January 25, 2000
Scientists Find Plutonium Has Aging Problem
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By WILLIAM J. BROAD
ussian scientists have discovered
that weapons-grade plutonium is far
more unstable in form than previously suspected, a finding that could
have implications for the aging and
reliability of America's arsenal of
10,000 or so nuclear warheads.
American scientists who recently
learned of the Russian discovery say
they did so in the course of an exchange program. But such teamwork has been curtailed by recent
espionage fears, which American
scientists fault as exaggerating the
risks of cooperation and ignoring
benefits like the plutonium insight.
The Russian disclosure has
prompted a number of new American studies of plutonium storage and
aging meant to pinpoint the potential
hazards.
"We're changing our experimental and theoretical approach to take
this information into account," Dr.
Siegfried S. Hecker, a leader in Russian cooperation at the Los Alamos
National Laboratory in New Mexico,
said in an interview. "We're doing
things today that we had not planned
to do one year ago."
Dr. Edward F. Hammel, a plutonium pioneer at Los Alamos, told a
laboratory forum last month that the
Russian insight could shake up the
way nuclear weapons were made,
handled, repaired and stored.
"The implications for the stockpile
are immense," Dr. Hammel said,
adding that the exchange visits with
foreign scientists had produced "tremendous benefits."
Plutonium is one of the most complex and mysterious elements known
to science. Only minute traces of it
exist in nature and most has been
made artificially, first in 1940, so
plutonium studies span little more
than half a century.
The silvery element yellows when
exposed to the air and is warm to the
touch because of energy released
through alpha-particle decay, a kind
of radiation. Its complexity stems in
part from its large number of stable
forms, or allotropes. Carbon has
three -- diamond, graphite and
amorphous. But plutonium exists in
six structural forms, all with differing densities and volumes, that vary
according to temperature. They
range from jumbled crystals typical
of minerals to the highly ordered
ones of metals, the latter allotrope
being the form used in weapons. But
keeping it in the metallic state can be
tricky.
"The ease with which plutonium
transforms itself from one crystallographic phase to another is exasperating," Dr. Hecker and Dr. Joseph C.
Martz, his Los Alamos colleague,
wrote recently. "The resultant
phases have such radically different
properties."
Added to such fickleness, plutonium is highly reactive with other metals and materials, especially oxygen
and hydrogen. That can make it difficult to cast and manipulate.
During the Manhattan Project, scientists at Los Alamos found that
plutonium behaved more reliably if
alloyed with elements like aluminum
and gallium. During cooling, these
helped the element stay in its most
metallic allotrope, known as delta
phase. Such alloys of plutonium behaved like normal metals, being relatively plastic, compressible and resistant to corrosion. Most important
for making weapons, they were easy
to cast and work into special shapes
because they had little or no brittleness.
For decades, the mandarins of
American science assumed that the
delta phase of plutonium alloyed
with gallium was rock stable, a kind
of Gibraltar on which the American
nuclear arsenal could be erected and
expected to weather centuries of
storms and service.
Hints of Russian disagreement
first emerged in 1975. While the
Americans held that the delta phase
of plutonium mixed with gallium was
in a stable equilibrium at room temperature, the Russians found that it
was poised to decompose to the alpha
form, which is more brittle than
ductile. And it is far denser, with a
smaller volume, raising the threat of
ruined mechanical assemblies and
perhaps changes in the critical mass
needed to start a nuclear chain reaction.
"The Russian work was not accepted in the West because insufficient detail was presented about the
precise nature of the experiments,"
Dr. Hecker and Dr. Martz wrote in a
manuscript for a forthcoming book.
But that changed last year at an
international conference in Oxford,
England. The basis for the disclosure, Dr. Hecker said, was years of
exchanges after the cold war between American and Russian nuclear arms designers, with the contacts
becoming increasingly cordial. At
the meeting, Dr. Lydia T. Timofeeva,
a Russian metallurgist, laid out the
experimental basis for the plutonium
insight for the first time, dazzling
American scientists with its care and
precision.
Dr. Matthew G. McKinzie, a former Los Alamos researcher now at
the Natural Resources Defense
Council, a group in Washington that
tracks nuclear arms, said Russia's
stockpiles of plutonium were older
and larger than those of the United
States, aiding such studies.
Plutonium instability, he said,
could sharply cut the lifetime of
weapon cores, in theory reducing
them from perhaps 70 years to as
little as 20 years. Rapid aging, he
added, could also mean the United
States would have to begin remanufacturing its nuclear arms sooner
than expected, increasing the effort's
scope and cost.
"The rate of aging has a tremendous impact on things like the size of
the production facility and its annual
throughput," Dr. McKinzie said.
Scientists at Los Alamos now believe the Russians are right, and are
doing experiments to seek confirmation. Specifically, they are taking delta phase plutonium and raising its
temperature slightly, and then monitoring it closely for signs of decomposition to the alpha form.
"We never would have done this
before," said Dr. Hecker, a former
director of Los Alamos. "In the Russian view, when you increase the
temperature, you increase the speed
at which things may decompose."
"Now," he added, "it's up to us to
apply that to our stockpile."
The repercussions, Dr. Hecker
said, could affect how plutonium for
warheads is made and stored, perhaps changing things like the spacing between plutonium cores. "If you
have such a transformation," he
said, "shape changes, and there's
distortion, and you'd have to account
for that."
"It doesn't mean it makes it more
dangerous," Dr. Hecker added. "But
you have to think about those kind of
things ahead of time."
The cooperative basis for such insights is under fire in Washington
after Congress last year asserted
that that Chinese spies had made off
with top American nuclear secrets.
In November, a federal moratorium went into effect that bars new
hiring of scientists at Los Alamos
from potentially unfriendly countries
like Russia and China. Foreign exchange programs and visits are also
down, said Dr. Hecker, who lamented
the trend.
"We're doing what no foreign nation could to do to us," he rued.
"We're crippling ourselves."
