View source for Helium-3

:''This article is about the elemental isotope.  For the record label Helium 3, see [[Muse (band)|Muse]] or [[A&E Records]]''.
{{infobox isotope
| background = #F99
| isotope_name = Helium-3
| alternate_names =Helium-3,He-3
| symbol =He
| mass_number =3
| mass =3.016
| num_neutrons =1
| num_protons =2
| abundance =0.000137%
| halflife =stable
| error_halflife =
| text_color =
| image =
| parent =Tritium
| parent_symbol =H
| parent_mass =3
| parent_decay =[[beta decay]] of tritium
| spin =1/2+
}}
'''Helium-3''' (He-3) is a light, non-[[radioactive]] [[isotope]] of [[helium]] with two protons and one neutron, rare on [[Earth]], sought for use in [[nuclear fusion]] research. The abundance of helium-3 is thought to be greater on the [[Moon]] (embedded in the upper layer of [[regolith]] by the [[solar wind]] over billions of years) and the [[solar system]]'s [[gas giant]]s (left over from the original [[solar nebula]]), though still low in quantity (28 [[parts per million|ppm]] of lunar regolith is helium-4 and 0.01 ppm is helium-3).<ref>http://www.moonminer.com/Lunar_regolith.html, [http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2175.pdf The estimation of helium-3 probable reserves in lunar regolith]</ref>  It is proposed to be used as a second-generation fusion power source.

The [[helion (chemistry)|helion]], the [[atomic nucleus|nucleus]] of a helium-3 atom, consists of two [[proton]]s but only one [[neutron]], in contrast to two neutrons in ordinary helium. Its existence was first proposed in 1934 by the Australian nuclear physicist [[Mark Oliphant]] while based at [[University of Cambridge|Cambridge University]]'s [[Cavendish Laboratory]], in an experiment in which fast [[deuteron]]s were reacted with other deuteron targets (the first demonstration of nuclear fusion). Helium-3, as an isotope, was postulated to be radioactive, until helions from it were accidentally identified as a trace "contaminant" in a sample of natural helium (which is mostly helium-4) from a gas well, by [[Luis Walter Alvarez|Luis W. Alvarez]] and [[Robert Cornog]] in a cyclotron experiment at the [[Lawrence Berkeley National Laboratory]], in 1939. <ref>[http://www.lbl.gov/Science-Articles/Research-Review/Magazine/1981/81fepi1.html Lawrence and His Laboratory: Episode: A Productive Error<!-- Bot generated title -->]</ref>

==Physical properties==
Helium-3's atomic mass of 3.0160293, being significantly lower than Helium-4's 4.0026, causes it to have significantly different properties. Helium-3 boils at 3.19 [[kelvin]]s compared to helium-4's 4.23&nbsp;K, and its [[critical point (thermodynamics)|critical point]] is also lower at 3.35&nbsp;K, compared to helium-4's 5.19&nbsp;K. It has less than half the density when liquid at its boiling point: 0.059&nbsp;g/ml compared to helium-4's 0.12473&nbsp;g/ml at one atmosphere. Its latent heat of vaporization is also considerably lower at 0.026&nbsp;[[Kilojoule per mole|kJ/mol]] compared to helium-4's 0.0829&nbsp;kJ/mol.<ref>[http://www.trgn.com/database/cryogen.html Teragon's Summary of Cryogen Properties] Teragon Research, 2005</ref>

