1.1 The Alchemists’ Dream

In the Middle Ages, the alchemists’ dream was to turn lead into gold. The only means of tackling this problem were essentially chemical ones, and these were doomed to failure. During the 19th century, the science of chemistry made enormous advances, and it became clear that lead and gold are different elements that cannot be changed into each other by chemical processes. However, the discovery of radioactivity at the very end of the 19th century led to the realization that sometimes elements do change spontaneously (or transmute) into other elements. Later, scientists discovered how to use high-energy particles, either from radioactive sources or accelerated in the powerful new tools of physics that were developed in the 20th century, to induce artificial nuclear transmutation in a wide range of elements. In particular, it became possible to split atoms (the process known as nuclear fission) or to combine them (the process known as nuclear fusion). The alchemists (Figure 1.1) did not understand that their quest was impossible with the tools they had at their disposal, but in one sense it could be said that they were the first people to search for nuclear transmutation.

Figure 1.1 An alchemist in search of the secret that would change lead into gold. Because alchemists had only chemical processes available, they had no hope of making the nuclear transformation required. From a painting by David Teniers the younger, 1610–1690.

What the alchemists did not realize was that nuclear transmutation was occurring before their very eyes, in the Sun and in all the stars of their night sky. The processes in the Sun and stars, especially the energy source that had sustained their enormous output for eons, had long baffled scientists. Only in the early 20th century was it realized that nuclear fusion is the energy source that runs the universe and that simultaneously it is the mechanism responsible for creating all the different chemical elements around us.

1.2 The Sun’s Energy

The realization that the energy radiated by the Sun and stars is due to nuclear fusion followed three main steps in the development of science. The first was Albert Einstein’s famous deduction in 1905 that mass can be converted into energy. The second step came a little over 10 years later, with Francis Aston’s precision measurements of atomic masses, which showed that the total mass of four hydrogen atoms is slightly larger than the mass of one helium atom. These two key results led Arthur Eddington and others, around 1920, to propose that mass could be turned into energy in the Sun and the stars if four hydrogen atoms combine to form a single helium atom. The only serious problem with this model was that, according to classical physics, the Sun was not hot enough for nuclear fusion to take place. It was only after quantum mechanics was developed in the late 1920s that a complete understanding of the physics of nuclear fusion became possible.

Having answered the question as to where the energy of the universe comes from, physicists started to ask how the different atoms arose. Again fusion was the answer. The fusion of hydrogen to form helium is just the start of a long and complex chain. It was later shown that three helium atoms can combine to form a carbon atom and that all the heavier elements are formed in a series of more and more complicated reactions. Nuclear physicists played a key role in reaching these conclusions. By studying the different nuclear reactions in laboratory accelerators, they were able to deduce the most probable reactions under different conditions. By relating these data to the astrophysicists’ models of the stars, a consistent picture of the life cycles of the stars was built up and the processes that give rise to all the different atoms in the universe were discovered.

1.3 Can We Use Fusion Energy?

When fusion was identified as the energy source of the Sun and the stars, it was natural to ask whether the process of turning mass into energy could be demonstrated on Earth and, if so, whether it could be put to use for man’s benefit. Ernest Rutherford, the famous physicist and discoverer of the structure of the atom, made this infamous statement to the British Association for the Advancement of Science in 1933: “We cannot control atomic energy to an extent that would be of any use commercially, and I believe we are not ever likely to do so.” It was one of the few times when his judgment proved wanting. Not everybody shared Rutherford’s view; H. G. Wells had predicted the use of nuclear energy in a novel published in 1914.1

The possibility of turning nuclear mass into energy became very much more real in 1939, when Otto Hahn and Fritz Strassman demonstrated that the uranium atom could be split by bombarding uranium with neutrons, with the release of a large amount of energy. This was fission. The story of the development of the fission chain reaction, fission reactors, and the atom bomb has been recounted many times. The development of the hydrogen bomb and the quest for fusion energy proved to be more difficult. There is a good reason for this. The uranium atom splits when bombarded with neutrons. Neutrons, so called because they have no electric charge, can easily penetrate the core of a uranium atom, causing it to become unstable and to split. For fusion to occur, two hydrogen atoms have to get so close to each other that their cores can merge; but these cores carry strong electric charges that hold them apart. The atoms have to be hurled together with sufficiently high energy to make them fuse.

1.4 Man-Made Suns

The fusion reaction was well understood by scientists making the first atomic (fission) bomb in the Manhattan Project. However, although the possibility that fusion could be developed as a source of energy was undoubtedly discussed, no practical plans were put forward. Despite the obvious technical difficulties, the idea of exploiting fusion energy in a controlled manner was seriously considered shortly after World War II, and research was started in the UK at Liverpool, Oxford, and London universities. One of the principal proponents was George Thomson, the Nobel Prize-winning physicist and son of J. J. Thomson, the discoverer of the electron. The general approach was to try to heat hydrogen gas to a high temperature so that the colliding atoms have sufficient energy to fuse together. By using a magnetic field to confine the hot fuel, it was thought that it should be possible to allow adequate time for the fusion reactions to occur. Fusion research was taken up in the UK, the US, and the Soviet Union under secret programs in the 1950s and subsequently, after being declassified in 1958, in many of the technically advanced countries of the world. The most promising reaction is that between the two rare forms of hydrogen, called deuterium and tritium. Deuterium is present naturally in water and is therefore readily available. Tritium is not available naturally and has to be produced in situ in the power plant. This can be done by using the products of the fusion reaction to interact with the light metal lithium in a layer surrounding the reaction chamber in a breeding cycle. Thus the basic fuels for nuclear fusion are lithium and water, both readily and widely available. Most of the energy is released as heat that can be extracted and used to make steam and drive turbines, as in any conventional power plant. A schematic diagram of the proposed arrangement is shown in Figure 1.2. The problem of heating and containing the hot fuel with magnetic fields (magnetic-confinement fusion) turned out to be much more difficult than at first envisaged.

Figure 1.2 Schematic diagram of a proposed nuclear fusion power plant. The deuterium and tritium fuel burns at a very high temperature in the central reaction chamber. The energy is released as charged particles, neutrons, X-rays, and ultraviolet radiation and it is absorbed in a lithium blanket surrounding the reaction chamber. The neutrons convert the lithium into tritium fuel. A conventional steam-generating plant is used to convert the nuclear energy to electricity. The waste product from the nuclear reaction is helium.

However, research on the peaceful use of fusion energy was overtaken in a dramatic way with the explosion of the hydrogen bomb in 1952. This stimulated a second approach to controlled fusion, based on the concept of heating the fuel to a sufficiently high temperature very quickly before it has time to escape. The invention of the laser in 1960 provided a possible way to do this; lasers can focus intense bursts of energy onto small targets. The idea is to rapidly heat and compress small fuel pellets or capsules in a series of mini-explosions. This is called inertial confinement because the fusion fuel is confined only by its own inertia. Initially, the expertise was limited to those countries that already had nuclear weapons, and some details still remain secret, although other countries have now taken it up for purely peaceful purposes. Apart from the heating and confinement of the fuel, the method of converting fusion energy into electricity will be very similar to that envisaged for magnetic confinement.

1.5 The Rest of the Story

The considerable scientific and technical difficulties encountered by the magnetic and inertial-confinement approaches have caused these programs to stretch over many years. The quest for fusion has proved to...