April 21, 2020
In 1933 the British physicists Ernest Rutherford and Mark Oliphant reported on a series of experiments in which they bombarded a thin film of the boron compound borax by a beam of protons (nuclei of hydrogen atoms) and registered the emission of high-energy alpha particles (nuclei of helium atoms).
This confirmed the earlier evidence of Cockcroft and Walton, that nuclear reactions were taking place between protons and boron nuclei, resulting in the transmutation of chemical elements: from hydrogen and boron, we get helium. This hydrogen-boron (or proton-boron) reaction was one of many nuclear reactions discovered in the 1930s.
Closer analysis revealed that the boron nucleus, having absorbed a proton, splits into three alpha particles, which fly off with enormous velocities. The total energy contained in their motion – their kinetic energy – turns out to be millions of times larger than the energy liberated per atom by any known chemical reaction.
This makes the hydrogen-boron reaction, alongside more familiar nuclear reactions such as the fission of uranium and fusion of the hydrogen isotopes deuterium and tritium, into a potential candidate for large-scale energy production. All the more so because boron is a readily available element.
Looking more closely, the reaction in question occurs only with a specific isotope of boron, called boron-11. No problem, boron-11 makes up 80% of naturally-occurring boron.
Although the hydrogen-boron reaction is commonly referred to as a form of fusion, it would perhaps be more accurate to describe it as a combination of a fusion and a fission process: A hydrogen nucleus (a proton, denoted p in the diagram) fuses with a B-11 nucleus to form an unstable, highly-exciting nucleus of the carbon isotope C-12; this excited carbon nucleus nearly instantaneously splits into three high-energy alpha particles which fly off at huge velocities. (That, at least, appears to be the generally accepted account of what happens.)
A number of reasons make the hydrogen-boron reaction especially attractive as an energy source.
One, already mentioned, is that plenty of boron is available, along with virtually endless amounts of hydrogen from ordinary water. On the basis of the hydrogen-boron reaction, a single gram of hydrogen-boron mixture would produce very roughly as much energy as is released by the combustion of three tons of coal. Present proven reserves of boron, contained in borax and other minerals, amount to over one billion tons. A bit of arithmetic shows us that this would be sufficient to supply world electricity consumption at present levels for a million years.
A second big advantage is that the hydrogen-boron nuclear reaction produces essentially no radioactivity. The products of the reaction, alpha particles – identical with the nuclei of ordinary helium atoms – are stable particles, which do not undergo radioactive decay. Also, alpha radiation (fast-flying alpha particles) has a very low penetrating power. Alpha particles rapidly give up their energy by elastic collisions with heavier nuclei when they interact with ordinary materials.
Even at the indicated high energies, alpha particles have a range of only a few centimeters in air and can be stopped by a few layers of paper. Alpha radiation is dangerous to health only if a person is exposed to it directly at a very short distance. Also, the radioactivity generated by secondary reactions, e.g. caused by rare reactions between the alpha particles and other nuclei, is negligible.
By contrast, the fusion reactions between the hydrogen isotopes deuterium (D) and tritium (T), which have been the main focus of fusion energy development until now, release penetrating gamma radiation and – most problematic – large numbers of neutrons. These neutrons are absorbed by the nuclei in the surrounding materials, transforming some of them into radioactive isotopes.
Although the problem of disposing of “activated” materials in D-T fusion reactors is relatively minor compared to the problem posed by radioactive waste from nuclear fission reactors, it imposes costs and makes the reactor more complicated. This problem does not exist for the hydrogen-boron reaction. It belongs to the class of so-called aneutronic nuclear reactions.
A third, very big advantage lies in the fact that alpha particles are electrically charged, carrying two units of positive electricity. We can think of a stream of fast-moving alpha particles as a high-voltage electric current. By making the particles traverse an electric field we can transform their energy of motion into electrical energy, with practically no loss. The heat exchangers, pumps and turbine systems, which account for much of the cost of fossil fuel or nuclear fission power stations, become superfluous.
All of this sounds wonderful. But now the trouble starts.
At present, practically all research into fusion energy has been focused on the deuterium-tritium reaction. D-T is by far the easiest to realize, in terms of the required combination of temperatures, pressures and “burn” durations. Nevertheless, despite over half a century of research and tens of billions of dollars of R&D, the goal of net energy generation from D-T reactions has still not been achieved.
Even then, the pathway from successful experiment to functioning reactor promises to be long and difficult. At least for the approaches which are getting most of the funding today. In recent years significant amounts of private money have been going into alternative pathways, some of them quite promising.
The hydrogen-boron reaction is incomparably more difficult than D-T, in terms of the required combination of physical parameters. Among other things, nuclei do not always react when they collide, but only with a certain probability, the so-called “cross-section” of the reaction. The reaction cross-section for hydrogen-boron – and thereby also the reaction rate – is about 300 times smaller for the hydrogen-boron reaction, than for the conventional D-T reaction.
The required temperature is also 10 times higher, and the energy obtained per hydrogen-boron reaction is half of that from a D-T reaction. The reason for these differences lies in the specifics of the nuclear structures and interactions involved. There are other issues such as potentially much larger energy losses due to the so-called bremsstrahlung, but that would be too technical to go into here.
In sum, the obstacles to exploiting the hydrogen-boron reaction as a practical energy source appear so immense, that it has hardly been considered a serious option. At least by the vast majority of the fusion community.
At this point, the reader might pose a naive question: If Rutherford and Oliphant already generated hydrogen-boron reactions with a rudimentary proton accelerator, 85 years ago, then why don’t we just scale that up? Their measurements showed, in fact, that the energy liberated when a fast-moving proton collides and reacts with a boron nucleus, is more than 10 times the original energy of the proton.
Simply irradiating a boron target with a beam of protons, looks like an easy way to generate energy. Compact proton accelerators are available on the market. What is the problem? Above all, what do we need billion-degree temperatures for?
It is easy to give reasons why the simple-minded beam-target approach, just described, doesn’t work. First and foremost, only a very tiny percentage of the protons in the beam actually trigger reactions – the “cross-section” problem. The rest of the protons bounce off, fly off or are otherwise lost, and with them, the energy expended to accelerate them. The overall efficiency is less than zero. This applies not only to the hydrogen-boron reaction but to the much “easier” deuterium-tritium reaction as well.
To get fusion to pay off in energetic terms, it doesn’t work to try to make reactions happen separately, one-by-one. We need some sort of collective process, in the broadest sense of the term.
The crudest approach would be to simply stuff as many of the mutually repelling particles as possible into a bottle (compression) and shake the mixture violently (heating) causing them to bounce around and collide with each other as often as possible. Keep that going until as many fusion reactions as possible take place. Hopefully you will end up with more energy than you put into the shaking process.
Ultimately, despite all their enormous technological sophistication, the mainline approaches to realizing fusion energy so far all boil down to this basic scenario. We could call it the “thermal” scenario insofar as heat – a disordered form of energy – mediates the whole process.
In the case of hydrogen-boron, for the reasons indicated above, the thermal scenario offers little hope. It would require a combination of temperature, density and so-called confinement time – the time during which the process is maintained in its compressed, heated state – far beyond the reach of present or readily foreseeable technology.
Fortunately, the advent of ultra-high-power lasers and “extreme light” opens the door to a short-cut method for realizing hydrogen-boron fusion, in which non-thermal, highly organized collective processes play the decisive role.
Jonathan Tennenbaum received his PhD in mathematics from the University of California in 1973 at age 22. Also a physicist, linguist and pianist, he’s a former editor of FUSION magazine. He lives in Berlin and travels frequently to Asia and elsewhere, consulting on economics, science and technology.