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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).