BOOK REVIEW
The Stuff of Bombs
Frank N. von Hippel
Plutonium: A History of the World's Most Dangerous Element.
Jeremy Bernstein. xii + 216 pp. Joseph Henry Press, 2007. $27.95.
Our solar system was originally endowed with plutonium as well as
uranium, but that plutonium is long gone. The half-life of the
element's most important isotope, plutonium239, is just 24,000
years—a very long time by any human measure, but short
compared with the age of the solar system. So almost all of the
2,000 metric tons or so of plutonium that exists on Earth today was
made in nuclear reactors; about 250 tons of it was created for use
in weapons, and the rest came into being as a by-product of the
operation of civilian nuclear-power reactors.
After an atom of uranium-238 absorbs a neutron, it decays within a
few days into plutonium-239. The plutonium can then be separated
chemically from the uranium to make bombs. The bomb dropped at
Nagasaki contained 6 kilograms of plutonium, of which 1 kilogram
fissioned. The International Atomic Energy Agency assumes that,
including the amount that would be lost during production, about 8
kilograms would be required to make a bomb. At that rate, 2,000 tons
of plutonium would be sufficient to make a quarter of a million
Nagasaki bombs!
Preventing additional states or terrorist groups from gaining the
ability to use plutonium in this way is the central challenge of
nuclear nonproliferation. Disposing of plutonium is very important
in this regard; it also helps to make arms reductions on the part of
existing nuclear powers largely irreversible.
In his short new book Plutonium, Jeremy Bernstein, a
physicist and veteran science journalist, tells the story of the
discovery of the element and its properties. He also sketches in the
larger background of the development of atomic and nuclear physics
during the first half of the 20th century and includes capsule
biographies of the atomic and nuclear physicists who made the big
discoveries—Henri Becquerel, Ernest Rutherford, Niels Bohr,
Enrico Fermi, Lise Meitner, Leo Szilard, Glenn Seaborg and others.
Their names are already familiar to physicists interested in nuclear
matters, but Bernstein's anecdotes reveal their human sides. He also
brings to life such lesser-known figures as William Zachariasen, who
determined the crystal phases of plutonium and its various compounds.
Plutonium was first made for nuclear bombs during the Manhattan
Project. Until late in the weapons program, the Los Alamos
scientists were convinced that they were in a nuclear-arms race with
their counterparts in Nazi Germany. It turned out, however, that
although the Germans understood the physics, they never got very far
either in making plutonium or in enriching uranium in the
chain-reacting isotope U-235, which is present in natural uranium at
a concentration of only 0.7 percent.
Bernstein raises—not for the first time—some interesting
questions about this one-sided nuclear arms race: What if Fermi had
realized that he was causing uranium fission in his neutron
experiments in 1934, four years before Lise Meitner and her nephew
Otto Frisch used fission to explain the puzzling chemical properties
of the products of German experiments with neutron irradiation of
uranium? Might World War II have been nuclear from the beginning?
Or what if the Nazis had penetrated the Manhattan Project (as the
Soviets did) and learned that it had been a mistake to reject the
use of graphite to slow fission neutrons? Then might Germany too
have built graphite-"moderated" plutonium-production
reactors instead of failing in its effort to acquire enough heavy
water from Norway to make possible a chain reaction in natural uranium?
If the Germans had succeeded in making plutonium, they would still
have had a large obstacle to overcome, however, in the actual design
of a nuclear weapon. Spontaneous fission of the small amount of
plutonium-240 contained in 6 kilograms of weapons-grade plutonium
emits a stream of neutrons at an average rate of one per 10
microseconds. As Bernstein points out, these neutrons make it
infeasible to adapt for plutonium the simple gun-type design that
brought together a supercritical mass of highly enriched uranium in
the Hiroshima bomb. During the hundreds of microseconds that this
assembly would be ramping up to full super-criticality, the neutrons
would start the plutonium chain reaction prematurely and the device
would blow itself apart with a very low explosive yield.
