A
pressurised heavy water reactor (PHWR) is a nuclear power reactor, commonly using unenriched
natural uraniumNatural uranium refers to refined uranium with the same isotopic ratio as found in nature. It contains 0.7 % uranium-235, 99.3 % uranium-238, and a trace of uranium-234 by weight. In terms of the amount of radioactivity, approximately 2.2 % comes from uranium-235, 48.6 % uranium-238, and 49.2 %...
as its fuel, that uses
heavy waterHeavy water is water that contains a higher proportion than normal of the isotope deuterium, as deuterium oxide, D2O or ²H2O, or as deuterium protium oxide, HDO or ¹H²HO. Its physical and chemical properties are somewhat similar to those of water, H2O...
(deuterium oxide D
2O) as its
coolantA coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...
and
moderatorIn nuclear engineering, a neutron moderator is a medium which reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
. The heavy water coolant is kept under pressure in order to raise its boiling point, allowing it to be heated to higher temperatures without boiling, much as in a
PWRPressurized water reactors comprise a majority of all western nuclear power plants and are one of two types of light water reactor , the other type being boiling water reactors . In a PWR the primary coolant is pumped under high pressure to the reactor core, then the heated water transfers...
. While heavy water is significantly more expensive than ordinary light water, it yields greatly enhanced
neutron economyNeutron economy is defined as the ratio of an adjoint weighted average of the excess neutron production divided by an adjoint weighted average of the fission production....
, allowing the reactor to operate without
fuel enrichment facilitiesIsotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes, for example separating natural uranium into enriched uranium and depleted uranium. This is a crucial process in the manufacture of uranium fuel for nuclear power stations, and is...
(mitigating the additional
capital costCapital costs are costs incurred on the purchase of land, buildings, construction and equipment to be used in the production of goods or the rendering of services. In other words, the total cost needed to bring a project to a commercially operable status. However, capital costs are not limited to...
of the heavy water) and generally enhancing the ability of the reactor to efficiently make use of alternate
fuel cyclesThe nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the front end, which are the preparation of the fuel, steps in the service period in which the fuel is used during reactor operation, and steps in...
.
The reactors are used in nuclear power plants to produce
nuclear powerNuclear power is power produced from controlled nuclear reactions. Commercial plants in use to date use nuclear fission reactions....
from
nuclear fuelNuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical fuel that is burned to derive energy. By far the most common type of nuclear fuel is heavy fissile elements that can be made to undergo nuclear fission chain reactions in a nuclear fission reactor;...
.
The first commercial PHWRs were of the Canadian CANDU design built by
AECLAECL is an acronym that can stand for:*Advanced Electronics Company Limited*Atomic Energy of Canada Limited*Australian Egg Corporation Limited...
.
A
pressurised heavy water reactor (PHWR) is a nuclear power reactor, commonly using unenriched
natural uraniumNatural uranium refers to refined uranium with the same isotopic ratio as found in nature. It contains 0.7 % uranium-235, 99.3 % uranium-238, and a trace of uranium-234 by weight. In terms of the amount of radioactivity, approximately 2.2 % comes from uranium-235, 48.6 % uranium-238, and 49.2 %...
as its fuel, that uses
heavy waterHeavy water is water that contains a higher proportion than normal of the isotope deuterium, as deuterium oxide, D2O or ²H2O, or as deuterium protium oxide, HDO or ¹H²HO. Its physical and chemical properties are somewhat similar to those of water, H2O...
(deuterium oxide D
2O) as its
coolantA coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...
and
moderatorIn nuclear engineering, a neutron moderator is a medium which reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
. The heavy water coolant is kept under pressure in order to raise its boiling point, allowing it to be heated to higher temperatures without boiling, much as in a
PWRPressurized water reactors comprise a majority of all western nuclear power plants and are one of two types of light water reactor , the other type being boiling water reactors . In a PWR the primary coolant is pumped under high pressure to the reactor core, then the heated water transfers...
. While heavy water is significantly more expensive than ordinary light water, it yields greatly enhanced
neutron economyNeutron economy is defined as the ratio of an adjoint weighted average of the excess neutron production divided by an adjoint weighted average of the fission production....
, allowing the reactor to operate without
fuel enrichment facilitiesIsotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes, for example separating natural uranium into enriched uranium and depleted uranium. This is a crucial process in the manufacture of uranium fuel for nuclear power stations, and is...
