This Following is a factfile on plutonium, which Japan says
has been found at very low levels at five locations at the
stricken Fukushima nuclear plant:
WHAT IS IT?
Plutonium is a highly radioactive silvery metal that is used
in nuclear bombs and nuclear power. It is a heavy and
essentially man-made element, derived from the
transformation of uranium through fission. Traces of two of
its atomic variants, or isotopes, exist naturally.
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HISTORY
Plutonium was found by US scientists in 1940. The new
element was named after Pluto, the god of the Roman
underworld, and assigned the abbreviation of Pu in the
periodic table. It is one of the most complex elements, for
it is a metal yet does not conduct heat or electricity well.
Only a slight rise in temperature makes it switch from solid
state similar to cast iron to plastic malleability.
Plutonium's military potential was swiftly recognised. It
provided the material for the first nuclear device,
detonated at Los Alamos in July 1945, and for the second
nuclear bomb, "Fat Man," which was dropped on Nagasaki the
following month. The first nuclear bomb, dropped on
Hiroshima, used uranium. Not until after World War II were
the discoverers of plutonium allowed to publish their
findings. Plutonium's very high energy density also brought
it into use in civilian nuclear power, especially after the
1970s oil shocks.
ISOTOPES
Plutonium has 15 isotopes. The longest-lived is plutonium
244, which takes 80.8 million years to decay to half its
level of radioactivity. The most commonly-used isotopes --
and those found at Fukushima -- are plutonium 238, with a
half life of 88 years; plutonium 239, with a half life of
24,000 years; and plutonium 240, with a half life of 6,500
years.
AIRBORNE HAZARD
Plutonium 238, 239 and 240 are highly radioactive but their
radiation is in alpha particles, which only travels very
short distances and cannot penetrate human skin. Where they
are highly dangerous is if they are inhaled. Their radiation
causes DNA damage in tissue, which then boosts the risk of
cancer. The bone marrow and liver, where plutonium is
transported through a blood protein called transferring, are
especially vulnerable. Just a dozen milligrams of plutonium
are lethal for a human, according to tests on lab animals
cited by France's Institute for Radiological Protection and
Nuclear Safety (IRSN).
Other dangers, but for which there is less data, come from
exposure to plutonium through ingestion or through an open
wound. Plutonium is only eliminated from the body very
slowly, though excretion. It takes around 50 years for
plutonium to be biologically removed from the skeleton and
about 20 years for it be eliminated from the liver, says the
IRSN.
PLUTONIUM LEVELS AT FUKUSHIMA
According to plant operator Tokyo Electric Power Company (TEPCO),
soil at five sites at the Fukushima plant was found to have
plutonium. At least two of these sites had isotopes where
there was a "high possibility" of a connection to the
accident. But no sample was of a level of contamination that
was hazardous for health, it said. France's Nuclear Safety
Authority (ASN) described the data as being in the same
category as "background levels" that are a legacy of
atmospheric nuclear bomb tests. Around four tonnes of
plutonium were released into the global environment before
atmospheric testing ended.
SOURCE OF FUKUSHIMA PLUTONIUM
This is unclear. Experts at France's Nuclear Safety
Authority (ASN) say it could have come from the No. 3
reactor, which uses mixed oxide, or MOX, which comprises
plutonium and uranium that has been extracted from spent
nuclear fuel and reprocessed. Alternatively, it could have
come as a fissile byproduct from burning uranium in the No.
1 and 2 reactors.
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