Designing a faster method of assembly for a plutonium explosive
became the central challenge for the weapons designers at Los
Alamos. Ultimately they had to turn to the implausible idea of
imploding an initially subcritical mass of plutonium to a
supercritical density. This was facilitated by the fact that weapons
plutonium is stabilized in the delta phase with a density of about
16 grams per cubic centimeter but is converted under pressure into
alpha phase, which has a density of about 20 grams per cubic centimeter.
Bernstein summarizes what happened after Nagasaki only briefly in a
final chapter, titled "Now What?" This is disappointing,
because "Now what?" is, of course, the question of primary
concern to those who worry about how to deal with all of the
plutonium that has been created.
In the year 2000, after several years of negotiations, Russia and
the United States sought to make their nuclear reductions more
assuredly permanent by agreeing that each would make at least 34
tons of their excess weapons plutonium largely
inaccessible—primarily by making it into fuel for power
reactors. Only a fraction of the plutonium would be fissioned; but
inside spent fuel, plutonium is protected from easy recovery by the
presence of fission products that release lethal gamma radiation.
(Plutonium itself only gives off alpha particles, which are not
energetic enough even to penetrate skin; hence, pure plutonium is
easy to transport and manipulate inside a glove box, which provides
protection against inhalation or ingestion of plutonium particles.)
The costs of the U.S. plutonium-disposition program have been
escalating rapidly, and Russia has made it clear that its own
program will go forward only if fully financed by the United States
and other interested countries. The future of both efforts is
currently in question. An alternative and potentially less costly
approach that has been considered from time to time in the United
States would be to mix the plutonium back into the fission-product
waste from which it had earlier been separated, and then encapsulate
the waste in glass for disposal.
In addition to the 250 tons of weapons plutonium that exists,
another 250 tons of plutonium has been separated from irradiated
civilian reactor fuel. Civilian plutonium separation began in the
1960s and 1970s in support of the huge but failed effort by the
industrialized world to commercialize plutonium "breeder"
reactors, so named because they make more fuel than they consume.
The separated civilian plutonium was to provide start-up fuel for
these reactors, which transform the abundant non-chain-reacting
isotope of uranium (uranium-238) into more plutonium reactor fuel.
Civilian spent-fuel reprocessing was abandoned in the United States,
however, after India conducted a "peaceful" nuclear
explosion in 1974 using the first plutonium it had separated in a
U.S.-supported reprocessing program. Attempts to commercialize
breeder reactors have failed in Europe and Japan, but the French and
Japanese nuclear establishments continue to reprocess and to talk of
a second effort to build breeders some decades hence.
Reprocessing is being abandoned in some countries more slowly than
are breeder reactors, in large part because nuclear utilities are
being pressed to show that they know what to do with their spent
fuel while not-in-my-backyard forces are blocking the establishment
of centralized storage sites. Reprocessing spent fuel is much more
costly than storing it but looks better than letting it accumulate
indefinitely at the nuclear-power plants.
A similar impasse over the licensing of a geological repository for
U.S. spent fuel under Yucca Mountain in Nevada has inspired the Bush
administration's Department of Energy to propose building a huge,
federally funded reprocessing plant to which U.S. utilities could
ship their spent fuel. After separation, however, most of the
plutonium would simply be stored until the uncertain day when
reactors are commercialized that can dispose of it more efficiently
than can current-generation reactors.
But it is absurd that one group in the Department of Energy is
proposing to spend tens of billions of dollars to separate plutonium
from spent civilian reactor fuel while another group in the same
agency is proposing to spend many billions to do the opposite:
dispose of excess separated plutonium in spent civilian reactor fuel!
Bernstein's book should play a useful role by helping to demystify
plutonium and by encouraging interested members of the public and
Congress to start constructing a more rational policy to deal with
the dangers posed by this man-made element.