A nuclear reactor produces and controls the release of energy from
splitting the atoms of certain elements.
In a nuclear power reactor, the energy released is used as heat to make
steam to generate electricity. Basic Components of
Most Types of Reactors:
Uranium is the basic fuel. Usually pellets of uranium oxide (UO2) are
arranged in tubes to form fuel rods. The rods are arranged into fuel
assemblies in the reactor core.
In a 1000 MWe class PWR there might be 51,000 fuel rods with over 18
In a new reactor with new fuel a neutron source is needed to get the
reaction going. Usually this is beryllium mixed with polonium, radium
or other alpha-emitter. Alpha particles from the decay cause a release
of neutrons from the beryllium as it turns to carbon-12. Restarting a
reactor with some used fuel may not require this, as there may be
enough neutrons to achieve criticality when control rods are removed.
In the core which slows down the neutrons released from fission so
that they cause more fission. It is usually water, but may be heavy
water or graphite.
These are made with neutron-absorbing material such as cadmium,
hafnium or boron, and are inserted or withdrawn from the core to
control the rate of reaction, or to halt it.
A fluid circulating through the core so as to transfer the heat from
Pressure vessel or pressure tubes
Usually a robust steel vessel containing the reactor core and
moderator/coolant, but it may be a series of tubes holding the fuel
and conveying the coolant through the surrounding moderator.
Part of the cooling system of pressurised water reactors (PWR &
PHWR) where the high-pressure primary coolant bringing heat from the
reactor is used to make steam for the turbine, in a secondary circuit.
Essentially a heat exchanger like in a family hot water system*
The structure around the reactor and associated steam generators which
is designed to protect it from outside intrusion and to protect those
outside from the effects of radiation in case of any serious
malfunction inside. It is typically a metre-thick concrete and steel
structure. Newer Russian and some other reactors install core melt
localisation devices or 'core catchers' under the pressure vessel to
catch any melted core material in the event of a major accident.
Pressurised water reactor (PWR)
This is the most common type, with about 300 operable reactors for power
generation and several hundred more employed for naval propulsion. The
design of PWRs originated as a submarine power plant. PWRs use ordinary
water as both coolant and moderator.
The design is distinguished by having a primary cooling circuit which
flows through the core of the reactor under very high pressure, and a
secondary circuit in which steam is generated to drive the turbine. In
Russia these are known as VVER types – water-moderated and -cooled.
Boiling water reactor (BWR)
This type of reactor has many similarities to the PWR, except that there
is only a single circuit in which the water is at lower pressure (about 75
times atmospheric pressure) so that it boils in the core at about 285°C.
The reactor is designed to operate with 12-15% of the water in the top
part of the core as steam, and hence with less moderating effect and thus
efficiency there. The steam passes through drier plates (steam separators)
above the core and then directly to the turbines, which are thus part of
the reactor circuit.
Since the water around the core of a reactor is always contaminated with
traces of radionuclides, it means that the turbine must be shielded and
radiological protection provided during maintenance.
Pressurised heavy water
The PHWR reactor has been developed in Canada as the CANDU.
PHWRs generally use natural uranium (0.7% U-235) oxide as fuel, hence
needs a more efficient moderator, in this case heavy water (D2O).**
The PHWR produces more energy per kilogram of mined uranium than other
The moderator is in a large tank called a calandria, penetrated by several
hundred horizontal pressure tubes which form channels for the fuel, cooled
by a flow of heavy water under high pressure (about 100 times atmospheric
pressure) in the primary cooling circuit, typically reaching 290°C.
As in the PWR, the primary coolant generates steam in a secondary circuit
to drive the turbines. The pressure tube design means that the reactor can
be refuelled progressively without shutting down, by isolating individual
pressure tubes from the cooling circuit.
CANDU reactors can accept a variety of fuels. They may be run on recycled
uranium from reprocessing LWR used fuel, or a blend of this and depleted
uranium left over from enrichment plants. About 4000 MWe of PWR might then
fuel 1000 MWe of CANDU capacity, with addition of depleted uranium.
Thorium may also be used in fuel.
Advanced gas-cooled reactor
These are the second generation of British gas-cooled reactors, using
graphite moderator and carbon dioxide as primary coolant. The fuel is
uranium oxide pellets, enriched to 2.5 - 3.5%, in stainless steel tubes.
The carbon dioxide circulates through the core, reaching 650°C and then
past steam generator tubes outside it, but still inside the concrete and
steel pressure vessel (hence 'integral' design).
Control rods penetrate the moderator and a secondary shutdown system
involves injecting nitrogen to the coolant. The high temperature gives it
a high thermal efficiency – about 41%.
graphite-moderated reactor (LWGR)
The main LWGR design is the RBMK, a Soviet design, developed from
plutonium production reactors. It employs long (7 metre) vertical pressure
tubes running through graphite moderator, and is cooled by water, which is
allowed to boil in the core at 290°C and at about 6.9 MPa, much as in a
Fuel is low-enriched uranium oxide made up into fuel assemblies 3.5 metres
long. With moderation largely due to the fixed graphite, excess boiling
simply reduces the cooling and neutron absorbtion without inhibiting the
fission reaction, and a positive feedback problem can arise, which is why
they have never been built outside the Soviet Union.
Fast neutron reactors (FNR)
Some reactors do not have a moderator and utilise fast neutrons,
generating power from plutonium while making more of it from the U-238
isotope in or around the fuel.
While they get more than 60 times as much energy from the original uranium
compared with the normal reactors, they are expensive to build.
If they are configured to produce more fissile material (plutonium) than
they consume they are called fast breeder reactors (FBR).