Patent Number: 
Section: claims

1. A control rod for inserting into a guide tube of a nuclear reactor fuel assembly of a nuclear reactor module, comprising:a first section including an active material for inserting into the guide tube and controlling a fission rate in a nuclear reactor core, the first section having a first outside diameter;a second section configured to attach to a head assembly of the nuclear reactor module;a dampening section located between the first section and the second section, wherein the dampening section has a second outside diameter larger than the first outside diameter such that, the dampening section reduces a separation distance between an outside surface of the control rod and an inside surface of the guide tube to decelerate the control rod when entering a top end of the guide tube; anda spring positioned inside the dampening section between the first section and the second section. 2. The control rod of claim 1, further comprising a cylindrical cladding including:a bottom portion at least partially defining the first section and retaining the active material, wherein the bottom portion has a first wall thickness; anda top portion at least partially defining the dampening section and having a second continuously increasing wall thickness. 3. The control rod of claim 2, wherein the spring is located in the top portion of the cladding above the active material. 4. The control rod of claim 1, wherein the second section has a third outside diameter, and wherein the second outside diameter of the dampening area continuously increases along the dampening section from the first outside diameter of the first section to the third outside diameter of the second section. 5. The control rod of claim 1, wherein the first section is shaped and sized to extend down to a bottom end of the guide tube when the dampening section enters a top end of the guide tube. 6. The control rod of claim 1, wherein the dampening section is shaped and sized to maintain a reduced separation distance when fully inserted into the top end of the guide tube.