A control rod for a nuclear reactor is provided to reduce swelling and heat in a neutron absorbing material of a dashpot region of a fuel assembly guide thimble when inserting a gray control rod into a core of the nuclear reactor completely. A long pipe cladding(40) includes an external size to match the most narrow size inside the hollow of the control rod guide thimble of the nuclear fuel assembly. A first end plug is slidingly received in the dashpot(44). A first neutron absorbing material is larger than the received length in the dashpot or the axial distance of the first neutron absorbing material is extended through the long pipe cladding. A second neutron absorbing material has a higher reactive value than the first neutron absorbing material. A second terminal plug is finished in the second end of the long pipe cladding. The inside of the hollow of the control rod guide thimble is composed of an upper end unit receiving the control rod and a lower end unit having a reduced inner diameter part serving as the dashpot.
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