HELIUM-3 (HE-3)

HELIUM-3      (HE-3)

Helium-3 (He-3, also written as ³He, see also helion) is a light, non-radioactive isotope of helium with two protons and one neutron(common helium having two protons and two neutrons). Its hypothetical existence was first proposed in 1934 by the Australian nuclear physicist Mark Oliphant while he was working at the University of Cambridge Cavendish Laboratory. Oliphant had performed experiments in which fast deuterons collided with deuteron targets (incidentally, the first demonstration of nuclear fusion). Helium-3 was thought to be a radioactive isotope until it was also found in samples of natural helium, which is mostly helium-4, taken both from the terrestrial atmosphere and from natural gas wells. Other than protium, helium-3 is the only stable isotope of any element with more protons than neutrons.

Helium-3 occurs as a primordial nuclide, escaping from the Earth's crust into the atmosphere and into outer space over millions of years. Helium-3 is also thought to be a natural nucleogenic and cosmogenic nuclide, one produced when lithium is bombarded by natural neutrons, which can be released by spontaneous fission and by nuclear reactions with cosmic rays. Some of the helium-3 found in the terrestrial atmosphere is also a relic of atmospheric and underwater nuclear weapons testing.

Much speculation has been made over the possibility of helium-3 as a future energy source. Unlike most other nuclear fusion reactions, the fusion of helium-3 atoms releases large amounts of energy without causing the surrounding material to become radioactive. However, the temperatures required to achieve helium-3 fusion reactions are much higher than in traditional fusion reactions.

The abundance of helium-3 is thought to be greater on the Moon than on Earth, having been embedded in the upper layer of regolith by the solar wind over billions of years, though still lower in abundance than in the solar system's gas giants.

General
Name, symbol	Helium-3, He-3,3He
Neutrons	1
Protons	2
Nuclide data
Natural abundance	0.000137% (% He on Earth)
Half-life	stable
Parent isotopes	3H (beta decay of tritium)
Isotope mass	3.0160293 u
Spin	​1⁄2

