Fusion

Fusion, and Fission which happens inside nuclear reactors, are both dependent on the stability of the nucleus of atoms – the neutrons and protons in the very middle. An atom is mostly empty space – just like our solar system is mostly empty space. The nucleus is in the middle and electrons whiz around a very long way, in terms of the relative sizes, from the nucleus. The scale of the nucleus is about 10,000 times smaller than the size of space where the electrons are likely to be found whizzing around. Nucleus size is of the order of 10^-15m, the “radius” of the fuzzy space where you would expect to find the electrons is about 5 x 10^-11m.

Chemistry is all to do with the behaviour of the electron cloud around the nucleus and the electrical attraction between the positively charged nucleus and the negatively charged electrons.

Fission and Fusion are dealing with the stability and attractions between the neutrons and protons in the nucleus. Neutrons are neutral and protons are positive electrically, but although you might expect protons to push each other apart electrically, all the protons and neutrons in the nucleus are attracted to each other because of another couple of forces called, imaginatively, the weak nuclear force and the strong nuclear force, which we do not observe in macroscopic terms, so we humans have no inherent experience or feel for those forces, but they are there. Places like CERN do experiments to help characterise these forces.

Looking at the nuclei of atoms, the most stable nucleus is that of iron with 26 protons and 30 neutrons, so that has the lowest energy per proton and neutron in the nucleus. Fusion is trying to push very light nuclei of hydrogen together so hard, or with so much pressure, that two of them fuse to produce a nucleus of a helium atom.  A helium nucleus is closer in weight and structure to Iron than hydrogen, so it has less energy per proton and neutron in its nucleus. Therefore, if you can persuade two hydrogen nuclei to fuse, a staggering amount of energy will be produced.

Experimentally, fusion is usually attempted between two hydrogen atoms, each with an extra neutron or two in its nucleus, to attempt to make a helium nucleus which has two protons and two neutrons. Hydrogen with one extra neutron is called deuterium, Hydrogen with two extra neutrons is called tritium. About 0.02% of sea water is deuterium, so it is rare, but not that rare.

It is fusion happening in the centre of our sun, which produces all the energy and sunlight coming from the sun. Fusion happens in the centres of all visible stars, and all elements bigger than hydrogen and up to iron can be produced from fusion in various sizes of star over the billions of years.  Young relatively light stars, like our sun, fuse hydrogen to helium, heavier and older stars go on to fuse bigger heavier nuclei. Some heavier elements than iron, and up to lead are produced as very old stars collapse in on themselves, gravitationally, creating shock-waves through the star, resulting in more energy-intense fusions than would otherwise occur. Many heavier elements have been produced from super-nova explosions at the end of a star’s life. The Supernova explosion is a rebound which follows a particularly violent gravitational collapse. The elements heavier than lead have been produced when neutron stars collide. Neutron stars are the remnants of old stars that have collapsed in on themselves completely but have not been big enough to become a super-nova, or the last bits of a star left after a super-nova explosion. They are typically about 20km across but up to about double the mass of our sun.  In the billions of years since the beginning of the universe, all the elements we know today have been formed from fusion in many generations of stars. So we are all made from star-dust – but that is another discussion.

The problem with fusion is that huge pressure is required to push the hydrogen or deuterium atoms together hard enough to make them fuse, and if they do fuse the amount of energy produced is so large that it would melt anything that the fusion products touched. It is essential to create a very strong magnetic ‘bubble’ to keep the resulting hot material away from any surfaces. This requires a large amount of energy. The whole system has to be housed in chambers lined with substances with very high melting points, like quartz. So far when fusion has been attempted, the energy “cost” of exerting the pressure and maintaining the magnetic bubble to stabilise the system, has exceeded any energy output.

Fusion has happened in an uncontrolled fashion: the H-bomb of the 1950s was created by instantaneous explosive compression of deuterium. The resulting uncontrolled fusion of the deuterium had massive destructive force.

To put it in context, to initiate natural fusion, a self-gravitating object at least a tenth the size of our sun is required, even Jupiter is not massive enough. So it is perhaps not surprising that we have not yet managed to get self-sustaining and controlled fusion to happen in a lab or reactor on earth.