(mitigating the additional
capital costCapital costs are costs incurred on the purchase of land, buildings, construction and equipment to be used in the production of goods or the rendering of services. In other words, the total cost needed to bring a project to a commercially operable status. However, capital costs are not limited to...
of the heavy water) and generally enhancing the ability of the reactor to efficiently make use of alternate
fuel cyclesThe nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the front end, which are the preparation of the fuel, steps in the service period in which the fuel is used during reactor operation, and steps in...
.
The reactors are used in nuclear power plants to produce
nuclear powerNuclear power is power produced from controlled nuclear reactions. Commercial plants in use to date use nuclear fission reactions....
from
nuclear fuelNuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical fuel that is burned to derive energy. By far the most common type of nuclear fuel is heavy fissile elements that can be made to undergo nuclear fission chain reactions in a nuclear fission reactor;...
.
History
The first commercial PHWRs were of the Canadian CANDU design built by
AECLAECL is an acronym that can stand for:*Advanced Electronics Company Limited*Atomic Energy of Canada Limited*Australian Egg Corporation Limited...
. Marketed world-wide, 29 are in currently in use as of 2008. The Nuclear Power Corporation of India Limited (NPCIL) has built and operates 13 PHWR units. Initially these indigenously built reactors were reverse-engineered from the CANDU design, but later models have diverged significantly.
SiemensSiemens AG is a German electrical and telecommunications companysiemens may refer to*siemens , the SI unit of electrical conductance, equivalent to 1 ampere/voltSiemens may also refer to:...
has also offered a PHWR design in the past, completing one unit in
ArgentinaArgentina, officially the Argentine Republic , is the second largest country in South America, constituted as a federation of 23 provinces and an autonomous city, Buenos Aires. It is the eighth largest country in the world by land area and the largest among Spanish-speaking nations, though Mexico,...
and partially completing a second, larger version at the same site.
Purpose of using heavy water
The key to maintaining a
nuclear reactionIn nuclear physics and nuclear chemistry, a nuclear reaction is the process in which two nuclei or nuclear particles collide to produce products different from the initial particles...
within a
nuclear reactorA nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate.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...
is to use the neutrons being released during
fissionIn nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts, often producing free neutrons and lighter nuclei, which may eventually produce photons...
to stimulate fission in other nuclei. With careful control over the geometry and reaction rates, this can lead to a self-sustaining
chain reactionA nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...
, a state known as "
criticalityA critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The...
".
Natural uranium consists of a mixture of various
isotopeIsotopes are different types of atoms of the same chemical element, each having a different number of neutrons. Correspondingly, isotopes differ in mass number but not in atomic number. The difference in the number of nucleons comes from a difference how many neutrons are in the atomic nucleus...
s, primarily
238UUranium-238 is the most common isotope of uranium found in nature. When hit by a neutron, it eventually becomes plutonium-239 ....
and a much smaller amount (about 0.72% by weight) of
235UUranium-235 is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238 it is fissile, i.e. it can sustain fission chain reaction...
.
238U can only be fissioned by neutrons that are fairly energetic, about 1
MeVMeV and meV are multiples and submultiples of the electron volt unit referring to 1,000,000 eV and 0.001 eV, respectively.Mev or MEV may refer to:In entertainment:* Musica Elettronica Viva, an Italian musical group...
or above. No amount of
238U can be made "critical", however, since it will tend to parasitically absorb more neutrons than it releases by the fission process.
235U, on the other hand, can support a self-sustained chain reaction, but due to the low natural abundance of
235U, natural uranium cannot achieve criticality by itself.
The "trick" to making a working reactor is to slow some of the neutrons to the point where their probability of causing nuclear fission in
235U increases to a level that permits a sustained chain reaction in the uranium as a whole. This requires the use of a
neutron moderatorIn nuclear engineering, a neutron moderator is a medium which reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
, which absorbs some of the neutrons'
kinetic energyThe kinetic energy of an object is the extra energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its current velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its...
, slowing them down to an energy comparable to the thermal energy of the moderator nuclei themselves (leading to the terminology of "thermal neutrons" and "thermal reactors"). During this slowing-down process it is beneficial to physically separate the neutrons from the uranium, since
238U nuclei have an enormous parasitic affinity for neutrons in this intermediate energy range (a reaction known as "resonance" absorption). This is a fundamental reason for designing reactors with discrete solid fuel separated by moderator, rather than employing a more homogeneous mixture of the two materials.
