Aneutronic fusion

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Aneutronic fusion is any form of fusion power where no more than 1% of the total energy released is carried by neutrons. Since the most-studied fusion reactions release up to 80% of their energy in neutrons, successful aneutronic fusion would greatly reduce problems associated with neutron radiation such as ionizing damage, neutron activation, and requirements for biological shielding, remote handling, and safety issues. Some proponents also see a potential for dramatic cost reductions by converting the energy of the charged fusion products directly to electricity. The conditions required to harness aneutronic fusion are much more extreme than those required for the conventional deuterium-tritium (DT) fuel cycle, and even these conditions have not yet been produced experimentally. Even if aneutronic fusion is one day shown to be scientifically feasible, it is still speculative whether power production could be made economical.

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[edit] 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 sections are these:

D + 3He →   4He (3.6 MeV) +   p (14.7 MeV)
D + 6Li → 2  4He + 22.4 MeV
p + 6Li →   4He (1.7 MeV) +   3He (2.3 MeV)
3He + 6Li → 2  4He   +   p + 16.9 MeV
3He + 3He →   4He   + 2  p  
p + 7Li → 2  4He + 17.2 MeV
p + 11B → 3  4He + 8.7 MeV

The first two of these use deuterium as a fuel, and D-D side reactions will 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 classify as aneutronic according to the 1% threshold.

The rates of the next two reactions (involving p, 3He, and 6Li) are not particularly high in a thermal plasma. When they are treated as a chain, however, they offer the possibility of an enhanced reactivity due to a non-thermal distribution. 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 naturally on the Earth in only minuscule amounts, so it would either have to be bred from reactions involving neutrons (counteracting the potential advantage of aneutronic fusion), or mined from extraterrestrial bodies. The top several meters of the surface of the Moon is relatively rich in 3He, 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 giant 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 last reaction, p-11B.

[edit] Technical challenges

[edit] 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 considered so formidable. To begin with, 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 keV (6.6 billion degrees Celsius or 6.6 gigakelvins) while D-T has a peak at around 66 keV (730 million degrees Celsius).[1]

[edit] Power balance

In addition, the peak reaction rate of p-11B is only one third that for D-T, so that better confinement of the plasma energy is required. The confinement is usually characterized by the time τ the energy must be retained so that the fusion power exceeds the power required to heat the plasma. Various requirements can be derived, the most commonly used being the product with the density, nτ, and the product with the pressure nTτ, both of which are called the Lawson criterion. The nτ required for p-11B is 45 times higher than that for DT. The nTτ required is 500 times higher.[2] (See here for more details). Since the confinement properties of conventional approaches to fusion such as the tokamak and laser pellet fusion are marginal, most proposals for aneutronic fusion are based on radically different confinement concepts.

In most fusion plasmas, bremsstrahlung radiation is a major channel of energy loss. (See here for more details.) For the p-11B reaction, some calculations indicate that the Bremsstrahlung 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 fusion reactor designs, whether based on magnetic confinement or inertial confinement concepts, the bremsstrahlung can easily escape the plasma and is considered a pure energy loss term. The outlook would be more favorable if the radiation could be reabsorbed by the plasma. Absorption occurs primarily via Thomson scattering on the electrons,[3] 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.[4] 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 the range of future inertial confinement systems.[5]

In very high magnetic fields, on the order of a megatesla, a quantum mechanical effect should suppress the transfer of energy from the ions to the electrons.[6] According to one calculation,[7] the Bremsstrahlung losses could be reduced to half the fusion power or less. In a strong magnetic field the cyclotron radiation 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),[8] 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 obtained in the laboratory, fields of 0.3 megatesla have been produced with high intensity lasers,[9] and fields of 0.02-0.04 megatesla have been observed with the dense plasma focus device.[10][11]

At much higher densities still (ne > 6.7×1034 m−3), the electrons will be Fermi degenerate, which will suppress the Bremsstrahlung losses, both directly and by reducing the energy transfer from the ions to the electrons.[12] If the necessary conditions could be attained, this would open up the possibility of net energy production from p-11B or D-3He fuel. The feasibility of a reactor based solely on this effect remains low, however, because the gain 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).

[edit] 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.[13] 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.

