Final answer:
An uncontrolled nuclear fission reaction leads to a nuclear explosion, unlike the controlled reactions used in nuclear power reactors to generate electricity.
Step-by-step explanation:
An uncontrolled nuclear fission reaction results in a nuclear explosion. Nuclear fission can be controlled to generate power within a nuclear power reactor, which is a facility designed to produce electricity. In these power reactors, a controlled chain reaction is maintained, where typically only one neutron released from each fission event causes another fission, allowing the reaction to continue in a regulated manner. This controlled process generates heat, which is then used to boil water, produce steam, and turn turbines to generate electricity. Countries like France, the US, and China utilize nuclear reactors extensively for their electricity needs. However, when the reaction is uncontrolled and causes more than one neutron to induce further fissions, the process increases exponentially and can lead to a nuclear explosion, much like the detonation of a nuclear weapon. This is fundamentally different from the controlled reactions within a power reactor.