Aneutronic fusion is a form of fusion power where no more than 1% of the total fusion energy released is carried by neutrons. It has long been a dream of both the conventional and alternative fusion communities because of problems associated with neutron radiation such as ionizing damage, and requirements for biological shielding, remote handling, and safety issues. Aneutronic fusion is potentially far more economical than other sources of electricity because it releases energy in the form of charged particles which can be converted directly to electricity. This avoids the very costly process, standard since Edison’s time, of producing heat, generating steam and using the steam to run turbines and generators.

Candidate aneutronic reactions

There are a few nuclear fusion reactions that have no neutrons as products on any of their branches. Those with the largest cross sections are these:

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.

The next two reactions are usually treated as a chain in the hope of attaining 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.

The second-to-last 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% by weight, but recovering presents significant difficulties in itself. Recovery of ³He from the atmospheres of the gas giant planets is a theoretical possiblity.

Residual radiation from a p-¹¹B reactor

For the above reasons, most advocates of aneutronic fusion concentrate on the last reaction, p-¹¹B. Even here, though, there are a number of side reactions that will produce neutrons or other radiation, among them the following:

as well as reactions with a possible ¹⁰B impurity fraction. Detailed calculations (Heindler and Kernbichler, Proc. 5th Intl. Conf. on Emerging Nuclear Energy Systems, 1989, pp. 177-82) show that at least 0.1% of the reactions in a thermal p-¹¹B plasma would produce neutrons. If left unshielded, this is still extremely intense radiation, as can be seen by the following simple calculation.

If we assume 0.1% of the energy is carried off by neutrons, even a "kitchen-sized" reactor with 30 kW of fusion power will produce 30 W 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. For an industrial size (100 MW) reactor under the same assumptions, the dose rate would be thousands of times higher, and anyone standing nearby would receive a fatal dose in a fraction of a second. The neutrons would also activate the structure so that remote maintenance and radioactive waste disposal would be necessary.

If we look at where these neutrons come from, they are dominated by the reaction

¹¹B + α → ¹⁴N + n + 157 keV

If we really want to eliminate neutrons, we see that we cannot tolerate fast alphas in the plasma. Usually, the product alphas are relied on to keep the fuel hot. If the alphas have to be extracted with their full energy, we will need very, very efficient processes to collect this power, transfer it, and drive whatever process maintains the plasma energy. The reaction itself produces only 157 keV, but the neutron will carry a large fraction of the alpha energy, which will be close to E_fusion/3 = 2.9 MeV.

Suppose we can do this, so that fast alpha reactions are suppressed by several orders of magnitude. We will always have the fuel ions, protons and borons. Of course, p+p doesn't do much, and boron-boron reactions can probably also be disregarded due to the large Coulomb barrier. The species can however react with one another in a number of ways to produce neutrons. These reactions are all endothermic. The smallest barrier is for the reaction

¹¹B + p → ¹¹C + n - 2.8 MeV

In a thermal plasma of a few hundred keV temperature, there is a sufficient number of protons in the high energy tail that this reaction is a significant source of neutrons. If the proton temperature is reduced below about 30 keV, then this process is suppressed, but there is also no longer any significant fusion. The only way around this dilemma is to produce a nearly mono-energetic proton energy distribution, that is, a beam. If the beam energy is chosen to be at the fusion resonance around 600 keV, then the reactivity is also about three times higher than the maximum for a thermal plasma.

Let us assume that we can produce and maintain such a non-Maxwellian distribution, so that (p,n) reactions are suppressed by several orders of magnitude. What is the next most serious source of neutrons? Probably those associated with fuel impurities. If the density of fast alphas and fast protons is controlled to suppress reactions with ¹¹B, then the reactions with impurity ¹⁰B should be similarly suppressed for the same reasons. The impurity deuterium density must be kept low enough to suppress D-D fusion. Since the fusion rate is proportional to the square of the deuterium density, this is presumably not too difficult. More serious is perhaps the reaction

¹¹B + d → ¹²C + n + 13.7 MeV

The cross section for this reaction should be similar to that for p-¹¹B fusion, so that it will be necessary to use very pure hydrogen fuel. Considering the factor 2 mass difference and the small amount of fuel needed, one might assume that it is technically and economically feasible to reduce the deuterium concentration several orders of magnitude below its natural abundance of 1.5×10^-4.

Let us assume that fuel of sufficient chemical and isotopic purity can be made. Any other elements getting into the plasma, for example through outgassing of the walls, are another potential source of neutrons. Any energetic fuel or product particles striking solid surfaces can also produce neutrons. It is difficult to estimate the severity of these reactions, even with a particular configuration in mind.

If, in addition to the other assumptions above, we assume that interactions between the plasma and the containment device can be adequately controlled, then neutron production will be suppressed by many orders of magnitude. What other types of radiation will be a concern? Bremsstrahlung will produce extremely large quantities of hard x-rays, which must and, presumably, can be shielded by a modest amount of metal. The fusion reaction

¹¹B + p → ¹²C + γ + 16.0 MeV

will produce 4, 12, and 16 MeV gammas with a branching probability relative to the primary fusion reaction of about 10^-4. With no shielding, this would be a tremendous radiation dose. The calculation above 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, so enough shielding must be installed to attenuate the hard gamma flux by well over three orders of magnitude. For an industrial reactor, the attenuation should be well over six orders of magnitude. Radiation shielding will be complicated by the possibility of (γ,n) reactions in the shield material, reintroducing the neutron problem.

While careful design will be necessary, it should be possible to reduce the occupational dose to operators to an acceptable, possibly even 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 a few tens of centimeters.

Due to the production of carbon 11(half-life 20.3 min), maintenance within the reactor will have to be delayed by several hours after shut-down to allow radiation levels to decline to background levels. While some long term radioactive waste is produced, the amount is trivial. After one year of operation, a small, 5 MW pB11 reactor will generate about 5 micro Curies of radioactive waste, about the same amount of radioactivity as contained in the bodies of a classroom of children. If the entire US electric power generation capacity were aneutronic fusion generators, 0.6 Curies of waste would be produced per year. By comparison conventional nuclear energy has so far generated nearly one hundred billion Curies of waste.

Power density and energy balance

The peak in the cross section for aneutronic reactions tends to occur at a relatively high temperature. More important than the peak in the cross section is the peak in σv/T² because this determines the power density at a given pressure and the required value of the Lawson criterion. (See here for more details). For aneutronic reactions compared to the D-T reaction, the peak is not only at a higher temperature, but the value of the maximum is much lower. Specifically, for p-¹¹B compared to D-T, the triple product nTτ required for ignition is 500 times higher and the power density is 2500 times lower.

The energy confinement time τ will depend on the particular confinement scheme and the size of the reactor, but one energy loss channel is practically universal, namely Bremsstrahlung radiation. (See here for more details.) For the p-¹¹B reaction, it has been shown 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.

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. According to one calculation^[1], the Bremsstrahlung losses could be reduced by about a factor of five, so that, in the idealized case, the bremsstrahlung losses would be much less than the fusion power. 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, 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. For this reason, high magnetic fields can be maintained for significant lengths of time, as has been observed in many experiments with the plasma focus device, for example.

notes

External link

• Focus Fusion Society