Why is fission easier than fusion?

Fission is the usual process in which a neutron is fired at a large and relatively unstable nucleus, which splits, releasing two (or more) smaller nuclei, usually one or more neutrons and a large amount of energy; however, fission can also happen spontaneously if a fissionable quantity of a fissionable element is concentrated in one location (a 'critical mass').

Fusion requires the collision of two positively charged small nuclei (hydrogen, usually deuterium, tritium, or helium) in order for the Strong Nuclear Force to take over and trigger the reaction. This requires enormous amounts of energy, similar to those found on the Sun, in order to overcome electrostatic repulsion and bring the nuclei close enough to one another.

This can happen in fusion bombs, also known as hydrogen bombs, which use a fission bomb to generate the pressure and temperature needed for fission to take place.

It is extremely difficult to make contained fusion work for power generation because the temperature of millions of degrees is higher than the melting point of any material that is known to exist, making it impossible to physically contain the fusion reaction.

Instead, current efforts concentrate on the very challenging task of creating and maintaining the necessary temperature and pressure, which is usually achieved with lasers, and magnetic fields to contain the fusion reaction.

Fission is easier than fusion because it involves splitting large atoms into smaller ones, a process that requires less energy and is more controllable than the fusion process of combining smaller atoms into larger ones. Additionally, fission reactions can occur at lower temperatures and pressures compared to the extreme conditions required for sustained fusion reactions.