What happens during nuclear fission?

During nuclear fission, a heavy atomic nucleus, such as that of uranium, absorbs a neutron. This absorption causes the nucleus to become unstable and split into two or more smaller nuclei, along with the release of a significant amount of energy. In the case of option B, firing a neutron into the nucleus of a uranium atom is the initiating event that triggers the fission reaction.

The process releases additional neutrons, which can then cause further fission events in a chain reaction, leading to a large release of energy. This is the principle behind both nuclear reactors and atomic bombs.

In the context of the other choices, option A inaccurately describes fusion, where lighter nuclei combine to form a heavier nucleus. Option C suggests that energy is absorbed, but in fission, energy is released. Option D implies that protons in the nucleus are absorbed, which is not a characteristic of nuclear fission as the focus is primarily on neutrons causing the splitting of the nucleus, not proton absorption.