The power generated by the nuclear reactor is maintained by the movement of control rods operated by reactivity control mechanisms. The control rod ejection due to any untoward forces is unacceptable under any circumstances in nuclear reactors as it will lead to positive reactivity insertion and considered as serious event. Opening of the vent line of reactivity control mechanism (RCM) can result in large differential pressure across the drive shaft which could eject the control rod cluster from the reactor core. If the provision to prevent rod ejection in reactivity control mechanism (RCM) malfunctions, rod ejection can happen during venting. Theoretical studies using CFD were carried out to estimate the forces acting on the drive shaft during venting. The drag forces were estimated for different clearances across the damper piston varying from 25 to 100 μm for the venting flow of 1.5–50 lpm for different types of mechanisms. The limiting venting flow rate for avoiding the rod ejection and size of flow restricting orifices at the downstream of the venting line to avoid the rod ejection during high pressure venting of RCMs are estimated for three different types of the damper piston arrangements.