What is radioactive
Iodine (chemical symbol I) is a
nonmetallic solid element. There are both radioactive and
non-radioactive isotopes of iodine. Iodine-129 and -131 are the most
important radioactive isotopes in the environment. Some isotopes of
iodine, such as I-123 and I-124 are used in medical imaging and
treatment, but are generally not a problem in the environment because
they have very short half-lives.
Where do iodine-129
and iodine-131 come from?
Both iodine-129 and iodine-131 are
produced by the fission of uranium atoms during operation of nuclear
reactors and by plutonium (or uranium) in the detonation of nuclear
What are the
properties of iodine-129 and iodine-131?
Radioactive iodines have the same
physical properties as stable iodine. However, radioactive iodines
decay with time
Iodine is a nonmetallic, purplish-black
crystalline solid. It has the unusual property of ‘sublimation,'
which means that it can go directly from a solid to a gas, without
first becoming liquid. It sublimes to a deep violet vapor at room
temperature. This vapor is irritating to the eyes, nose and throat.
Iodine dissolves in alcohol and in water. It melts at 236 °F.
Iodine reacts easily with other
chemicals, and isotopes of iodine are found as compounds rather than
as a pure elemental nuclide. Thus, iodine-129 and -131 found in
nuclear facilities and waste treatment plants quickly form compounds
with the mixture of chemicals present. However, iodine released to the
environment from nuclear power plants is usually a gas.
Iodine-129 has a half-life of 15.7
million years; iodine-131 has a half-life of about 8 days. Both emit
beta particles upon radioactive decay.
How do iodine-129 and
iodine-131 get into the environment?
Iodine-129 and iodine-131 are gaseous
fission products that form within fuel rods as they fission. Unless
reactor chemistry is carefully controlled, they can build up too fast,
increasing pressure and causing corrosion in the rods. As the rods
age, cracks or wholes may breach the rods.
Cracked rods can release radioactive
iodine into the water that surrounds and cools the fuel rods. There,
it circulates with the cooling water throughout the system, ending up
in the airborne, liquid, and solid wastes from the reactor. From time
to time, reactor gas capture systems release gases, including iodine,
to the environment under applicable regulations.
Anywhere spent nuclear fuel is handled,
there is a chance that iodine-129 and iodine-131 will escape into the
environment. Nuclear fuel reprocessing plants dissolve the spent fuel
rods in strong acids to recover plutonium and other valuable
materials. In the process, they also release iodine-129 and -131 into
the airborne, liquid, and solid waste processing systems. In the U.S.,
spent nuclear fuel is no longer reprocessed, because of concerns about
nuclear weapons proliferation.
Currently, spent nuclear fuel remains
in temporary storage at nuclear power plants around the country. If
the nuclear waste repository at Yucca
Mountain opens, it will provide permanent disposal for spent
nuclear fuel and other high-level radioactive wastes. Wherever spent
nuclear fuel is stored, the short-lived iodine-131 it contains will
decay away quickly and completely. However, the long-lived iodine-129
will remain for millions of years. Keeping it from leaking into the
environment, requires carefully designed, long-term safeguards.
The detonation of nuclear weapons also
releases iodine-129 into the environment. Atmospheric testing in the
1950's and 60's released radioactive iodine to the atmosphere which
has disseminated around the world, and is now found at very low levels
in the environment. Most I-129 in the environment came from weapons
How do iodine-129 and
iodine-131 change in the environment?
Radioactive iodine can disperse rapidly
in air and water, under the right conditions. However, it combines
easily with organic materials in soil. This is known as ‘organic
fixation' and slows iodine's movement in the environment. Some soil
minerals also attach to, or adsorb, iodine, which also slows its
The long half-life of iodine-129, 15.7
million years, means that it remains in the environment. However,
iodine-131's short half-life of 8 days means that it will decay away
completely in the environment in a matter of months. Both decay with
the emission of a beta particle, accompanied by weak gamma radiation.
Iodine-131 is of great interest because it is produced in
relatively large quantities by nuclear fission and has a half-life of
only eight days. This
means any 131I that is detected would be from a recent
release of fission products.