[edit] Current research

There are a number of efforts aimed at achieving hydrogen-boron fusion, using different fusion devices. One approach, using the dense plasma focus, has been funded by NASA’s Jet Propulsion Laboratory, the Air Force Research Laboratory and the Chilean Nuclear Energy Commission, among others.[14] In 2001, Lawrenceville Plasma Physics (LPP) Inc. reported achieving ion energies of over 100 keV using a plasma focus device at Texas A&M University.[15] A test of this approach, also known as “focus fusion” is now underway, in a joint project with LPP, at the Thermonuclear Plasma Laboratory in Santiago Chile.[16] Researchers from University of Illinois and from the Air Force Research Laboratory have described how a dense plasma focus device using hydrogen-boron fuel can be used for space propulsion.[17]

A second approach uses a Field-Reversed Configuration device with high energies supplied by ion beams. This effort is privately financed by TriAlpha Energy, Inc.[18] In yet another approach, pioneered by Robert W. Bussard, and until recently funded by the US Navy, an inertial electrostatic confinement device is used.[19][20]

None of the efforts noted here has yet actually 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.

While the z-pinch device has not been mentioned as a possible hydrogen-boron reactor, ion energies of interest to such reactions, up to 300 keV, were reported by researchers for the Z-machine at Sandia National Laboratory.[21] In 2005, a Russian team produced hydrogen-boron aneutronic fusions using a picosecond laser.[22] However, the number of the resulting α particles (around 103 per laser pulse) was extremely low.

[edit] 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.[23]

These neutrons come primarily from the reaction[24]

11B + α → 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 MeV. 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-rays will be produced by bremsstrahlung, and 4, 12, and 16 MeV gamma rays will be produced by the fusion reaction

11B + p → 12C + γ + 16.0 MeV

with a branching probability relative to the primary fusion reaction of about 10−4.[25]

Finally, isotopically 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.[26]

[edit] 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 technology 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.[citation needed]

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.[27] 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.[28] Some of this energy could also be converted directly to electricity. X-rays passing though an array of conducting foils would transfer some of their energy to electrons, which can then be captured electrostatically.[citation needed] 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.