Z →	0	1	2
n ↓	n	H	He	3	4	5
0		1H		Li	Be	B	6
1	1n	2H	3He	4Li	5Be	6B	C	7
2		3H	4He	5Li	6Be	7B	8C	N	8
3		4H	5He	6Li	7Be	8B	9C	10N	O	9
4		5H	6He	7Li	8Be	9B	10C	11N	12O	F	10			13
5		6H	7He	8Li	9Be	10B	11C	12N	13O	14F	Ne	11	12	Al
6		7H	8He	9Li	10Be	11B	12C	13N	14O	15F	16Ne	Na	Mg	19Al	14
		7	9He	10Li	11Be	12B	13C	14N	15O	16F	17Ne	18Na	19Mg	20Al	Si	15
		8	10He	11Li	12Be	13B	14C	15N	16O	17F	18Ne	19Na	20Mg	21Al	22Si	P	16
			9	12Li	13Be	14B	15C	16N	17O	18F	19Ne	20Na	21Mg	22Al	23Si	24P	S	17
				10	14Be	15B	16C	17N	18O	19F	20Ne	21Na	22Mg	23Al	24Si	25P	26S	Cl	18
				11	15Be	16B	17C	18N	19O	20F	21Ne	22Na	23Mg	24Al	25Si	26P	27S	28Cl	Ar	19
				12	16Be	17B	18C	19N	20O	21F	22Ne	23Na	24Mg	25Al	26Si	27P	28S	29Cl	30Ar	K	20
						13	19C	20N	21O	22F	23Ne	24Na	25Mg	26Al	27Si	28P	29S	30Cl	31Ar	32K	Ca
						14	20C	21N	22O	23F	24Ne	25Na	26Mg	27Al	28Si	29P	30S	31Cl	32Ar	33K	34Ca	21	22
							15	22N	23O	24F	25Ne	26Na	27Mg	28Al	29Si	30P	31S	32Cl	33Ar	34K	35Ca	Sc	Ti	23
								16	24O	25F	26Ne	27Na	28Mg	29Al	30Si	31P	32S	33Cl	34Ar	35K	36Ca	37Sc	38Ti	V	24
									17	26F	27Ne	28Na	29Mg	30Al	31Si	32P	33S	34Cl	35Ar	36K	37Ca	38Sc	39Ti	40V	Cr	25	26
									18	27F	28Ne	29Na	30Mg	31Al	32Si	33P	34S	35Cl	36Ar	37K	38Ca	39Sc	40Ti	41V	42Cr	Mn	Fe	27	28
									19	28F	29Ne	30Na	31Mg	32Al	33Si	34P	35S	36Cl	37Ar	38K	39Ca	40Sc	41Ti	42V	43Cr	44Mn	45Fe	Co	Ni
									20	29F	30Ne	31Na	32Mg	33Al	34Si	35P	36S	37Cl	38Ar	39K	40Ca	41Sc	42Ti	43V	44Cr	45Mn	46Fe	47Co	48Ni
									21	30F	31Ne	32Na	33Mg	34Al	35Si	36P	37S	38Cl	39Ar	40K	41Ca	42Sc	43Ti	44V	45Cr	46Mn	47Fe	48Co	49Ni	29
									22	31F	32Ne	33Na	34Mg	35Al	36Si	37P	38S	39Cl	40Ar	41K	42Ca	43Sc	44Ti	45V	46Cr	47Mn	48Fe	49Co	50Ni	Cu	30
										23	33Ne	34Na	35Mg	36Al	37Si	38P	39S	40Cl	41Ar	42K	43Ca	44Sc	45Ti	46V	47Cr	48Mn	49Fe	50Co	51Ni	52Cu	Zn	31
										24	34Ne	35Na	36Mg	37Al	38Si	39P	40S	41Cl	42Ar	43K	44Ca	45Sc	46Ti	47V	48Cr	49Mn	50Fe	51Co	52Ni	53Cu	54Zn	Ga	32
											25	36Na	37Mg	38Al	39Si	40P	41S	42Cl	43Ar	44K	45Ca	46Sc	47Ti	48V	49Cr	50Mn	51Fe	52Co	53Ni	54Cu	55Zn	56Ga	Ge	33
											26	37Na	38Mg	39Al	40Si	41P	42S	43Cl	44Ar	45K	46Ca	47Sc	48Ti	49V	50Cr	51Mn	52Fe	53Co	54Ni	55Cu	56Zn	57Ga	58Ge	As
												27	39Mg	40Al	41Si	42P	43S	44Cl	45Ar	46K	47Ca	48Sc	49Ti	50V	51Cr	52Mn	53Fe	54Co	55Ni	56Cu	57Zn	58Ga	59Ge	60As
												28	40Mg	41Al	42Si	43P	44S	45Cl	46Ar	47K	48Ca	49Sc	50Ti	51V	52Cr	53Mn	54Fe	55Co	56Ni	57Cu	58Zn	59Ga	60Ge	61As
													29	42Al	43Si	44P	45S	46Cl	47Ar	48K	49Ca	50Sc	51Ti	52V	53Cr	54Mn	55Fe	56Co	57Ni	58Cu	59Zn	60Ga	61Ge	62As	34	35
														30	44Si	45P	46S	47Cl	48Ar	49K	50Ca	51Sc	52Ti	53V	54Cr	55Mn	56Fe	57Co	58Ni	59Cu	60Zn	61Ga	62Ge	63As	Se	Br
															31	46P	47S	48Cl	49Ar	50K	51Ca	52Sc	53Ti	54V	55Cr	56Mn	57Fe	58Co	59Ni	60Cu	61Zn	62Ga	63Ge	64As	65Se	66Br	36
																32	48S	49Cl	50Ar	51K	52Ca	53Sc	54Ti	55V	56Cr	57Mn	58Fe	59Co	60Ni	61Cu	62Zn	63Ga	64Ge	65As	66Se	67Br	Kr	37
