- LWR redirects here. See also: LWR
The
light water reactor or
LWR is
a type of
thermal reactor that uses
light water as a coolant and
neutron
moderator (water,
H2O) as opposed to heavy water as a
coolant/moderator (
deuterium oxide,
2H2O). Thermal
reactors are the most common type of
nuclear reactor, and light water reactors
are the most common type of thermal reactor.
There are three varieties of light water reactors; the
pressurized water reactor (PWR),
the
boiling water reactor
(BWR), and the
supercritical
water reactor (SWR). The Russian abbreviation for LWR is
VVR (or sometimes WWR), meaning
water water
reactor. Similarly, the Russian term for a PWR is
VVER, meaning
water water energy
reactor.
Overview
The family of nuclear reactors known as light water reactors (LWR),
cooled and moderated using ordinary water, tend to be simpler and
cheaper to build than other types of nuclear reactor, and are well
known to possess excellent safety and stability characteristics;
due to these factors, they make up the vast majority of civil
nuclear reactors and naval propulsion reactors in service
throughout the world as of
2009. LWRs can be
subdivided into three categories - pressurized water reactors
(PWRs), boiling water reactors (BWRs), and supercritical water
reactors (SWRs).
Various agencies of the United
States
Federal Government
were responsible for the initial development of the PWR and
BWR. An effort by the United States Navy, starting immediately
after the end of World War II, and lead
by (then) Captain Hyman Rickover,
developed the first pressurized water reactors in the early 1950s,
building the first nuclear submarine (the USS Nautilus) while researcher Samuel Untermyer II lead the effort to
develop the BWR at the US National Reactor Testing
Station (now the Idaho National Laboratory
) in a series of tests called the BORAX experiments. The former Soviet Union
also independently developed their version of the
PWR in the late 1950s, and it became known as the VVER; because of this, Russian-designed PWRs are known
in the West as VVERs, to denote their independent origin, and
certain national design distinctions from Western PWRs. The
SWR remains hypothetical as of
2009; it is a
Generation IV design that is still a
light water reactor, but it is only partially moderated by light
water and exhibits certain characteristics of a
fast neutron reactor.
The
leaders in national experience with PWRs, offering reactors for
export, are the United States (which offers the passively-safe AP1000, a Westinghouse design, as well
as several smaller, modular, passively-safe PWRs, such as the
Babcock and Wilcox MPower, and the NuScale MASLWR), the Russian Federation
(offering both the VVER-1000 and the VVER-1200 for
export), the Republic of
France
(offering the AREVA EPR for export), and Japan
(offering
the Mitsubishi Advanced Pressurized Water
Reactor for export); in addition, both the People's
Republic of China
and the Republic of Korea
are both noted to be rapidly ascending into the
front rank of PWR-constructing nations as well, with the Chinese
being engaged in a massive program of nuclear power expansion, and
the Koreans currently designing and constructing their second
generation of indigenous designs. The leaders in
national experience with BWRs, offering reactors for export, are
the United
States and Japan, with the
alliance of General Electric (of
the US) and Hitachi (of Japan), offering
both the ABWR and the ESBWR for construction and export, in addition,
Toshiba also offers an ABWR variant for construction in Japan, as well.
Though the
Federal Republic
of Germany
was once a major player with BWRs, that nation has
moved towards other pursuits, such as their massive expansion of
coal power plants. The other types of
nuclear reactor in use for power generation are the heavy water moderated reactor,
built by Canada
(CANDU) and the Republic of India
(AHWR), the advanced gas cooled reactor
(AGCR), built by the United Kingdom
, the liquid
metal cooled reactor (LMFBR), built by the Russian
Federation, the Republic of France, and Japan, and the
graphite-moderated, water-cooled reactor
(RBMK), found exclusively within the Russian Federation and former Soviet states.
Though
Electricity generation
capabilities are comparable between all these types of reactor, due
to the aforementioned features, and the extensive experience with
operations of the LWR, it is favored in the vast majority of new
nuclear power plants, though the CANDU/AHWR has a comparatively
small (but quite dedicated) following. In addition, light water
reactors make up the vast majority of reactors that power
naval nuclear powered vessels.
