Nuclear
Reactor Technology
Just as many conventional thermal power
stations generate electricity by harnessing the thermal energy released from
burning fossil fuels, nuclear power plants convert the energy
released from the nucleus of an atom, typically via nuclear fission.
When a relatively large fissile atomic
nucleus (usually uranium-235 or plutonium-239) absorbs a neutron, a fission of
the atom often results. Fission splits the atom into two or more smaller nuclei
with kinetic energy (known as fission products) and also releases gamma
radiation and free neutrons. A portion of these neutrons may later be absorbed
by other fissile atoms and create more fissions, which release more neutrons,
and so on.
This nuclear chain reaction can be
controlled by using neutron poisons and neutron moderators to change the
portion of neutrons that will go on to cause more fissions. Nuclear
reactorsgenerally have automatic and manual systems to shut the fission
reaction down if unsafe conditions are detected.
A cooling system removes heat from the
reactor core and transports it to another area of the plant, where the thermal
energy can be harnessed to produce electricity or to do other useful work.
Typically the hot coolant will be used as a heat source for a boiler, and the
pressurized steam from that boiler will power one or more steam turbine driven
electrical generators.
There are many different reactor designs,
utilizing different fuels and coolants and incorporating different control
schemes. Some of these designs have been engineered to meet a specific need.
Reactors for nuclear submarines and large naval ships, for example, commonly
use highly enriched uranium as a fuel. This fuel choice increases the reactor's
power density and extends the usable life of the nuclear fuel load, but is more
expensive and a greater risk to nuclear proliferation than some of the other
nuclear fuels.
A number of new designs for nuclear
power generation, collectively known as the Generation IV reactors, are the
subject of active research and may be used for practical power generation in
the future. Many of these new designs specifically attempt to make fission
reactors cleaner, safer and/or less of a risk to the proliferation of nuclear
weapons. Passively safe plants (such as the ESBWR) are available to be built
and other designs that are believed to be nearly fool-proof are being pursued.
Fusion reactors, which may be viable in the future, diminish or eliminate many
of the risks associated with nuclear fission.
There
are two types of nuclear power in current use:
● The Radioisotope thermoelectric generator produces
heat through passive radioactive decay. Some radioisotope thermoelectric
generators have been created to power space probes (for example, the Cassini
probe), some lighthouses in the former Soviet Union, and some pacemakers. The
heat output of these generators diminishes with time; the heat is converted to
electricity utilising the thermoelectric effect.
● Nuclear fission reactors produce heat through a
controlled nuclear chain reaction in a critical mass of fissile material. All
current nuclear power plants are critical fission reactors, which are the focus
of this article. The output of fission reactors is controllable. There are
several subtypes of critical fission reactors, which can be classified as
Generation I, Generation II and Generation III. All reactors will be compared
to the Pressurized Water Reactor (PWR), as that is the standard modern reactor
design.
Pressurized Water Reactors (PWR)
These reactors use a pressure vessel to contain
the nuclear fuel, control rods, moderator, and coolant. They are cooled and
moderated by high pressure liquid water. The hot radioactive water that leaves
the pressure vessel is looped through a steam generator, which in turn heats a
secondary (non-radioactive) loop of water to steam that can run turbines. They
are the majority of current reactors, and are generally considered the safest
and most reliable technology currently in large scale deployment. This is a
thermal neutron reactor design, the newest of which are the VVER-1200, Advanced
Pressurized Water Reactor and the European Pressurized Reactor. United States
Naval reactors are of this type.
Boiling Water Reactors (BWR)
A BWR is like a PWR without the steam
generator. A boiling water reactor is cooled and moderated by water like a PWR,
but at a lower pressure, which allows the water to boil inside the pressure
vessel producing the steam that runs the turbines. Unlike a PWR, there is no
primary and secondary loop. The thermal efficiency of these reactors can be
higher, and they can be simpler, and even potentially more stable and safe.
This is a thermal neutron reactor design, the newest of which are the Advanced
Boiling Water Reactor and the Economic Simplified Boiling Water Reactor.
Pressurized Heavy Water Reactor (PHWR)
A Canadian design (known as CANDU), these
reactors are heavy-water-cooled and -moderated Pressurized-Water reactors.
