A pressurized heavy-water reactor (PHWR) is a
nuclear reactor
A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nu ...
that uses
heavy water (
deuterium oxide D
2O) as its
coolant
A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosio ...
and
neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, ideally without capturing any, leaving them as thermal neutrons with only minimal (thermal) kinetic energy. These thermal neutrons are immensely mo ...
. PHWRs frequently use
natural uranium
Natural uranium (NU or Unat) refers to uranium with the same isotopic ratio as found in nature. It contains 0.711% uranium-235, 99.284% uranium-238, and a trace of uranium-234 by weight (0.0055%). Approximately 2.2% of its radioactivity comes fr ...
as fuel, but sometimes also use
very low enriched uranium. The heavy water coolant is kept under pressure to avoid boiling, allowing it to reach higher temperature (mostly) without forming steam bubbles, exactly as for
pressurized water reactor
A pressurized water reactor (PWR) is a type of light-water reactor, light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants (with notable exceptions being the UK, Japan and Canada). In a PWR, the primary ...
. While
heavy water is very expensive to isolate from ordinary water (often referred to as ''light water'' in contrast to ''heavy water''), its low absorption of neutrons greatly increases the
neutron economy
Neutron 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.
The distribution of neutron energies in a nuclear reactor differs from the f ...
of the reactor, avoiding the need for
enriched fuel. The high cost of the heavy water is offset by the lowered cost of using natural uranium and/or
alternative fuel cycles. As of the beginning of 2001, 31 PHWRs were in operation, having a total capacity of 16.5 GW(e), representing roughly 7.76% by number and 4.7% by generating capacity of all current operating reactors.
Purpose of using heavy water
The key to maintaining a
nuclear chain reaction
In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series of these reactions. The specific nu ...
within a
nuclear reactor
A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nu ...
is to use, on average, ''exactly'' one of the neutrons released from each
nuclear fission event to stimulate another nuclear fission event (in another fissionable nucleus). With careful design of the reactor's geometry, and careful control of the substances present so as to influence the
reactivity, a self-sustaining
chain reaction
A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events.
Chain reactions are one way that syst ...
or "
criticality" can be achieved and maintained.
Natural uranium consists of a mixture of various
isotope
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...
s, primarily
238U and a much smaller amount (about 0.72% by weight) of
235U.
238U can only be fissioned by neutrons that are relatively energetic, about 1
MeV
In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacu ...
or above. No amount of
238U can be made "critical" 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 achieving
criticality using only natural or low enriched uranium, for which there is no "bare" ''critical mass'', is to slow down the emitted neutrons (without absorbing them) to the point where enough of them may cause further nuclear fission in the small amount of
235U which is available. (
238U which is the bulk of natural uranium is also fissionable with fast neutrons.) This requires the use of a
neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, ideally without capturing any, leaving them as thermal neutrons with only minimal (thermal) kinetic energy. These thermal neutrons are immensely mo ...
, which absorbs virtually all of the neutrons'
kinetic energy
In physics, the kinetic energy of an object is the energy that 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 stated velocity. Having gained this energy during its accele ...
, slowing them down to the point that they reach thermal equilibrium with surrounding material. It has been found beneficial to the
neutron economy
Neutron 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.
The distribution of neutron energies in a nuclear reactor differs from the f ...
to physically separate the neutron energy moderation process from the uranium fuel itself, as
238U has a high probability of absorbing neutrons with intermediate kinetic energy levels, a reaction known as "resonance" absorption. This is a fundamental reason for designing reactors with separate solid fuel segments, surrounded by the moderator, rather than any geometry that would give a homogeneous mix of fuel and moderator.
Water makes an excellent moderator; the
ordinary hydrogen or ''protium'' atoms in the water molecules are very close in mass to a single neutron, and so their collisions result in a very efficient transfer of momentum, similar conceptually to the collision of two billiard balls. However, as well as being a good moderator, ordinary water is also quite effective at absorbing neutrons. And so using ordinary water as a moderator will easily absorb so many neutrons that too few are left to sustain a chain reaction with the small isolated
235U nuclei in the fuel, thus precluding criticality in natural uranium. Because of this, a
light-water reactor
The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron react ...
will require that the
235U isotope be concentrated in its uranium fuel, as
enriched uranium
Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238 ...