																33	49S	50Cl	51Ar	52K	53Ca	54Sc	55Ti	56V	57Cr	58Mn	59Fe	60Co	61Ni	62Cu	63Zn	64Ga	65Ge	66As	67Se	68Br	69Kr	Rb
																	34	51Cl	52Ar	53K	54Ca	55Sc	56Ti	57V	58Cr	59Mn	60Fe	61Co	62Ni	63Cu	64Zn	65Ga	66Ge	67As	68Se	69Br	70Kr	71Rb
																		35	53Ar	54K	55Ca	56Sc	57Ti	58V	59Cr	60Mn	61Fe	62Co	63Ni	64Cu	65Zn	66Ga	67Ge	68As	69Se	70Br	71Kr	72Rb
																			36	55K	56Ca	57Sc	58Ti	59V	60Cr	61Mn	62Fe	63Co	64Ni	65Cu	66Zn	67Ga	68Ge	69As	70Se	71Br	72Kr	73Rb
																			37	56K	57Ca	58Sc	59Ti	60V	61Cr	62Mn	63Fe	64Co	65Ni	66Cu	67Zn	68Ga	69Ge	70As	71Se	72Br	73Kr	74Rb
																					38	59Sc	60Ti	61V	62Cr	63Mn	64Fe	65Co	66Ni	67Cu	68Zn	69Ga	70Ge	71As	72Se	73Br	74Kr	75Rb
																					39	60Sc	61Ti	62V	63Cr	64Mn	65Fe	66Co	67Ni	68Cu	69Zn	70Ga	71Ge	72As	73Se	74Br	75Kr	76Rb
																						40	62Ti	63V	64Cr	65Mn	66Fe	67Co	68Ni	69Cu	70Zn	71Ga	72Ge	73As	74Se	75Br	76Kr	77Rb	38
																						41	63Ti	64V	65Cr	66Mn	67Fe	68Co	69Ni	70Cu	71Zn	72Ga	73Ge	74As	75Se	76Br	77Kr	78Rb	Sr	39
																							42	65V	66Cr	67Mn	68Fe	69Co	70Ni	71Cu	72Zn	73Ga	74Ge	75As	76Se	77Br	78Kr	79Rb	80Sr	Y	40
																								43	67Cr	68Mn	69Fe	70Co	71Ni	72Cu	73Zn	74Ga	75Ge	76As	77Se	78Br	79Kr	80Rb	81Sr	82Y	Zr	41
																									44	69Mn	70Fe	71Co	72Ni	73Cu	74Zn	75Ga	76Ge	77As	78Se	79Br	80Kr	81Rb	82Sr	83Y	84Zr	Nb	42
																										45	71Fe	72Co	73Ni	74Cu	75Zn	76Ga	77Ge	78As	79Se	80Br	81Kr	82Rb	83Sr	84Y	85Zr	86Nb	Mo	43
																										46	72Fe	73Co	74Ni	75Cu	76Zn	77Ga	78Ge	79As	80Se	81Br	82Kr	83Rb	84Sr	85Y	86Zr	87Nb	88Mo	Tc	44
																											47	74Co	75Ni	76Cu	77Zn	78Ga	79Ge	80As	81Se	82Br	83Kr	84Rb	85Sr	86Y	87Zr	88Nb	89Mo	90Tc	Ru	45
																											48	75Co	76Ni	77Cu	78Zn	79Ga	80Ge	81As	82Se	83Br	84Kr	85Rb	86Sr	87Y	88Zr	89Nb	90Mo	91Tc	92Ru	Rh	46
																												49	77Ni	78Cu	79Zn	80Ga	81Ge	82As	83Se	84Br	85Kr	86Rb	87Sr	88Y	89Zr	90Nb	91Mo	92Tc	93Ru		Pd	47
																												50	78Ni	79Cu	80Zn	81Ga	82Ge	83As	84Se	85Br	86Kr	87Rb	88Sr	89Y	90Zr	91Nb	92Mo	93Tc	94Ru	95Rh		Ag	48
																													51	80Cu	81Zn	82Ga	83Ge	84As	85Se	86Br	87Kr	88Rb	89Sr	90Y	91Zr	92Nb	93Mo	94Tc	95Ru	96Rh	97Pd		Cd
																														52	82Zn	83Ga	84Ge	85As	86Se	87Br	88Kr	89Rb	90Sr	91Y	92Zr	93Nb	94Mo	95Tc	96Ru	97Rh	98Pd	99Ag	100Cd	49
																														53	83Zn	84Ga	85Ge	86As	87Se	88Br	89Kr	90Rb	91Sr	92Y	93Zr	94Nb	95Mo	96Tc	97Ru	98Rh	99Pd	100Ag	101Cd	In	50
																															54	85Ga	86Ge	87As	88Se	89Br	90Kr	91Rb	92Sr	93Y	94Zr	95Nb	96Mo	97Tc	98Ru	99Rh	100Pd	101Ag	102Cd		Sn	51
																															55	86Ga	87Ge	88As	89Se	90Br	91Kr	92Rb	93Sr	94Y	95Zr	96Nb	97Mo	98Tc	99Ru	100Rh	101Pd	102Ag	103Cd	104In		Sb	52

