Recent advances in design methodology for the sensitivity field of capacitive control rod position detectors in small modular reactors

The control rod drive mechanism (CRDM) actuates the control rods to perform critical operational functions such as holding rod positions, incremental upward (step-up), downward (step-down) movements, and rapid drops (scram). Control rod assemblies typically include neutron-absorbing materials. By the precise adjustment of the axial positions of these assemblies within the reactor core, a nuclear reactor's reactivity can be effectively regulated. Therefore, the positions of the control rods serve as key indicators of the reactor's reactivity state and constitute one of the primary parameters requiring continuous real-time monitoring during operation. Rod position detectors are critical components that directly measure the actual positions of control rods, providing essential signals to the reactor control and protection system. Consequently, the accuracy and reliability of these detectors directly influence the safety performance of reactor operation.

Inductive rod position detectors are the most widely used type in pressurized water reactors (PWRs) and small modular reactors (SMRs). Their configuration typically includes a primary coil and multiple secondary coil groups. As the control rod's measuring rod advances in discrete mechanical steps, the magnetic coupling between the primary and secondary coils varies, modulating the output voltage signals from the secondary coils. These detectors are specifically designed to interface with step-driven CRDMs. By encoding the voltage outputs from individual secondary coil groups, the detector produces discrete rod position "step" signals, each representing an integer multiple of the control rod's fundamental mechanical step distance. Accordingly, inductive rod position detectors provide only discrete positional information and cannot deliver continuous analog data. For reactors requiring precise rod position control, inductive detectors are inadequate due to their limited measurement accuracy. However, capacitance sensing technology has the potential to achieve continuous recognition of control rod positions, thus serving as a benchmark for precise regulation of nuclear reactor reactivity.

The nonuniformity of the sensitivity field is the key factor affecting the accuracy of capacitive rod position detectors. Measurement errors in detectors designed for SMRs are proportional to the product of the sensitivity field's nonuniformity and the insertion depth of the measuring rod. The nonuniformity of the sensitivity field depends on the radial displacement and the deflection angle of the measuring rod. Thus, to ensure the accuracy of capacitive control rod position detectors, the sensitivity fields must be properly designed and regulated. The sensitivity mechanism and rod position recognition model of the detectors are described herein, and the characteristics of the sensitivity fields are summarized. In addition, the design strategies for the sensitivity field are discussed. Future research directions for this technology are also proposed.