==Fusion reactions==
{| class="wikitable" style="float:right;"
|+ Fusion reactions involving Helium-3<ref> {{cite web | url = http://members.tm.net/lapointe/IEC_Fusion.html | title = Inertial Electrostatic Confinement Fusion | accessdate = 2007-05-06 }} </ref><ref> {{cite web | url = http://www.lancs.ac.uk/ug/suttond1/#fusion | title = Nuclear Fission and Fusion | accessdate = 2007-05-06}} </ref><ref> {{cite web | url = http://library.thinkquest.org/28383/nowe_teksty/htmla/2_37a.html | title = The Fusion Reaction | accessdate = 2007-05-06}} </ref><ref> {{cite web | url = http://fti.neep.wisc.edu/pdf/fdm1291.pdf | title = A Strategy for D - <sup>3</sup>He Development | author = John Santarius | date = June, 2006 | accessdate = 2007-05-06}} </ref><ref>{{cite web | url = http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/nucrea.html | title = Nuclear Reactions | accessdate = 2007-05-06}}</ref>
|- 
! Reactants
! 
! Products
! ''Q''
|-
! First Generation Fuels
|
|
|
|- style="text-align: center;"
| [[Deuterium|<sup>2</sup><sub>1</sub>H]] + [[Deuterium|<sup>2</sup><sub>1</sub>H]]
| →
| [[Helium-3|<sup>3</sup><sub>2</sub>He]] + <sup>1</sup><sub>0</sub>n
| 3.268 [[MeV]]
|- style="text-align: center;"
| [[Deuterium|<sup>2</sup><sub>1</sub>H]] + [[Deuterium|<sup>2</sup><sub>1</sub>H]]
| →
| [[Tritium|<sup>3</sup><sub>1</sub>H]] + <sup>1</sup><sub>1</sub>p
| 4.032 [[MeV]]
|- style="text-align: center;"
| [[Deuterium|<sup>2</sup><sub>1</sub>H]] + [[Tritium|<sup>3</sup><sub>1</sub>H]]
| →
| [[Helium-4|<sup>4</sup><sub>2</sub>He]]  + <sup>1</sup><sub>0</sub>n
| 17.571 [[MeV]]
|- style="text-align: center;" 
! Second Generation Fuel
|
|
|
|- style="text-align: center;"
| [[Deuterium|<sup>2</sup><sub>1</sub>H]] + [[Helium-3|<sup>3</sup><sub>2</sub>He]]
| →
| [[Helium-4|<sup>4</sup><sub>2</sub>He]] + <sup>1</sup><sub>1</sub>p
| 18.354 [[MeV]]
|- style="text-align: center;"
! Third Generation Fuel
|
|
|
|- style="text-align: center;"
| [[Helium-3|<sup>3</sup><sub>2</sub>He]]   +   [[Helium-3|<sup>3</sup><sub>2</sub>He]]
| →
| [[Helium-4|<sup>4</sup><sub>2</sub>He]]+ 2<sup>1</sup><sub>1</sub>p
| 12.86 [[MeV]]
|}
Some fusion processes produce highly energetic neutrons which render reactor components [[radioactive]] with their bombardment, and [[power generation]] must occur through thermal means. However, the appeal of helium-3 fusion stems from the nature of its reaction products. Helium-3 itself is non-radioactive. The lone high-energy [[proton]] produced can be contained using electric and magnetic fields, which results in direct electricity generation.<ref> {{cite web | url = http://fti.neep.wisc.edu/presentations/jfs_ieee0904.pdf | title = Lunar <sup>3</sup>He and Fusion Power | author = John Santarius | date = [[September 28]], [[2004]] | accessdate = 2007-05-06}} </ref>

However, since both reactants need to be mixed together to fuse, side reactions ([[Deuterium|<sup>2</sup><sub>1</sub>H]] + [[Deuterium|<sup>2</sup><sub>1</sub>H]] and [[Helium-3|<sup>3</sup><sub>2</sub>He]]+ [[Helium-3|<sup>3</sup><sub>2</sub>He]]) will occur, the first of which is not aneutronic. Therefore in practice this reaction is unlikely to ever be completely 'clean', thus negating some of its attraction. Also, due to the higher [[Coulomb barrier]], the temperatures required for [[Deuterium|<sup>2</sup><sub>1</sub>H]] + [[Helium-3|<sup>3</sup><sub>2</sub>He]]  fusion are much higher than those of conventional [[Deuterium|<sup>2</sup>H]] + [[Tritium|<sup>3</sup><sub>1</sub>H]] ([[deuterium]] + [[tritium]]) fusion.

The amounts of helium-3 needed as a replacement for [[fossil fuel|conventional fuel]]s should not be underestimated. The total amount of energy produced in the ³He + [[Deuterium|<sup>2</sup><sub>1</sub>H]]+  reaction is 18.4 M[[electronvolt|eV]], which corresponds to some 493 [[watt-hour|megawatt-hour]]s (4.93x10<sup>8</sup> Wh) per three [[gram]]s (one [[mole (chemistry)|mole]]) of ³He. Even if that total amount of energy could be converted to electrical power with 100% efficiency (a physical impossibility), it would correspond to about 30 minutes of output of a thousand-megawatt electrical plant; a year's production by the same plant would require some 17.5 kilograms of helium-3.

