What Is Nuclear Fusion?

Abstract

During the five years I spent at ITER I discovered that people visiting ITER, despite very different origins and backgrounds, have one thing in common: the vast majority of them confuse nuclear fission and nuclear fusion. However, the difference between fusion and fission is indeed fundamental. In modern nuclear fission power plants large atomic nuclei such as uranium or plutonium are split apart releasing large amounts of energy. This energy is stored in the strong bonds that hold the protons and neutrons together in the nucleus; therefore, breaking the nucleus apart releases the energy. In a fusion reactor the opposite process takes place: light atomic nuclei such as hydrogen are heated to several million degrees and will then have enough kinetic energy to overcome their electrostatic repulsion and “fuse” with each other. This releases even larger amounts of energy. Although fusion and fission are fundamentally very different technologies, they are unified under the adjective “nuclear”. To achieve fusion on Earth one must create astronomical temperatures of tens or even hundreds of millions of degrees. For example, the H-bomb (a.k.a. hydrogen bomb or thermonuclear bomb) is actually a double bomb. It contains a primary fission A-bomb (made of uranium or plutonium) that explodes only to compress and heat the gas inside (tritium, deuterium, or lithium deuteride) up to a very high temperature of about 100 million degrees. This triggers hydrogen fusion reactions that constitute the thermonuclear explosion of the bomb. This became clear in the 1950s when scientists realized that fusion holds huge potential for peaceful applications and controlled (nonexplosive) systems. Although the fusion pioneers did not master all the science and technology at that time, it was clear that fusion would be a vastly superior energy source compared with fission. However, these visionary scientists clearly underestimated the many difficulties and technical hurdles they would encounter on the road to fusion that complicated, if not prevented, the road to peaceful application of the technology … This chapter will introduce the principles of nuclear fusion (without, however, let me reassure you right away, turning into a physics handbook) and look into the “tokamak” technology, invented by Russian scientists in the early 1950s, which is currently the most promising to produce fusion energy.