Aneutronic fusion is any form of
fusion powerFusion power is the power generated by nuclear fusion processes. In fusion reactions two light atomic nuclei fuse together to form a heavier nucleus . In doing so they release a comparatively large amount of energy arising from the binding energy due to the strong nuclear force which is manifested...
where
neutronThe 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 carry no more than 1% of the total released energy. The most-studied fusion reactions release up to 80% of their energy in neutrons. Successful aneutronic fusion would greatly reduce problems associated with
neutron radiationNeutron radiation is a kind of ionizing radiation which consists of free neutrons. A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from atoms, and these free neutrons react with nuclei of other atoms to form new isotopes, which, in turn, may produce...
such as
ionizing damageIonizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
,
neutron activationNeutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus often decays immediately by emitting particles such as neutrons, protons, or alpha...
, and requirements for biological shielding, remote handling, and safety.
Some proponents also see a potential for dramatic cost reductions by converting energy directly to electricity. However, the conditions required to harness aneutronic fusion are much more extreme than those required for the conventional
deuteriumDeuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
–
tritiumTritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
(DT) fuel cycle.
Candidate aneutronic reactions
There are a few fusion reactions that have no neutrons as products on any of their branches. Those with the largest
cross sectionThe nuclear cross section of a nucleus is used to characterize the probability that a nuclear reaction will occur. The concept of a nuclear cross section can be quantified physically in terms of "characteristic area" where a larger area means a larger probability of interaction...
s are these:
DDeuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
|
3He Helium-3 is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on Earth, and is sought for use in nuclear fusion research...
|
→ |
|
4He Helium-4 is a non-radioactive isotope of helium. It is by far the most abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on earth. Its nucleus is the same as an alpha particle, consisting of two protons and two neutrons. Alpha decay of heavy...
|
(3.6 MeV In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
) |
+ |
|
p |
(14.7 MeV) |
| D |
6Li |
→ |
2 |
4He |
22.4 MeV
pThe 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....
|
6Li |
→ |
|
4He |
(1.7 MeV) |
+ |
|
3He |
(2.3 MeV) |
| 3He |
6Li |
→ |
2 |
4He |
|
+ |
|
p |
16.9 MeV
| 3He |
3He |
→ |
|
4He |
|
+ |
2 |
p |
+12.86 MeV |
pThe 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....
|
7Li |
→ |
2 |
4He |
17.2 MeV
| p |
11B |
→ |
3 |
4He |
8.7 MeV
| p |
15N |
→ |
|
12C |
4He
5.0 MeV
|}
The first two of these use deuterium as a fuel, and D–D side reactions produce some neutrons. Although these can be minimized by running hot and deuterium-lean, the fraction of energy released as neutrons will probably be several percent, so that these fuel cycles, although neutron-poor, do not qualify as aneutronic according to the 1% threshold.
The next two reactions' rates (involving p, 3He, and 6Li) are not particularly high in a thermal plasma. When treated as a chain, however, they offer the possibility of an enhanced reactivity due to a non-thermal distributionIn molecular kinetic theory in physics, a particle's distribution function is a function of seven variables, f, which gives the number of particles per unit volume in phase space. It is the number of particles per unit volume having approximately the velocity near the place and time...
. The product 3He from the first reaction could participate in the second reaction before thermalizing, and the product p from the second reaction could participate in the first reaction before thermalizing. Unfortunately, detailed analyses have not shown sufficient reactivity enhancement to overcome the inherently low cross section.
The pure 3He reaction suffers from a fuel-availability problem. 3He occurs in only minuscule amounts naturally on Earth, so it would either have to be bred from neutron reactions (counteracting the potential advantage of aneutronic fusion), or mined from extraterrestrial sources. The top several meters of the surface of the Moon is relatively rich in 3HeHelium-3 is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on Earth, and is sought for use in nuclear fusion research...
, on the order of 0.01 parts per million by weight, but mining this resource and returning it to Earth would be very difficult and expensive. 3He could in principle be recovered from the atmospheres of the gas giantA gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
planets, but this would be even more challenging.
The p –7Li reaction has no advantage over p –11B. On the contrary, its cross section is somewhat lower.
For the above reasons, most studies of aneutronic fusion concentrate on the reaction, p –11B.