The amount of fuel needed for large-scale applications can also be put in terms of total consumption: According to the US Energy Information Administration, "Electricity consumption by 107 million U.S. households in 2001 totaled 1,140 billion kWh" (1.114x10<sup>15</sup> Wh). Again assuming 100% conversion efficiency, 6.7 tonnes of helium-3 would be required just for that segment of one country's energy demand, 15 to 20 tonnes given a more realistic end-to-end conversion efficiency. {{Fact|date=May 2007}}

==Neutron detection==
'''Helium-3''' is a most important isotope in instrumentation for [[neutron detection]]. It has a high absorption cross section for thermal [[neutron radiation|neutron]] beams and is used as a converter gas in neutron detectors. The neutron is converted through the nuclear reaction
:n + <sup>3</sup>He  → <sup>3</sup>H + <sup>1</sup>H + 0.764 MeV
into charged particles [[tritium]] (T, <sup>3</sup>H) and [[proton]] (p, <sup>1</sup>H) which then are detected by creating a charge cloud in the stopping gas of a [[proportional counter]] or a [[Geiger-Müller tube]]. <ref>[http://www.lanl.gov/quarterly/q_sum03/neutron_detect.shtml A Modular Neutron Detector | Summer 2003| Los Alamos National Laboratory<!-- Bot generated title -->]</ref>

Furthermore, the absorption process is strongly [[Spin (physics)|spin]]-dependent, which allows a [[Spin polarization|spin-polarized]] helium-3 volume to transmit neutrons with one spin component while absorbing the other. This effect is employed in [[Polarized neutron scattering|neutron polarization analysis]], a technique which probes for magnetic properties of matter.<ref>[http://www.ncnr.nist.gov/AnnualReport/FY2002_html/pages/neutron_spin.htm NCNR Neutron Spin Filters<!-- Bot generated title -->]</ref> <ref>[http://physics.nist.gov/Divisions/Div846/Gp3/Helium/applications/neutronApps/PolarAnalysis.html Polarization Analysis using Polarized 3He<!-- Bot generated title -->]</ref>

==Cryogenics==
A [[helium-3 refrigerator]] uses helium-3 to achieve temperatures of 0.2 to 0.3 [[kelvin]]. A [[dilution refrigerator]] uses a mixture of helium-3 and helium-4 to reach [[cryogenics|cryogenic]] temperatures as low as a few thousandths of a [[kelvin]]. <ref>[http://na47sun05.cern.ch/target/outline/dilref.html Dilution Refrigeration<!-- Bot generated title -->]</ref>

An important property of helium-3, which distinguishes it from the more common helium-4, is that its nucleus is a [[fermion]] since it contains an odd number of spin 1/2 particles. Helium-4 nuclei are [[boson]]s, containing an even number of spin 1/2 particles. This is a direct result of the [[Angular momentum quantum number#Addition of quantized angular momenta|addition rules]] for quantized angular momentum. At low temperatures (about 2.17 K), helium-4 undergoes a [[phase transition]]: A fraction of it enters a [[superfluid]] [[phase (matter)|phase]] that can be roughly understood as a type of [[Bose-Einstein condensate]]. Such a mechanism is not available for helium-3 atoms, which are fermions. However, it was widely speculated that helium-3 could also become a superfluid at much lower temperatures, if the atoms formed into ''pairs'' analogous to [[Cooper pair]]s in the [[BCS theory]] of [[superconductivity]]. Each Cooper pair, having integer spin, can be thought of as a boson. During the [[1970s]], [[David Morris Lee]], [[Douglas Osheroff]] and [[Robert Coleman Richardson]] discovered two phase transitions along the melting curve, which was soon realized to be the two superfluid phases of helium-3.<ref>{{cite journal |last=Osheroff |first=D. D. |authorlink= |coauthors=Richardson, R. C.; Lee, D. M. |year=1972 |month= |title=Evidence for a New Phase of Solid He<sup>3</sup> |journal=[[Physical Review Letters]] |volume=28 |issue=14 |pages=885-888 |doi=10.1103/PhysRevLett.28.885 |url= |accessdate= |quote= }}</ref><ref>{{cite journal |last=Osheroff |first=D. D. |authorlink= |coauthors=Gully, W. J.; Richardson, R. C.; Lee, D. M. |year=1972 |month= |title=New Magnetic Phenomena in Liquid He<sup>3</sup> below 3 mK |journal=Physical Review Letters |volume=29 |issue=14 |pages=920-923 |doi=10.1103/PhysRevLett.29.920 |url= |accessdate= |quote= }}</ref> The transition to a superfluid occurs at 2.491 millikelvins on the melting curve. They were awarded the [[1996]] [[Nobel Prize in Physics]] for their discovery. [[Anthony James Leggett|Tony Leggett]] won the [[2003]] Nobel Prize in Physics for his work on refining understanding of the superfluid phase of helium-3.<ref>{{cite journal |last=Leggett |first=A. J. |authorlink= |coauthors= |year=1972 |month= |title=Interpretation of Recent Results on He<sup>3</sup> below 3 mK: A New Liquid Phase? |journal=Physical Review Letters |volume=29 |issue=18 |pages=1227-1230 |doi=10.1103/PhysRevLett.29.1227 |url= |accessdate= |quote= }}</ref>

