Since the formation of the first stars, fusion of hydrogen into helium has provided abundant energy, coming to us as sunshine. Further fusion reactions in exploding stars have produced all the naturally occurring elements making up the planets. For the past 4.5 billion years, our sun has provided light and heat to Earth, making possible the evolution of life powered by photosynthesis. Some of the Earth’s internal energy comes from the natural nuclear fission of unstable radioactive elements in the core.
Despite Ernest Rutherford’s dismissive comment in 1933, ”Anyone who expects a source of power from the transformation of the atom is talking moonshine,” humans have been able to control nuclear fission, first in immensely destructive atomic bombs, and then, for over 60 years, generating electricity in nuclear power stations. However, harnessing nuclear fusion, the energy source of our sun, has proved elusive (apart from the H-bomb), despite 70 years of research.
The problem is that, while fission reactions occur spontaneously once a sufficient quantity (critical mass) of uranium-235 has been assembled, fusion reactions require the sorts of temperatures and pressures found in the centre of the Sun. This has been achieved in hydrogen bombs by the drastic expedient of compressing and heating the fuel, not ordinary hydrogen but a mixture of deuterium (D) and tritium (T),1 with an atomic bomb “trigger”. This is obviously unsuitable for safe power generation.
The strategy for safe fusion is to find ways of confining the fuel for long enough so it can reach the required temperature and ensuring that it reaches that temperature. The main problem is that, at such a temperature, the charge becomes a plasma, a gas of ions which tends to escape before the pressure is high enough. There are two approaches to this problem, the tokamak and inertial confinement. In tokamaks,2 a toroidal (ring-doughnut-shaped) vacuum chamber, permeated by an enormous magnetic field, contains a plasma of fast-moving D and T ions and electrons heated to 100 million ºC, hot enough for fusion reactions to occur. With inertial confinement, a tiny sphere containing D and T is blasted with lasers, compressing and heating the fuel.
All designs require an enormous amount of energy input but success requires a greater output. The ratio of output to input energy (Q) therefore has to exceed 1 by a large amount for a commercially viable reactor. So far, the Joint European Torus (JET) has achieved Q = 0.67 while recently producing power for 5 seconds, albeit at a lower Q. The much bigger International Thermonuclear Experimental Reactor (ITER) is projected to produce sustained power at Q greater than 1 from 2035. Meanwhile, the National Ignition Facility (NIF) in California, using inertial confinement, had achieved a Q of 0.7 in 2021 but only for an infinitesimal time.
Last month, in a first for fusion, the NIF achieved a Q of 1.5, the first gain in energy in a nuclear fusion experiment and a positive step on the road to commercial fusion power. However, the details give an idea of how far there is still to go. The experiment involved a tiny peppercorn-sized glass sphere containing the D and T fuel inside a gold capsule. The latter was blasted with 192 lasers, heating it so much that it emitted X-rays which caused the fuel pellet to implode, heating and pressurising the fuel. Inertia meant that there was a tiny fraction of a second for fusion to occur before the gold, glass and fusion fuel flew apart.
Commercial IC reactors would require a steady stream of such fuel pellets for a useful rate of energy production and, while gold isn’t cheap, tritium costs about 500 times as much. The NIF experiment produced about 1 megajoule (MJ) of energy: at 10 a second, that would be 10 megawatts (MW) of power, 1/100th the output of an average nuclear power plant. No doubt, Q and the throughput of fuel pellets could be increased, but there is still the problem of harvesting the heat to generate useful electricity.
It is easier to conceive of a commercial tokamak with a continuous injection of D and T into its chamber. ITER, described as the most expensive science experiment of all time and the most complicated engineering project in human history, is planned to produce 500 MW of heat for up to 10 minutes a time. Problems to be examined include the potential damage to the walls of the tokamak from bombardment by high-speed particles, mostly neutrons, which must be captured so that they yield their heat energy. Rare and expensive tritium will also need to be generated from lithium in the walls of the tokamak; the electromagnets will have to be the strongest ever, made of superconducting wires cooled by liquid helium close to absolute zero. The temperature differential between the magnets and the hot plasma at some 150 million ºC will likely be the greatest in the universe.
It is difficult to see fusion becoming a significant contributor to carbon-free energy any time soon but many of its discoveries and innovations may find applications in other areas, as has been the case with the CERN particle-collider.
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Where does fusion energy come from?
The famous E = mc2 equation (Einstein, 1905) acquired a startling implication when atomic masses could be measured accurately. One pioneer of this was Francis Aston, inventor of mass spectrometry, who used his machine to confirm the existence of isotopes. Atomic masses, relative to a standard atom (now the carbon-12 isotope with an exact mass of 12 units (u)), seemed to be whole numbers: this was logical because atomic nuclei contain whole numbers of protons and neutrons, each with a relative mass of 1.
However, more precise measurements came up with puzzling discrepancies. Most astonishing was that the mass of hydrogen-1 (with just 1 proton in its nucleus) was 1.0078 u while that of helium-4 (with 2 protons and 2 neutrons) was not (4 x 1.0078 =) 4.0312 but 4.0026 u. Its mass was 0.0286 u or 0.7% less than it should have been, referred to as a mass defect.
It was quickly realised that the mass lost was equivalent to the energy released if a more stable He-4 nucleus was made by fusing 4 H-1 nuclei. If 1 kg of hydrogen could be fused into helium, enough energy would be released to boil about 200 million kettles, while burning 1 kg of hydrogen would release enough energy to boil about 200 kettles.
Conversely, for heavy atoms, mass is lost when nuclei split (or fission) to form lighter atoms. For uranium-235, about 0.1% of mass is converted to energy on fission (which can be controlled) so 1 kg of U-235 could release enough energy to boil about 30 million kettles.
As Aston said 100 years ago, “… if hydrogen is transformed into helium, a certain quantity of mass must be annihilated in the process. The cosmical importance of this [is] greater in fact than any suggested before by science in the whole history of the human race. … Should … some means [be discovered] of releasing this energy in a [useful] form …, the human race will have at its command powers beyond the dreams of scientific fiction.”
This possibility was largely discounted at the time, notably by Ernest Rutherford, discoverer of the atomic nucleus, in his moonshine comment (see above). However, it was realised that fusion of hydrogen into helium explained the energy output of the stars, particularly after PhD student Cecilia Payne3 concluded that stars were overwhelmingly composed of hydrogen, with helium in second place. So Rutherford should have been talking about sunshine.
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Footnotes
1Deuterium (heavy hydrogen) and tritium (heavier still but radioactive) are easier to get to fuse than hydrogen. In practice, lithium deuteride is used as it produces more tritium when irradiated during the explosion and boosts the yield.
2Andrei Sakharov, inventor of the tokamak (and father of the Soviet H-bomb), was a consistent and brave opponent of the Soviet regime, campaigning along with his wife, Yelena Bonner, for nuclear disarmament, democracy and human rights, despite persecution. He was awarded the Nobel Peace Prize in 1975.
3Her discovery (“the most brilliant PhD thesis ever written in astronomy”) was rejected by her supervisor who subsequently took the credit for it.
Random Walk in Science 