Reliability analysis of control rod drive mechanisms of FBTR for reactor startup and power control

Fast Breeder Test Reactor (FBTR) is a 40 MWt/13.2 MWe, mixed carbide fuelled, sodium cooled, loop type reactor. Its main aim is to generate experience in operation of fast reactors and sodium systems and to serve as an irradiation facility for development of fuels and materials for fast reactors. Control rods are used for rapid /controlled reactor shut down and regulation of reactor power. The above two functions are realized through mechanisms called control rod drive mechanism (CRDM). Each of the six control rods is provided with an independent mechanism which holds the control rod. The CRDM is in two parts, an upper part which is supported at the top and a mobile part which is coupled to upper part by an electro magnet. Interlocks are provided for CRDMs which ensure operation of drive motor of any of the mechanisms at a time to get either upward or downward movement of control rod. The electrical drive is used to raise the control rods one by one to predetermined level for taking reactor from shut down state to desired power level and regulation of power is done there after by manual control of control rods through CRDMs. For shutdown function, insertion of any two rods into the core is sufficient where as for reactor start up and regulation all the six rods and their drives are essential. Availability of control rods depends upon performance of various electrical & electronic components and mechanical components & its mechanisms. This paper details the salient observation made from reliability analysis of CRDMs for reactor operation of FBTR. Reliability analysis software "ISOGRAPH" was used for this analysis. The reliability, availability and the performance of CRDMs for reactor start up and power control is good.