Performance Test Results for AHR CRDM Damping Mechanism

The control absorber unit or control rod drive mechanism(CRDM) is the part of the reactor regulating system, and its function is to insert, withdraw or maintain neutron absorbing material at any required position within the reactor core, in order to control the reactivity of the core. The time required to insert the control rod into the core, generally refereed to as scram time, must be very important and optimized. A control rod with a fast dropping velocity would impose a substantial impact to reactor structure and to the rod itself. It is good to have a minimum drop time of control rod from the view point of safe shutdown of the reactor. But it is important to optimize the drop time considering the structural integrity and the life time due to the drop impact. The purpose of this experimental test is to verify the performance of the new concept of the CRDM proposed for the Advanced HANARO Research Reactor(AHR). The new concept for the CRDM damping mechanism for AHR is a dual-hydraulic cylinder type to reduce the excessive impact force by the distribution of the drop energy into the total stroke. The stroke times at interest points and the shock responses for the drop impact were repeatedly measured to obtain the rigorous results of the gravity falling of the control rod in a test pool. And then the results were also compared with those of the existing HANARO case. In this report, it will be studied and depicted the distinct phenomena for deceleration of shock responses and the increase of damping effects caused by the openings and a number of orifices on the piston rod. The shock response values for the AHR CRDM are only 15-20% of that for the HANARO, and we finally determined the optimum values for the design parameters