, generally between 3% to 5%
235U by weight (the by-product from this process enrichment process is known as
depleted uranium
Depleted uranium (DU; also referred to in the past as Q-metal, depletalloy or D-38) is uranium with a lower content of the fissile isotope than natural uranium.: "Depleted uranium possesses only 60% of the radioactivity of natural uranium, hav ...
, and so consisting mainly of
238U, chemically pure). The degree of enrichment needed to achieve ''criticality'' with a ''light-water'' moderator depends on the exact geometry and other design parameters of the reactor.
One complication of this approach is the need for uranium enrichment facilities, which are generally expensive to build and operate. They also present a
nuclear proliferation
Nuclear proliferation is the spread of nuclear weapons, fissionable material, and weapons-applicable nuclear technology and information to nations not recognized as " Nuclear Weapon States" by the Treaty on the Non-Proliferation of Nuclear Wea ...
concern; the
same systems used to enrich the
235U can also be used to produce much more "pure"
weapons-grade
Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon or has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nucle ...
material (90% or more
235U), suitable for producing a
nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bom ...
. 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 water, or deuterium-oxide. Although it reacts dynamically with the neutrons in a fashion similar to light water (albeit with less energy transfer on average, given that heavy hydrogen, or
deuterium
Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two Stable isotope ratio, stable isotopes of hydrogen (the other being Hydrogen atom, protium, or hydrogen-1). The atomic nucleus, nucleus of a deuterium ato ...
, is about twice the mass of hydrogen), it already has the extra neutron that light water would normally tend to absorb.
Advantages and disadvantages
![U235 Fission cross section](https://upload.wikimedia.org/wikipedia/commons/e/e0/U235_Fission_cross_section.png)
Advantages
The use of heavy water as the moderator is the key to the PHWR (pressurized heavy water reactor) system, enabling the use of natural uranium as the fuel (in the form of ceramic UO
2), which means that it can be operated without expensive uranium enrichment facilities. The mechanical arrangement of the PHWR, which places most of the moderator at lower temperatures, is particularly efficient because the resulting thermal neutrons have lower energies (
neutron temperature
The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium with ...
after successive passes through a moderator roughly equals the temperature of the moderator) than in traditional designs, where the moderator normally is much hotter. The
neutron cross section
In nuclear physics, the concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. The neutron cross section σ can be defined as the area in cm2 for which the number of ...
for fission is higher in the lower the neutron temperature is, and thus lower temperatures in the moderator make successful interaction between neutrons and fissile material more likely. These features mean that a PHWR can use natural uranium and other fuels, and does so more efficiently than
light water reactor
The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron react ...
s (LWRs). CANDU type PHWRs are claimed to be able to handle fuels including
reprocessed uranium
Reprocessed uranium (RepU) is the uranium recovered from nuclear reprocessing, as done commercially in France, the UK and Japan and by nuclear weapons states' military plutonium production programs. This uranium makes up the bulk of the material s ...
or even
spent nuclear fuel
Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor an ...
from "conventional"
light water reactor
The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron react ...
s as well as
MOX fuel
Mixed oxide fuel, commonly referred to as MOX fuel, is nuclear fuel that contains more than one oxide of fissile material, usually consisting of plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an alt ...
and there is ongoing research into the ability of CANDU type reactors to operate exclusively on such fuels in a commercial setting. (More on that in the article on the
CANDU
The CANDU (Canada Deuterium Uranium) is a Canadian pressurized heavy-water reactor design used to generate electric power. The acronym refers to its deuterium oxide ( heavy water) moderator and its use of (originally, natural) uranium fuel. C ...
reactor itself)
Disadvantages
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. 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
Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor a ...
than in LWRs employing enriched uranium. Since unenriched uranium fuel accumulates a lower density of
fission products than enriched uranium fuel, however, it generates less heat, allowing more compact storage. While deuterium has a ''lower'' neutron capture cross section than
Protium, this value isn't ''zero'' and thus part of the heavy water moderator will inevitably be converted to
tritiated water
Tritiated water is a radioactive form of water in which the usual protium atoms are replaced with tritium. In its pure form it may be called tritium oxide (T2O or 3H2O) or super-heavy water. Pure T2O is corrosive due to self-radiolysis. Dilut ...
