Big tech and the future of energy: nuclear power vs. nuclear fusion

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Big Tech wants to use nuclear power plants to satisfy the energy hunger of its AI developments. In this context, the argument keeps coming up that they would be better off investing the money in nuclear fusion research. After all, fusion is seen as a clean alternative to nuclear fission. But its implementation is complex. We answer the most important questions about nuclear power and nuclear fusion.

Successful nuclear fusion of light elements releases an enormous amount of energy – just like the fission of heavy atomic nuclei. A – reasonably – clear explanation of this is provided by a curve that shows the binding energy per nucleon – i.e. proton or neutron – as a function of the number of nucleons in the nucleus. The binding energy is the energy that is released during the binding of nuclear components. The curve initially rises steeply for small numbers of nucleons, reaches a maximum at 30 to 40 and then falls flat. The sum of the binding energy per nucleon of two deuterium nuclei is smaller than that of a helium nucleus, so energy is released during their fusion – in the same way as heat is released during a chemical bond.

One gram of fuel could generate 90,000 kilowatt hours of energy in a power plant – the combustion heat of eleven tons of coal.

Has controlled nuclear fusion ever been achieved?

Yes, in 1997 researchers succeeded for the first time in igniting a real deuterium-tritium fusion in a tokamak reactor with a diameter of around 15 meters and a height of 12 meters. One measure for assessing such reactors is the energy confinement time. This is the average time it takes for a hot plasma particle to travel from the center of the plasma to the cold outer regions. In the case of the Joint European Torus (JET), which is located in Culham, UK, this time was one second.

The researchers achieved this in two further experiments. In 2021, they fused 0.2 milligrams of fuel and achieved an energy yield of 59 megajoules. In October 2023, shortly before the JET reactor was decommissioned, they were also able to produce a fusion lasting 5.2 seconds with 69 megajoules of fusion energy. However, the JET reactor was never intended to be a complete reactor, but only for preparatory experiments.

What is so difficult about building a fusion reactor?

Quite a lot. The main problem: if the temperature of the fuel is high enough, the electrons separate from the nuclei and a plasma is formed. Protons repel each other. But fusion can only be achieved if two atomic nuclei come so close together that the strong nuclear force overcomes the electrostatic repulsion. This is achieved by heating the fuel for fusion – usually deuterium and tritium – to extreme temperatures and compressing the resulting plasma.

The conditions for a "burning plasma" are reached when the product of temperature, density and energy confinement time exceeds a critical value – Experts call this the "Lawson criterion". Typically, these are plasmas with a temperature of around 100 million degrees Celsius that remain stable for a few tens of seconds – or are correspondingly denser with a shorter lifetime. There are a number of ideas as to how this could be realized technically:

• by confinement in magnetic fields
• by bombardment with powerful lasers
• by pressure waves

Magnetic confinement in a tokamak

Most researchers are focusing on the construction of a so-called tokamak reactor – a hollow, torus-shaped device that is to be filled with a gas containing deuterium and tritium atoms. In addition, an extremely strong magnet in the center of the donut induces an electric current that further heats up the gas via the ohmic resistance. However, a tokamak like the ITER can only be operated in pulsed mode due to its design.

Small spherical reactors

However, possible alternatives were already being considered around 20 years ago. However, this was only theoretical for a long time, as it would require much stronger magnets than before. New materials have been available for a few years now: high-temperature superconductors. With a small demonstrator including a superconductor coil, the British start-up Tokamak Energy was able to show that the idea works in principle. At the same time, the British company worked on larger coils made from high-temperature superconductors. In the USA, Commonwealth Fusion Systems is a leader in the construction of such magnets.

Stellarators

Tokamaks and spherical reactors can only be operated in pulsed mode because they rely on a current in the plasma to generate a magnetic field that additionally stabilizes the plasma. In stellarators, the magnetic field around the fusion chamber is twisted. The basic idea sounds simple, but generating such a magnetic field is technically very complex. The world's largest research reactor built according to this principle is the Wendelstein 7x, which is located in Germany.

Approach: laser fusion

However, the vast majority of public and private projects to date have relied on fusion plasma that is enclosed in magnetic fields (magnetic fusion). Until recently, laser fusion was considered an exotic outsider in an exotic field of research. This is now changing. Federal Research Minister Bettina Stark-Watzinger (FDP) also has high hopes for the idea.

However, the indirect approach is of course not the only way to create laser fusion. The start-up Focused Energy is convinced that firing directly at the target works better.

The team led by Markus Roth from TU Darmstadt has been working with the Lawrence Livermore National Laboratory (LLNL) for many years in the field of the civilian use of fusion. However, they are pursuing the principle of "direct-drive", in which the capsule is driven directly by laser beams – and is not located in a cavity. Focused Energy also relies on a specially designed target, into which the laser energy should couple better, and on several fast laser pulses in succession: the fuel already compressed by the first pulse is fired again with a short-pulse petawatt laser. This could simplify the ignition process and initiate a fusion reaction more quickly.

The Munich-based start-up Marvel Fusion wants to ignite a boron-proton fusion with laser bombardment. This is actually even more difficult than the fusion of deuterium and hydrogen because it requires higher temperatures, but has technical advantages in terms of utilizing the fusion energy released.

A powerful laser fires a very, very short pulse – focused on a spot just a few micrometers in diameter. The pellet is "nanostructured" to better couple the energy. What exactly happens inside the pellet is not fully understood physically. According to current computer simulations, the script looks something like this: If the laser pulse is fast and intense enough, its electric field accelerates electrons in the pellet. This charge shift in turn generates a strong electric field that pushes protons from the surface of the pellet inwards, while the boron nuclei hardly move at all due to their high mass. If the intensity of the laser pulse is high enough, the protons receive enough energy to overcome the repulsion of the boron nuclei and fuse with them.

Note: This article first appeared on t3n.de.

(mma)