Sensor development for position detection of diverse safety rod of fast reactor

Abstract

The 500MWe sodium cooled prototype fast breeder reactor (PFBR) presently under advanced stage of construction has two diverse and independent shut-down systems. Each shutdown system comprises of independent sensing systems, control logic and drive mechanisms. The drive mechanism belonging to the first shutdown system is used for controlling reactor power and also for shutting down the reactor. It is known as control and safety drive rod mechanism (CSRDM). The mechanism belonging to the second shutdown system is used only for shutting down the reactor and is called diverse safety rod drive mechanism (DSRDM). CSRDM has potentiometer which is used to determine the position of the control and safety rod (CSR). The free fall time of control and safety rod can also be measured with the help of the potentiometer. Constraint of putting similar type of instrument in DSRDM has prohibited position detection of DSR as well as measurement of free fall time of diverse safety rod (DSR). However center has embarked on development of three different techniques to detect the position and free fall time of DSR. These techniques are Kalman filtering technique using neutronic signal to detect position of DSR, measurement of acoustic signal which is produced when DSR hits the dashpot and the eddy current based technique. Description of development of eddy current based sensor to detect position and free fall time of DSR is the main aim of this paper. The sensor consists of five coils which are inductively coupled. The change in position of DSR causes change in inductance of the coil and hence voltage induced in the coil. The measurement of coil voltage reveals the position of the DSR. Sodium testing of the sensor has been carried out on a 1:1 model of the sensor. The performance of the sensor at various temperatures and frequencies has been studied and found satisfactory. The sodium testing has shown encouraging results and have proven the feasibility of using such a sensor for the reactor application.The piston of the DSR is made of modified 9Cr–1Mo magnetic steel. A sensor winding is placed around the dashpot, the inductance of the winding changes when the DSR is deposited in the dashpot. Transformer coupling between two windings is then used to measure the inductance change of the sensor winding to facilitate non-contact measurement. The winding configuration has been chosen such that the signal can be picked-up even when there is misalignment between the coils due to thermal and radiation induced dimensional changes.