In a nuclear fission reactor, each fission of a uranium nucleus is accompanied by the emission of one or more high-speed neutrons, which travel through the surrounding material. If one of theneutrons is captured in another uranium nucleus, it can trigger fission, which produces more fast neutrons, which could make possible a chain reaction

The nuclear fission reaction of Uraniumand $\left(U\right)$ and Selenium $\left(Se\right)$neutrons.

The nuclear fission is the splitting of a heavy atomic nucleus, such as uranium or plutonium, into two roughly equal mass fragments. A tremendous amount of energy is released as a result of the process. average nucleon binding energy as a function of mass number

Because the mass of the uranium nucleus $\left(U-235\right)$is $235$times more than the mass of a neutron, fast neutrons passing through a block of uranium suffer minimal change in speed. A neutron colliding with a uranium nucleus is an example of a moving low-mass object colliding with a stationary high-mass object; the high-mass object has a very small change in velocity after the collision, and the low-mass object rebounds backward with nearly the same speed as before the collision (as the example from section $10.4$when the ping-pong ball hits bowling ball, the ping-pong ball rebounds backward with the same speed and the bowling ball has a small, non-zero velocity after the collision).