Organization, driving themes: The inherent stability of American nuclear reactors.

1. Nuclear reactors use the energy released when a neutron splits uranium-235 into smaller atoms and a few neutrons: nuclear fission.
2. The fission process is controlled by diluting the density of uranium-235 with non-fissioning uranium-238 that absorbs some of the neutrons.

3. If the reactor temperature were to increase, non-fissioning uranium-238 would absorb more neutrons and decrease the fission process, thus lowering the temperature: a process called fuel-temperature negative feedback.
4. Water lowers the energy of neutrons into an energy range where fissioning uranium-235 is enhanced; the neutrons are said to be moderated.
5. If the reactor temperature were to increase, water would expands and be less efficient at moderating neutrons, thus decreasing the rate of fission: a process called moderator feedback.

Nuclear accidents: Three Mile Island compared to Chernobyl

6. Even a shutdown reactor generates heat from the radioactive decay of the fission products, a heat that, if not removed, could melt the reactor core.
7. The potential danger of a reactor-core meltdown is avoided in two ways: the moderator water is continuously circulated to remove the heat; a steel-reinforced concrete building could contain even a molten reactor core.
8. At Three Mile island, even with repeated operator mistakes, the reactor shuts itself off stopping the fission process; even tho the core melted, all the radioactivity was contained.
9. The non-US Chernobyl incident resulted from a lack of inherent stability -- the use of carbon moderator that provided no feedback -- and no containment building to prevent radioactivity escaping.