Physical properties[edit]

Because of its low atomic mass of 3.02 atomic mass units, helium-3 has some physical properties different from those of helium-4, with a mass of 4.00 atomic mass units. Because of the weak, induced dipole–dipole interaction between the helium atoms, their microscopic physical properties are mainly determined by their zero-point energy. Also, the microscopic properties of helium-3 cause it to have a higher zero-point energy than helium-4. This implies that helium-3 can overcome dipole–dipole interactions with less thermal energy than helium-4 can.

The quantum mechanical effects on helium-3 and helium-4 are significantly different because with two protons, two neutrons, and two electrons, helium-4 has an overall spin of zero, making it a boson, but with one fewer neutron, helium-3 has an overall spin of one half, making it a fermion.

Helium-3 boils at 3.19 K compared with helium-4 at 4.23 K, and its critical point is also lower at 3.35 K, compared with helium-4 at 5.2 K. Helium-3 has less than half the density of Helium-4 when it is at its boiling point: 59 gram per liter compared to the 125 gram per liter of helium-4—at a pressure of one atmosphere. Its latent heat of vaporization is also considerably lower at 0.026 kilojoules per mole compared with the 0.0829 kilojoules per mole of helium-4.^[7][8]

Fusion reactions[edit]

Comparison of neutronicity of reactions^{[9][10][11][12][13]}

Reactants		Products	Q	n/MeV
First-generation fusion fuels
21H + 21H (D-D)	→	32He + 10n	3.268 MeV	0.306
21H + 21H (D-D)	→	31H + 11p	4.032 MeV	0
21H + 31H (D-T)	→	42He + 10n	17.571 MeV	0.057
Second-generation fusion fuel
21H + 32He (D-3He)	→	42He + 11p	18.354 MeV	0
Third-generation fusion fuels
32He + 32He	→	42He+ 211p	12.86 MeV	0
115B + 11p	→	3 42He	8.68 MeV	0
Net result of D burning (sum of first 4 rows)
6D	→	2(4He + n + p)	43.225 MeV	0.046
Current nuclear fuel
235U + n	→	2 FP+ 2.5n	~200 MeV	0.001

³He can be produced by the low temperature fusion of ²H + ¹p (D-p) → ³He + γ + 4.98 MeV. If the fusion temperature is below that for the helium nuclei to fuse, the reaction produces a high energy alpha particle which quickly acquires an electron producing a stable light helium ion which can be utilized directly as a source of electricity without producing dangerous neutrons.

The fusion reaction rate increases rapidly with temperature until it maximizes and then gradually drops off. The DT rate peaks at a lower temperature (about 70 keV, or 800 million kelvins) and at a higher value than other reactions commonly considered for fusion energy.

³He can be used in fusion reactions by either of the reactions ²H + ³He →   ⁴He +  ¹p + 18.3 MeV, or ³He + ³He → ⁴He   + 2 ¹p+ 12.86 MeV.

