For hundreds of thousands of years, mankind has released stored energy by burning hydrocarbons, such as wood, natural gas, and oil. Heat is pro­duced, as are chemical compounds such as water and carbon dioxide. This is a simple chemical process, employing the weak forces that hold together the electron structures of atoms. In the case of all energy conver­sion, there is direct matter-to-energy conversion. The products of com­bustion weigh less than the original components, but the effect is so slight it is not measurable. A much more obvious matter-to-energy conversion employs the forces that bind together the atomic nucleus and not just the atom. The nuclear forces are a million times greater than the electron forces, and the energy release is accordingly larger on an atom-by-atom basis.

Anything that will burn in air will give a combustion energy release, but to get a nuclear energy release is not so simple. The Sun and stars release energy by nuclear conversion, but the stellar process is difficult to scale down. One way that nuclear energy can be released with practical effect is to use fissile materials. Only a few special species, or isotopes, of a few elements are fissile, meaning that they have nuclei that will blow in half upon capture of a passing neutron, and further that the process of fission releases multiple neutrons. The neutron is a special, subnuclear particle, and its presence can affect nuclear properties. The products of

a nuclear fission definitely weigh less than the original components, and the effect is measurable. Uranium-235, a rare isotope of uranium, is fissile.

The fission process releases energy at a very efficient level, more than a million times greater per atom than the best chemical process, and it is sustainable. Each fission is initiated by the capture of a free neutron, and each fission releases more than two new neutrons. These neutrons can then cause further fissions in other U-235 nuclei, and there are neutrons left over to waste. If there were only one neutron released for every neu­tron captured, then nuclear fission would never sustain. Out of trillions of fissions per second, if only one neutron were lost then the process would become nonsustaining and shut down. The existence of excess fission neu­trons allows some to be lost by nonproductive capture and leakage from the assembly of uranium.

Energy release by fission uses a plentiful fuel, uranium. Using advanced fuel-breeding technology, there is enough uranium in the Earth’s crust to supply the energy needs of mankind for thousands of years. An important advantage of nuclear energy release is that it results in no greenhouse gases, and the volume of the waste product is millions of times less than the waste product of any technology that involves combustion. An impor­tant disadvantage to nuclear energy release is that the waste products are dangerously radioactive. The elements produced by the splitting of a ura­nium nucleus are unnaturally neutron heavy. They tend to revert to a more natural state, and in doing so they release heat and ionizing radia­tion. These materials must be handled with unusual care and attention.

The special handling of waste and the extraordinary detail with which every aspect of this new energy conversion process must be conducted have taken time to work into the industrial culture. Becoming accustomed to the cost of such a fastidious process has been a challenge, but this is a new century with modified expectations, requirements, and anticipations. As mankind and civilization have matured, so has the concept of energy conversion and its relationship with the biosphere.