. While
tritium
Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus o ...
, a radioactive isotope of hydrogen, is also produced as a fission product in minute quantities in other reactors, tritium can more easily escape to the environment if it is also present in the cooling water, which is the case in those PHWRs which use heavy water both as moderator and as coolant. Some CANDU reactors separate out the tritium from their heavy water inventory at regular intervals and sell it at a profit, however.
While with typical
CANDU
The CANDU (Canada Deuterium Uranium) is a Canadian pressurized heavy-water reactor design used to generate electric power. The acronym refers to its deuterium oxide ( heavy water) moderator and its use of (originally, natural) uranium fuel. C ...
derived fuel bundles, the reactor design has a slightly ''positive''
Void coefficient
In nuclear engineering, the void coefficient (more properly called void coefficient of reactivity) is a number that can be used to estimate how much the reactivity of a nuclear reactor changes as voids (typically steam bubbles) form in the reactor ...
of reactivity, the Argentina designed CARA fuel bundles used in
Atucha I, are capable of the preferred negative coefficient.
Nuclear proliferation
While prior to India's development of nuclear weapons (see below), the ability to use natural uranium (and thus forego the need for
uranium enrichment
Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238U ...
which is a
dual use
In politics, diplomacy and export control, dual-use items refers to goods, software and technology that can be used for both civilian and military applications. technology) was seen as ''hindering'' nuclear proliferation, this opinion has changed drastically in light of the ability of several countries to build atomic bombs out of plutonium, which can easily be produced in heavy water reactors. Heavy-water reactors may thus pose a ''greater'' risk of
nuclear proliferation
Nuclear proliferation is the spread of nuclear weapons, fissionable material, and weapons-applicable nuclear technology and information to nations not recognized as " Nuclear Weapon States" by the Treaty on the Non-Proliferation of Nuclear Wea ...
versus comparable
light-water reactor
The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron react ...
s due to the low neutron absorption properties of heavy water, discovered in 1937 by
Hans von Halban
Hans Heinrich von Halban (24 January 1908 – 28 November 1964) was a French physicist, of Austrian- Jewish descent.
Family
He was descended on his father's side from Polish Jews, who left Kraków for Vienna in the 1850s. His grandfather, Hei ...
and
Otto Frisch.
Occasionally, when an atom of
238U is exposed to
neutron radiation
Neutron radiation is a form of ionizing radiation that presents as free neutrons. Typical phenomena are nuclear fission or nuclear fusion causing the release of free neutrons, which then Neutron capture, react with Atomic nucleus, nuclei of other ...
, its nucleus will capture a
neutron
The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...
, changing it to
239U. The
239U then rapidly undergoes two
β− decays — both emitting an
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
and an
antineutrino
A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is ...
, the first one transmuting the
239U into
239Np, and the second one transmuting the
239Np into
239Pu. Although this process takes place with other moderators such as ultra-pure graphite or beryllium, heavy water is by far the best.
The
Manhattan Project
The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project w ...
ultimately used graphite moderated reactors to produce plutonium, while the
German wartime nuclear project wrongfully dismissed graphite as a suitable moderator due to overlooking impurities and thus made unsuccessful attempts using heavy water (which they correctly identified as an excellent moderator). The
Soviet nuclear program
The Soviet atomic bomb project was the classified research and development program that was authorized by Joseph Stalin in the Soviet Union to develop nuclear weapons during and after World War II.
Although the Soviet scientific community disc ...
likewise used graphite as a moderator and ultimately developed the graphite moderated
RBMK
The RBMK (russian: реактор большой мощности канальный, РБМК; ''reaktor bolshoy moshchnosti kanalnyy'', "high-power channel-type reactor") is a class of graphite-moderated nuclear power reactor designed and buil ...
as a reactor capable of producing both large amounts of electric power and
weapons grade plutonium
Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon or has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nucle ...
without the need for heavy water or - at least according to initial design specifications -
uranium enrichment
Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238U ...
