A nuclear power plant can be basically divided into the nuclear part
and the conventional part. In the nuclear part the fission energy is
converted into heat which is used to produce steam. In the
conventional part this steam runs the turbine connected to the
generator. The conventional part of a nuclear power plant is very
similar to a conventional thermal power plant from the boiler
onwards. The nuclear part is also called the Nuclear Steam Supply
System (NSSS); its main component is the nuclear reactor where the
nuclear fission chain reaction takes place.
The reactor contains nuclear fuel, which consists of uranium
(sometimes mixed with plutonium). There are, however, two main
kinds of uranium atom, called isotopes: the majority (99.3%) of the
atoms are uranium-238 or 238U, and a small minority (0.7%) is
uranium-235 or 235U. Only 235U can sustain a nuclear fission chain
reaction. Nuclear fuel can be made from natural uranium (with only
0.7% of 235U) or enriched uranium where the share of 235U is
artificially increased (usually up to 5 %).
The chain reaction is maintained by subatomic particles called
neutrons. The neutrons born during fission are very energetic and are
called fast neutrons. For inducing further fissions, however, the most
efficient are slow or thermal neutrons which have low kinetic energy
(0.025 eV). A reactor must therefore have means to slow down
neutrons. This is the role of the moderator, a material made of light
elements. The most usual moderator is ordinary water, also called
light water (H2O).
If ordinary hydrogen 1H is replaced with its heavy isotope deuterium
2H (denoted also D), its compound with oxygen is called heavy water
D2O. Heavy water is a more efficient moderator than light water,
because it absorbs almost no neutrons in the process of slowing them
down. A reactor with heavy water can use natural uranium as fuel,
while a light water reactor requires enriched uranium.
A third possible moderator is graphite which is a form of carbon.
Similarly to heavy water, graphite-moderated reactors can also use
natural uranium fuel. In fact, the first nuclear reactor, built by Enrico
Fermi in 1942, used graphite as the moderator.
Nuclear reactions produce large quantities of heat which must be
transferred from the fuel. This is the role of the reactor coolant. The
coolant must be in liquid or gaseous form and should not absorb
neutrons substantially. In many cases the reactor coolant and
moderator are the same substance, but this is not necessary. In
particular, graphite-moderated reactors are cooled with gas (CO2, He)
or light water.
Each reactor also has a control system, which is used to start-up the
reactor, to shut it down, and to adjust the reactor power level. The
control system contains materials that are strong neutron absorbers
(boron, indium, cadmium…).
There are several classes (types) of nuclear power plant. The basic
distinction between them is defined by the type of fuel, the moderator
and the coolant. Enjoy<3

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