In zero magnetic field, there are two distinct superfluid phases of <sup>3</sup>He, the A-phase and the B-phase. The B-phase is the low-temperature, low-pressure phase which has an isotropic energy gap. The A-phase is the higher temperature, higher pressure phase that is further stabilized by a magnetic field and has two point nodes in its gap. The presence of two phases is a clear indication that <sup>3</sup>He is an unconventional superfluid (superconductor), since the presence of two phases requires an additional symmetry, other than gauge symmetry, to be broken. In fact, it is a ''p''-wave superfluid, with spin one, '''S'''=1, and angular momentum one, '''L'''=1.  The ground state corresponds to total angular momentum zero, '''J'''='''S'''+'''L'''=0 (vector addition).  Excited states are possible with non-zero total angular momentum, '''J'''>0, which are excited pair collective modes. Because of the extreme purity of superfluid <sup>3</sup>He (since all materials except <sup>4</sup>He have solidified and sunk to the bottom of the liquid <sup>3</sup>He and any <sup>4</sup>He has phase separated entirely, this is the most pure condensed matter state), these collective modes have been studied with much greater precision than in any other unconventional pairing system.

==Manufacturing==
Due to the rarity of helium-3 on Earth, it is typically manufactured instead of recovered from natural deposits. Helium-3 is a byproduct of [[tritium]] decay, and tritium can be produced through neutron bombardment of [[lithium]], [[boron]], or [[nitrogen]] targets. Current supplies of helium-3 come, in part, from the dismantling of nuclear weapons where it accumulates<ref>http://afci.lanl.gov/aptnews/aptnews.mar1_98.html</ref>; approximately 150 kilograms of it have resulted from decay of US tritium production since 1955, most of which was for warheads<ref>[http://www.ieer.org/sdafiles/vol_5/5-1/tritium.html IEER: Science for Democratic Action Vol. 5 No. 1<!-- Bot generated title -->]</ref>. However, the production and storage of huge amounts of the gas tritium is probably uneconomical, as roughly eighteen tons of tritium stock are required for each ton of helium-3 produced annually by decay (production rate '''dN/dt''' from number of moles or other unit mass of tritium '''N''' is '''N γ''' = '''N * [ln2/t<sub>½</sub>]''' where the value of '''t<sub>½</sub>/(ln2)''' is about 18 years; see [[radioactive decay]]). If commercial fusion reactors were to use helium-3 as a fuel, they would require tens of tons of helium-3 each year to produce a fraction of the world's power.<ref name="wittenberg1994">{{cite paper
  |author=L.J. Wittenberg
  |date=July 1994
  |url=http://fti.neep.wisc.edu/pdf/fdm967.pdf
  |format=pdf
  |title=Non-Lunar <sup>3</sup>He Resources
  |publisher=
  |version=
  |accessdate=2006-08-26
 }}</ref>  Breeding tritium with lithium-6 consumes the neutron, while breeding with lithium-7 produces a low energy neutron as a replacement for the consumed fast neutron. Note that any breeding of tritium on Earth requires the use of a high neutron flux, which proponents of helium-3 nuclear reactors hope to avoid. {{Fact|date=May 2007}}

