Subcritical reactor

Subcritical reactor

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A subcritical reactor is a nuclear fission
Nuclear fission
In 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 photons , and releasing a tremendous amount of energy...

 reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

 that produces fission without achieving criticality
Critical mass
A 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 reaction
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....

, a subcritical reactor uses additional neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s from an outside source. The neutron source
Neutron source
A Neutron source is a device that emits neutrons. There is a wide variety of different sources, ranging from hand-held radioactive sources to neutron research facilities operating research reactors and spallation sources...

 can be a nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

 machine or a particle accelerator
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...

 producing neutrons by spallation
Spallation
In general, spallation is a process in which fragments of material are ejected from a body due to impact or stress. In the context of impact mechanics it describes ejection or vaporization of material from a target during impact by a projectile...

.

Such a device with a reactor coupled to an accelerator is called an Accelerator-Driven System (ADS).

Motivation


A subcritical reactor can be used to destroy heavy isotopes contained in the used fuel from a conventional nuclear reactor, while at the same time producing electricity. The long-lived transuranic elements in nuclear waste can in principle be fission
Nuclear fission
In 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 photons , and releasing a tremendous amount of energy...

ed, releasing energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 in the process and leaving behind the fission products which are shorter-lived. This would shorten considerably the time for disposal of radioactive waste
Radioactive waste
Radioactive wastes are wastes that contain radioactive material. Radioactive wastes are usually by-products of nuclear power generation and other applications of nuclear fission or nuclear technology, such as research and medicine...

. However, some isotopes have threshold fission cross section
Cross section (physics)
A cross section is the effective area which governs the probability of some scattering or absorption event. Together with particle density and path length, it can be used to predict the total scattering probability via the Beer-Lambert law....

s and have a small effective fraction of delayed neutron
Delayed neutron
In nuclear engineering, a delayed neutron is a neutron emitted after a nuclear fission event by one of the fission products anytime from a few milliseconds to a few minutes later....

s 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 neptunium-237, americium-241 and americium-243. The nuclear weapon
Nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...

 material plutonium-239
Plutonium-239
Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in...

 is also suitable although it can be expended in a cheaper way as MOX fuel
MOX fuel
Mixed oxide fuel, commonly referred to as MOX fuel, is nuclear fuel that contains more than one oxide of fissile material. MOX fuel contains plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an alternative to the low-enriched uranium fuel used in the...

 or inside existing fast reactors.

Besides nuclear waste incineration, there's interest in this type reactor because they are seen as safer than normal fission reactors.http://www.roanoke.com/news/nrv/wb/203552 In most critical reactors, the nuclear chain reaction can potentially increase exponentially until the heat destroys the reactor, causing an expensive and potentially dangerous accident (see the chernobyl disaster
Chernobyl disaster
The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine , which was under the direct jurisdiction of the central authorities in Moscow...

 as an example). With a subcritical reactor, the reaction will stop automatically unless continually fed neutrons from an outside source.

Principle


Most current ADS designs propose a high-intensity proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

  accelerator
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...

 with an energy of about 1 GeV
GEV
GEV or GeV may stand for:*GeV or gigaelectronvolt, a unit of energy equal to billion electron volts*GEV or Grid Enabled Vehicle that is fully or partially powered by the electric grid, see plug-in electric vehicle...

, directed towards a spallation
Spallation
In general, spallation is a process in which fragments of material are ejected from a body due to impact or stress. In the context of impact mechanics it describes ejection or vaporization of material from a target during impact by a projectile...

 target made of thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....

 that is cooled by liquid lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

-bismuth
Bismuth
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...

 in the core
Nuclear reactor core
A nuclear reactor core is the 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 neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s are created to irradiate the surrounding fuel. Thus, the neutron balance can be regulated so that the reactor would be below criticality
Criticality
Criticality may refer to:* Critical thinking in education* Critical reflection in adult education* Critical mass in nuclear reactions* Criticality accident* Criticality matrix* Nuclear Criticality Safety...

 if the additional neutrons by the accelerator were not provided. The main advantage is inherent safety
Inherent safety
Inherent safety is a concept particularly used in the chemical and process industries. An inherently safe process has a low level of danger even if things go wrong. It is used in contrast to safe systems where a high degree of hazard is controlled by protective systems...

, even if the nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...

 under consideration lack Uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

's self-regulating properties—like delayed neutrons and Doppler
Fuel temperature coefficient of reactivity
Fuel temperature coefficient of reactivity is the change in reactivity of the nuclear fuel per degree change in the fuel temperature. The coefficient quantifies the amount of neutrons that the nuclear fuel absorbs from the fission process as the fuel temperature increases. It is a measure of the...

 coefficient—that make standard nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

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 data
Nuclear data
Nuclear 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 Rubbia
Carlo Rubbia
Carlo Rubbia Knight Grand Cross is an Italian particle physicist and inventor who shared the Nobel Prize in Physics in 1984 with Simon van der Meer for work leading to the discovery of the W and Z particles at CERN.-Biography:...

, a nuclear physicist
Physicist
A 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 CERN
CERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...

, was one of the first to conceive a design of a subcritical reactor, the so-called "energy amplifier
Energy amplifier
In 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 power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...

 and as a means of transmutation
Nuclear transmutation
Nuclear transmutation is the conversion of one chemical element or isotope into another. In other words, atoms of one element can be changed into atoms of other element by '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 disposal
Deep geological repository
A deep geological repository is a nuclear waste repository excavated deep within a stable geologic environment...

 or transmutation in fast-neutron reactors. However, the solution of a subcritical reactor might be favoured for a better public acceptance
Public opinion
Public 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 reactor
Light water reactor
The light water reactor is a type of thermal reactor that uses normal water as its coolant and neutron moderator. Thermal reactors are the most common type of nuclear reactor, and light water reactors are the most common type of thermal reactor...

 design, other proposals have been made that incorporate an ADS into other generation IV reactor
Generation IV reactor
Generation 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...

 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 coefficient
Void coefficient
In 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.

See also

  • Alternative energy
    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
    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
    Spallation
    In general, spallation is a process in which fragments of material are ejected from a body due to impact or stress. In the context of impact mechanics it describes ejection or vaporization of material from a target during impact by a projectile...

  • Cosmic ray spallation
    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 protons, alpha particles, and nuclei...

  • Spallation Neutron Source
    Spallation Neutron Source
    The Spallation Neutron Source is an accelerator-based neutron source facility that provides the most intense pulsed neutron beams in the world for scientific research and industrial development...

  • PSI Spallation Neutron Source (SINQ)
  • ISIS neutron source
    ISIS neutron source
    ISIS is a pulsed neutron and muon source. It is situated at the Rutherford Appleton Laboratory on the Harwell Science and Innovation Campus in Oxfordshire, United Kingdom and is part of the Science and Technology Facilities Council...

  • Hybrid nuclear fusion
    Hybrid nuclear fusion
    Hybrid nuclear fusion-fission is a proposed means of generating power by use of a combination of nuclear fusion and fission processes...