Development of Control Rod Position Indicator Using Seismic-Resistance Reed Switches

The system-integrated reactor currently under development at the Korea Atomic Energy Research Institute is being designed with a soluble boron free operation and the use of a nuclear heating for the reactor start-up. These design features require a Control Element Drive Mechanism (CEDM) for the integral reactor to have a fine-step movement capability as well as a high reliability for a fine reactivity control. Also the reliability and accuracy of the information for the control rod position is very important to the reactor safety as well as the design of the core protection system. The position indicator is classified as a Class 1E component because the rod position signal of the position indicator is used in the safety related systems. Therefore it will be separated from the control systems to the extent that a failure of any single control system component of a channel and will have sufficient independence, redundancy, and testability to perform its safety functions assuming a single failure. The position indicator is composed of a permanent magnet, reed switches and a voltage divider. Four independent position indicators around the upper pressure housing provide an indication of the position of a control rod comprising of a permanent magnet with a magnetic field concentrator which moves with the extension shaft connected to the control rod. The arranged reed switches are positioned along a line parallel to the path of the movement of the permanent magnet and it is activated selectively when the permanent magnet passes by. The traditional reed switch is sealed in a glass tube filled with an inert gas. So it is easily damaged during an assembly and test. Therefore, this research adopts seismic-resistance reed switches to the position indicator in order to reduce the damages or impacts during the handling of the position indicator and earthquake. The control rod position indicator having a seismic-resistance characteristic for the integral reactor was developed and tested. The performance test of a position indicator with a compact arrangement of the seismic-resistance reed switches for a high resolution was successfully carried out. The results of this study have shown that a position indicator including seismic-resistance reed switches and a compact arrangement can be applied effectively to the integral reactor CEDM after the environmental tests.