==Medical lung imaging==
Polarized helium-3 may be produced directly with lasers of the appropriate power, and with a thin layer of protective Cs metal on the inside of cylinders, the magnetized gas may be stored at pressures of 10 atm for up to 100 hours. When inhaled, mixtures containing the gas can be imaged with an MRI-like scanner which produces breath by breath images of lung ventilation, in real-time. Applications of this experimental technique are just beginning to be explored<ref>[http://www.cerncourier.com/main/article/41/8/14 Take a deep breath of nuclear spin - CERN Courier<!-- Bot generated title -->]</ref>.

==Terrestrial occurrence==
{{main|isotope geochemistry}}
<sup>3</sup>He is a primordial substance in the Earth's [[mantle (geology)|mantle]], considered to have become entrapped within the Earth during planetary formation. The ratio of <sup>3</sup>He to <sup>4</sup>He within the Earth's crust and mantle is less than that for assumptions of solar disk composition as obtained from meteorite and lunar samples, with terrestrial materials generally containing lower <sup>3</sup>He/<sup>4</sup>He ratios due to ingrowth of <sup>4</sup>He from radioactive decay.

<sup>3</sup>He is present within the mantle, in the ratio of 200-300 parts of <sup>3</sup>He to a million parts of <sup>4</sup>He. Ratios of <sup>3</sup>He/<sup>4</sup>He in excess of atmospheric are indicative of a contribution of <sup>3</sup>He from the mantle. Crustal sources are dominated by the [[helium-4|<sup>4</sup>He]] which is produced by the decay of radioactive elements in the crust and mantle.

<sup>3</sup>He is also present in the [[Earth's atmosphere]]. The natural abundance of <sup>3</sup>He in naturally occurring helium gas is 1.38{{e|-6}}. The partial pressure of helium in the Earth's atmosphere is about 4 millitorr, and thus 5.2 parts per million of helium. It has been proven that the Earth's atmosphere contains approximately 4000 tons of <sup>3</sup>He.

<sup>3</sup>He is produced on Earth from three sources: lithium [[spallation]], [[cosmic rays]], and decay of tritium (<sup>3</sup>H). The contribution from cosmic rays is negligible within all except the oldest regolith materials, and lithium spallation reactions are a lesser contributor than the production of <sup>4</sup>He by [[alpha particle]] emissions.

The total amount of helium-3 in the mantle may be in the range of 100 thousand to a million [[tonne]]s. However, this mantle helium is not directly accessible. Some of it leaks up through deep-sourced [[Hotspot (geology)|hotspot]] volcanoes such as those of the [[Hawaii]]an islands, but only 300 grams per year is emitted to the atmosphere. [[Mid-ocean ridge]]s emit another 3 kilogram per year. Around [[subduction zone]]s, various sources produce helium-3 in [[natural gas]] deposits which possibly contain a thousand tonnes of helium-3 (although there may be 25 thousand tonnes if all ancient subduction zones have such deposits). Crustal natural gas sources may have only half a tonne total. There may be another four thousand tonnes in [[interplanetary dust]] particles on the ocean floors. Extracting helium-3 from these sources consumes more energy than fusion would release.  Extraction from the most efficient source, natural gas, consumes ten times the energy available from fusion reactions. <ref name="wittenberg1994" />