[edit] References

  1. ^ For confinement concepts that are pressure limited, the optimum operating temperatures are about 5 times lower, but the ratio is still roughly ten-to-one.
  2. ^ Both of these figures assume the electrons have the same temperature as the ions. If operation with cold electrons is possible, as discussed below, the relative disadvantage of p-11B would be a factor of three smaller, as calculated here.
  3. ^ Lecture 3 : Accelerated charges and bremsstrahlung, lecture notes in astrophysics from Chris Flynn, Tuorla Observatory
  4. ^ mi/σT = 2.5×(1.67×10−24 g)/(6.65×10−25 cm²) = 6.28 g/cm²
  5. ^ Robert W. B. Best. "Advanced Fusion Fuel Cycles". Fusion Technology, Vol. 17 (July 1990), pp. 661-5.
  6. ^ G.S. Miller, E.E. Salpeter, and I. Wasserman, Deceleration of infalling plasma in the atmospheres of accreting neutron stars. I. Isothermal atmospheres, Astrophysical Journal, 314: 215-233, 1987 March 1. In one case, they report an increase in the stopping length by a factor of 12.
  7. ^ E.J. Lerner, Prospects for p11B fusion with the Dense Plasma Focus: New Results (Proceedings of the Fifth Symposium on Current Trends in International Fusion Research), 2002, http://www.arxiv.org/ftp/physics/papers/0401/0401126.pdf
  8. ^ Assuming 1 MT field strength. This is several times higher than solid density.
  9. ^ http://jasmine.kues.kyoto-u.ac.jp/pps/PPSProceedings/05_Beiersdorfer_LaserPPS.pdf
  10. ^ Bostick, W.H. et al, Ann. NY Acad. Sci., 251, 2 (1975)
  11. ^ The magnetic pressure at 1 MT would be 4×1011 MPa. For comparison, the tensile strength of stainless steel is typically 600 MPa.
  12. ^ S.Son, N.J.Fisch, Aneutronic fusion in a degenerate plasma, Physics Letters A 329 (2004) 76-82 or online
  13. ^ Comparing two different types of power systems involves many factors in addition to the power density. Two of the most important are the volume in which energy is produced in comparison to the total volume of the device, and the cost and complexity of the device. In contrast, the comparison of two different fuel cycles in the same type of machine is generally much more robust.
  14. ^ JPL Contract 959962, JPL Contract 959962
  15. ^ E.J. Lerner, Prospects for p11B fusion with the Dense Plasma Focus: New Results (Proceedings of the Fifth Symposium on Current Trends in International Fusion Research), 2002, http://www.arxiv.org/ftp/physics/papers/0401/0401126.pdf
  16. ^ http://focusfusion.org/log/index.php/site/article/lpp_cchen_collaboration_announcement/
  17. ^ Thomas, Robert; Yang, Yang; Miley, G. H.; Mead, F. B Advancements in Dense Plasma Focus (DPF) for Space Propulsion SPACE TECHNOLOGY AND APPLICATIONS INT.FORUM-STAIF 2005:. AIP Conference Proceedings, Volume 746, pp. 536-543 (2005)
  18. ^ Cheung, A.; Binderbauer, M.; Liu, F.; Qerushi, A.; Rostoker, N.; Wessel, F. J. Colliding Beam Fusion Reactor Space Propulsion System SPACE TECHNOLOGY AND APPLICATIONS INTERNAT.FORUM-STAIF AIP Conference Proceedings, Volume 699, pp. 354-361 (2004)
  19. ^ Bussard, R. W. & Jameson L. W., Inertial-Electrostatic-Fusion Propulsion Spectrum: Air-Breathing to Interstellar Flight, Journal of Propulsion and Power Vol. 11, No. 2, March - April 1995
  20. ^ Should Google go Nuclear? - A video of Dr. Bussard presenting his concept to an audience at Google
  21. ^ Malcolm Haines et al, Viscous Heating of Ions through Saturated Fine-Scale MHD Instabilities in a Z-Pinch at 200-300 keV Temperature; Phys. Rev. Lett. 96, 075003 (2006)
  22. ^ V.S. Belyaev et al, Observation of neutronless fusion reactions in picosecond laser plasmas, Physical Review E 72 (2005), or online, mentioned in news@nature.com on August 26, 2005 : Lasers trigger cleaner fusion
  23. ^ Heindler and Kernbichler, Proc. 5th Intl. Conf. on Emerging Nuclear Energy Systems, 1989, pp. 177-82. Even though 0.1% is a small fraction, the dose is rate still high enough to require very good shielding, as illustrated by the following calculation. Assume we have a very small reactor producing 30 kW of total fusion power (a full-scale power reactor might produce 100,000 times more than this) and 30 W in the form of neutrons. If there is no significant shielding, a worker in the next room, 10 m away, might intercept (0.5 m²)/(4 pi (10 m)2) = 4×10−4 of this power, i.e., 0.012 W. With 70 kg body mass and the definition 1 gray = 1 J/kg, we find a dose rate of 0.00017 Gy/s. Using a quality factor of 20 for fast neutrons, this is equivalent to 3.4 millisieverts. The maximum yearly occupational dose of 50 mSv will be reached in 15 s, the fatal (LD50) dose of 5 Sv will be reached in half an hour. If very effective precautions are not taken, the neutrons would also activate the structure so that remote maintenance and radioactive waste disposal would be necessary.
  24. ^ W. Kernbichler, R. Feldbacher, M. Heindler. "Parametric Analysis of p-11B as Advanced Reactor Fuel" in Plasma Physics and Controlled Nuclear Fusion Research (Proc. 10th Int. Conf., London, 1984) IAEA-CN-44/I-I-6. Vol. 3 (IAEA, Vienna, 1987).
  25. ^ As with the neutron dose, shielding is essential with this level of gamma radiation. The neutron calculation in the previous note would apply if the production rate is decreased a factor of ten and the quality factor is reduced from 20 to 1. Without shielding, the occupational dose from a small (30 kW) reactor would still be reached in about an hour.
  26. ^ El Guebaly, Laial, A., Shielding design options and impact on reactor size and cost for the advanced fuel reactor Aploo, Proceedings- Symposium on Fusion Engineering, v.1, 1989, pp.388-391. This design refers to D-He3, which actually produces more neutrons than p-11B fuel.
  27. ^ Miley, G.H., et al, Conceptual design for a B-3He IEC Pilot plant, Proceedings--Symposium on Fusion Engineering, v. 1, 1993, pp. 161-164; L.J. Perkins et al, Novel Fusion energy Conversion Methods, Nuclear Instruments and Methods in Physics Research, A271, 1988, pp. 188-96
  28. ^ Quimby, D.C., High Thermal Efficiency X-ray energy conversion scheme for advanced fusion reactors, ASTM Special technical Publication, v.2, 1977, pp. 1161-1165

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