Temperature
Despite the suggested advantages of aneutronic fusion, the vast majority of fusion research effort has gone toward D–T fusion because the technical challenges of hydrogen–boron (p–11B) fusion are so formidable. Hydrogen–boron fusion requires ion energies or temperatures almost ten times higher than those for D–T fusion. For any given densities of the reacting nuclei, the reaction rate for hydrogen boron achieves its peak rate at around 600 keVKev can refer to:*Kev Hawkins, a fictional character.*Kevin, a given name occasionally shortened to "Kev".*Kiloelectronvolt, a unit of energy who symbol is "KeV".* Krefelder Eislauf-VereinKEV can refer to:...
(6.6 billion degrees Celsius or 6.6 gigakelvins) while D–T has a peak at around 66 keV (730 million degrees Celsius).
Power balance
In addition, the peak reaction rate of p–11B is only one third that for D–T, requiring better plasma confinement. Confinement is usually characterized by the time τ the energy must be retained so that the fusion power released exceeds the power that went into the heating of the plasma. Various requirements can be derived, most commonly the product with the density, nτ, and the product with the pressure nTτ, both of which are called the Lawson criterionIn nuclear fusion research, the Lawson criterion, first derived on fusion reactors by John D. Lawson in 1955 and published in 1957, is an important general measure of a system that defines the conditions needed for a fusion reactor to reach ignition, that is, that the heating of the plasma by the...
. The nτ required for p–11B is 45 times higher than that for DT. The nTτ required is 500 times higher. (See also neutronicity, confinement requirement, and power density.) Since the confinement properties of conventional fusion approaches such as the tokamakA tokamak is a device using a magnetic field to confine a plasma in the shape of a torus . Achieving a stable plasma equilibrium requires magnetic field lines that move around the torus in a helical shape...
and laser pellet fusion are marginal, most aneutronic fusion proposals use radically different confinement concepts.
In most fusion plasmas, bremsstrahlungBremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...
radiation is a major energy loss channel. (See also bremsstrahlung losses in quasineutral, isotropic plasmas.) For the p–11B reaction, some calculations indicate that the bremsstrahlungBremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...
power will be at least 1.74 times larger than the fusion power. The corresponding ratio for the 3He-3He reaction is only slightly more favorable at 1.39. This is not applicable to non-neutral plasmas, and different in anisotropic plasmas.
In conventional reactor designs, whether based on magnetic confinementMagnetic confinement fusion is an approach to generating fusion power that uses magnetic fields to confine the hot fusion fuel in the form of a plasma. Magnetic confinement is one of two major branches of fusion energy research, the other being inertial confinement fusion. The magnetic approach is...
or inertial confinementInertial confinement fusion is a process where nuclear fusion reactions are initiated by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium....
, the bremsstrahlung can easily escape the plasma and is considered a pure energy loss term. The outlook would be more favorable if the plasma could reabsorb the radiation. Absorption occurs primarily via Thomson scatteringThomson scattering is the elastic scattering of electromagnetic radiation by a free charged particle, as described by classical electromagnetism. It is just the low-energy limit of Compton scattering: the particle kinetic energy and photon frequency are the same before and after the scattering...
on the electrons, which has a total cross section of σT = 6.65×10−29 m². In a 50–50 D–T mixture this corresponds to a range of 6.3 g/cm². This is considerably higher than the Lawson criterion of ρR > 1 g/cm², which is already difficult to attain, but might not be out of reach in future inertial confinement systems.
In very high magnetic fields, on the order of a megatesla, a quantum mechanicalQuantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
effect may suppress energy transfer from the ions to the electrons. According to one calculation, the bremsstrahlung losses could be reduced to half the fusion power or less. In a strong magnetic field the cyclotron radiationCyclotron radiation is electromagnetic radiation emitted by moving charged particles deflected by a magnetic field. The Lorentz force on the particles acts perpendicular to both the magnetic field lines and the particles' motion through them, creating an acceleration of charged particles that...
is even larger than the bremsstrahlung. In a megatesla field, an electron would lose its energy to cyclotron radiation in a few picoseconds if the radiation could escape. However, in a sufficiently dense plasma (ne > 2.5×1030 m−3, a density greater than that of a solid), the cyclotron frequency is less than twice the plasma frequency. In this well-known case, the cyclotron radiation is trapped inside the plasmoid and cannot escape, except from a very thin surface layer.
While megatesla fields have not yet been achieved in the laboratory, fields of 0.3 megatesla have been produced with high intensity lasers, and fields of 0.02–0.04 megatesla have been observed with the dense plasma focusA dense plasma focus is a machine that produces, by electromagnetic acceleration and compression, a short-lived plasma that is so hot and dense that it can cause nuclear fusion and emit X-rays. The electromagnetic compression of the plasma is called a pinch. It was invented in the early 1960s by...
device.
