A
subcritical reactor is a nuclear
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...
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...
that produces fission without achieving
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...
. Instead of a sustaining
chain reactionA 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...
, a subcritical reactor uses additional
neutronThe neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.Neutron are usually found in atomic nuclei. The nuclei of most atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of protons in a...
s from an outside source. A reactor coupled to a
particle acceleratorA particle accelerator is a device that uses electric fields to propel ions or charged subatomic particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator...
to produce neutrons by
spallationIn general, spallation is a process in which fragments of material are ejected from a body due to impact or stress. In nuclear physics, it is the process in which a heavy nucleus emits a large number of nucleons as a result of being hit by a high-energy particle, thus greatly reducing its atomic...
is called an
Accelerator-Driven System (
ADS).
Motivation
The long-lived transuranic elements in nuclear waste can in principle be
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...
ed, releasing
energyIn physics, energy is a scalar physical quantity that describes the amount of work that can be performed by a force, an attribute of objects and systems that is subject to a conservation law...
in the process and leaving behind the fission products which are shorter-lived. This would shorten considerably the time for disposal of radioactive waste. However, some isotopes have
threshold-Film and television:* Threshold , an adaptation of the 1958 science fiction film It! The Terror from Beyond Space* Threshold , an American science fiction drama series* "Threshold" , an episode of the TV series...
fission
cross sectionIn nuclear and particle physics, the concept of a cross section is used to express the likelihood of interaction between particles.When particles are thrown against a foil made of a certain substance, the cross section is a hypothetical area measure around the target particles that represents a...
s and have a small effective fraction of delayed neutrons and therefore require a fast reactor for being fissioned, and for safety reasons preferably a subcritical reactor if they constitute a significant fraction of the fuel. The three most important long-term radioactive isotopes that could advantageously be handled that way are
NeptuniumNeptunium is a chemical element with the symbol Np and atomic number 93. A radioactive metallic element, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production and it...
-237,
AmericiumAmericium is a synthetic element that has the symbol Am and atomic number 95. A radioactive metallic element, americium is an actinide that was obtained in 1944 by Glenn T. Seaborg who was bombarding plutonium with neutrons and was the fourth transuranic element to be discovered. It was named for...
-241 and Americium-243. The
nuclear weaponA nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion...
material
PlutoniumPlutonium is a rare transuranic radioactive element. It is an actinide metal of silvery-white appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. It reacts with carbon, halogens, nitrogen and...
-239 is also suitable although it can be expended in a cheaper way as
MOXMOX might be a name or acronym for:*Malaysian Oxygen Berhad - A Malaysian company that is specializes in providing total gas solutions.*Mixed Oxide Fuel, from nuclear reprocessing*An alien race in the TimeSplitters 2 video game, the Mox...
fuel or inside existing fast reactors.
Principle
Most current ADS designs propose a high-intensity
protonThe proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H
+...
acceleratorA particle accelerator is a device that uses electric fields to propel ions or charged subatomic particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator...
with an energy of about 1
GeVGEV or GeV may stand for:*Generalized extreme value distribution*GeV or gigaelectronvolt, a unit of energy equal to billion electron volts*Ground effect vehicle*G.E.V., a tabletop game by Steve Jackson games, based on OGRE...
, directed towards a
spallationIn general, spallation is a process in which fragments of material are ejected from a body due to impact or stress. In nuclear physics, it is the process in which a heavy nucleus emits a large number of nucleons as a result of being hit by a high-energy particle, thus greatly reducing its atomic...
target made of
thoriumThorium is a chemical element with the symbol Th and atomic number 90. It is a naturally occurring, slightly radioactive metal. Thorium is estimated to be about three to four times more abundant than uranium in the earth's crust...
that is cooled by liquid
leadLead is a main-group element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal, also considered to be one of the heavy metals. Lead has a bluish-white color when freshly cut, but tarnishes to a dull grayish color when exposed to air...
-
bismuthBismuth is a chemical element that has the symbol Bi and atomic number 83. This trivalent poor metal chemically resembles arsenic and antimony. Bismuth is heavy and brittle; it has a silvery white color with a pink tinge owing to the surface oxide. Bismuth is the most naturally diamagnetic of all...
in the
coreA nuclear reactor core is that portion of a nuclear reactor containing the nuclear fuel components where the nuclear reactions take place.- Description :...
of the reactor. In that way, for each proton interacting in the target, an average 20
neutronThe neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.Neutron are usually found in atomic nuclei. The nuclei of most atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of protons in a...
s are created to irradiate the surrounding fuel. Thus, the neutron balance can be regulated such as the reactor would be below
criticalityCriticality may refer to:*Critical mass, critical mass in nuclear reactors;*Self-organized criticality;*Criticality matrix;*Criticality accident*Nuclear Criticality Safety...
if the additional neutrons by the accelerator were not provided. The main advantage is inherent safety, even if the
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;...
under consideration lack
UraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. Besides its 92 protons, a uranium nucleus can have between 141 and 146 neutrons. The most common uranium isotopes are U-238 and U-235 . A uranium atom has...
's nice self-regulating properties—like delayed neutrons and doppler coefficient—that make standard
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...
s safe. Whenever the neutron source is turned off, the reaction ceases.