The conventional deuterium + tritium ("D-T") fusion process produces energetic neutrons which render reactor components radioactive with activation products. The appeal of helium-3 fusion stems from the aneutronic nature of its reaction products. Helium-3 itself is non-radioactive. The lone high-energy by-product, the proton, can be contained using electric and magnetic fields. The momentum energy of this proton (created in the fusion process) will interact with the containing electromagnetic field, resulting in direct net electricity generation.^[14]

Because of the higher Coulomb barrier, the temperatures required for ²₁H + ³₂He fusion are much higher than those of conventional D-T fusion. Moreover, since both reactants need to be mixed together to fuse, reactions between nuclei of the same reactant will occur, and the D-D reaction (²₁H + ²₁H) does produce a neutron. Reaction rates vary with temperature, but the D-³He reaction rate is never greater than 3.56 times the D-D reaction rate (see graph). Therefore, fusion using D-3He fuel at the right temperature and a D-lean fuel mixture, can produce a much lower neutron flux than D-T fusion, but is not clean, negating some of its main attraction.

The second possibility, fusing ³₂He with itself (³₂He + ³₂He), requires even higher temperatures (since now both reactants have a +2 charge), and thus is even more difficult than the D-³He reaction. However, it does offer a possible reaction that produces no neutrons; the charged protons produced can be contained using electric and magnetic fields, which in turn results in direct electricity generation. ³₂He + ³₂He fusion is feasible as demonstrated in the laboratory and has immense advantages, but commercial viability is many years in the future.^[15]

The amounts of helium-3 needed as a replacement for conventional fuels are substantial by comparison to amounts currently available. The total amount of energy produced in the ²₁H + ³₂He reaction is 18.4 MeV, which corresponds to some 493 megawatt-hours (4.93×10⁸ W·h) per three grams (one mole) of ³He. If the 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 gigawatt electrical plant per mole of ³He. Thus, a year's production (at 6 grams for each operation hour) would require 52.5 kilograms of helium-3. The amount of fuel needed for large-scale applications can also be put in terms of total consumption: electricity consumption by 107 million U.S. households in 2001^[16] totaled 1,140 billion kW·h (1.14×10¹⁵ W·h). Again assuming 100% conversion efficiency, 6.7 tonnes per year of helium-3 would be required for that segment of the energy demand of the United States, 15 to 20 tonnes per year given a more realistic end-to-end conversion efficiency.^{[citation needed]}

Neutron detection[edit]

Helium-3 is a most important isotope in instrumentation for neutron detection. It has a high absorption cross section for thermal neutron beams and is used as a converter gas in neutron detectors. The neutron is converted through the nuclear reaction

n + ³He → ³H + ¹H + 0.764 MeV

into charged particles tritium (T, ³H) and protium (p, ¹H) which then are detected by creating a charge cloud in the stopping gas of a proportional counter or a Geiger-Müller tube.^[17]

Furthermore, the absorption process is strongly spin-dependent, which allows a spin-polarized helium-3 volume to transmit neutrons with one spin component while absorbing the other. This effect is employed in neutron polarization analysis, a technique which probes for magnetic properties of matter.^{[18][19][20][21]}

The United States Department of Homeland Security had hoped to deploy detectors to spot smuggled plutonium in shipping containers by their neutron emissions, but the worldwide shortage of helium-3 following the drawdown in nuclear weapons production since the Cold War has to some extent prevented this.^[22] As of 2012, DHS determined the commercial supply of boron-10 would support converting its neutron detection infrastructure to that technology.^[23]

Cryogenics[edit]

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 cryogenic temperatures as low as a few thousandths of a kelvin.^[24]

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 bosons, containing an even number of spin ​¹⁄₂ particles. This is a direct result of the 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 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 pairs in the BCS theory of superconductivity. Each Cooper pair, having integer spin, can be thought of as a boson. During the 1970s, David Lee, Douglas Osheroff and Robert Coleman Richardson discovered two phase transitions along the melting curve, which were soon realized to be the two superfluid phases of helium-3.^[25][26] 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. Tony Leggett won the 2003 Nobel Prize in Physics for his work on refining understanding of the superfluid phase of helium-3.^[27]

In a zero magnetic field, there are two distinct superfluid phases of ³He, 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 ³He 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 ³He (since all materials except ⁴He have solidified and sunk to the bottom of the liquid ³He and any ⁴He 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.