At much higher densities (ne > 6.7×1034 m−3), the electrons will be Fermi degenerate, which suppresses bremsstrahlung losses, both directly and by reducing energy transfer from the ions to the electrons. If necessary conditions can be attained, net energy production from p–11B or D–3He fuel may be possible. The probability of a feasible reactor based solely on this effect remains low, however, because the gainThe fusion energy gain factor, usually expressed with the symbol Q, is the ratio of fusion power produced in a nuclear fusion reactor to the power required to maintain the plasma in steady state. The condition of Q = 1 is referred to as breakeven.In a fusion power reactor a plasma must be...
is predicted to be less than 20, while more than 200 is usually considered to be necessary. (There are, however, effects that might improve the gain substantially.)
Power density
In every published fusion power plant design, the part of the plant that produces the fusion reactions is much more expensive than the part that converts the nuclear power to electricity. In that case, as indeed in most power systems, the power density is a very important characteristic. If the power density can be doubled without changing the design too much, then the cost of electricity will be at least halved. In addition, the confinement time required depends on the power density.
It is, however, not trivial to compare the power density produced by two different fusion fuel cycles. The case most favorable to p–11B relative to D–T fuel is a (hypothetical) confinement device that only works well at ion temperatures above about 400 keV, where the reaction rate parameter <σv> is equal for the two fuels, and that runs with low electron temperature. In terms of confinement time required, p–11B would even have an advantage, because the energy of the charged products of that reaction is two and a half times higher than that for D–T. As soon as these assumptions are relaxed, for example by considering hot electrons, by allowing the D–T reaction to run at a lower temperature, or by including the energy of the neutrons in the calculation, the power density advantage shifts back to D–T.
The most common assumption is to compare the power densities at the same pressure, with the ion temperature for each reaction chosen to maximize the power density, and with the electron temperature equal to the ion temperature. Although confinement schemes can be and sometimes are limited by other factors, most well-investigated schemes have, not surprisingly, some kind of pressure limit. Under these assumptions, the power density for p–11B is about 2100 times smaller than that for D–T. If the device runs with cold electrons, the ratio is still about 700. These numbers are another indication that aneutronic fusion power will not be possible with any mainline confinement concept.
Current research
- Lawrenceville Plasma Physics has published initial results and outlined a theory and experimental program for aneutronic fusion with the Dense Plasma Focus
A dense plasma focus is a machine that produces, by electromagnetic acceleration and compression, a short-lived plasma that is so hot and dense that it can cause nuclear fusion and emit X-rays. The electromagnetic compression of the plasma is called a pinch. It was invented in the early 1960s by...
(DPF), building on earlier discussions. The private effort was initially funded by NASA’s Jet Propulsion LaboratoryJet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...
. Support for other DPF aneutronic fusion investigations has come from the Air Force Research LaboratoryThe Air Force Research Laboratory is a scientific research organization operated by the United States Air Force Materiel Command dedicated to leading the discovery, development, and integration of affordable aerospace warfighting technologies; planning and executing the Air Force science and...
.
- Polywell
A polywell device is a type of fusion reactor that was originated by Robert Bussard under a U.S. Navy research contract. It traps electrons in a magnetic confinement inside its hollow center. The negatively charged electrons then accelerate positively charged ions for the purpose of achieving...
fusion was pioneered by Robert W. BussardRobert W. Bussard was an American physicist who worked primarily in nuclear fusion energy research. He was the recipient of the Schreiber-Spence Achievement Award for STAIF-2004. He was also a fellow of the International Academy of Astronautics and held a Ph.D...
and funded by the US Navy, uses inertial electrostatic confinementInertial electrostatic confinement is a concept for retaining a plasma using an electrostatic field. The field accelerates charged particles radially inward, usually in a spherical but sometimes in a cylindrical geometry. Ions can be confined with IEC in order to achieve controlled nuclear fusion...
.
- The Z-machine
The Z machine is the largest X-ray generator in the world and is designed to test materials in conditions of extreme temperature and pressure. Operated by Sandia National Laboratories, it gathers data to aid in computer modeling of nuclear weapons...
at Sandia National Laboratory, a z-pinchIn fusion power research, the Z-pinch, also known as zeta pinch or Bennett pinch , is a type of plasma confinement system that uses an electrical current in the plasma to generate a magnetic field that compresses it...
device, can produce ion energies of interest to hydrogen–boron reactions, up to 300 keV. Non-equilibrium plasmas usually have an electron temperature higher than their ion temperature, but the plasma in the Z machine has a special, reverted non-equilibrium state, where ion temperature is 100 times higher than electron temperature. This data represents a new research field, and would indicate that Bremsstrahlung losses could be in fact lower than expected in such a design.
