What happens when bundles of fuel rods are bombarded by neutrons?

When bundles of fuel rods are bombarded by neutrons, a nuclear chain reaction occurs, leading to the splitting (fission) of uranium atoms in the fuel rods. This releases energy in the form of heat and more neutrons, which can trigger additional fission reactions in neighboring fuel rods, sustaining the chain reaction. This process is controlled in nuclear reactors to generate heat for electricity production.