The generic term for all kinds of energy produced by nuclear processes. This includes nuclear fission and nuclear fusion.
Fission and Fusion
Nuclear fission is a well known concept used in nuclear power generation by different types of nuclear reactors since the 1950s.
Nuclear fusion is a well known process in physics and used by our sun to produce the energy which keeps all processes on earth including our existence alive. Heat generation by fusion processes is tested in some fusion reactors which have an early experimental stage which will lead to a first prototype power plant in the mid of 21st century.
The whole chain
The use of nuclear energy depends on nuclear fuels like uranium-233, uranium-235, thorium-232 or -- for nuclear fusion -- on deuterium, tritium or helium-3.
Each use of nuclear energy produces radioactive waste consisting of a mixture of more or less instable isotopes wich produce nuclear radiation for days, months, years or several millions of years.
Problems and the need for discussions
The safe storage of the waste of todays fission reactors has to be quaranteed for hundreds of thousands of years which is impossible. A future use of fission reactors depends strongly on methods to avoid the long living nuclear waste by new mixtures of nuclear fuels and advanced reactor designs. An option is the destruction of critical isotopes by transmutation or specialized reactors. For nuclear fission reactors less critical nuclear waste is expected: Mainly the structural parts of a hypothetical fusion reactor were degraded by the bombardment with neutrons during operation. They have to be exchanged from time to time and deposited. These parts of the reactor emit low amounts of radiation and the isotopes are short-living.
There is a strong debate about the usage of nuclear energy: Nuclear fission has many risks -- mainly proliferation, the risk of the misuse of nuclear material for dirty and real nuclear bombs and the radioactive waste disposal. But the increasing climate change needs action to reduce the carbon dioxide emissions dramatically to reduce the greenhouse effect in earths atmosphere.