Areva - Reference Document 2016

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BUSINESS OVERVIEW 06
A FEW FUNDAMENTAL CONCEPTS FOR AN
UNDERSTANDING OF NUCLEAR POWER
Since the beginning of this century, energy has been a centerpiece of many of
our society’s challenges, which may be summed up as the need to continue
to produce and consume energy without threatening the climate. If the share
of fossil fuels in the global energy mix is to be reduced from its current level
of more than 80%, low-carbon energy sources that do not affect the climate
must be developed, including nuclear power, capable of producing massive
quantities of electricity on demand, and renewable energies.
Using fission energy in nuclear power plants
A nuclear power plant is an electric generating station with one or more
reactors. Like all conventional thermal power plants, each reactor unit
consists of a steam supply system that converts water into steam, among
other things. The steam drives a turbine, which in turn drives a generator,
producing electricity.
A “nuclear reactor” is an industrial facility that produces heat from the energy
released by the fission of combustible atoms during a controlled chain
reaction. A “nuclear steam supply system” is the combination of equipment
used to produce steam from fission energy. A “nuclear island” is the system
encompassing the nuclear steam supply system and the fuel-related facilities,
as well as the equipment required for the system’s operation and safety. A
“conventional island” consists of the alternating current turbogenerator
coupled to the nuclear island, along with the equipment required for its
operation. A nuclear power plant thus consists primarily of a nuclear island
and a conventional island. The reactor is enclosed in a solid, leaktight building
meeting nuclear safety requirements. The three main components needed
to sustain, control and cool the fission process in the reactor core are fuel,
a moderator and a coolant. The combination of these three components
determines the reactor type or model. Several combinations have been tested,
but only a few of them have gone beyond the prototype stage to commercial
operation.
A heat source and a cooling source
Like all other thermal power plants, a nuclear power plant has a heat source
(the nuclear steam supply systemwith its fuel core and heat exchangers) and
a cooling source designed to condense steam after it has passed through
the turbine. That is why power plants are usually built near the sea or a river
– the water is used to cool the steam. Some power plants are also equipped
with cooling towers in which cooling water is dispersed like rain so that it will
evaporate, improving the efficiency of cooling and reducing the environmental
impacts (reduced withdrawal of water and elimination of thermal releases
to rivers).
A moderator and a coolant
During the fission process, neutrons are released at very high speed. As they
strike light atoms (hydrogen contained in water) and slow down, they react
much more with the uranium-235 atoms. That is how “thermal neutron” or
“slow” reactors function. The level of uranium-235 enrichment required for
the chain reaction is much lower than for “fast” reactors. In water reactors, the
water is used as a moderator, i.e. to slow the neutrons released by nuclear
fission, but it also serves as a coolant, i.e. the fluid that circulates in the reactor
core to extract heat.
2016 AREVA REFERENCE DOCUMENT 41