Sensors and measurement technologies enable nuclear power plant operators to gather real-time and accurate information about the reactor core performance, the operation of the auxiliary systems, and the environmental safety of the nuclear facility.
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This article provides an overview of the advanced sensor technology used to monitor and control modern nuclear power reactors.
Sensors and control systems are critical for the functioning of both research and power-generating nuclear plants. Modern nuclear reactors utilize a wide range of monitoring equipment to provide the operators with a feedback mechanism, ensuring that the nuclear reactor operates safely and within specification limits.
The control instrumentation enables the operators to monitor the status of the nuclear reactor power output and to prevent the reactor operation outside of normal conditions, thus protecting the reactor against accidents and their potential effects, such as reactor core melt and radioactive release.
Safe operation of the plant relies on nuclear reactor sensors that interact with the physical processes occurring in the reactor and its auxiliary systems, measuring process variables, such as temperature, pressure, gas and fluid flow, radiation levels, and neutron flux.
The control instrumentation of a typical commercial reactor can incorporate up to 20 neutron detectors, 60 resistive temperature sensors, 100 thermocouples, and between 500 and 2500 pressure sensors. These are integrated into communication and control infrastructure that includes human-system interfaces, surveillance and diagnostic systems, and actuators, such as valves and motors, that operate the control and safety components of the reactor.
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Nuclear Reactor Sensors are Critical to Nuclear Safety
Nuclear reactor sensors can be classified into four categories.
- Nuclear sensors measure the parameters of the nuclear chain reaction, such as neutron flux density, thus providing information about the reactor power.
- Process sensors are used to monitor non-nuclear processes, such as reactor coolant pressure, temperature and flow, containment pressure, and others.
- Radiation monitoring sensors for monitoring radiation levels in coolant lines, gas effluents, and the environment around the reactor.
- Special sensors that monitor seismic activity, vibration, hydrogen concentration, water conductivity, and many others.
In most nuclear reactor designs, the most important and safety-critical sensors measure temperature, pressure, coolant level and flow, and neutron flux. Such nuclear reactor sensors often need to operate sufficiently fast in harsh environments with high sensitivity to accurately and accurately communicate the measured variables in real-time.
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Sensors for Neutron Flux Monitoring
Neutron detection is of importance in various fields, such as monitoring the core activity in research and commercial reactors, nuclear fusion research, and medical applications involving low neutron flux sources.
Fission chambers and self-powered neutron detectors (SPNDs) are the most widely used types of sensors that can deliver instantaneous data for reactor monitoring and control of nuclear power plants. Semiconductor-based detectors or scintillator systems can be used to measure moderate-to-low neutron flux.
The fission chamber sensors consist of two electrodes covered by a layer of fissile material (natural or enriched uranium or other radioactive metals). The chamber is filled with an argon-nitrogen mixture and, when neutron-induced fission in the electrode coating creates high-energy products, the filling gas is ionized, and the resulting ions and electrons are detected by applying a polarization voltage on the electrodes.
The SPND sensors have a coaxial structure consisting of a central metallic emitter surrounded by a mineral insulator and enclosed in a metallic sheath. The neutron capture in the emitter leads to activating a short-lived beta-emitting radioisotope. Each emitted beta particle with sufficiently high energy to cross the insulator contributes to a net current between the emitter and the sheath. Typical currents amount to a few μA at a neutron flux of 1014 cm-2 s-1 with a response time of the order of a few minutes.
In the last two decades, different types of SPNDs were developed with a nearly-instantaneous response where the current is generated by gamma radiation upon neutron capture in the emitter. Such SPNDs are particularly suited for detecting local changes in neutron flux distribution in liquid-metal-cooled fast neutron reactors as they can provide dynamic information on the neutron flux in the reactor core.
Temperature Sensors in Nuclear Reactors
The main specificities of the temperature measurements in nuclear reactor environments are exposure to radiation damaging the sensor components, elevated operating temperatures, and aggressive fluids, like molten metal-based coolants. In addition, the temperature sensors in nuclear reactors must be highly accurate and with short response times.
For example, resistance temperature detectors (RTDs), key components in nuclear reactor safety systems, are expected to provide 0.1% accuracy and respond to a temperature change in less than 4 seconds.
To minimize the temperature drift exhibited by the traditional RTDs and thermocouples, and the need for frequent calibration and maintenance of the temperature sensors, researchers are exploiting the so-called Johnson noise thermometry. The method uses the thermal motion of the electrons in a conductor as an indication of its temperature and does not require calibration.
The final goal is to develop a practical device capable of drift-free temperature sensors with an accuracy better than 1 °C and a response time of a few seconds.
Pressure and Flow Sensors for Nuclear Reactors
Depending on its size and configuration, a commercial nuclear reactor can employ up to a few thousand electromechanical transducers that convert fluid pressure to analog or digital signals. Such transducers can measure either absolute or differential pressure.
The electromechanical pressure transducer combines a mechanical system (usually involving an elastic sensing element, like a diaphragm or bellows, that deforms in response to the applied pressure) and an electronic sensor that converts displacement into an electrical signal proportional to the pressure.
Differential pressure sensors are suited for measuring coolant levels in pressurized vessels or flow rates in cooling lines by monitoring the pressure difference at different locations of the reactor cooling circuit.
Along with the Johnson noise thermometers, researchers and engineers are developing ultrasonic and magnetic sensors for fluid flow and level measurements suitable for future nuclear reactor designs operating at much higher temperatures and pressures.
References and Further Reading
INTERNATIONAL ATOMIC ENERGY AGENCY (2011) Core Knowledge on Instrumentation and Control Systems in Nuclear Power Plants, IAEA Nuclear Energy Series No. NP-T-3.12, IAEA, Vienna. Available at: https://www-pub.iaea.org/MTCD/Publications/PDF/Pub1495_web.pdf
Qiu-kuan, W., et al. (2012) Advanced Measuring (Instrumentation) Methods for Nuclear Installations: A Review. Science and Technology of Nuclear Installations, 2012, 672876. Available at: https://doi.org/10.1155/2012/672876
Korsah, K., et al. (2017) Assessment of sensor technologies for advanced reactors - 250. United States: American Nuclear Society - ANS. Available at: https://info.ornl.gov/sites/publications/files/Pub68822.pdf