Technical challenges
There are technical difficulties to overcome before ADS can become economical and eventually be integrated into future nuclear waste management. The accelerator must provide a high intensity and be highly reliable. There are concerns about the window separating the protons from the spallation target, which is expected to be exposed to stress under extreme conditions. The chemical separation of the transuranic elements and the fuel manufacturing, as well as the structure materials, are important issues. Finally, the lack of
nuclear dataNuclear data groups all experimental data relevant for nuclear physics and nuclear applications. It includes a large and non-exhaustic number of physical quantities, like scattering and reaction cross sections , nuclear structure and nuclear decay parameters, etc...
at high neutron energies limits the efficiency of the design.
Some laboratory experiments and many theoretical studies have demonstrated the theoretical possibility of such a plant.
Carlo RubbiaCarlo Rubbia is an Italian particle physicist who won the Nobel Prize in Physics in 1984 for work leading to the discovery of the W and Z particles at CERN.-Biography:...
, a nuclear
physicistA physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...
and former director of
CERNThe European Organization for Nuclear Research , known as CERN , , is the world's largest particle physics laboratory, situated in the northwest suburbs of Geneva on the Franco–Swiss border, established in 1954...
, was one of the first to conceive a design of a subcritical reactor, the so-called "
energy amplifierIn nuclear physics, an energy amplifier is a novel type of nuclear power reactor, a subcritical reactor, in which an energetic particle beam is used to stimulate a reaction, which in turn releases enough energy to power the particle accelerator and leave an energy profit for power generation...
". In 2005, several large-scale projects are going on in Europe and Japan to further develop subcritical reactor technology.
Economics and public acceptance
Subcritical reactors have been proposed both as a means of generating
electric powerElectric power is defined as the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt.When electric current flows in a circuit, it can transfer energy to do mechanical or thermodynamic work...
and as a means of transmutation of nuclear waste, so the gain is twofold. However, the costs for construction, safety and maintenance of such complex installations are expected to be very high, let alone the amount of research needed to develop a practical design (see above). There exist cheaper and reasonably safe waste management concepts, such as the long-term
geological disposalA deep geological repository is a nuclear waste repository excavated below 300 m within a salt dome or bedrock. It entails a combination of waste form, waste package and engineered seals that is designed to provide a high level of long-term storage without future maintenance.- Principles and...
. However, the solution of a subcritical reactor might be favoured for a better
public acceptancePublic opinion is the aggregate of individual attitudes or beliefs held by the adult population. Public opinion can also be defined as the complex collection of opinions of many different people and the sum of all their views...
—it is considered more acceptable to burn the waste than to bury it for hundreds of thousands of years. For future waste management, a few transmutation devices could be integrated into a large-scale nuclear program, hopefully increasing only slightly the overall costs.
Subcritical Hybrid Systems
While originally thought that an ADS would be a part of a
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...
design, other proposals have been made that incorporate an ADS into other
generation IV reactorGeneration IV reactors are a set of theoretical nuclear reactor designs currently being researched. Most of these designs are generally not expected to be available for commercial construction before 2030, with the exception of a version of the Very High Temperature Reactor called the Next...
concepts.
One such proposal calls for a gas cooled fast reactor that is fueled primarily by Plutonium and Americium. The neutronic properties of Americium make it difficult to use in any critical reactor due to neutronic properties that tend to make the
moderator temperature coefficientIn nuclear engineering, the void coefficient is a number that can be used to estimate how much the reactivity of a nuclear reactor changes as voids form in the reactor moderator or coolant...
more positive, decreasing stability. The inherent safety of an ADS, however, would allow Americium to be safely burned. These materials also have good neutron economy, allowing the pitch-to-diameter ratio to be large, which allows for improved natural circulation and economics.
External links
See also
- Alternative energy
Alternative energy is an umbrella term that refers to any source of usable energy intended to replace fuel sources without the undesired consequences of the replaced fuels...
- Spall
Spall are flakes of a material that are broken off a larger solid body and can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure...
, or spalling
- Spallation
In general, spallation is a process in which fragments of material are ejected from a body due to impact or stress. In nuclear physics, it is the process in which a heavy nucleus emits a large number of nucleons as a result of being hit by a high-energy particle, thus greatly reducing its atomic...
- Cosmic ray spallation
Cosmic ray spallation is a form of naturally occurring nuclear fission and nucleosynthesis. It refers to the formation of elements from the impact of cosmic rays on an object. Cosmic rays are highly energetic charged particles from outside of Earth ranging from stray electrons to alpha particles....
- Spallation Neutron Source
The Spallation Neutron Source is an accelerator-based neutron source in Oak Ridge, Tennessee , USA, at the site of Oak Ridge National Laboratory by the U.S. Department of Energy . SNS was designed and constructed by a unique partnership of six DOE national laboratories: Argonne, Lawrence...
- PSI Spallation Neutron Source (SINQ)
- ISIS neutron source
ISIS is a pulsed neutron and muon source. It is situated at the Rutherford Appleton Laboratory in Oxfordshire, United Kingdom and is part of the Science and Technology Facilities Council...