Fission reactors, popularly called “nuclear energy”, operate by allowing neutrons to split or “fission” nuclei of certain elements such as uranium, plutonium or thorium. The fission releases more neutrons to drive a “chain reaction”. Fission reactors have some inherent drawbacks. First, they are expensive—they must be relatively complex to run safely and they produce heat to run large and expensive steam turbines and generators, ase fossil fuel plants do. Second, they are liable to serious accidents because they store year’s worth of fission fuel within the reactor, so an accident can cause the reaction of that fuel, sometimes producing a melt-down of the fuel and relapse of radioactivity to the environment. Third they produce radioactive waste—radioactive material whose radioactivity is at a high enough level to endanger human life and which lasts for many years.
In contrast, focus fusion stores very little fuel within the generator and only a tiny bit of that fuels is heated to the billion of degrees needed for fusion reactions in each pulse. A complex sequence of events is needed to generate the next pulse, so that an uncontrolled release of energy is simply impossible. If something happens to the generator, it just stops.