Four out
of the five great powers with nuclear
naval propulsion capacity use light water reactors exclusively: the
British
Royal Navy, the Chinese People's
Liberation Army Navy, the French Marine nationale,
and the United
States Navy. Only the Russian
Federation's Navy has used a
relative handful of liquid-metal cooled reactors in
production vessels, specifically the Alfa class submarine, which used
lead-bismuth eutectic as a
reactor moderator and coolant, but the vast majority of Russian
nuclear-powered boats and ships use light water reactors
exclusively. The reason for near exclusive LWR use aboard
nuclear naval vessels is the level of inherent safety built in to
these types of reactors. Since light water is used as both a
coolant and a neutron moderator in these reactors, if one of these
reactors suffers damage due to military action, leading to a
compromise of the reactor core's integrity, the resulting release
of the light water moderator will act to stop the nuclear reaction
and shut the reactor down. This capability is known as a
negative void coefficient of
reactivity.
Currently-offered LWRs include the following:
LWR Statistics
Data from
the International Atomic Energy
Agency
.
| Reactors in operation. |
359 |
| Reactors under construction. |
27 |
| Number of countries with LWRs. |
27 |
| Generating capacity (Gigawatt). |
328.4 |
Reactor design
The light water reactor produces heat by controlled
nuclear fission. The nuclear
reactor core is the portion of a
nuclear reactor where the nuclear reactions
take place. It mainly consists of
nuclear
fuel and
control elements. The
pencil-thin nuclear fuel rods, each about 12 feet (3.7 m) long, are
grouped by the hundreds in bundles called fuel assemblies. Inside
each fuel rod, pellets of
uranium, or more
commonly
uranium oxide, are stacked
end to end. The control elements, called control rods, are filled
with pellets of substances like
hafnium or
cadmium that readily capture neutrons. When
the control rods are lowered into the core, they absorb neutrons,
which thus cannot take part in the
chain
reaction. On the converse, when the control rods are lifted out
of the way, more neutrons strike the fissile
uranium-235 or
plutonium-239 nuclei in nearby fuel rods, and
the chain reaction intensifies. All of this is enclosed in a
water-filled steel
pressure vessel,
called the
reactor vessel.
In the
boiling water reactor,
the heat generated by fission turns the water into steam, which
directly drives the power-generating turbines. But in the
pressurized water reactor, the
heat generated by fission is transferred to a secondary loop via a
heat exchanger. Steam is produced in the secondary loop, and the
secondary loop drives the power-generating turbines. In either
case, after flowing through the turbines, the steam turns back into
water in the condenser.
The water required to cool the condenser is taken from a nearby
river or ocean. It is then pumped back into the river or ocean, in
warmed condition. The heat could also be dissipated via a cooling
tower into the atmosphere.
The United States
uses LWR reactors for electric power production, in
comparison to the heavy water
reactors used in Canada.
Control
Control rods are usually combined into control rod assemblies —
typically 20 rods for a commercial pressurized water reactor
assembly — and inserted into guide tubes within a fuel element. A
control rod is removed from or inserted into the
central core of a nuclear reactor in
order to control the number of neutrons which will split further
uranium atoms. This in turn affects the thermal power of the
reactor, the amount of steam generated, and hence the electricity
produced. The control rods are partially removed from the core to
allow a
chain reaction to occur. The
number of control rods inserted and the distance by which they are
inserted can be varied to control the reactivity of the
reactor.
Usually there are also other means of controlling reactivity. In
the PWR design a soluble neutron absorber, usually
boric acid, is added to the reactor coolant
allowing the complete extraction of the control rods during
stationary power operation ensuring an even power and flux
distribution over the entire core. Operators of the BWR design use
the coolant flow through the core to control reactivity by varying
the speed of the reactor recirculation pumps. An increase in the
coolant flow through the core improves the removal of steam
bubbles, thus increasing the density of the coolant/moderator with
the result of increasing power.
Coolant
The light water reactor also uses ordinary water to keep the
reactor cooled. The cooling source, light water, is circulated past
the reactor core to absorb the heat that it generates. The heat is
carried away from the reactor and is then used to generate steam.
Most reactor systems employ a cooling system that is physically
separate from the water that will be boiled to produce pressurized
steam for the
turbines, like the
pressurized water reactor. But in some reactors the water for the
steam turbines is boiled directly by the reactor core, for example
the boiling water reactor.
Many other reactors are also light water cooled, notably the
RBMK and some military
plutonium production reactors. These are not
regarded as LWRs, as they are moderated by
graphite, and as a result their nuclear
characteristics are very different. Although the coolant flow rate
in commercial PWRs is constant, it is not in nuclear reactors used
on
U.S. Navy
ships.
Fuel
Nuclear fuel pellets that are ready
for fuel assembly completion.
