Summary Resources are naturally occurring substances of use to humans. Reserves are known deposits from which materials can be extracted profitably with existing technology under present economic conditions. Coal is the world’s most abundant fossil fuel. The supply of oil, like all fossil fuels, is limited. Natural gas is another major source of fossil-fuel energy, but transport of natural gas to consumers is problematic.

Summary Nuclear fission is the splitting of the nucleus of an atom. All reactors contain a core with fuel, control rods to control the rate of the reaction, and a cooling mechanism to prevent the reactor from overheating.

Summary Although accidents at Three Mile Island and Chernobyl raised safety concerns for a time, rising energy prices have stimulated increased building of nuclear power plants in many countries. Disposal of waste is expensive and controversial. Long-term storage in geologically stable regions is supported. Russia, Japan, and the UK operate nuclear reprocessing facilities to reduce future long-term storage needs.