Medical lung imaging[edit]

Helium-3 nuclei have an intrinsic nuclear spin of ​¹⁄₂, and a relatively high magnetogyric ratio. Helium-3 can be hyperpolarized using non-equilibrium means such as spin-exchange optical pumping.^[28] During this process, circularly polarized infrared laser light, tuned to the appropriate wavelength, is used to excite electrons in an alkali metal, such as caesium or rubidium inside a sealed glass vessel. The angular momentum is transferred from the alkali metal electrons to the noble gas nuclei through collisions. In essence, this process effectively aligns the nuclear spins with the magnetic field in order to enhance the NMR signal. The hyperpolarized gas may then be stored at pressures of 10 atm, for up to 100 hours. Following inhalation, gas mixtures containing the hyperpolarized helium-3 gas can be imaged with an MRI scanner to produce anatomical and functional images of lung ventilation. This technique is also able to produce images of the airway tree, locate unventilated defects, measure the alveolar oxygen partial pressure, and measure the ventilation/perfusion ratio. This technique may be critical for the diagnosis and treatment management of chronic respiratory diseases such as chronic obstructive pulmonary disease (COPD), emphysema, cystic fibrosis, and asthma.^[29]

Industrial production[edit]

Production, sales and distribution of helium-3 in the United States are managed by the US Department of Energy (DOE) Isotope Program.^[30] Virtually all helium-3 used in industry today is produced from the radioactive decay of tritium. Tritium is a radioactive isotope of hydrogen and is typically produced by bombarding lithium-6 with neutrons in a nuclear reactor. The lithium nucleus absorbs a neutron and splits into helium-4 and tritium. Tritium decays into helium-3 with a half-life of 12.3 years, so helium-3 can be produced by simply storing the tritium until it undergoes radioactive decay.

Tritium is a critical component of nuclear weapons and historically it was produced and stockpiled primarily for this application. The decay of tritium into helium-3 reduces the explosive power of the fusion warhead, so periodically the accumulated helium-3 must be removed from warhead reservoirs and tritium in storage. Helium-3 removed during this process is marketed for other applications.

For decades this has been, and remains, the principal source of the world's helium-3.^[31] However, since the signing of the START I Treaty in 1991 the number of nuclear warheads that are kept ready for use has decreased^[32][33] This has reduced the quantity of helium-3 available from this source. Helium-3 stockpiles have been further diminished by increased demand,^[34] primarily for use in neutron radiation detectors and medical diagnostic procedures. US industrial demand for helium-3 reached a peak of 70,000 liters (approximately 8 kg) per year in 2008. Price at auction, historically about $100/liter, reached as high as $2000/liter^[35] Since then, demand for helium-3 has declined to about 6000 liters per year due to the high cost and efforts by the DOE to recycle it and find substitutes.

The DOE recognized the developing shortage of both tritium and helium-3. and began producing tritium by lithium irradiation at the Tennessee Valley Authority's Watts Bar Nuclear Generating Station in 2010.^[34] In this process tritium-producing burnable absorber rods (TPBARs) containing lithium in a ceramic form are inserted into the reactor in place of the normal boron control rods^[36] Periodically the TPBARs are replaced and the tritium extracted.

Currently only one reactor is used for tritium production but the process could, if necessary, be vastly scaled up to meet any conceivable demand simply by utilizing more of the nation's power reactors. Substantial quantities of tritium and helium-3 could also be extracted from the heavy water moderator in CANDU nuclear reactors.^[34][37]

Natural abundance[edit]

Solar nebula (primordial) abundance[edit]

One early estimate of the primordial ratio of ³He to ⁴He in the solar nebula has been the measurement of their ratio in the atmosphere of Jupiter, measured by the mass spectrometer of the Galileo atmospheric entry probe. This ratio is about 1:10,000,^[38] or 100 parts of ³He per million parts of ⁴He. This is roughly the same ratio of the isotopes in lunar regolith, when it contains 28 ppm helium-4 and 2.8 ppb helium-3 (which is at the lower end of actual sample measurements, which vary from about 1.4 to 15 ppb). However, terrestrial ratios of the isotopes are lower by a factor of 100, mainly due to enrichment of helium-4 stocks in the mantle by billions of years of alpha decay from uranium and thorium.

