What is the role of control rods in nuclear fission?

They absorb some or all of the neutrons released in the fission reaction, therefore controlling how many further fission reactions will occur.

Nuclear fission is where the nucleus of a bigger unstable element breaks down. When it does so, normally three neutrons are released hitting into other nuclei. This breaks down three more nuclei which in turn releases 3 more neutrons each. If this isn't slowed down it would spiral out of control, temperatures would soar and we'd have a nuclear meltdown. The job of the control rods is to absorb some of these neutrons. They are placed along side the nuclear material so a steady reaction can occur giving out a steady level of heat.

Control rods play an important role in controlling the rate of fission. They are often made of boron, and they absorb neutrons. The nuclear chain reaction can be slowed down and even stopped by inserting the rods. Removing them slightly will speed up the rate of fission. They are needed because excess neutrons are produced during fission. When a neutron collides with uranium-235 and splits into smaller nuclei, it also releases two or three neutrons. These neutrons will then collide with different uranium nuclei. However, only one neutron is needed to maintain the chain reaction at a steady rate, so the control rods absorb these extra neutrons.