WEBSITE: TECHNOLOGY
A Siona and Liron Benjamin
Contribution:
What is a Nuclear Reactor?
By
Siona Benjamin
1-What is a Nuclear
Reactor
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A nuclear reactor
is a system that contains and controls sustained nuclear chain reactions.
Reactors are used for generating electricity, producing radionuclides
(for industry and medicine), conducting research, and military purposes. All of
the various designs of power-producing reactors accomplish the same simple
task: spinning a generator. Many commercial reactors pass water over
heat-producing fuel rods to generate steam and run a turbine. Some designs call
for the passage of helium over a pile of heat-producing fuel pebbles. Yet
another design uses liquid sodium as a coolant.
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Components of a
Nuclear Reactor
Main
components
The core of the reactor contains all of the nuclear fuel
and generates all of the heat. It contains low-enriched uranium (<5%
U-235), control systems, and structural materials. The core can contain
hundreds of thousands of individual fuel pins. - The coolant is the material that passes through the core, transferring
the heat from the fuel to a turbine. It could be water, heavy-water,
liquid sodium, helium, or something else. In the
- The turbine transfers the heat from the coolant to
electricity, just like in a fossil-fuel plant.
- The containment is the structure that separates the reactor from
the environment. These are usually dome-shaped, made of high-density,
steel-reinforced concrete.
- Cooling towers are needed by some plants to dump the excess
heat that cannot be converted to energy due to the laws of thermodynamics.
These are the hyperbolic icons of nuclear energy. They emit only clean
water vapor.
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Animated
Reactor System
This image shows a nuclear reactor heating up water
and spinning a generator to produce electricity. It captures the essence of the
sytem well. The water coming into the condenser and
then going right back out would be water from a river, lake, or ocean. It goes
out the cooling towers. As you can see, this water does not go near the
radioactivity, which is in the reactor vessel.
2-The Nuclear Core
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Fuel pins
The smallest unit
of the reactor is the fuel pin. These are typically uranium-oxide (UO2).
They are surrounded by a zirconium clad to keep fission products from escaping
into the coolant.
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Fuel assembly
Fuel assemblies
are bundles of fuel pins. Fuel is put in and taken out of the reactor in
assemblies. Click here to see a 3-D blowup diagram of an
assembly.
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Full core
This is a full
core, made up of several hundred assemblies. Some assemblies are control
assemblies. Various fuel assemblies around the core have different fuel in
them. They vary in enrichment and age, among other parameters. The assemblies
may also vary with height, with different enrichments at the top of the core
from those at the bottom.
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3-Types of Nuclear
Reactors
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Pressurized Water
Reactor
The most
common type of reactor -- the PWR uses regular old water as a coolant. The
primary cooling water is kept at very high pressure so it does not boil. It
goes through a heat exchanger, transferring heat to a secondary coolant loop,
which then spins the turbine. These use oxide fuel pellets stacked in zirconium
tubes. They could possibly burn thorium or plutonium fuel as well.
Pros:
- Strong negative void coefficient --
reactor cools down if water starts bubbling
- Secondary loop keeps radioactive stuff
away from turbines, making maintenance easy.
Cons:
- Pressurized coolant escapes rapidly if a
pipe breaks, necessitating lots of back-up cooling systems.
- Can’t breed new fuel -- susceptible to
"uranium shortage"
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Sodium Cooled Fast
Reactor
The
first electricity-producing nuclear reactor in the world was SFR (the EBR-1 in
Pros:
- Can breed its own fuel, effectively
eliminating any concerns about uranium shortages (see what is a fast reactor?)
- Can burn its own waste
- Metallic fuel and excellent thermal properties
of sodium allow for passively safe operation -- the reactor will shut
itself down without any backup-systems working (or people around), only
relying on physics (gravity, natural circulation, etc.).
Cons:
- Sodium coolant is explosively reactive with
air, water. Thus, leaks in the pipes results in sodium fires. These can be
engineered around (by making a pool and eliminating pipes, etc.) but are a
major setback for these nice reactors.
- To fully burn waste, these require
reprocessing facilities which can also be used for nuclear proliferation.
- Positive void coefficients are inherent to
all fast reactors. This is a safety concern.
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Liquid Fluoride Cooled
Thorium Reactor
LFTRs have gotten a lot of attention lately in the media.
They are unique so far in that they use molten fuel. So there's no worry of
meltdown because they’re already melted. The folks over at Energy from
thorium are totally stoked about this technology.
Pros:
- Can constantly breed new fuel, eliminating
concerns over energy resources
- Can be maintained online with chemical
fission product removal, eliminating the need to shut down during
refueling.
- No cladding means less neutron-absorbing
material in the core, which leads to better neutron efficiency and thus
higher fuel utilization
Cons:
- Radioactive gaseous fission products are
everywhere, ready to escape at the first breach of containment. This
violates the common practice of defense-in-depth where there are multiple
levels of protection. All liquid fuel reactors have this problem.
- The presence of an online reprocessing
facility with incoming pre-melted fuel is a concern. The operator could easily divert Pa-233 to
provide a stream of nearly pure weapons-grade U-233. Thus, anyone who
operates this kind of reactor will have easy access to bomb material.
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Boiling Water Reactor
Second
most common, the BWR is similar to the PWR in many ways. However, they only
have one coolant loop. The hot nuclear fuel boils water as it goes out the top
of the reactor, where the steam heads over to the turbine to spin it.
Pros:
- Simpler plumbing reduces costs
- Power levels can be increased simply by
speeding up the pumps, giving less boiled water and more moderation. Thus,
load following is fun.
Cons:
- With liquid and gaseous water in the
system, many weird transients are possible, making safety analysis difficult
- Primary coolant is in direct contact with
turbines, so if a fuel rod had a leak, radioactive material could be
placed on the turbine. This complicates maintenance as the staff must be
dressed for radioactive environments.
- Can’t breed new fuel -- susceptible to
"uranium shortage"
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High Temperature Gas
Cooled Reactor
HTGRs use little pellets of fuel backed into either
hexagonal compacts or into larger pebbles (in the prismatic and pebble-bed
designs). Gas such as helium or carbon dioxide is passed through the reactor
rapidly to cool it.
Pros:
- Can operate at very high temperatures,
leading to great thermal efficiency (near 50%!) and the ability to create
process heat for things like oil refineries, water desalination plants,
hydrogen fuel cell production, and much more.
- Each little pebble of fuel has its own
containment structure, adding yet another barrier between radioactive
material and the environment.
Cons:
- High temperature has a bad side too.
Materials that can stay structurally sound in high temperatures and with
many neutrons flying through them are hard to come by.
- If the gas stops flowing, the reactor
heats up very quickly. Backup cooling systems are necessary.
References
- http://www.whatisnuclear.com/articles/nucreactor.html
- http://www.cameco.com/uranium_101/uranium_science/nuclear_reactors/
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