How a nuclear reactor makes electricity
A nuclear reactor produces and controls the release of energy
from splitting the atoms of uranium.
Uranium-fuelled nuclear power is a clean and efficient way of
boiling water to make steam which drives turbine generators. Except
for the reactor itself, a nuclear power station works like most
coal or gas-fired power stations.
The Reactor Core
Several hundred fuel assemblies containing thousands of small
pellets of ceramic uranium oxide fuel make up the core of a
reactor. For a reactor with an output of 1000 megawatts
(MWe), the core would contain about 75 tonnes of enriched
In the reactor core the U-235 isotope fissions or splits,
producing a lot of heat in a continuous process called a chain
reaction. The process depends on the presence of a moderator
such as water or graphite, and is fully controlled.
The moderator slows down the neutrons produced by fission of the
uranium nuclei so that they go on to produce more fissions.
Some of the U-238 in the reactor core is turned into plutonium
and about half of this is also fissioned similarly, providing about
one third of the reactor's energy output.
The fission products remain in the ceramic fuel and undergo
radioactive decay, releasing a bit more heat. They are the
main wastes from the process.
The reactor core sits inside a steel pressure vessel, so that
water around it remains liquid even at the operating temperature of
over 320°C. Steam is formed either above the reactor core or
in separate pressure vessels, and this drives the turbine to
produce electricity. The steam is then condensed and the
PWRs and BWRs
The main design is the pressurised water reactor (PWR) which has
water in its primary cooling/heat transfer circuit, and generates
steam in a secondary circuit. The less popular boiling water
reactor (BWR) makes steam in the primary circuit above the reactor
core, though it is still under considerable pressure. Both
types use water as both coolant and moderator, to slow
To maintain efficient reactor performance, about one-third or
half of the used fuel is removed every year or two, to be replaced
with fresh fuel.
The pressure vessel and any steam generators are housed in a
massive containment structure with reinforced concrete about 1.2
metres thick. This is to protect neighbours if there is a
major problem inside the reactor, and to protect the reactor from
Because some heat is generated from radioactive decay even after
the reactor is shut down, cooling systems are provided to remove
this heat as well as the main operational heat output.
Natural Prehistoric Reactors
The world's first nuclear reactors operated naturally in a
uranium deposit about two billion years ago in what is now
Gabon. These were in rich
uranium orebodies in the Earth's crust and moderated by percolating
rainwater. (At that time the uranium-235 isotope was more concentrated than it is today.)
Nuclear energy's contribution to global electricity supply
Nuclear energy supplies some 12% of the world's electricity.
Today 31 countries use nuclear energy to generate up to three
quarters of their electricity, and a substantial number of these
depend on it for one-quarter to one-third of their supply. Over 15,000 reactor-years of operational experience have been
accumulated since the 1950s by the world's 440 nuclear power
reactors (and nuclear reactors powering naval vessels have clocked
up a similar amount).