Nuclear fusion explained

The processes by which stars, such as the Sun, produce energy is well-known to be based on nuclear fusion, and there has been a long-held ambition to reproduce this on Earth.

The terrestrial abundance of the isotope of heavy-hydrogen, deuterium, makes this an attractive proposition for sustainable energy production. However, despite the Sun being able to readily, and naturally, generate energy from fusion, it is taking a huge scientific and engineering effort to realise.

Nevertheless, and maybe because of this, 2021 saw investments of close to $3billion in fusion power developments.

Solar fusion

Solar fusion proceeds through a three-stage process. First, two protons (hydrogen atoms stripped of their electrons) react to produce a deuteron, a positron and a neutrino. Then the deuteron, ²H, captures a proton to form the helium isotope ³He (helium-3), and then two ³He nuclei fuse to form ⁴He with the emission of two protons. In this way, the reaction catalyses the conversion of 4 protons into a ⁴He (helium-4) nucleus, an alpha-particle. The energy produced is small and owes its origin to the mass difference between ⁴He and 4 protons via 𝐸=𝑚𝑐², and accounts for 0.7 per cent of the mass of the original protons. Nevertheless, the rate at which this is happening, given the mass of the sun, generates a huge amount of energy, 3.8x10¹⁷ gigawatts (GW). To put this into context, a typical nuclear power plant is ~1 GW.

As witnessed in the use of atomic weapons, nuclear reactions tend to be rapid and explosive. The fact that the Sun has managed to burn controllably for 4.5 billion years is related to two key features. First the fusion process proceeds through a quirk of quantum mechanics. Although the Sun is hot, the kinetic energy of the protons is very low compared to the Coulomb repulsion that arises from the two positive proton charges.

Classically, the kinetic energy required to make two protons fuse is about 1000 times greater than they have inside the Sun. There is a repulsive barrier, called the Coulomb barrier. However, in quantum mechanics there is a probability of particles being able to tunnel through this barrier, although classically this would be forbidden. The Sun exploits this small probability.

The second factor is that the first reaction proceeds via the weak interaction, not the strong nuclear force. It is the weak interaction which results in the production of the neutrino and positron. The weak interaction being weak, limits the reaction rate. It is these effects which allows the quiescent burning of nuclear matter inside the Sun. 

Terrestrial fusion

The first terrestrial nuclear fusion experiment is attributed to Mark Oliphant who discovered ³He and ³H (tritium) via fusion of deuterons (see footnote 1). The fusion of two deuterons releases 5.5 megaelectron volts (MeV) of energy (compared to the 26.7 MeV produced in the stellar fusion), whereas the fusion of deuterons with tritium (²H+³H, D+T) releases 17.6 MeV and creates a ⁴He nucleus and a neutron. Given that the neutron is lighter than the alpha-particle it carries most of this energy; 14.2 MeV. Almost a century later the challenge to create a terrestrial fusion D+T reactor continues.  

One of the impediments of the solar fusion has been removed, the D+T reaction is mediated by the strong and not the weak force, but the reaction is still strongly attenuated by the need to tunnel through the Coulomb barrier. Naively, terrestrial fusion should be easier. However, the Sun has one massive advantage and that is its mass. The ball of hydrogen of which the Sun is largely formed is held together by gravity. The gravitational force on the surface of the Sun is 28 times that of Earth and prevents the hydrogen atoms escaping (unlike on Earth).

This holds the hot plasma in space. The quest to contain a similar plasma on Earth is challenged by the fact the plasma mass is vanishingly small and the benefits of the gravitation force vanish – other means are required. 

Approaches: tokamak and laser-based fusion

There are typically two approaches to fusion which have been historically pursued: i) Tokamak and ii) laser driven.

The Tokamak reactor is a Russian invention which holds a doughnut ring of D+T plasma using magnetic fields and pumps energy into the plasma, using beams, heating it to the point where it is hot enough that the tunnelling probability is high enough for a sufficient rate of reactions to occur.

In laser-based fusion a D+T frozen pellet is irradiated with X-rays, generated from lasers, ablating the surface and driving the compression of the pellet to instigate fusion. The first method is that made famous by the UK-based Joint European Torus (JET) experiment and the development of the ITER reactor in France and the second by the National Ignition Facility (NIF) in the US. 

