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Nuclear Fusion

Fusion is seen as the key to the future: its fuels are near exhaustible; it is inarguably safe, unlike its fission counterpart, and it is available all year round. With the current drive to abolish fossil fuels, huge investment has been focused into it, facilitating huge jumps in the area and the potential unlocking of fusion.

The idea behind fusion is to fuse atoms together, like how hydrogen nuclei fuse to create helium within the intense heat and pressure of stars, and harness the substantial energy released by the exothermic reaction. The most common type of fusion is that of the two hydrogen isotopes deuterium and tritium. It occurs at lower temperatures than other elements, has a high energy yield, and deuterium is reasonably common, making up about 1 of every 5, 000 hydrogen atoms in the sea, so just a gallon of seawater could provide the energy equivalent of 300 gallons of gasoline. However, tritium is radioactive, with a half-life of 12.32 years: only trace amounts formed by the interaction of the atmosphere with cosmic rays can naturally be found on Earth. Fortunately, tritium can be manufactured from lithium, and though the U.S. stopped doing so in 1998 (choosing instead to recycle tritium from dismantled nuclear weapons) it now plans to resume, exposing lithium to energetic neutrons within a pressurised water reactor to create new supplies of tritium.

The ideal scenario though, is deuterium–deuterium fusion. Experiments within the JET (Joint European Torus) have proved its feasibility, but the 400-500 million °C temperatures needed (rather than the 150-200 million °C temperatures required for deuterium–tritium fusion) will make it difficult to facilitate.

Fusion temperatures are so high that the resulting plasma (an ionised gas) can easily vaporise vessel walls. There are two main containment systems: inertial (ICF) and magnetic confinement (MCF). ICF compresses and heats targets filled with thermonuclear fuel, by depositing energy onto the target’s outer layer (typically by the use of laser beams), which causes an outward explosion. This explosion creates a reaction force against the remainder of the target, that accelerates inwards, compressing the fuel through shock waves and heating it at the same time. The aim is for the fuel to continue fusing after the compression stops, and the world’s biggest operational ICF facility, NIF, has come tantalisingly close, with an experiment that reached 70% efficiency in 2021. This is close to ‘ignition’ (100% and above efficiency), at which the excess energy necessary for fusion to be a viable power option is apparent. MCF, creates magnetic pressure on the outside of a ring of fusion plasma and injects fuel throughout the reaction, keeping the fusion fairly constant. It is the system used in tokamaks, of which there are 250 worldwide, making them the most common fusion reactor design.

Tokamaks also use magnetic fields to heat the plasma to fusion temperatures, and then divert the excess heat created into other parts of the reactor to create turbine-turning steam.

The currents are produced by the transformer, a central solenoid. By reducing the transformer’s size, plasma could be created in a smaller space and higher pressures applied. This has two huge benefits. Firstly, it will majorly increase energy output, as fusion power should increase per square of pressure and secondly, the use of smaller coils diminishes costs, as it is no longer necessary to supercool the tokamak conducting coils.

TAE technologies, the world’s largest private fusion company, has already announced that it will have a commercially viable nuclear fusion reactor by 2030 showing the era of fusion is drawing even closer. JET has succeeded in generating 16MW of fusion power, for 24MW used to heat to plasma, setting a record, and fusion is estimated to one day, provide 10% of the world’s energy needs. When this happens, civilisation will find its houses lit up with the power of stars.

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