.
239Pu is a
fissile material
In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of thermal energy. The predominant neutron energy may be ty ...
suitable for use in
nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bom ...
s. As a result, if the fuel of a heavy-water reactor is changed frequently, significant amounts of
weapons-grade plutonium
Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon or has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nucle ...
can be chemically extracted from the irradiated natural uranium fuel by
nuclear reprocessing
Nuclear reprocessing is the chemical separation of fission products and actinides from spent nuclear fuel. Originally, reprocessing was used solely to extract plutonium for producing nuclear weapons. With commercialization of nuclear power, the ...
.
In addition, the use of heavy water as a moderator results in the production of small amounts of
tritium
Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus o ...
when the
deuterium
Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two Stable isotope ratio, stable isotopes of hydrogen (the other being Hydrogen atom, protium, or hydrogen-1). The atomic nucleus, nucleus of a deuterium ato ...
nuclei in the heavy water absorb neutrons, a very inefficient reaction. Tritium is essential for the production of
boosted fission weapon
A boosted fission weapon usually refers to a type of nuclear bomb that uses a small amount of fusion fuel to increase the rate, and thus yield, of a fission reaction. The neutrons released by the fusion reactions add to the neutrons released ...
s, which in turn enable the easier production of
thermonuclear weapon
A thermonuclear weapon, fusion weapon or hydrogen bomb (H bomb) is a second-generation nuclear weapon design. Its greater sophistication affords it vastly greater destructive power than first-generation nuclear bombs, a more compact size, a lowe ...
s, including
neutron bomb
A neutron bomb, officially defined as a type of enhanced radiation weapon (ERW), is a low-yield thermonuclear weapon designed to maximize lethal neutron radiation in the immediate vicinity of the blast while minimizing the physical power of the b ...
s. This process is currently expected to provide (at least partially) tritium for
ITER
ITER (initially the International Thermonuclear Experimental Reactor, ''iter'' meaning "the way" or "the path" in Latin) is an international nuclear fusion research and engineering megaproject aimed at creating energy by replicating, on Earth ...
.
The proliferation risk of heavy-water reactors was demonstrated when India produced the
plutonium
Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibi ...
for
Operation Smiling Buddha
Operation Smiling BuddhaThis test has many code names. Civilian scientists called it "Operation Smiling Buddha" and the Indian Army referred to it as ''Operation Happy Krishna''. According to United States Military Intelligence, ''Operation H ...
, its first nuclear weapon test, by extraction from the spent fuel of a heavy-water research reactor known as the
CIRUS reactor.
See also
*
CANDU reactor
The CANDU (Canada Deuterium Uranium) is a Canadian pressurized heavy-water reactor design used to generate electric power. The acronym refers to its deuterium oxide ( heavy water) moderator and its use of (originally, natural) uranium fuel. C ...
: The predominant type of PHWR
*
IPHWR-700 and
IPHWR-220
The IPHWR-220 (Indian Pressurized Heavy Water Reactor-220) is an Indian pressurized heavy-water reactor designed by the Bhabha Atomic Research Centre. It is a Generation II reactor developed from earlier CANDU based RAPS-1 and RAPS-2 reactors b ...
, PHWR types developed in India
*
List of nuclear reactors
A ''list'' is any set of items in a row. List or lists may also refer to:
People
* List (surname)
Organizations
* List College, an undergraduate division of the Jewish Theological Seminary of America
* SC Germania List, German rugby union ...
*
Pressurized water reactor
A pressurized water reactor (PWR) is a type of light-water reactor, light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants (with notable exceptions being the UK, Japan and Canada). In a PWR, the primary ...
References
Economics of Nuclear Power from Heavy Water ReactorsNuclear Power Program – Stage1 – Pressurised Heavy Water ReactorIAEA - Technical Reports Series No. 407
External links
Official website of AECL
{{Nuclear fission reactors
Nuclear power reactor types