When a relatively large fissile atomic nucleus (usually uranium-235or plutonium-239)
absorbs a neutron,
a fission of the atom often results. Fission splits the atom into two or more
smaller nuclei with kinetic
energy (known as fission
products) and also releases gamma
radiation and free neutrons. A portion of these neutrons may later be absorbed by other fissile atoms and
create more fissions, which release more neutrons, and so on.
This nuclear chain reaction can be controlled
by using neutron poisons and neutron moderators to change the portion of
neutrons that will go on to cause more fissions. Nuclear reactors generally have automatic and manual systems to shut the
fission reaction down if unsafe conditions are detected.
Three nuclear powered ships, (top to bottom)
nuclear cruisers USS Bainbridge and USS Long Beach with USS Enterprise the first nuclear powered
aircraft carrier in 1964. Crew members are spelling out Einstein's
mass-energy equivalence formula E = mc2
on the flight deck.
There are many different reactor designs, utilizing different fuels and
coolants and incorporating different control schemes. Some of these designs
have been engineered to meet a specific need. Reactors for nuclear
submarines and large naval ships, for example, commonly use highly enriched uranium as a fuel. This
fuel choice increases the reactor's power density and extends the usable life
of the nuclear fuel load, but is more expensive and a greater risk to nuclear
proliferation than some of the other nuclear fuels.
A number of new designs
for nuclear power generation, collectively known as the Generation IV reactors, are the subject of
active research and may be used for practical power generation in the future.
Many of these new designs specifically attempt to make fission reactors
cleaner, safer and/or less of a risk to the proliferation of nuclear weapons. Passively safe plants (such as the ESBWR) are available to be
built and other designs that are believed to be nearly fool-proof are being
pursued. Fusion reactors, which may be viable in the
future, diminish or eliminate many of the risks associated with nuclear
fission. There are trades to be made between safety, economic and technical
properties of different reactor designs for particular applications.
Historically these decisions were often made in private by scientists,
regulators and engineers, but this may be considered problematic, and since
Chernobyl and Three Mile Island, many involved now consider informed consent
and morality should be primary considerations.
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