What is Nuclear Fusion? And How Does it Work?

Aligning with recent breakthroughs in the field, nuclear fusion is being increasingly discussed in the media. But what is nuclear fusion and how does it differ from nuclear fission? And why are so many companies and states investing in fusion research and development? 

We answer these questions here before highlighting the enabling role that vacuum plays in achieving nuclear fusion.

Currently, nuclear power plants produce energy by exploiting the fission process. Nuclear fission involves taking a large, unstable atom such as uranium isotope U-235, and splitting it into two lighter atoms. This process releases a tremendous amount of useable energy. 

Fission is initiated by bombarding U-235 with neutrons. As these neutrons smash into U-235 atoms, they force additional neutrons to release which will interact with more U-235 and create a chain reaction. This reaction can also produce electromagnetic radiation. To maximize energy production, the number of neutrons used in fission must be controlled to prevent the chain reaction from dying or running away.

Fission reactors function in the same way as conventional power plants. Fission occurs in a reactor vessel, moderated by control rods. The heat from the reaction is used to heat water and produce steam. This steam then turns a turbine to produce electricity. Additional steps are required to safely dispose of spent fuel as they are radioactive and can have a half-life of over 10,000 years. In many cases, decommissioning costs are up to 15% of the initial capital cost.

To create fuel for nuclear fission, enrichment is first required to increase uranium-235 concentration from the 0.7% abundance found naturally in uranium ore to the 4-5% needed for the fission process. This is done using 1,000-2,000 centrifuge stages. To do this, uranium ore is converted to UF6 gas, processed, and then converted into fuel pellets after the enrichment process. The centrifugal process is carefully controlled under specific vacuum parameters.

Around 440 nuclear power reactors operate in 33 countries. These reactors produce roughly 10% of the world’s electricity. Additionally, about 55 nuclear power reactors are under construction. Uranium ore is sourced in 10 countries and has an energy density of 70,000 times that of fossil fuels. The cost of construction of a nuclear power plant can be between $2 and $20 billion, though the development of lower-cost, smaller units is feasible.

Fusion reactions of light elements power the stars and produce virtually all elements in a process called nucleosynthesis.

Nuclear fusion is a developing technology that sits at the forefront of current scientific research. With the potential to revolutionize energy production, fusion is the focus of a significant number of national and international projects. 

In the nuclear fusion process, two light atoms are ‘fused’ together to create a larger atom. In this way, nuclear fusion is the opposite of fission: instead of splitting atoms, atoms combine to form a larger one. This results in the release of energy. 

Fusion combines hydrogen isotopes such as deuterium and tritium. Deuterium, also called heavy hydrogen, is similar to a ‘regular’ hydrogen isotope like protium, but deuterium contains an additional neutron. For comparison, tritium is a hydrogen isotope containing two extra neutrons. 

Ideal fusion reactions use a 50:50 mix of deuterium and tritium (reaction 3 in the illustration above) with only a few grams needed at any time. However, this ideal has been difficult to achieve. Currently, all but one fusion device operates using only deuterium (reactions 1 and 2). This is because tritium is rare in nature and must be produced artificially. 

Light atoms can only fuse if they are ‚squeezed‘ together for a long enough time. At nucleus radii distances, the attractive nuclear force is stronger than the repulsive electrostatic force. Based on these constraints, the main technical difficulty for fusion is getting the nuclei close enough to fuse. This is why a form of either magnetic or inertial confinement is needed. 

Massive temperatures are needed for both types of confinement: over 4x10⁷ K for deuterium–tritium (DT) fusion. That’s 10 x hotter than the Sun!

Many different approaches that have been proposed to achieve fusion. However, there are only two main kinds of confinement. 

Magnetic confinement fusion (MCF) is arguably the most encouraging for commercial energy generation. It uses a series of very powerful magnets, with liquid helium being utilised to cool the magnets and improve device performance.

The most common magnetic confinement devices types are the Stellarator and Tokamak, though some facilities have experimented with devices that use magnetic mirrors. All three of these device types consist of a central vacuum vessel, where the plasma is contained and fusion reaction occurs. The vacuum vessel is surrounded by a cryostat, where the superconducting magnets are housed and cooled. The Stellarator and Tokamak have a toroidal shape, whereas the mirror machine has a cylindrical shape. The magnetic field continually forces the atoms together and the high temperature causes ignition. 

The second common confinement type is inertial confinement fusion (ICF). This technique extracts fusion energy by bombarding small pellets (1 to 3 mm) of DT fuel with lasers or charge particle beams to compress and heat the mixture to ignition. Here, ignition refers to the condition that all alpha particles are stopped, which causes the plasma to heat.