None of these efforts has yet tested its device with hydrogen–boron fuel, so the anticipated performance is based on extrapolating from theory, experimental results with other fuels and from simulations.
- A picosond laser produced hydrogen–boron aneutronic fusions for a Russian team in 2005. However, the number of the resulting α particles (around 103 per laser pulse) was extremely low.
Residual radiation from a p–11B reactor
Detailed calculations show that at least 0.1% of the reactions in a thermal p–11B plasma would produce neutrons, and the energy of these neutrons would account for less than 0.2% of the total energy released.
These neutrons come primarily from the reaction
- 11B + α
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
→ 14N + n + 157 keV
The reaction itself produces only 157 keV, but the neutron will carry a large fraction of the alpha energy, which will be close to Efusion/3 = 2.9 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...
. Another significant source of neutrons is the reaction
- 11B + p → 11C + n − 2.8 MeV
These neutrons will be less energetic, with an energy comparable to the fuel temperature. In addition, 11C itself is radioactive, but will decay to negligible levels within several hours as its half life is only 20 minutes.
Since these reactions involve the reactants and products of the primary fusion reaction, it would be difficult to further lower the neutron production by a significant fraction. A clever magnetic confinement scheme could in principle suppress the first reaction by extracting the alphas as soon as they are created, but then their energy would not be available to keep the plasma hot. The second reaction could in principle be suppressed relative to the desired fusion by removing the high energy tail of the ion distribution, but this would probably be prohibited by the power required to prevent the distribution from thermalizing.
In addition to neutrons, large quantities of hard X-rayX-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s will be produced by bremsstrahlungBremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...
, and 4, 12, and 16 MeV gamma rayGamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
s will be produced by the fusion reaction
- 11B + p → 12C
Carbon-12 is the more abundant of the two stable isotopes of the element carbon, accounting for 98.89% of carbon; it contains 6 protons, 6 neutrons, and 6 electrons....
+ γGamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
+ 16.0 MeV
with a branching probability relative to the primary fusion reaction of about 10−4.
Finally, isotopicallyIsotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
pure fuel will have to be used and the influx of impurities into the plasma will have to be controlled to prevent neutron-producing side reactions like these:
- 11B + d → 12C + n + 13.7 MeV
- d + d → 3He + n + 3.27 MeV
Fortunately, with careful design, it should be possible to reduce the occupational dose of both neutron and gamma radiation to operators to a negligible level. The primary components of the shielding would be water to moderate the fast neutrons, boron to absorb the moderated neutrons, and metal to absorb X-rays. The total thickness needed should be about a meter, most of that being water.
Direct conversion of energy
Aneutronic fusion reactions produce the overwhelming bulk of their energy in the form of charged particles instead of neutrons. This means that energy could be converted directly into electricity by various techniques. Many proposed direct conversion techniques are based on mature technologyA mature technology is a technology that has been in use for long enough that most of its initial faults and inherent problems have been removed or reduced by further development...
derived from other fields, such as microwave technology, and some involve equipment that is more compact and potentially cheaper than that involved in conventional thermal production of electricity.
In contrast, fusion fuels like deuterium-tritium (DT), which produce most of their energy in the form of neutrons, require a standard thermal cycle, in which the neutrons are used to boil water, and the resulting steam drives a large turbine and generator. This equipment is sufficiently expensive that about 80% of the capital cost of a typical fossil-fuel electric power generating station is in the thermal conversion equipment.
Thus, fusion with DT fuels could not significantly reduce the capital costs of electric power generation even if the fusion reactor that produces the neutrons were cost-free. (Fuel costs would, however, be greatly reduced.) But according to proponents, aneutronic fusion with direct electric conversion could, in theory, produce electricity with reduced capital costs.
Direct conversion techniques can either be inductive, based on changes in magnetic fields, or electrostatic, based on making charged particles work against an electric field. If the fusion reactor worked in a pulsed mode, inductive techniques could be used.
A sizable fraction of the energy released by aneutronic fusion would not remain in the charged fusion products but would instead be radiated as X-rays. Some of this energy could also be converted directly to electricity. Because of the photoelectric effectIn the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...
, X-rays passing through an array of conducting foils would transfer some of their energy to electrons, which can then be captured electrostatically. Since X-rays can go through far greater thickness of material than electrons can, many hundreds or even thousands of layers would be needed to absorb most of the X-rays.
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