Terrestrial abundance[edit]

Main article: isotope geochemistry

³He is a primordial substance in the Earth's mantle, considered to have become entrapped within the Earth during planetary formation. The ratio of ³He to ⁴He 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 ³He/⁴He ratios due to ingrowth of ⁴He from radioactive decay.

³He has a cosmological ratio of 300 atoms per million atoms of ⁴He (at. ppm),^[39] leading to the assumption that the original ratio of these primordial gases in the mantle was around 200-300 ppm when Earth was formed. A lot of ⁴He was generated by alpha-particle decay of uranium and thorium, and now the mantle has only around 7% primordial helium,^[40]lowering the total ³He/⁴He ratio to around 20 ppm. Ratios of ³He/⁴He in excess of atmospheric are indicative of a contribution of ³He from the mantle. Crustal sources are dominated by the ⁴He which is produced by the decay of radioactive elements in the crust and mantle.

The ratio of helium-3 to helium-4 in natural Earth-bound sources varies greatly.^[41][42] Samples of the lithium ore spodumene from Edison Mine, South Dakota were found to contain 12 parts of helium-3 to a million parts of helium-4. Samples from other mines showed 2 parts per million.^[41]

Helium is also present as up to 7% of some natural gas sources,^[43] and large sources have over 0.5% (above 0.2% makes it viable to extract).^[44] The fraction of ³He in helium separated from natural gas in the U.S. was found to range from 70 to 242 parts per billion.^[34][45] Hence the US 2002 stockpile of 1 billion normal m^3[44] would have contained about 12 to 43 kilograms of helium-3. According to one expert, about 26 m³ or almost 5 kg of ³He is available annually for separation from the US natural gas stream. If the process of separating out the ³He could employ as feedstock the liquefied helium typically used to transport and store bulk quantities, estimates for the incremental energy cost range from US$34 to $300 per liter NTP, excluding the cost of infrastructure and equipment.^[34] Algeria's annual gas production is assumed to contain 100 million normal cubic metres^[44] and this would contain between 7 and 24 m³ of helium-3 (about 1 to 4 kilograms) assuming a similar ³He fraction.

³He is also present in the Earth's atmosphere. The natural abundance of ³He in naturally occurring helium gas is 1.38×10⁻⁶ (1.38 parts per million). The partial pressure of helium in the Earth's atmosphere is about 0.52 Pa, and thus helium accounts for 5.2 parts per million of the total pressure (101325 Pa) in the Earth's atmosphere, and ³He thus accounts for 7.2 parts per trillion of the atmosphere. Since the atmosphere of the Earth has a mass of about 5.14×10¹⁵ tonnes,^[46] the mass of ³He in the Earth's atmosphere is the product of these numbers, or about 37,000 tonnes of ³He.

³He is produced on Earth from three sources: lithium spallation, cosmic rays, and beta decay of tritium (³H). 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 ⁴He by alpha particle emissions.