==Extraterrestrial supplies==
The [[Moon]]'s surface contains helium-3 at concentrations on the order of 0.01 [[Parts-per notation|ppm]].<ref>[http://fti.neep.wisc.edu/Research/he3_pubs.html FTI Research Projects :: 3He Lunar Mining<!-- Bot generated title -->]</ref><ref>{{cite web | url= http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2175.pdf | title = The estimation of helium-3 probable reserves in lunar regolith | author =                                       E. N. Slyuta and A. M. Abdrakhimov, and E. M. Galimov | work = Lunar and Planetary Science XXXVIII | year=2007}}</ref>  A number of people, starting with [[Gerald Kulcinski]] in 1986,<ref>{{cite news | url = http://www.thespacereview.com/article/536/1 | title = A fascinating hour with [[Gerald Kulcinski]] | author=Eric R. Hedman | date = January 16, 2006 | work = The Space Review}}</ref> have proposed to [[Exploration of the Moon|explore the moon]], mine lunar [[regolith]] and using the helium-3 for [[Nuclear fusion|fusion]]. Because of the low concentrations of helium-3, any mining equipment would need to process large amounts of regolith,<ref>{{cite web | title = The challenge of mining He-3 on the lunar surface: how all the parts fit together | author = I.N. Sviatoslavsky | date = November 1993 | url = http://fti.neep.wisc.edu/pdf/wcsar9311-2.pdf }} Wisconsin Center for Space Automation and Robotics Technical Report WCSAR-TR-AR3-9311-2.</ref> and some proposals have suggested that helium-3 extraction be piggybacked onto a larger mining and development operation.{{Fact|date=August 2007}}

[[Cosmochemistry|Cosmochemist]] and [[geochemist]] [[Ouyang Ziyuan]] from the [[Chinese Academy of Sciences]]  who is now in charge of the [[Chang'e program|Chinese Lunar Exploration Program]] has already stated on many occasions that one of the main goals of the program would be the mining of helium-3, from where "each year three space shuttle missions could bring enough fuel for all human beings across the world."<ref>[http://www.chinadaily.com.cn/cndy/2006-07/26/content_649325.htm He asked for the moon-and got it<!-- Bot generated title -->]</ref>

In January 2006 the Russian space company [[RKK Energiya]] announced that it considers lunar helium-3 a potential economic resource to be mined by 2020,<ref>[http://www.space.com/news/ap_060126_russia_moon.html SPACE.com - Russian Rocket Builder Aims for Moon Base by 2015, Reports Say<!-- Bot generated title -->]</ref> if funding can be found.<ref>{{cite web | url = http://www.thespacereview.com/article/551/1 | title = Moonscam: Russians try to sell the Moon for foreign cash | author = James Oberg | date = February 6, 2006}}</ref><ref>{{cite web | url = http://www.thespacereview.com/article/824/1 | title = Death throes and grand delusions | author = [[Dwayne A. Day]] | work = [[The Space Review]] | date=March 5, 2007}}</ref>

Mining [[gas giant]]s for helium-3 has also been proposed.<ref>{{cite web | title = Atmospheric Mining in the Outer Solar System | author = Bryan Palaszewski | url = http://gltrs.grc.nasa.gov/reports/2006/TM-2006-214122.pdf | }} NASA Technical Memorandum 2006-214122.  AIAA–2005–4319. Prepared for the 41st Joint Propulsion Conference and Exhibit cosponsored by AIAA, ASME, SAE, and ASEE, Tucson, Arizona, July 10–13, 2005.</ref> The [[British Interplanetary Society]]'s hypothetical [[Project Daedalus]] interstellar probe design was fueled by helium-3 mines on the planet [[Jupiter (planet)|Jupiter]], for example. Jupiter's high gravity makes this a less energetically favorable operation than extracting helium-3 from the other gas giants of the solar system, however.

==Power generation==
A second-generation approach to controlled [[nuclear fusion|fusion]] power involves combining helium-3 ([[Helium-3|<sup>3</sup><sub>2</sub>He]]) and [[deuterium]] ([[Deuterium|<sup>2</sup><sub>1</sub>H]]). This reaction produces an [[helium-4]] ion ([[Helium-4|<sup>4</sup><sub>2</sub>He]]) (like an [[alpha particle]], but of different origin) and a high-energy [[proton]] (positively charged hydrogen ion) (<sup>1</sup><sub>1</sub>p). The most important potential advantage of this fusion reaction for power production as well as other applications lies in its compatibility with the use of [[electrostatic]] fields to control fuel [[ion]]s and the fusion protons. Protons, as positively charged particles, can be converted directly into [[electricity]], through use of [[solid-state]] conversion materials as well as other techniques. Potential conversion efficiencies of 70 percent may be possible, as there is no need to convert proton energy to heat in order to drive [[turbine]]-powered [[generator]]s.