Challenges

There are plenty of challenges in these approaches. In the Tokamak method, containing a stable plasma has been the focus for many years, and in both getting more energy out from the nuclear reactions than is put in, either to heating the plasma or from the lasers creating the photons irradiating the pellet.  There have been a host of records claimed. In 2022 JET recorded 59 megajoules (MJ) of energy production and NIF recorded an input energy of 2 MJ of laser power 3 MJ of energy from the fusion process. 

These are colossal scientific achievements, but some perspective may be useful. To boil a kettle takes approximately 0.2 MJ. A nuclear fission power station of 1.2 GW (such as Sizewell B) produces 1,200 MJ each second. The 59 MJ from JET was for a 5 second period and the laser event was for a few millionths of a second. Commercial operation requires more or less continuous operation.

Second, account needs to be taken of not just the energy into the plasma or laser and energy produced by fusion, but also the overall energy efficiency of the whole system. For example, the lasers used have an efficiency of ~10 per cent and then the efficiency of a steam based electricity production of up to 90%. To be viable, energy losses need to be compensated for in the energy gain of the fusion process. Further development is needed.  

Future development

To date, a great deal of the focus has been on the fusion process itself. However, as fusion devices develop there is a need to begin to think about how a fusion reactor is manufactured and the energy might be extracted. The object which emerges from fusion which carries most of the energy is the neutron. The neutron being neutral is not confined by the magnetic fields of a Tokamak reactor.

Neutrons being neutral also means they pass rather readily through most materials. Capturing the energy of these particles as they steam away has relied on nuclear reactions which are exothermic. The proposal is to use a lithium blanket around the reactor.

The advantage is that the isotope lithium ⁶Li, Lithium-6,  can capture the neutron producing ⁷Li which then decays into ⁴He+³H and 4.8 MeV. This is one mechanism for the deposition of energy of the neutron, but one which has the advantage of creating tritium, ³H, which is the reactor fuel. So the reactor has a virtuous circle of creating energy and its own fuel.  

The theory is attractive, but the practical realisation remains a challenge which needs addressing as the next generation of fusion reactors are proposed to be energy generating. The manufacture of a reactor, which can operate with a duty cycle that is commercially attractive, means that the flux of neutrons that pass through the fusion reactor will be colossal.

When neutrons pass through a material they scatter from the nuclei inside the atoms with sufficient energy imparted that the atoms are displaced from their lattice sites. This knocking of the atoms around inside the material degrades the material properties and will limit the operational time of the reactor. The problem is that the interaction of 14.2 MeV neutrons with most materials is not known. As such there is a push towards creating facilities which can generate enough high energy neutrons to test the materials which will be used. Both energy production and materials development are major programmes of research. 

There is also a concern regarding the availability of the tritium fuel that is required ignite a reactor. It may generate its own fuel in a steady state, but needs some to start with. It is estimated that the European DEMO reactor would need between 5 and 14 kilograms (see footnote 2).

The issue is that most of the tritium available comes from the Canadian style CANDU nuclear fission reactors which use heavy water (deuterium) and create tritium. The ceasing of operation of these reactors and the fact that tritium has a half-life of 12.3 years means that the global supply of tritium will decline and by 2050 the available amount of tritium could be below the 14 kilograms required. The clock to get the reactors up and running is ticking. This is not an insurmountable problem, but points to the need for a broader strategy for fusion energy. 

Development in the UK

The UK is just embarking on the development of the Spherical Tokamak for Energy Production (STEP) development. This will create a fusion power plant around a compact UK designed Spherical Tokamak reactor design. The target is an operational plant by 2040. This is an exciting development which will galvanise and focus the research community around the challenges highlighted. There is no doubt there is a lot to do, but success would have huge implications for sustainable, green, energy beyond 2050. 

Footnotes

1. Transmutation Effects Observed with Heavy Hydrogen, Oliphant, M. L. E., Harteck, P. and Rutherford, E. Proc. R. Soc. A 144, 692 (1934)
2. Out of gas: A shortage of tritium fuel may leave fusion energy with an empty tank