A nuclear reactor is a machine in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate,
as opposed to a nuclear bomb, in which the chain reaction occurs in a fraction of a second and is uncontrolled.
The most significant use of nuclear reactors is as an energy source for the generation of electrical power and for the
power in some ships. This is usually accomplished by methods that involve using heat from the nuclear reaction
to power steam turbines.
The key components common to most types of nuclear power plants are the following:
- Nuclear fuel
- Neutron moderator
- Control rods
- Pressure vessel
- Emergency Core Cooling Systems (ECCS)
- Reactor Protective System (RPS)
- Steam generators (not in BWRs)
- Containment building
- Boiler feedwater pump
- Steam turbine
- Electricity generator
Conventional thermal power plants all have a fuel source to provide heat. Examples are gas, coal, or oil.
For a nuclear power plant, this heat is provided by nuclear fission inside the nuclear reactor. When a relatively
large fissile atomic nucleus (usually uranium-235 or plutonium-239) is struck by a neutron it forms two or more smaller
nuclei as fission products, releasing energy and neutrons in a process called nuclear fission. The neutrons then
trigger further fission, and so on. When this nuclear chain reaction is controlled, the energy released can
be used to heat water, produce steam and drive a turbine that generates electricity. It should be noted that a
nuclear explosive involves an uncontrolled chain reaction, and the rate of fission in a reactor is not capable of reaching
sufficient levels to trigger a nuclear explosion because commercial reactor grade nuclear fuel is not enriched to a high
enough level. Enriched uranium is uranium in which the percent composition of uranium-235 has been increased
from that of uranium found in nature. Natural uranium is only 0.72% uranium-235, with the rest being mostly
uranium-238 (99.2745%) and a tiny fraction is uranium-234 (0.0055%).
Nuclear Reactors are classified by several methods:
Classification by type of nuclear reaction Most reactors, and all commercial ones, are based on nuclear
fission. They generally use uranium
as fuel, but research on using thorium is ongoing. This article assumes that the technology is nuclear fission
unless otherwise stated. Fission reactors can be divided roughly into two classes, depending on the energy of
the neutrons that are used to sustain the fission chain reaction:
- Thermal reactors use slow or thermal neutrons. Most power reactors are of this type. These are
characterized by neutron moderator materials that slow neutrons until they approach the average kinetic energy of the
surrounding particles; that is, until they are thermalized. Thermal neutrons have a far higher probability of
fissioning uranium-235, and a lower probability of capture by uranium-238 than the faster neutrons that result from
fission. In addition to the moderator, thermal reactors have fuel (fissionable material), containments, pressure
vessels, shielding, and instrumentation to monitor and control the reactor's systems.
- Fast neutron reactors use fast neutrons to sustain the fission chain reaction. They are characterized by
an absence of moderating material. Initiating the chain reaction requires enriched uranium (and/or enrichment
with plutonium 239), due to the lower probability of fissioning U-235, and a higher probability of capture by
U-238 (as compared to a moderated, thermal neutron). In general, fast reactors will produce less waste
and the waste they do produce will have a vastly shorter halflife, but they are more difficult to build and more
expensive to operate. Overall, fast reactors are less common than thermal reactors in most applications.
Some early power stations were fast reactors, as are some Russian naval propulsion units.
Classification by moderator material used by thermal reactors.
- Graphite moderated reactors
- Water moderated reactors
- Heavy Water moderated reactors
- Light water moderated reactors (LWRs). Light water reactors use ordinary water to moderate and cool
the reactors. When at operating temperatures if the temperature of the water increases, its density drops,
and fewer neutrons passing through it are slowed enough to trigger further reactions. That negative feedback
stabilizes the reaction rate. Graphite and heavy water reactors tend to be more thoroughly thermalised
than light water reactors. Due to the extra thermalization, these types can use natural uranium/unenriched fuel.
Classification by coolant.
- Water cooled reactor
- Pressure water reactor
- Most commercial and naval reactors use pressure vessels. Pressure vessels are almost always lined up
to reactors and are only isolated from reactors during special maintenance or tests.
- Pressurised channels. Channel-type reactors can be refuelled under load.
- Boiling water reactor
- Pool-type reactor
- Liquid metal cooled reactor. Since water is as a moderator, it cannot be used as a coolant in a fast reactor.
All fast neutron reactors that have been used for power generation have been liquid metal cooled reactors,
but research continues in gas cooled reactors.
- Gas cooled reactor are cooled by a circulating inert gas, usually helium. Nitrogen and carbon dioxide have
also been used. Utilization of the heat varies, depending on the reactor. Some reactors run hot enough that the
gas can directly power a gas turbine. Older designs usually run the gas through a heat exchanger to make
steam for a steam turbine.
Classification by phase of fuel
- Solid fueled
- Fluid fueled
- Gas fueled
Classification by use
- Power plants
- Nuclear marine propulsion
- Heat for domestic and industrial heating
- Hydrogen production for use in a hydrogen economy
- Production reactors for transmutation of elements:
- Fast breeder reactors are capable of enriching Uranium during the fission chain reaction
by converting fertile U-238 to Pu-239, which allows an operational fast reactor to generate more fissile material
than it consumes. Thus, a breeder reactor can be refueled with natural or even depleted uranium.