There have been many claims about the capabilities of Helium-3 power plants. According to proponents, fusion power plants operating on [[deuterium]] and helium-3 would offer lower capital and [[operating cost]]s than their competitors due to less technical complexity, higher conversion efficiency, smaller size, the absence of radioactive fuel, no air or water [[pollution]], and only low-level [[radioactive]] waste disposal requirements. Recent estimates suggest that about $6 billion in [[investment]] [[Capital (economics)|capital]] will be required to develop and construct the first helium-3 fusion [[power plant]]. Financial breakeven at today's wholesale [[electricity]] prices (5 cents per [[kilowatt-hour]]) would occur after five 1000-[[megawatt]] plants were on line, replacing old conventional plants or meeting new demand.<ref> {{cite news | url = http://www.popularmechanics.com/science/air_space/1283056.html?page=4 | title = Mining The Moon | author = Paul DiMare | date = [[October]] [[2004]] | accessdate =  2007-05-06 }} </ref>

The reality is not so clean-cut. The most advanced fusion programs in the world are [[inertial confinement fusion]] (such as [[National Ignition Facility]]) and [[magnetic confinement fusion]] (such as [[ITER]] and other [[tokamak]]s). In the case of the former, there is no solid roadmap to power generation. In the case of the latter, commercial power generation is not expected until around [[2050]]<ref>{{cite news | url = http://www.iter.org/Future-beyond.htm | title = Beyond ITER | accessdate = 2007-05-07}}</ref>. In both cases, the type of fusion discussed is the simplest: D-T fusion. The reason for this is the very low [[Coulomb barrier]] for this reaction; for D+He3, the barrier is much higher, and He3-He3 higher still. The immense cost of reactors like [[ITER]] and [[National Ignition Facility]] are largely due to their immense size, yet to scale up to higher plasma temperatures would require reactors far larger still. The 14.7 MeV proton and 3.6 MeV alpha particle from D-He3 fusion, plus the higher conversion efficiency, means that more electricity is obtained per kilogram than with D-T fusion (17.6 MeV), but not that much more. As a further downside, the rates of reaction for [[Aneutronic fusion#Candidate aneutronic reactions|He3 fusion reactions]] are not particularly high, requiring a reactor that is larger still or more reactors to produce the same amount of electricity.

To attempt to work around this problem of massively large power plants that may not even be economical with D-T fusion, let alone the far more challenging D-He3 fusion, a number of other reactors have been proposed -- the [[Fusor]], [[Polywell]], [[Focus fusion]], and many more. These generally attempt to achieve fusion in thermal disequilibrium, something that could potentially prove impossible<ref>{{cite news | url = http://dspace.mit.edu/handle/1721.1/29869 | title = A general critique of inertial-electrostatic confinement fusion systems | author = Todd Rider | date = accessdate = 2007-05-07}}</ref>, and consequently, these long-shot programs tend to have trouble garnering funding despite their low budgets.  Unlike the "big", "hot" fusion systems, however, if such systems were to work, they could scale to the higher barrier "[[aneutronic fusion|aneutronic]]" fuels.  However, these systems would scale well enough that their proponents tend to promote [[Aneutronic fusion#Technical challenges|p-B fusion]], which requires no exotic fuels like He-3.

==See also==
*[[Moon]]
==References==
{{reflist|2}}

==External links==
*[http://nobelprize.org/physics/laureates/2003/presentation-speech.html The Nobel Prize in Physics 2003, presentation speech]
*[http://www.bbc.co.uk/sn/tvradio/programmes/horizon/broadband/tx/moonsale/ Moon for Sale: A BBC Horizon Documentary on the possibility of Lunar mining for Helium-3]

{{Isotope|element=Helium
|lighter=[[Diproton]]
|heavier=[[Helium-4]]
|before=[[Lithium-4]] '''([[proton emission|p]])<br />'''[[Hydrogen-3]] '''([[beta decay|β−]])
|after=Stable
}}

[[Category:Isotopes of helium]]
[[Category:Nuclear fusion fuels]]
[[Category:Space exploration]]

[[ca:Heli-3]]
[[es:Helio-3]]
[[fr:Hélium 3]]
[[it:Elio-3]]
[[lv:Hēlijs-3]]
[[pt:Hélio 3]]
[[ru:Гелий-3]]
[[sv:Helium-3]]
[[tr:Helyum-3]]
[[zh:氦-3]]