Register  /  Login      

A small Sun on the Earth: Energy from the Nuclear Fusion – TechNick

By CoperNick

Valerio Pagliarino

 

December 16, 2019

 

1) A MANDATORY STEP TO COMPLETE THE TRANSITION TO RENEWABLE ENERGY

Can you imagine what it means to live in a modern city for a week without electricity? It is hard to figure out, especially if we consider critical users like hospitals, public transports, telecommunication infrastructures etc. The reliability of the modern electrical system is fundamental and stands on two crucial ingredients:

  • Abundant electrical power production;
  • Sources that can provide energy when it is needed.

The latter point strongly discourages the complete transition to renewable energy: even if a large surface of solar panel or a wind turbine array can be sized for considerable output power, we cannot have the control on the constancy of sunlight and wind. Nowadays the problem is mitigated by hydroelectric plants which, pumping back the water, produce electricity storage in the form of mechanical potential energy; but for a future without fossil fuels, there is the demand for new technologies, and the nuclear fusion could be an answer.

 

2) Understanding fusion inside stars…
To understand complex physical phenomena such as nuclear fusion in the stars, general physical principles represent a first level of approximation. So let’s start with two famous laws: 1) the mass-energy equivalence:

 

 

 

    \[ \Delta {mc^2}= \Delta E \]

 (1) 

 

And, even more generally, 2) the trend of every physical system to move toward a lower-energy state (i.e. a more stable state). Nucleons (protons and neutrons) bonded inside atomic nuclei have a different energy compared to free ones or other nuclear species; they tend to follow exothermic transformations, and thereby we can explain – in first approximation – why some light atoms tend to undergo fusion (i.e. to give rise to heavier nuclei, after giving a large amount of energy necessary to break the current equilibrium) and why some heavy nuclei tend to undergo fission (i.e. to split up into lighter nuclei).

 

 

Examining more closely the hydrogen atom, in nature we can find three famous isotopes:

 

  1. Protium (A = 1): only a proton, stable, about 99.985% of the total hydrogen on the Earth
  2. Deuterium (A = 2): a proton and a neutron, stable, is quite easy to obtain by processing a large amount of H2O
  3. Tritium (A = 3): a proton a two neutrons, unstable and radioactive, with a half-life of 12.32 years, decays emitting an electron (and an antineutrino). It is quite rare in nature where it is produced by cosmic rays in the higher layers of the atmosphere from nitrogen, but it can also be synthetized artificially from lithium or deuterium through the following nuclear reactions:

 

In stars the huge quantity of energy, heat and radiation produced is mainly produced by two kinds of nuclear fusion reactions: the proton-proton chain (Fig. 3) and the carbon-oxygen-nitrogen cycle. Both produce 4Helium starting from 1Hydrogen, the most abundant element in the universe.

 

The first process is the main source of energy in stars such as the Sun. It is, however, significantly slowed down in its first stage by the weak interaction that produces the emission of the positron (it is a beta+ decay, such as the one of 18fluorine). The CNO cycle becomes relevant in stars with a mass at least 20% higher than the Sun, or in the red giant phase of the stellar cycle, after the star has depleted the hydrogen fuel in its core.

3) … and bringing it on the Earth

First, why should it be so interesting to artificially reproduce nuclear fusion on the Earth, instead of focusing on other energy sources? The mass-energy equation (1) explains why nuclear plants are so useful: firstly, they employ a fuel with an energy density not comparable with any chemical reaction (e.g. combustion); moreover, in parity of fuel mass, a fusion plant would produce 4-times more energy than a fission one (all currently in use nuclear plants are fission-based). From the safety point of view, a fusion reactor can operate only in strictly controlled physical conditions, otherwise the plasma cools down stopping the reaction: so a Fukushima-type accident cannot happen. Finally, the life of radioactive waste is sensibly lower in respect to a fission plant.

 

As quickly presented in the previous paragraph, there are different kinds of fusion reactions and they can start only after having heated the reactants to an extremely high temperature, which is necessary to leave the current nuclear equilibrium (one of system energy local minima).  It is not trivial to induce a fusion reaction, but it is a real challenge to be able to extract from the plasma an energy amount greater than the initial heating consumption: this is what a collaboration of 35 nations is trying to do in the ITER (International Thermonuclear Experimental Reactor) project in southern France.

 

To minimize the energy required to start the process, we cannot consider reproducing the proton-proton chain or the CNO cycle as inside stars. Instead (please see fig. 5-6), we have to look at the D-T reaction (deuterium-tritium) which has higher probability to occur at lower energy. The plasma will, in any case, reach a temperature of about 150 million degrees.

 

At ITER, a combination of heating systems is used to inject into the reactor sufficient energy to turn on the fusion: this combination consists of neutral beam injection, ion beam injection, ohmic heating (inducing currents inside the plasma with radiofrequency sources). During a normal operation, the energy is produced primarily in the form of high-temperature neutrons: they heat high-pressure water which produces the electricity that moves a steam turbine.

Looking now at the required reactants, as anticipated before, while the deuterium is quite easy to obtain, tritium is present on the Earth only in a very small amount and it is very dangerous to transport from a production plant up to the tokamak (the fusion reactor with the toroidal scheme displayed in Fig. 7).  A way to solve the problem would be to produce the required tritium directly inside the tokamak by means of a blanket – a shielding device on chamber walls that, in addition to slowing down neutrons while heating the water, hosts nuclear reactions for the production of tritium from lithium (as in equation 5).

 

4) Yet, a very ambitious goal!

We should consider that building a fusion reactor means working with a 150-million-degree fused plasma, at a few metres from electrical apparatuses and cryogenic magnets at extremely low temperature: it is an incredibly hard technological challenge. In fact, in the current state, all the engineering effort is to stabilise and confine the plasma inside the tokamak, without it touching the chamber walls. This is possible because charged ions can be bent by powerful magnetic fields (please see the Lorentz force) generated by an array of superconducting coils, as illustrated in Fig. 5.

Despite these important steps forward, to bring fusion technology to an adequate level of maturity, efficiency, and stability, a global effort is required, and the path seems to be yet very long.

I hope that spreading ecological awareness will have among its effects also that of bringing new enthusiasm and resources to scientific research projects towards new energy sources; and fusion seems to be an exciting opportunity to explore.

 

This essay was only an extremely simple and not exhaustive introduction to the topic, if interested in learning more about fusion, please find more detailed information here:

https://www.iter.org/proj/inafewlines

https://www.euro-fusion.org/

Bibliography

 

Picture credits

 

READ MORE ARTICLES AT THIS LINK