The use of ordinary water makes it necessary to do a certain amount
of enrichment of the uranium fuel before the necessary criticality
of the reactor can be maintained. The light water reactor uses
uranium 235 as a fuel, enriched to
approximately 3 percent. Although this is its major fuel, the
uranium 238 atoms also contribute to the
fission process by converting to
plutonium
239; about one-half of which is consumed in the reactor.
Light-water reactors are generally refueled every 12 to 18 months,
at which time, about 25 percent of the fuel is replaced.
The enriched UF
6 is converted into
uranium dioxide powder that is then
processed into pellet form. The pellets are then fired in a
high-temperature, sintering furnace to create hard, ceramic pellets
of
enriched uranium. The
cylindrical pellets then undergo a grinding process to achieve a
uniform pellet size. The uranium oxide is dried before inserting
into the tubes to try to eliminate moisture in the ceramic fuel
that can lead to corrosion and hydrogen embrittlement. The pellets
are stacked, according to each nuclear core's design
specifications, into tubes of
corrosion-resistant metal alloy. The
tubes are sealed to contain the fuel pellets: these tubes are
called fuel rods.
The finished fuel rods are grouped in special fuel assemblies that
are then used to build up the nuclear fuel core of a power reactor.
The metal used for the tubes depends on the design of the reactor -
stainless steel was used in the
past, but most reactors now use a
zirconium alloy. For the most common types of
reactors the tubes are assembled into bundles with the tubes spaced
precise distances apart. These bundles are then given a unique
identification number, which enables them to be tracked from
manufacture through use and into disposal.
Pressurized water reactor fuel consists of
cylindrical rods put into bundles. A uranium
oxide ceramic is formed into pellets and inserted into
Zircaloy tubes that are bundled together. The
Zircaloy tubes are about 1 cm in diameter, and the fuel cladding
gap is filled with helium gas to improve the conduction of heat
from the fuel to the cladding. There are about 179-264 fuel rods
per fuel bundle and about 121 to 193 fuel bundles are loaded into a
reactor core. Generally, the fuel
bundles consist of fuel rods bundled 14x14 to 17x17. PWR fuel
bundles are about 4 meters in length. The Zircaloy tubes are
pressurized with
helium to try to minimize pellet cladding interaction
which can lead to fuel rod failure over long periods.
In boiling water reactors, the fuel is similar to PWR fuel except
that the bundles are "canned"; that is, there is a thin tube
surrounding each bundle. This is primarily done to prevent local
density variations from effecting
neutronics and
thermal hydraulics of the nuclear core on
a global scale. In modern BWR fuel bundles, there are either 91,
92, or 96 fuel rods per assembly depending on the manufacturer. A
range between 368 assemblies for the smallest and 800 assemblies
for the largest U.S. BWR forms the reactor core. Each BWR fuel rod
is back filled with helium to a pressure of about three atmospheres
(300 kPa).
Moderator
A neutron moderator is a medium which reduces the velocity of
fast neutrons, thereby turning them
into
thermal neutrons capable of
sustaining a nuclear chain reaction involving uranium-235. A good
neutron moderator is a material full of atoms with light nuclei
which do not easily absorb neutrons. The neutrons strike the nuclei
and bounce off. After sufficiently many such impacts, the velocity
of the neutron will be comparable to the thermal velocities of the
nuclei; this neutron is then called a thermal neutron.
The light water reactor uses ordinary
water,
also called
light water, as its neutron
moderator. The light water absorbs too many neutrons to be used
with unenriched natural uranium, and therefore
uranium enrichment or
nuclear reprocessing becomes necessary
to operate such reactors, increasing overall costs. This
differentiates it from a
heavy water
reactor, which uses
heavy water as a
neutron moderator. While ordinary water has some heavy water
molecules in it, it is not enough to be important in most
applications. In practice all LWRs are also water cooled. In
pressurized water reactors the coolant water is used as a moderator
by letting the neutrons undergo multiple collisions with light
hydrogen atoms in the water, losing speed in the process. This
moderating of neutrons will happen more often when the water is
denser, because more collisions will occur.
The use of water as a moderator is an important safety feature of
PWRs, as any increase in temperature causes the water to expand and
become less dense; thereby reducing the extent to which neutrons
are slowed down and hence reducing the reactivity in the reactor.
Therefore, if reactivity increases beyond normal, the reduced
moderation of neutrons will cause the chain reaction to slow down,
producing less heat. This property, known as the negative
temperature coefficient of
reactivity, makes PWR reactors very stable. In event of a
loss of coolant accident, the
moderator is also lost and the reaction will stop.
See also
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