Nuclear Fusion is the energy source of the sun. For the reaction to occur, atomic nuclei must be brought into close proximity requiring extremely high temperatures. In the sun, the reacting nuclei are mainly hydrogen while in proposals for terrestrial fusion reactors the fuel is usually deuterium and tritium.

Nuclear Fission is a spontaneous reaction where a nucleus splits. Natural Nuclear Fission reactions take place in the core of the Earth and have been present in the crust of the Earth billions of years ago. 

Both fission and fusion produce energy by conversions into different elements which have lower masses, as indicated in the figure below.

Practicalities of fusion and fission nuclear reactors

In fission reactors, the nuclear processes are started spontaneously by the radioactive decay of an initial uranium nuclei which yields neutrons. These neutrons then go on to break up further uranium nuclei and produce a self sustaining chain reaction. This takes place in fuel rods in the reactor core which is typically cooled by water going to an electricity turbine. 

In Fusion reaction, the interactions do not start spontaneously and require extremely high temperature (about 100 million °C) to initiate. This happens in a gas mixture of deuterium and tritium from which the atomic electrons have been stripped, called a "plasma". The plasma must be contained and heated to a high temperature. For a sustainable fusion reactions to occur, the plasma environment must have specific parameters of density, temperature and containment time, as described on the page: Requirements for Energy from Fusion. Once the reactions are started, the heat is transported out of the plasma by neutrons that are stopped in a region of the reactor called the "Breeder  Blanket", which is cooled by a fluid going on to a turbine. 

The complexity of making a suitable plasma environment, as well as the difficulties of getting the heat out, are the reasons why fusion reactors are still not available more than 70 years after the first ideas. The first fission reactor was operational only a few year after the principles were first conceived.

Pros and cons of fusion and fission 

Below is a, subjective, comparison of fusion and fission power plants.

StatusR&D for 70 yrsOperational for 70 yrs
ReliabilityMajor doubt>80% of time
CO2 EmissionNoneNone
Fuel availabilityTritium scarceUranium abundant
RadiationMostly short lifetimeShort and long lifetime
SafetyComplex systemshistoric accidents

The cost numbers here are indicative and this critical item is discussed in a later section: Reactor Costs, Fusion compared to Fission .

Not all entries in this table are universally accepted by advocates of fusion. The present website defends the comparisons given in this table in the subsequent pages.