Water makes an excellent moderator; the hydrogen atoms in the water molecules are very close in mass to a single neutron, and thus have a potential for high energy transfer, similar conceptually to the collision of two billiard balls. However, in addition to being a good moderator, water is also fairly effective at absorbing neutrons. Using water as a moderator will absorb enough neutrons that there will be too few left over to react with the small amount of
235U in the fuel, again precluding criticality in natural uranium. Instead,
light water reactorThe light water reactor or LWR is a type of thermal reactor that uses light water as a coolant and neutron moderator as opposed to heavy water as a coolant/moderator...
s first enhance the amount of
235U in the uranium, producing
enriched uraniumEnriched uranium is a kind of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.711% of its weight...
, which generally contains between 3% and 5%
235U by weight (the waste from this process is known as
depleted uraniumDepleted uranium is uranium primarily composed of the isotope uranium-238 . Natural uranium is about 99.27 percent U-238, 0.72 percent U-235, and 0.0055 percent U-234. U-235 is used for fission in nuclear reactors and nuclear weapons. Uranium is enriched in U-235 by separating the isotopes by mass...
, consisting primarily of
238U). In this enriched form there
is enough
235U to react with the water-moderated neutrons to maintain criticality.
One complication of this approach is the requirement to build an uranium enrichment facility, which are generally expensive to build and operate. They also present a
nuclear proliferationNuclear proliferation is a term now used to describe the spread of nuclear weapons, fissile material, and weapons-applicable nuclear technology and information, to nations which are not recognized as "Nuclear Weapon States" by the Treaty on the Nonproliferation of Nuclear Weapons, also known as the...
concern; the same systems used to enrich the
235U can also be used to produce much more "pure"
weapons-gradeA weapons-grade substance is one that is pure enough to be used to make a weapon or has properties that make it suitable for weapons use. Weapons-grade plutonium and uranium are the most common examples, but it may also be used to refer to chemical and biological weapons...
material (90% or more
235U), suitable for producing a nuclear bomb. This is not a trivial exercise, by any means, but feasible enough that enrichment facilities present a significant nuclear proliferation risk.
An alternative solution to the problem is to use a moderator that does
not absorb neutrons as readily as water. In this case potentially all of the neutrons being released can be moderated and used in reactions with the
235U, in which case there
is enough
235U in natural uranium to sustain criticality. One such moderator is
heavy waterHeavy water is water that contains a higher proportion than normal of the isotope deuterium, as deuterium oxide, D2O or ²H2O, or as deuterium protium oxide, HDO or ¹H²HO. Its physical and chemical properties are somewhat similar to those of water, H2O...
, or deuterium-oxide. Although it reacts dynamically with the neutrons in a similar fashion to light water (albeit with less energy transfer on average, given that heavy hydrogen, or
deuteriumDeuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in of hydrogen...
, is about twice the mass of hydrogen), it already has the extra neutron that light water would normally tend to absorb.
Advantages and Disadvantages
The use of heavy water moderator is the key to the PHWR system, enabling the use of natural uranium as fuel (in the form of ceramic UO
2), which means that it can be operated without expensive uranium enrichment facilities. Additionally, the mechanical arrangement of the PHWR, which places most of the moderator at lower temperatures, is particularly efficient because the resulting thermal neutrons are "more thermal" than in traditional designs, where the moderator normally runs hot. This means that a PHWR is not only able to "burn" natural uranium and other fuels, but tends to do so more efficiently as well.
Pressurised heavy water reactors do have some drawbacks. Heavy water generally costs hundreds of dollars per kilogram, though this is a trade-off against reduced fuel costs.
It is also notable that the reduced energy content of natural uranium as compared to enriched uranium necessitates more frequent replacement of fuel; this is normally accomplished by use of an on-power refuelling system. The increased rate of fuel movement through the reactor also results in higher volumes of spent fuel than in reactors employing enriched uranium; however, as the unenriched fuel was less reactive, the heat generated is less, allowing the spent fuel to be stored much more compactly.
See also
- CANDU
- List of nuclear reactors
- Heavy water reactor
Heavy water reactors use heavy water as a neutron moderator. Heavy water is deuterium oxide, D2O. Neutrons in a nuclear reactor that uses uranium must be slowed down so that they are more likely to split other atoms and get more neutrons released to split other atoms...
- Pressurized water reactor
Pressurized water reactors comprise a majority of all western nuclear power plants and are one of two types of light water reactor , the other type being boiling water reactors . In a PWR the primary coolant is pumped under high pressure to the reactor core, then the heated water transfers...