A damping area or dash pot on the upper ends of control rods absorb energy from dropped control rod assemblies without narrowing the diameter of guide tubes. As a result, coolant can freely flow through the guide tubes reducing boiling water issues. The dampening area reduces a separation distance between an outside surface of the control rod and an inside surface of the guide tubes decelerating the control rods when entering a top end of the guide tubes. In another example, the dampening area may be located on a drive shaft. The dampening area may have a larger diameter than an opening in a drive shaft support member that decelerates the drive shaft when dropped by a drive mechanism.

A damping area or dash pot on the upper ends of control rods absorb energy from dropped control rod assemblies without narrowing the diameter of guide tubes. As a result, coolant can freely flow through the guide tubes reducing boiling water issues. The dampening area reduces a separation distance between an outside surface of the control rod and an inside surface of the guide tubes decelerating the control rods when entering a top end of the guide tubes. In another example, the dampening area may be located on a drive shaft. The dampening area may have a larger diameter than an opening in a drive shaft support member that decelerates the drive shaft when dropped by a drive mechanism.

A nuclear reactor comprising: a core of nuclear fuel having a height; a pressure vessel surrounding the core of nuclear fuel to allow circulation of gas there through; a set of neutron-absorbing control rods movable for insertion and withdrawal into and out of the core along a respective axis for control of a nuclear reaction in the core, each of the neutron-absorbing control rods comprising mutually sliding elements moving relative to each other between an extended position separated along the axis in a first direction over a first length and a compacted position overlapping over a second length less than the first length and less than 51% of the core height; and a control rod mechanism communicating with the control rods to move them for insertion and withdraw into and out of the core.

A nuclear reactor comprising: a core of nuclear fuel having a height; a pressure vessel surrounding the core of nuclear fuel to allow circulation of gas there through; a set of neutron-absorbing control rods movable for insertion and withdrawal into and out of the core along a respective axis for control of a nuclear reaction in the core, each of the neutron-absorbing control rods comprising mutually sliding elements moving relative to each other between an extended position separated along the axis in a first direction over a first length and a compacted position overlapping over a second length less than the first length and less than 51% of the core height; and a control rod mechanism communicating with the control rods to move them for insertion and withdraw into and out of the core.