- Creating various radioactive isotopes, such as americium for use in smoke detectors, and cobalt-60,
molybdenum-99 and others, used for imaging and medical treatment.
- Pressurized Water Reactors (PWR) :
These are reactors cooled and moderated by high pressure and temperature liquid water. They are
the majority of current reactors, and are generally considered the safest and most reliable technology.
- Boiling Water Reactors (BWR) :
These are reactors cooled and moderated by water, under slightly lower pressure. The water is allowed to boil in the
reactor. The thermal efficiency of these reactors can be higher, and they can be simpler, and even potentially more
stable and safe. However, the boiling water puts more stress on many of the components, and increases the
risk that radioactive water may escape in an accident. These reactors make up a substantial percentage of modern
- Pressurized Heavy Water Reactor (PHWR) :
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 because it allows for precise flux control
in the core.
- Reaktor Bolshoy Moshchnosti Kanalniy (RBMK = High Power Channel Reactor ) :
A Soviet Union design built to produce plutonium as well as power. RBMKs are water cooled with a graphite moderator.
RBMKs are in some respects similar to PHWR in that they are refuelable On-Load and employ a pressure tube design
instead of a PWR-style pressure vessel. However, unlike the PHWR they are very unstable and too large to have
containment buildings making them dangerous in the case of an accident. A series of critical safety flaws have also
been identified with the RBMK design, though some of these were corrected following the Chernobyl accident.
RBMK reactors are generally considered one of the most dangerous reactor designs in use. The Chernobyl plant
had four RBMK reactors.
- Gas Cooled Reactor (GCR) and Advanced Gas Cooled Reactor (AGCR) :
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. This is a thermal
neutron reactor design.
- 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.
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. 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.
- 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 a leak of
superheated fluid from a PWR.
Nuclear fuel cycleWiki n.p.
Thermal reactors generally depend on refined and enriched uranium. Some nuclear reactors can operate with a mixture
of plutonium and uranium. The process by which uranium ore is mined, processed, enriched, used,
reprocessed and disposed of is known as the nuclear fuel cycle.
Under 1% of the uranium found in nature is the easily fissionable U-235 isotope and as a result most reactor designs
require enriched fuel. Enrichment involves increasing the percentage of U-235 and is usually done by means of gaseous
diffusion or gas centrifuge. The enriched result is then converted into uranium dioxide powder, which is pressed and
fired onto pellet form. These pellets are stacked into tubes which are then sealed and called fuel rods. Many of these
fuel rods are used in each nuclear reactor.
Most BWR and PWR commercial reactors use uranium enriched to about 4% U-235. Many research reactors use highly
enriched, or weapons grade uranium, while some commercial reactors with a high neutron economy do not require the
fuel to be enriched at all.
Fissionable U-235 and non-fissionable U-238 are both used in the fission process. U-235 is
fissionable by thermal, i.e. slow-moving, neutrons. A thermal neutron is one which is moving about the same speed
as the atoms around it. Since all atoms vibrate proportional to their absolute temperature, a thermal neutron has the
best opportunity to fission U-235 when it is moving at this same vibrational speed. On the other hand, U-238 is more
likely to capture a neutron when the neutron is moving very fast. This U-239 atom will soon decay into plutonium-239,
which is another fuel. Pu-239 is a viable fuel and must be accounted for even when a highly enriched uranium fuel is
used. Plutonium fissions will dominate the U-235 fissions in some reactors, especially after the initial loading of U-235
is spent. Plutonium is fissionable with both fast and thermal neutrons, which make it ideal for either nuclear reactors
or nuclear bombs.
Most reactor designs in existence are thermal reactors and typically use water as a neutron moderator
and as a coolant. However, in a fast breeder reactor, some
other kind of coolant is used which will not moderate or slow the neutrons down much. This enables fast neutrons to
dominate, which can effectively be used to constantly replenish the fuel supply. By merely placing cheap unenriched
uranium into such a core, the non-fissionable U-238 will be turned into Pu-239, "breeding" fuel.
The amount of energy in the nuclear reactor fuel is expressed in terms of "full-power days," which is
the number of 24-hour periods days a reactor is scheduled for operation at full power output for the generation of
heat energy. The number of full-power days in a reactor's operating cycle between refueling outage times is related
to the amount of fissile uranium-235 contained in the fuel assemblies at the beginning of the cycle. A higher
percentage of U-235 in the core at the beginning of a cycle will permit the reactor to be run for a greater number of
The amount of energy extracted from nuclear fuel is called its "burn up," which is expressed in terms of the heat
energy produced per initial unit of fuel weight. Burn up is commonly expressed as megawatt days thermal per metric
ton of initial heavy metal.
At the end of the operating cycle, the fuel in some of the assemblies is "spent," and is discharged and replaced with
new fuel assemblies. It is the buildup of reaction poisons in nuclear fuel that determines
the lifetime of nuclear fuel in a reactor. Long before all possible fissions have taken place, the buildup of long-lived
neutron absorbing fission products damps out the chain reaction. The fraction of the reactor's fuel core replaced during
refueling is typically 1/4 for a boiling-water reactor and 1/3 for a pressurized-water reactor.
Not all reactors need to be shut down for refueling. In some reactors, fuel can be shifted through the reactor
while it is running.