The Cast of Characters

The atomic number of an element is the number of protons in the nucleus. Uranium, element 92, has atomic number 92. Fissionable elements all have atomic number 92 or higher.75 The mass number is the total number of protons and neutrons in the nucleus. So uranium 235 has 92 protons and 143 (=235 — 92) neutrons. The atomic weight is a loosely used term which is essentially the mass number but differs by a fraction because protons and neutrons do not weigh exactly the same; they are bound with different energies; and energy and mass are interchangeable, according to Einstein. The symbol for uranium 235 is 92U235, but we shall write it is U235 because the 92 is already specified by “U.” Elements can have the same atomic number but different mass numbers; these are called isotopes. Here are a few isotopes of importance in fission:

U238: The normal isotope of uranium in nature.

U235: The fissionable isotope of uranium, with an abundance of only 0.7% in nature.

P239: Plutonium (element 94) is generated in a reactor and fissions easily.

U239: Uranium 239 decays76 in 23 min.

Np239: Neptunium 239 (element 93) decays in 2.4 days.

Cs137: Cesium 137 (element 55) decays in 30 years.

I131: Iodine 131 (element 53) decays in eight days.

The first group of three contains the isotopes we will be discussing. The next two are intermediate states in the transformation of uranium into plutonium in a reactor. The last two are the most dangerous reaction products when released into the air in an accident. The decay times here are half-lives. Isotopes never com­pletely disappear. Half of what is left goes away in a half-life. Note that only iso­topes with odd mass numbers are fissionable.77 What is not given here is the tremendous amount of energy that nuclei can give. A single-fuel pellet, the size of a AAA battery, can make as much electricity as 6 tons of coal.78