The fission of one atom of uranium-235 releases () inside the reactor. That corresponds to 19.54 TJ/
mol, or 83.14 TJ/kg. Another 8.8 MeV escapes the reactor as anti-neutrinos. When nuclei are bombarded with neutrons, one of the many fission reactions that it can undergo is the following (shown in the adjacent image):
Heavy water reactors and some
graphite moderated reactors can use natural uranium, but
light water reactors must use
low enriched uranium because of the higher
neutron absorption of light water.
Uranium enrichment removes some of the uranium-238 and increases the proportion of uranium-235.
Highly enriched uranium (HEU), which contains an even greater proportion of uranium-235, is sometimes used in the reactors of
nuclear submarines,
research reactors and
nuclear weapons. If at least one
neutron from uranium-235 fission strikes another nucleus and causes it to fission, then the chain reaction will continue. If the reaction continues to sustain itself, it is said to be
critical, and the mass of 235U required to produce the critical condition is said to be a critical mass. A critical chain reaction can be achieved at low concentrations of 235U if the neutrons from fission are
moderated to lower their speed, since the probability for fission with
slow neutrons is greater. A fission chain reaction produces intermediate
mass fragments which are highly
radioactive and produce further energy by their
radioactive decay. Some of them produce neutrons, called
delayed neutrons, which contribute to the fission chain reaction. The power output of
nuclear reactors is adjusted by the location of
control rods containing elements that strongly absorb neutrons, e.g.,
boron,
cadmium, or
hafnium, in the reactor core. In
nuclear bombs, the reaction is uncontrolled and the large amount of
energy released creates a
nuclear explosion.
Nuclear weapons The
Little Boy gun-type atomic bomb dropped on Hiroshima on August 6, 1945, was made of highly enriched uranium with a large
tamper. The nominal spherical critical mass for an untampered 235U nuclear weapon is , which would form a sphere in diameter. The material must be 85% or more of 235U and is known as
weapons grade uranium, though for a crude and inefficient weapon 20% enrichment is sufficient (called
weapon(s)-usable). Even lower enrichment can be used, but this results in the required
critical mass rapidly increasing. Use of a large tamper,
implosion geometries, trigger tubes,
polonium triggers,
tritium enhancement, and
neutron reflectors can enable a more compact, economical weapon using one-fourth or less of the nominal critical mass, though this would likely only be possible in a country that already had extensive experience in engineering nuclear weapons. Most modern nuclear weapon designs use
plutonium-239 as the fissile component of the primary stage; however, HEU (highly enriched uranium, in this case uranium that is 20% or more 235U) is frequently used in the secondary stage as an igniter for the fusion fuel. == Decay ==