Reactor Types
Listed below are a number of nuclear reactors
currently in use around the world today:
Pressurised Water Reactor
(PWR)
These reactors are the most widely used reactors
in the world for power generation. They are also
used for propulsion of nuclear submarines by generating
heat to turn a high speed turbine.
Features include:
- Fuel - Enriched uranium dioxide (UO2),
which contains about 3.2% uranium-235 (U235)
- Moderator - Water
- Control Rods - Boron
- Coolant - Water
- Thermal Efficiency (ratio of electricty generated/heat
generated) - approx 32%
Operation: Water in the reactor core reaches
about 325 DegC but remains in liquid form under
about 150 times atmospheric pressure to prevent
it boiling. This pressure is maintained in the
reactor vessel by the steam in a pressuriser.
This water then passes its heat on to water in
a secondary circuit causing this water to boil
and produce steam to turn the turbine.
A safety feature of the PWR is the negative void
coefficient. If the reactor core gets too hot,
the water in the moderator turns to steam and
therefore there is no moderator left to slow the
neutrons down and hence the fission reaction would
stop. This negative feedback effect is one of
the advantages of Pressurised Water Reactors.
Diagram taken from: http://www.nrc.gov/reading-rm/basic-ref/students/animated-pwr.html
Boiling Water Reactor (BWR)
These reactors have many similar features to
the Pressurised Water Reactors and are a popular
design in countries such as Japan, Sweden and
the USA.
Features include:
- Fuel - Enriched uranium dioxide (UO2)
- Moderator - Water
- Control Rods - Boron
- Coolant - Water
- Thermal Efficiency - greater than that for
a PWR
Operation: Unlike the PWR, the BWR has
no secondary circuit and the steam that turns
the turbine is produced in the reactor core rather
than in a steam generator. This water in the reactor
core boils at about 285oC. Reactor power can be
controlled by inserting or withdrawing control
rods but also by changing the amount of water
flowing through the reactor core. As the amount
of liquid water in the core increases, neutron
moderation is increased and hence reactor power
increases. However, as the amount of liquid water
in the core decreases, neutron moderation decreases,
fewer neutrons are slowed down, and reactor power
decreases.
Advantages of BWR compared to PWR is that they
produce a greater thermal efficiency operating
at the same temperature, there is less heat exchange
equipment needed, and the pressure inside the
containment structure is lower. However, disadvantages
include contamination of the turbine due to the
water being in contact with the fuel and higher
and more frequent maintenance needed for these
reactors.
Diagram taken from: http://www.nrc.gov/reading-rm/basic-ref/students/animated-bwr.html
CANDU Reactor
CANDU (or CANada Deuterium Uranium) reactors
are a pressurised heavy water reactor that uses
unenriched natural uranium as its fuel source.
Therefore, in order to increase its efficiency,
it uses a more efficient moderator in heavy water
(deuterium oxide D2O).
Whilst heavy water is expensive, the reactor can
operate without expensive fuel enrichment facilities
thus balancing the costs. All reactors in Canada
are of the CANDU type, but these reactors have
been marketed overseas as well.
Features include:
- Fuel - Natural uranium dioxide (UO2)
- Moderator - Heavy Water (deuterium oxide D2O)
- Control Rods - Cadmium
- Coolant - Heavy Water (deuterium oxide D2O)
- Fuel Use - Is the most efficient of any reactor
type
Operation: The heavy water moderator
is contained in a large tank called a calandria.
Several hundred horizontal pressure tubes that
form channels for the fuel penetrate the calandria.
As in the PWR, the primary coolant generates steam
in a secondary circuit to drive the turbines.
This reactor has the least down-time of any known
type. This is due to the unique fuel-handling
system. The pressure tubes containing the fuel
rods can be individually opened, and the fuel
rods changed without taking the reactor out of
service.
Diagram taken from: http://www.uic.com.au/nip64.htm
RBMK (High Power Channel
Reactor)
These reactors were designed in the Soviet Union
and are a pressurised water reactor with individual
fuel channels. These reactors were designed and
used for both plutonium production and power generation.
Features include:
- Fuel - Low-enriched uranium dioxide (UO2),
which contains about 1.8% uranium-235 (U235)
- Moderator - Graphite
- Control Rods - Boron carbide
- Coolant - Water
Operation: The structure of the reactor
consists of a large graphite core containing around
1700 vertical channels, each containing enriched
uranium dioxide fuel. Heat is removed from the
fuel by pumping water up through the channels
where it is allowed to boil and pass into steam
drums to drive electrical turbine-generators.
The combination of graphite moderator and water
coolant is found in no other power reactors. The
design characteristics of the reactor mean that
it is unstable at low power levels, and this was
shown in the Chernobyl accident. The instability
is due primarily to control rod design and a positive
void coefficient. The water that becomes steam
tends to increase the rate at which the nuclear
reaction proceeds. In a water-moderated reactor,
this effect is countered by the reduction in moderation,
but in the RBMK the moderating effect of the graphite
continues to slow down neutrons, and hence as
more steam is created, there is a further increase
in power generation. This is known as the positive
void coefficient.
Diagram taken from: http://www.fatherryan.org/nuclearincidents/rbmk.htm
Magnox Reactor
The Magnox reactors were the world's first commercial
scale nuclear reactor built in the United Kingdom
(U.K.) in the 1950's through to the 1970's. A
total of 26 Magnox reactors were built in the
U.K. Currently eight remain in operation, but
they will be decommissioned by 2010.
Features include:
- Fuel - Natural uranium, which contains about
0.7% of the fissile isotope uranium-235 (U235)
and around 99.2% uranium-238 (U238).
- Moderator - Graphite
- Control Rods - Boron steel
- Coolant - Gaseous carbon dioxide (CO2)
- Thermal Efficiency - Initially very low 22%
but was improved to 28%
Advanced Gas Cooled Reactor
(AGR)
These reactors are the second generation of British
gas-cooled reactors using graphite as the neutron
moderator and carbon dioxide as the coolant. The
AGR was adopted as the standard reactor in the
United Kingdom and 14 were built starting operation
between 1976-1989. These reactors are exclusive
to the UK.
Features include:
- Fuel -enriched uranium oxide pellets, which
contain between 2.5-3.5% uranium-235 (U235)
- Moderator - Graphite
- Control Rods - Boron
- Coolant - Carbon dioxide (CO2)
- Thermal Efficiency - very high at 41%
Operation: The carbon dioxide circulates
through the core, reaching 640°C and then
passes through steam generator tubes, which are
still within the concrete and steel pressure vessel.
Control rods penetrate the moderator and a secondary
shutdown system involves injecting nitrogen into
the coolant.
The reactor core is usually larger than that
of a PWR in order to produce the same power output.
Whilst this type of reactor has the best thermal
efficiency, this advantage is shadowed by its
fuel efficiency which tends to be less than other
reactors.
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