Control rod drop-time reduction in typical swimming pool research reactors

Abstract

Control rods play an important role in the safety of nuclear reactors, due to the fact that in the emergency situations, they are to be inserted into the reactor core and scram the reactor secure and fast. As far as safety concerns, drop-time of control rods should be minimized as much as possible. In this paper as a case study, TRR (Tehran Research Reactor) data is chosen as a typical swimming pool reactor and drop-time of control safety rod is to be minimized. Present fork type control rods are more prone to entanglement than former tube type. Methods to ease this problem are considered. Several approaches are studied and proposed in this work aimed to decrease the drop-time of control rods especially in case of emergency. Changing control rod shape into a more aerodynamic form, helps to reduce drag force and thus decreasing drop-time in an aqueous medium. Further improvement is achieved by provision of several holes in the head section. As fluid dynamic computations suggest, drop-time may reduce as much as 6% due to geometry modification. Focusing on electronic part, improved design on magnet (Coil and its core together) could reduce the lag time between scram signal issuance till control rod release. Using magnet with modified air gaps showed that existing drop-time from about 800 ms, could be reduced by as much as 34%. Moreover, by using amorphous core instead of existing one, drop-time is expected to reduce from 800 ms by as much as 30%.Measurements showed that existing electromechanical circuitry of relays, has an inherent 3 ms delay which is not so much compared to 800 ms total scram time. Therefore, working on this part is not a big deal with respect to other modifications and therefore is disregarded.