Around 1900 Sir Arthur Eddington was trying to understand how stars work, what makes them shine. It was understood by that time that the solar system and sun are billions of years old. What process could keep stars shining that long? Eddington thought of nuclear fusion, since when two light nuclei collide they can fuse into a system with much higher binding energy, releasing particles with kinetic energies of several MeV. But the problem is that stars, and in fact the entire universe, consist of 75% H and 25% He (everything else is negligible). So stars form by gravitational collapse of denser clumps in the universe-filling clouds of gas and dust. [This process is of course still going on and can be directly observed.] But two protons cannot be fused (²He does not exist), a proton and ⁴He cannot be fused (⁵B does not exist) and two helium nuclei cannot be fused (⁸Be does not exist). WHAT'S GOING ON?????   Hans Bethe finally figured it out!

Hans Bethe (1906 - 2005), Nobel Prize 1967

What Bethe realized was that if the first step in energy production in stars was nuclear fusion, stars could not exist... as soon as they began to form they would blow apart, in a few seconds! The trick is that, under the conditions of density, temperature and pressure at the center of a star like our sun, there is a 50% chance in about 5 billion years that a proton will weak-decay into a neutron, positron and neutrino. Essentially the instant the neutron appears it fuses with a proton to form ²H, a deuteron. Essentially the instant a deuteron is formed, a long chain of fusions will begin, that will eventually result in a final state of the very tightly bound ⁴He!  Each of these processes begins with 4 protons and ends with 2, so that in 5 billion years half the protons at the center of the star are consumed.  That is why a star like the sun will survive for 10 billion years before beginning its death throes.  [But most of the matter in the universe has never been inside a star, so how does the universe manage to consist of 25% He?  Physicists quickly realized that the entire universe must at some point have been hot and dense enough to make He by fusion, somehow.]

Could fusion be done on earth, to generate electricity somehow? Physicists have wondered about this for 100 years. The problem is that only two light nuclei have a high probability of fusion, ²H and ³H, and (1) fusion will take place, because of the Coulomb barrier, only at temperatures of roughly a billion Kelvin, and (2) ³H is highly radioactive with a short half life and so does not exist in nature. It can be made nucleus by nucleus by neutron capture on ²H, but that requires something like a nuclear reactor, to produce intense floods of neutrons.  To penetrate the Coulomb barrier requires a plasma temperature of about a billion Kelvin, or kinetic energy of 100 keV!

There have been many approaches to the seemingly unattainable goal of practical, power-plant-style fusion, but the main ones are magnetic confinement and inertial confinement. In magnetic confinement a plasma of deuterium and tritium is confined by enormously strong magnetic fields and heated by a powerful electric current. This highly unstable situation would have to be sustained for several seconds to extract more power from the plasma than is delivered to it. In inertial confinement, drops of supercold liquid deuterium and tritium are zapped by a gigantic bank of lasers when they reach the center of a combustion chamber... the high density of the liquid means that the system has to be confined for only a tiny fraction of a second to get out more power than is input. Despite what you see on the news media almost every week, no real progress has been made using either approach.

Nuclear fission was discovered in December 1938 by chemists Otto Hahn and Fritz Strassmann, and quickly figured out by physicists Lise Meitner and Otto Robert Frisch. When Meitner and Frisch realized that this splitting of a heavy nucleus into two roughly equal pieces was induced by capture of a neutron, physicists instantly understood how a power plant could be built using natural uranium (mainly ²³⁸U and its naturally occurring isotope ²³⁵U). They also realized that if it were possible to separate out the ²³⁵U isotope, you could build an incredibly powerful bomb using it. Enrico Fermi in early December 1942 built the first nuclear reactor, underneath the abandoned football stadium on the campus of the University of Chicago! A crash program had also been started the year before, to try to construct a fission bomb... the famous Manhattan Project.  In the early 1950s, nuclear power plants for submarines were designed and built, and when scaled up, were sold as the basis of commercial nuclear electric power plants.