3.2: Nuclear Fusion

Extraordinary circumstances, such as the high temperature and pressure found in the sun and stars, are needed to change one element into another. Nearly all of the elements were originally hydrogen atoms (1 proton) that fused to form atoms with more than one proton (all the other elements).

Figure 3.1: Diagram of Some Atoms and Covalent and Ionic Bonds

The nuclei of atoms must come very close together before they can fuse. Nuclear fusion is extremely difficult because the nuclei are positively charged, and similarly charged particles strongly repel each other.

The extremely high temperature and pressure in the sun and stars cause atoms to move and collide such that their nuclei come close enough to fuse and become the heavier elements. In the sun (about 15 million °C), about 657 million tons of hydrogen atoms (1 proton each) fuse to make about 653 million tons of helium atoms (2 protons each) per second:

The atomic weight of a helium atom is slightly less than that of two hydrogen atoms. The “missing mass” of 4 (657653) million tons is converted into the heat and energy that sustains life on earth. Mass is converted to energy according to Einstein’s E = mc² (E = energy, m = mass, c = the speed of light).

This conversion of mass to energy is what keeps the sun shining, a discovery that led to a 1967 Nobel Prize for Hans Bethe. Yes, the sun will eventually burn itself out, but not for billions of years.

All the heavy elements (carbon, oxygen, calcium, etc.) were—and still are—made by nuclear fusion in stars and in the extreme conditions when stars explode. We are truly made in heaven—every atom in our bodies was once inside a star that exploded (a supernova explosion).

Hydrogen is the most abundant element on earth and a potential source of virtually unlimited energy. The hydrogen bomb, first exploded in 1952, works by uncontrolled fusion of hydrogen atoms. (The atomic bombs dropped over Hiroshima and Nagasaki in 1945 released energy by splitting very heavy atoms—nuclear fission.)

For decades, physicists have been trying to fuse hydrogen atoms in a controlled way to provide a sustained source of energy. They try to create conditions like those in the sun (hot fusion) using contraptions that are huge, complex, and expensive—quite unlike “cold fusion in a jar” (see Chap. 2).

Producing energy in a controlled manner by nuclear fusion would be revolutionary. Unlike the burning of fossil fuels (e.g., coal, gas), we’d have a virtually unlimited supply of energy, without climate-changing consequences. For example, the carbon dioxide produced when fossil fuels are burned contributes to the greenhouse effect* and isn’t produced in fusion. And unlike today’s nuclear fission power plants (which split big atoms like uranium), fusion plants wouldn’t produce radioactive byproducts, or melt-down danger (radiation release from accidental melting of fuel rods).

An international effort (35-nation, $25 billion) to build a fusion reactor to test the viability of fusion as a major source of energy is in progress. The International Thermonuclear Experimental Reactor (iter.org), being built in France, is expected to be completed by 2025.

^{*The warming of the earth’s surface and lower atmosphere due to the trapping of the sun’s heat by more carbon dioxide (CO₂) and other greenhouse gases in the atmosphere. CO₂ made by animal life is normally offset by plant life using CO₂ (Figure 3.2). The balance can be upset by making more CO₂ (e.g., burning of oil, gasoline, wood, coal) or absorbing less CO₂ from the atmosphere (e.g., by destroying forests).}