The total amount of helium-3 in the mantle may be in the range of 0.1–1 million tonnes. However, most of the mantle is not directly accessible. Some helium-3 leaks up through deep-sourced hotspot volcanoes such as those of the Hawaiian Islands, but only 300 grams per year is emitted to the atmosphere. Mid-ocean ridges emit another 3 kilogram per year. Around subduction zones, 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). Wittenberg estimated that United States crustal natural gas sources may have only half a tonne total.^[47] Wittenberg cited Anderson's estimate of another 1200 metric tonnes in interplanetary dust particles on the ocean floors.^[48] In the 1994 study, extracting helium-3 from these sources consumes more energy than fusion would release.^[49]

Extraction from extraterrestrial sources[edit]

Materials on the Moon's surface contain helium-3 at concentrations between 1.4 and 15 ppb in sunlit areas,^[50][51] and may contain concentrations as much as 50 ppb in permanently shadowed regions.^[6] A number of people, starting with Gerald Kulcinski in 1986,^[52] have proposed to explore the moon, mine lunar regolith and use the helium-3 for fusion. Because of the low concentrations of helium-3, any mining equipment would need to process extremely large amounts of regolith (over 150 million tonnes of regolith to obtain one ton of helium 3),^[53] and some proposals have suggested that helium-3 extraction be piggybacked onto a larger mining and development operation.^{[citation needed]}

The primary objective of Indian Space Research Organisation's first lunar probe called Chandrayaan-I, launched on October 22, 2008, was reported in some sources to be mapping the Moon's surface for helium-3-containing minerals.^[54] However, no such objective is mentioned in the project's official list of goals, although many of its scientific payloads have noted helium-3-related applications.^[55][56]

Cosmochemist and geochemist Ouyang Ziyuan from the Chinese Academy of Sciences who is now in charge of the 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 which operation "each year, three space shuttle missions could bring enough fuel for all human beings across the world."^[57] To "bring enough fuel for all human beings across the world",^[58] more than one Space Shuttle load (and the processing of 4 million tonnes of regolith) per week, at least 52 per year, would be necessary.^{[citation needed][dubious – discuss]}

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,^[59] if funding can be found.^[60][61]

Mining gas giants for helium-3 has also been proposed.^[62] The British Interplanetary Society's hypothetical Project Daedalus interstellar probe design was fueled by helium-3 mines in the atmosphere of 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.

Not all authors feel the extraterrestrial extraction of helium-3 is feasible. Dwayne Day, writing in The Space Review, identifies some major obstacles to helium-3 extraction from extraterrestrial sources for use in fusion, and questions the feasibility of extraterrestrial extraction when compared to production on Earth.^[63]

Several science fiction works have featured helium-3 extraction on the moon, including the films Moon (2009) and Iron Sky (2012), the manga and corresponding anime Planetes, the video game Anno 2205 (2015) and the novel Luna: New Moon (2015). The novel Morning Star (Pierce Brown, 2016) features helium-3 mining on Phobos (a moon of Mars), while his novel Red Rising (2014) features helium-3 extraction from Mars itself.

Power generation[edit]

This section contains weasel words: vague phrasing that often accompanies biased or unverifiable information. Such statements should be clarified or removed. (March 2013)

A second-generation approach to controlled fusion power involves combining helium-3 (³₂He) and deuterium (²₁H). This reaction produces a helium-4 ion (⁴₂He) (like an alpha particle, but of different origin) and a high-energy proton (positively charged hydrogen ion) (¹₁p). 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 ions and the fusion protons. High speed protons, as positively charged particles, can have their kinetic energy converted directly into electricity, through use of solid-state conversion materials as well as other techniques. Potential conversion efficiencies of 70% may be possible, as there is no need to convert proton energy to heat in order to drive a turbine-powered electrical generator^{[citation needed]}.

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 costs 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 will be required to develop and construct the first helium-3 fusion power plant. Financial breakeven at today's wholesale electricity prices (5 US cents per kilowatt-hour) would occur after five 1-gigawatt plants were on line, replacing old conventional plants or meeting new demand.^[64]

The reality is not so clear-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 Wendelstein 7-X). 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.^[65] 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+³He, the barrier is much higher, and it is even higher for ³He–³He. 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–³He 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 helium-3 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–³He fusion, a number of other reactors have been proposed – the Fusor, Polywell, Focus fusion, and many more, though many of these concepts have fundamental problems with achieving a net energy gain, and generally attempt to achieve fusion in thermal disequilibrium, something that could potentially prove impossible,^[66] 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" fuels, and therefore their proponents tend to promote p-B fusion, which requires no exotic fuels like helium-3.