Why is fission easier than fusion?
There are two reasons:
- It takes much more energy to bring nuclei together than to break them apart, for reasons described below.
- Fusion releases much more energy per nucleon, making it harder to contain.
In fission, a neutron is typically 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. Some radioactive decays release neutrons too, so if a fissionable quantity of a fissionable element is put together in one place (a 'critical mass'), fission can occur spontaneously.
Fusion requires two small nuclei (hydrogen - often deuterium or tritium, or helium) have to collide. Since both are positively charged, it requires huge amounts of energy - temperatures like those occurring on the Sun - to overcome electrostatic repulsion and bring them close enough to each other for the Strong Nuclear Force to take over and cause the reaction to happen.
This can occur in fusion bombs (hydrogen bombs), which use a fission bomb to create the conditions of temperature and pressure required for fission to occur.
Making contained fusion work for power generation is incredibly challenging: millions of degrees is higher than the melting point of any material we know, so there is no way to contain the fusion reaction in a physical container.
Current efforts are focused on using magnetic fields instead to contain the fusion reaction, and usually lasers to create the required temperature and pressure. This is very difficult to create and maintain.