **Figure 1**, an integrated control system may automatically maneuver reactor, feedwater, and turbine systems to match megawatts generated to megawatts demanded by adjusting control rod positions, valve positions, and pump speeds.

In comparison to reactor control systems, reactor safety systems are used to shut down and maintain safe shutdown of a reactor in the event a reactor safety limit

#### **Figure 1.**

*A notional automatic integrated reactor control system for an LWR.*

is met or exceeded to assure reasonable protection against uncontrolled release of radioactivity. For example, reactor safety systems in an LWR include Reactor Protection Systems (RPS), Engineered Safety Feature Actuation Systems (ESFAS), and diverse actuation or diverse trip systems. Reactor safety systems often use either two-out-of-four or two-out-of-three logic. For instance, an RPS may have four redundant instrumentation channels that monitor key parameters, such as reactor power, reactor coolant temperature, reactor coolant pressure, reactor coolant flow, reactor building pressure, reactor pump status, and steam generator level. If any design limits are exceeded on two separate channels, an automatic trip signal is sent to the control rod system to shut down the reactor. A notional representation of an RPS is shown in **Figure 2**.

In an LWR, an ESFAS is designed to provide emergency core cooling for the reactor and to reduce the potential for offsite release of radiation. Comparable to an RPS, ESFAS uses multiple channels of equipment in two-out-of-three logic (or similar) to monitor signals such as reactor coolant pressure and containment pressure. Based upon the specific coincident actuation signals received, ESFAS will start the required safety system, such as emergency core cooling systems, emergency feedwater, containment isolation and ventilation, containment spray, or emergency diesel generators.