Instead of using a single large pressure vessel as in a PWR, the fuel is
contained in hundreds of pressure tubes. These reactors are fueled with natural
uranium and are thermal neutron reactor designs. PHWRs can be refueled while at
full power, which makes them very efficient in their use of uranium (it allows
for precise flux control in the core). CANDU PHWRs have been built in Canada,
Argentina, China, India (pre-NPT), Pakistan (pre-NPT), Romania, and South
Korea. India also operates a number of PHWRs, often termed 'CANDU-derivatives',
built after the Government of Canada halted nuclear dealings with India
following the 1974 Smiling Buddha nuclear weapon test.
Reaktor Bolshoy Moschnosti Kanalniy
(High Power Channel Reactor) (RBMK)
A Soviet design, built to produce plutonium
as well as power. RBMKs are water cooled with a graphite moderator. RBMKs are
in some respects similar to CANDU in that they are refuelable during power
operation and employ a pressure tube design instead of a PWR-style pressure
vessel. However, unlike CANDU they are very unstable and large, making
containment buildings for them expensive. A series of critical safety flaws have
also been identified with the RBMK design, though some of these were corrected
following the Chernobyl accident. Their main attraction is their use of light
water and un-enriched uranium. As of 2010, 11 remain open, mostly due to safety
improvements and help from international safety agencies such as the DOE.
Despite these safety improvements, RBMK reactors are still considered one of
the most dangerous reactor designs in use. RBMK reactors were deployed only in
the former Soviet Union.
Gas Cooled Reactor (GCR) and Advanced
Gas Cooled Reactor (AGR)
These are generally graphite moderated and
CO2 cooled. They can have a high thermal efficiency compared
with PWRs due to higher operating temperatures. There are a number of operating
reactors of this design, mostly in the United Kingdom, where the concept was
developed. Older designs (i.e. Magnox stations) are either shut down or will be
in the near future. However, the AGCRs have an anticipated life of a further 10
to 20 years. This is a thermal neutron reactor design. Decommissioning costs
can be high due to large volume of reactor core.
Liquid Metal Fast Breeder Reactor
(LMFBR)
This is a reactor design that is cooled by
liquid metal, totally unmoderated, and produces more fuel than it consumes.
They are said to "breed" fuel, because they produce fissionable fuel
during operation because of neutron capture. These reactors can function much
like a PWR in terms of efficiency, and do not require much high pressure
containment, as the liquid metal does not need to be kept at high pressure,
even at very high temperatures. BN-350 and BN-600 in USSR and Superphénix in
France were a reactor of this type, as was Fermi-I in the United States. The
Monju reactor in Japan suffered a sodium leak in 1995 and is pending restart
earliest in February 2010. All of them use/used liquid sodium. These reactors
are fast neutron, not thermal neutron designs. These reactors come in two types:
Lead cooled
Using lead as the liquid metal provides
excellent radiation shielding, and allows for operation at very high
temperatures. Also, lead is (mostly) transparent to neutrons, so fewer neutrons
are lost in the coolant, and the coolant does not become radioactive. Unlike
sodium, lead is mostly inert, so there is less risk of explosion or accident,
but such large quantities of lead may be problematic from toxicology and
disposal points of view. Often a reactor of this type would use a lead-bismuth
eutectic mixture. In this case, the bismuth would present some minor radiation
problems, as it is not quite as transparent to neutrons, and can be transmuted
to a radioactive isotope more readily than lead. The Russian Alfa class
submarine uses a lead-bismuth-cooled fast reactor as its main power plant.
Sodium cooled
Most LMFBRs are of this type. The sodium is
relatively easy to obtain and work with, and it also manages to actually
prevent corrosion on the various reactor parts immersed in it. However, sodium
explodes violently when exposed to water, so care must be taken, but such
explosions wouldn't be vastly more violent than (for example) a leak of
superheated fluid from a SCWR or PWR. EBR-I, the first reactor to have a core
meltdown, was of this type.
Pebble Bed Reactors (PBR)
These use fuel molded into ceramic balls,
and then circulate gas through the balls. The result is an efficient,
low-maintenance, very safe reactor with inexpensive, standardized fuel. The
prototype was the AVR.
Molten Salt Reactors
These dissolve the fuels in fluoride salts,
or use fluoride salts for coolant. These have many safety features, high
efficiency and a high power density suitable for vehicles. Notably, they have
no high pressures or flammable components in the core. The prototype was the
MSRE, which also used Thorium's fuel cycle to produce 0.1% of the radioactive
waste of standard reactors.
Aqueous Homogeneous Reactor (AHR)
These reactors use soluble nuclear salts
dissolved in water and mixed with a coolant and a neutron moderator.