Fission and Fusion: Vive La Difference!

The energy of the nucleus can be tapped two ways: by splitting large nuclei into smaller ones (fission) or by combining small nuclei into larger ones (fusion). The first yields what we know as atomic or nuclear (fission) energy, together with its dangers and storage problems. The second gives fusion energy, which is basically solar power, since that is the way the sun and stars generate their energies. Fusion is much safer than fission and requires as fuel only a little bit of water (in the form of D2O instead of H2O, as will soon be clear). Fission is a well-developed technology, while fusion is still being perfected as an energy source. The object of this book is to show how far fusion research has gone, how much further there is to go, and what we will gain when we get there.

Binding Energy

How can we get energy by fusing two nuclei when normally we have to split them? To understand this, we have to remember that atomic nuclei are composed of protons and neutrons, each of which weighs about the same1 but has a different electric charge: +1 for protons and 0 for neutrons. When these nucleons (a general term for protons and neutrons) are assembled into a nucleus, they hold themselves together with a nuclear force measured by the so-called binding energy. The size of this binding energy varies from element to element in the periodic table, as shown in Fig. 4.1. There we see that elements near the middle of the periodic table are more tightly bound than those at either end. At the peak of the curve, with the highest binding energy, is iron. It is labeled as Fe56, 56 being its atomic number, meaning that this is the number of nucleons in its nucleus.

Energy is released when elements are transmuted into other elements which have higher binding energy. Starting with a heavy element like uranium, one has to

Number of nucleons in nucleus

Fig. 4.1 Binding energy vs. atomic number for all elements from hydrogen to uranium (redrawn from Wikipedia. com). The energy units will be explained later split it to get atoms with lower atomic number. If one starts with a light element like hydrogen, one has to fuse two nuclei together to get higher atomic number and move toward the peak of the curve. As labeled, fission goes from right to left, and fusion goes from left to right.

You may wonder why binding energy is increased in both fission and fusion. Would not that require an input of energy rather than yield an output of energy? Yes, it is confusing; but to move forward without such distractions, the explanation is relegated to Box 4.1. Figure 4.1 would make more sense if we turn it upside down and plot binding energy downwards. This is done in Fig. 4.2. There we see that both fission and fusion go downhill, generating energy in the process.