By 1934, physicists understood the nucleus to be a cluster of protons and neutrons. They were now focused on researching how this entity would behave under all manner of circumstances. Enrico Fermi, the Italian physicist who first postulated the existence of the neutron, was experimenting in Rome with bombarding uranium nuclei with neutrons. The results were somewhat puzzling to all.
In Germany, Lise Meitner, Otto Hahn and Fritz Strassmann sought to solve the mystery of Fermi’s results and conducted their own neutron bombardment experiments. As Europe continued its descent into darkness, Meitner fled to Sweden but continued to correspond with Hahn so she could make a contribution to his work. In 1938, she received a very interesting letter from Hahn informing her that he and Strassmann had discovered that a product of the neutron bombardment of uranium was barium.
This was as puzzling as Fermi’s original results. Barium has half the mass of uranium and for it to have been produced from uranium would mean the entire nucleus would have had to practically split in half. Frisch was sceptical and suggested Hahn had made a mistake but Meitner had too much faith in Hahn’s skill as a chemist. So at the beginning of 1939, Frisch conducted his own cloud chamber experiments and confirmed that uranium had indeed split as a result of being impacted by a neutron. Meitner and Frisch immediately sent papers to England, interpreting these results as nuclear fission.
Over the next decade, a lot of research across the world was done on nuclear fission giving enough understanding for the creation of the first fission reactors. When a nucleus capable of undergoing induced fission is struck by a neutron it will split into two, sometimes three, fragments, generically called fission products. In the process, it will release two or three high energy neutrons as well as a huge amount of heat.
Since neutrons are the trigger for nuclear fission in fissile nuclei, their production within a nuclear reactor is very important. At start-up, an external neutron source is often brought in to introduce free neutrons into the reactor core. One of these neutrons will strike a fissile nucleus and cause it to fission, releasing two or so more neutrons. These two neutrons will strike two more fissile nuclei. They will fission releasing a combined total of four neutrons. The four go on to cause fission in four more fissile nuclei, releasing a total of eight neutrons and the pattern continues.
A chain reaction is in progress.
As the rate of fission increases, so too does the power level of the reactor. Eventually, the core will approach its target power level and the time will come to level off the rate of fission. This is done by introducing materials into the reactor, such as control rods, which absorb neutrons. The intention is to absorb enough neutrons so that on average, only one neutron produced from a fission event will go on to cause another fission. The rest will either escape from the core to be absorbed in the biological shield or be absorbed in either the control rods or some other reactor component.
When the reactor is at a steady power, it is described as critical. One neutron from every fission event causes another fission event. The various mechanisms at work in the core, both active and passive, will seek to maintain this power level. If the power level decreases due to too much neutron absorption, the reactor becomes sub-critical, the system will respond by decreasing absorption to return the core to critical. If the power level increases due to not enough neutron absorption, the reactor becomes super-critical, the system will similarly respond by increasing absorption.
The yellow stuff
By far the most common fuel today in fission reactors is the actinide metal, uranium. Uranium is a fairly ubiquitous metal, found across the world in one form or another. The highest grade ores are found in Australia, Kazakhstan and Canada. Almost all natural uranium is composed of 99.3% uranium-238 and 0.7% uranium-235. Uranium-235 is described as fissile because it can be made to undergo fission with neutrons of any energy. Uranium-238 is described as fissionable because it can be made to undergo fission only with neutrons of high energy.
Most working fission reactors depend on slow moving neutrons, known as thermal neutrons, to cause fission in the uranium-235. However, the neutrons released from fission always have a lot of energy, so-called fast neutrons. In order to slow down to thermal energies, thermal reactors include a component called a moderator, which essentially is a material in which fast neutrons literally bounce between its nuclei, losing energy. Light water is the most common form of moderator, not least of which because it is cheap. Graphite and heavy water also seen and although they are expensive, they are actually much more efficient moderators.
Reactors moderated by light water cannot physically be made critical if natural uranium was used. There is simply not enough fissile material. So these reactors depend on a process called enrichment. Enrichment aims to increases proportion of uranium-235 in uranium by removing excess uranium-238 (the excess, once removed, becomes known as depleted uranium). An average light water reactor uses 3-4% enriched uranium, meaning that it is composed of 3-4% uranium-235 rather than the natural 0.7%.
Fast vs slow
Fast reactors have their advantages. A nucleus undergoing fission from a fast neutron would tend to release more neutrons than from a thermal neutron. This means that there is a greater excess of neutrons in a fast reactor, which can be used for in a process called breeding. Breeding means a non-fissile nucleus changing into a fissile one through neutron absorption. The most common example is uranium-238, a non-fissile isotope, absorbing a neutron to become uranium-239, which decays by beta emission to neptunium-239 and then again to plutonium-239, which is fissile.
This kind of transmutation happens in any reactor containing uranium-238 (which is why about 1% of spent fuel from a light water reactor is plutonium), but in a properly configured fast reactor, plutonium-239 can be bred faster than the original fissile material is consumed. This means that it is possible for more fissile fuel to emerge from the reactor than went in. A reactor so configured is called a fast breeder reactor.
However, thermal reactors, despite obviously having poorer fuel utilisation, have the benefit of being easier to build and maintain. They can make use of cheaper and more manageable materials such as water rather than having to be engineered to carry molten metals or salts. These engineering factors and the cheap price of uranium means that thermal reactors have dominated the nuclear scene since its inception and are expected to do so in the coming several decades.