Performance Analysis of Oil Dashpot in Control and Safety Rod Drive Mechanism of Prototype Fast Breeder Reactor

摘要

In Prototype Fast Breeder Reactor, there are two independent, fast acting, diverse shutdown systems each comprising of sensors, logic, circuit, Drive Mechanisms and neutron absorber rods having B_4C pellets. The absorber rod of the first system is known as Control &Safety Rod (CSR) and its drive mechanism is known as Control & Safety Rod Drive Mechanism (CSRDM). The functions of CSRDM are to facilitate start-up, reactor control, controlled shutdown and emergency shutdown by scram (Safety Control Rod Accelerated Movement) action. During normal operation of the reactor the CSRs are all partially inserted in the core. During emergency shutdown the scram release electromagnet gets de-energised and the Mobile Assembly (MA) of CSRDM along with CSR drops under gravity into the active core. The maximum drop height is 1085 mm. At the end of free fall there is an oil dashpot which acts as a damper to absorb the kinetic energy of the falling mass. Proper functioning and performance of the oil dashpot is critical for the healthiness of the shutdown system. In the present study, Characterisation of flow parameters in the oil dashpot using a commercial Computational Fluid Dynamics (CFD) code to obtain a relation between velocity and pressure drop in the dashpot and implementation of the flow characteristics in the performance analysis code to obtain the performance characteristics of the dashpot has been done. The performance analysis is performed by mathematical modeling of the dashpot system as spring-mass-damper two degree of freedom system. This paper gives a brief description of Control and Safety Rod Drive Mechanism and oil dashpot, the methodology of modeling, flow characterisation in dashpot and presents the results of performance analysis of oil dashpot.