Fission Reactors: Components
The fuel: The fissile material that is used in the
chain reaction to produce heat. The main fuels used in nuclear
reactors are uranium-235 and plutonium-293. These fuels are mixed
with other elements to create fuel that exhibits certain properties
which depend on the design of the reactor in which the fuel will be
used.
Moderator: The moderator serves to slow down the
neutrons that are emitted in the nuclear chain reaction. Some common
moderators are graphite, light and heavy water (which can also serve
as coolant).
Coolant: The coolant is also sometimes refereed to
as the working fluid, this gets heated up during the sustained chain
reaction within the core and is then usually circulated away to a
heat exchanger where energy is removed from the coolant and used to
power a steam turbine. The cooler coolant is then circulated back
through the get heated up again and the cycle is repeated. Some
common coolants are:
Light Water
Most currently operating nuclear reactors use light water under
high pressure as working fluid. Around one third of these light
water reactors are boiling water reactors where the steam is used to
spin turbines directly. The other two thirds of light water reactors
pressurize the water even further to keep it from boiling off, thus
having to pump the water away to a turbine under higher pressure
conditions. A third type of light water working fluid,
super-critical water was proposed which would be able to utilize
higher temperatures and pressures, thus increasing efficiency.
Heavy Water
Heavy water is deuterium oxide which is almost the same as light
water, expect for the fact that it serves as a better moderator.
Molten Metal
Molten metal is used in fast neutron reactors since these reactors
do not need neutron moderation. The most common metal that is used
is sodium. Lead has also been used.
Gas
Gases such as helium, and carbon dioxide are also used as coolants
since they are largely inert to the nuclear chemical reaction.
Control Rods: Control rods, also known as nuclear
poisons, are used when a reactor is getting too hot, the control
rods can be raised and lowered within the reactor core to manipulate
the heat of the reactor. Control rods are usually made out of boron,
silver, indium, or cadmium and have the ability to absorb neutrons
without fissioning, thus slowing the chain reaction down. The
material used in a control rod is dependent on the reactor it is
designed for. A common control rod consists of a
silver-indium-cadmium alloy in a 80/15/5 ratio respectively. The
pros and cons of various control materials is as follows:
Hafnium: Excellent control for
water-cooled, water-moderated reactors due to neutronic, mechanical,
and physical properties. However, suffers limited availability and
high cost. [4]
Silver-Indium-Cadmium Alloys:
Highly effective neutron absorber. Control effectiveness in
water-moderated reactors is close to hafnium. Used in
pressurized-water reactors. Easily fabricated and adequate in
strength. However, the alloy must be enclosed in stainless steel
tubing to protect it from corrosion. [4]
Boron: Very high
thermal-absorption cross-section and low cost. Commonly used in
thermal reactors for control rods and burnable poison. However,
boron is nonmetallic and must be incorporated into a metallic
material to be used as a control rod. [4]
Concrete shielding: Concrete walls are built
around the reactor to protect people and the environment from the
radiation that is produced by the nuclear chain reaction.
Next: Fission Reactors: Common Designs
Previous: Fission Reactors: Overview
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