The compression and heating is done very rapidly with short pulse (1 to 10 ns) high power lasers, X-rays or ion beams which intersect the fuel from many directions. Direct-drive devices deliver compressive energy pulses to the fuel pellet directly. Indirect-drive ICF techniques have the fuel pellet placed inside a small metal vessel (hohlraum/burning chamber). Lasers are then focused on the hohlraum, heating it into a plasma which radiates X-rays that, in turn, are absorbed by the pellet. The pellet then compresses in the same way as ‘direct-drive’. Compared with direct-drive devices, the indirect-drive approach is significantly less efficient. But as thermonuclear bombs use a similar technique, the indirect-drive technique is used in nuclear weapon simulations. 

To create electricity from ICF, fuel pellets are dropped into a vacuum reaction chamber where they are ignited. The radiation from the plasma and fusion neutrons is absorbed by liquid lithium. Heat produced is used in conventional steam generation and tritium is produced to be recycled as new fuel.

Fusion has the potential to produce massive amounts of energy from only a very small amount of fuel — only around 250kg a year in a commercial reactor — once sustained ignition is achieved. 

The main advantages of fusion are the abundance of fuel compared with hydrocarbon sources, the lack of harmful by-products such as greenhouse gases, and its intrinsic safety compared with nuclear fission. 

Compared with fission, nuclear fusion has additional advantages: 

• While the fusion reaction releases neutrons that are dangerous to humans, these neutrons are contained and the production of neutrons ceases almost as soon as the plant is turned off.  
  
• Fusion is inherently safer as there are no chain reactions, run-aways, or melt-downs.
  
• Fusion results in extreme temperatures but if any plasma were to escape, then the heat would quickly dissipate without causing harm.
  
• Neutrons can be indirectly ionizing but are easily stopped.
  
• While nuclear fusion does produce radioactive waste, this radioactivity is short-lived — especially when compared with fission-produced waste. This includes the neutron bombardment vessel.  
  
• Waste produced through fusion does not need to be transported for disposal, storage or reprocessing.
  
• Tritium decays via very weak beta particles, so the isotope is not an external radiation hazard. However, tritium can pose an internal radiation hazard if tritiated water vapour is inhaled or absorbed through the skin. At the same time, tritium is less strongly absorbed by the body, whether through the lungs or the skin.

The vacuum requirement in fusion reactors depends on the type of confinement and the size of the reactor.

The largest need for vacuum in magnetic confinement is in the central vacuum chamber and surrounding cryostatic chamber. This can be over 100 m³ which must be under UHV (around 10^-9 mbar) to prevent contamination of the plasma. 

Vacuum is also needed for the production of tritium, pumping helium and fuel recirculation. In the case of inertial confinement fusion, all the laser or particle beam lines must be evacuated along with the reaction chamber. As with magnetic confinement, this can be a very large volume requiring UHV to prevent losses and aberration in the beams. Turbomolecular and cryogenic pumps are the most commonly used and they must be able to cope with the localized peak in radiation and high magnetic fields. Similar concerns apply to primary pumps sets, gauges and valves.

1. Evacuating reaction vessel itself to UHV: needed to remove larger gas species and prevent plasma contamination.
   
2. Cryogenics (cryostat): the cooling system itself requires vacuum.
   
3. Tritium production: tritium is most commonly produced by colliding neutrons with lithium which requires vacuum and pumping for coolant gas. Neutrons are also produced from fission reactions or linear accelerators which both require vacuum to operate.
   
4. Plasma heating: vacuum is needed in the neutral beam injection system for heating the plasma
   
5. Vacuum testing: equipment is tested under a vacuum and using tools such as outgassing rigs and leak detectors.
   
6. Pressure gauges and instrumentation: for monitoring equipment and process performance
   
7. Cooling: pumping is needed for helium used to cool superconductors
   
8. Recirculation: unused fuel and helium is pumped from the chamber, separated and recirculated
   
9. Fusion fuel: powerful pumps are needed to inject the fusion fuels into the chamber

1. Evacuating target chamber to UHV: needed to remove larger gas species and prevent plasma contamination
   
2. Target positioning systems: systems used to target the lasers on the fuel require vacuum
   
3. Beamline vacuum: several beamlines are used for the lasers and these are all required to be under vacuum
   
4. Spatial filter: spatial filters are used to ‘clean up’ the laser beam and function under vacuum
   
5. Pressure gauges and instrumentation: for monitoring equipment and process performance

One of the key requirements for vacuum pumps is the ability to handle tritium. This means that metal seals are preferred and pumps should be hydrocarbon free.

In the case of magnetic confinement, the pumps must also be able to operate under high magnetic fields and shielding is often required. Space around the reaction chambers is very limited and shutdowns are extremely expensive, so pumps must have a small footprint and be highly reliable with minimal service requirements.