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Dangerous Water
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Hitler's Sunken Secret homepage
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To Norwegian Resistance fighters during World War II, heavy water was a
mysterious substance considered so perilous that they were willing, under
orders from the Special Operations Executive in London, to sacrifice the lives
of their countrymen in order to keep it out of Nazi hands. Learn more about
heavy water—its discovery, uses, and why it remains a threat
today—by clicking on the images above or simply scrolling
down.—Susan K. Lewis
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What is it?
Heavy water is a form of water with a unique atomic structure and properties
coveted for the production of nuclear power and weapons. Like ordinary
water—H20—each molecule of heavy water contains two
hydrogen atoms and one oxygen atom. The difference, though, lies in the
hydrogen atoms. In ordinary water, each hydrogen atom has just a single proton
in its nucleus. In heavy water, each hydrogen atom is indeed heavier, with a
neutron as well as a proton in its nucleus. This isotope of hydrogen is called
deuterium, and heavy water's more scientific name is deuterium oxide,
abbreviated as D20.
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With the weight of extra neutrons, heavy-water ice will sink to the bottom of a
beaker filled with ordinary H20.
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How is it Used in Nuclear Power Plants?
Nuclear power plants harness the energy of countless atoms of uranium splitting
apart, or fissioning, in a chain reaction. Heavy water can help keep such a
chain reaction going. As each uranium atom breaks apart, it shoots out neutrons
that can go on to split other atoms. But the neutrons are much more likely to
trigger new fission events if they are slowed down. Like traffic cops, heavy
water's deuterium atoms effectively curb the pace of neutrons without capturing
them.
Nuclear reactors that use heavy water can employ a form of uranium commonly
found in nature (U-238) rather than requiring so-called enriched uranium, which
contains a higher percentage of easily split uranium atoms (U-235) but is
expensive to produce.
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Heavy-water nuclear reactors generate electricity in China (above), Canada, and
India. No commercial plants in the U.S. use heavy water.
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Why is it Dangerous?
In its natural state, common uranium (U-238) can't generate destructive nuclear
explosions. It either must be enriched—made more concentrated in a rare
form of uranium (U-235)—or converted into plutonium (Pu-239). Heavy water
can play a role in breeding weapons-grade plutonium from common uranium. In a
heavy-water nuclear reactor, when neutrons bombard U-238, some uranium atoms
absorb an additional neutron and are transformed into Pu-239.
On the eve of World War II, scientists both in Germany and Great Britain
realized that heavy water could be used in this way to make nuclear weapons.
And because this potential still exists today, the International Atomic Energy
Agency and various national governments monitor the production and distribution
of heavy water.
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Heavy water provides a path to turn common uranium into plutonium, one of the
easily split or "fissile" materials that fuels nuclear bombs.
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What Countries Make It?
When Norsk Hydro began producing heavy water in 1934, Norway became the first
country with a commercial heavy-water plant. The Nazi invasion of Norway in
1940 transferred control of the plant—and most of the world's heavy
water—to Germany. In the early 1940s, Allied countries joined the race
for heavy water, and by 1944, the Manhattan Project had made 20 tons of the
precious liquid, more than enough to fill the first heavy-water nuclear
reactor.
America's atomic weapons program ultimately relied more on graphite than on heavy
water in nuclear reactors, but the United States has continued to produce heavy
water for military use ever since the '40s. Today, Canada and India, which both
rely on heavy-water nuclear power plants for electricity, make the most heavy
water. Other countries with heavy-water production facilities include
Argentina, Iran, Romania, and Russia.
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Satellite images taken in February 2005 reveal a heavy-water plant in Arak,
Iran.
Iran claims the facility will help the country produce electricity, not
plutonium for bombs.
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How Was it Discovered?
In 1913, chemists Arthur Lamb and Richard Leen at New York University tried to
find a definitive value for the density of pure water, but despite meticulous
experiments, they kept getting varying results. Their "failure" was, in
retrospect, important evidence for the existence both of heavy water and of
isotopes—atoms of an element that have the same number of protons but a
different number of neutrons and, therefore, different weights. That same year,
independently, the notion of an isotope was proposed for the first time.
By 1931, the existence of isotopes was firmly established, and Harold Urey at
Columbia University, together with colleague George Murphy, first glimpsed
hydrogen's heavier isotope deuterium using a technology called spectroscopy.
They later distilled deuterium from liquid hydrogen, clinching proof of its
existence. Urey won the Nobel Prize in Chemistry in 1934 for discovering
deuterium, the key component of heavy water.
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Gilbert Lewis, a renowned chemist at U.C. Berkeley, isolated the first sample of essentially pure heavy water from ordinary
water in 1933.
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How is it Made?
Heavy water is naturally present in ordinary water, so it's more accurate to
speak of "isolating" rather than "making" it. Separating out significant
quantities, though, is no easy trick because heavy water constitutes only one
part in 4,500. Gilbert Lewis isolated the first samples in 1933 using
electrolysis—sending an electric current through water to separate it
into its elements. His technique relied on the fact that H20 breaks
apart more readily than D20, and the residual water left after
electrolysis is relatively rich in D20. By reprocessing the residual
water over and over again, he could eventually purify heavy water. With his lab
equipment, however, Lewis's process was time-consuming and expensive.
Norsk Hydro, which already used electrolytic cells in the early 1930s to make
fertilizer, seized the chance to make heavy water on an industrial scale. By
1935, the Norwegian company was shipping heavy water to scientists throughout
Europe who wanted it for physics, chemistry, and biomedical research. Today,
heavy water is isolated in a variety of ways, including a distillation method
akin to making brandy from wine. Other methods exploit the different affinities
that deuterium and hydrogen have for various compounds.
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After the Nazis took control of the Norsk Hydro plant in 1940, they expanded the number of electrolytic cells from nine to 18, doubling the plant's production of heavy water.
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