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The basic know-how of atom-bomb-building is half a century old, and adequate
recipes have cropped up in physics term papers and high school science
projects. The simplest design entails taking a lump of highly enriched
uranium, about the size of a cantaloupe, and firing it down a big gun barrel
into a second lump. Theodore Taylor, the nuclear physicist who designed both
the smallest and the largest American nuclear-fission warheads before
becoming a remorseful opponent of all things nuclear, told me he recently
looked up ''atomic bomb'' in the World Book Encyclopedia in the upstate New
York nursing home where he now lives, and he found enough basic information
to get a careful reader started. ''It's accessible all over the place,'' he
said. ''I don't mean just the basic principles. The sizes, specifications,
things that work.''

Most of the people who talk about the ease of assembling a nuclear weapon, of
course, have never actually built one. The most authoritative assessment I
found was a paper, ''Can Terrorists Build Nuclear Weapons?'' written in 1986
by five experienced nuke-makers from the Los Alamos weapons laboratory. I was
relieved to learn that fabricating a nuclear weapon is not something a lone
madman -- even a lone genius -- is likely to pull off in his hobby room. The
paper explained that it would require a team with knowledge of ''the
physical, chemical and metallurgical properties of the various materials to
be used, as well as characteristics affecting their fabrication; neutronic
properties; radiation effects, both nuclear and biological; technology
concerning high explosives and/or chemical propellants; some hydrodynamics;
electrical circuitry; and others.'' Many of these skills are more difficult
to acquire than, say, the ability to aim a jumbo jet.

The schemers would also need specialized equipment to form the uranium, which
is usually in powdered form, into metal, to cast it and machine it to fit the
device. That effort would entail months of preparation, increasing the risk
of detection, and it would require elaborate safeguards to prevent a mishap
that, as the paper dryly put it, would ''bring the operation to a close.''

Still, the experts concluded, the answer to the question posed in the title,
while qualified, was ''Yes, they can.''

David Albright, who worked as a United Nations weapons inspector in Iraq,
says Saddam Hussein's unsuccessful crash program to build a nuclear weapon in
1990 illustrates how a single bad decision can mean a huge setback. Iraq had
extracted highly enriched uranium from research-reactor fuel and had, maybe,
barely enough for a bomb. But the manager in charge of casting the metal was
so afraid the stuff would spill or get contaminated that he decided to melt
it in tiny batches. As a result, so much of the uranium was wasted that he
ended up with too little for a bomb.

''You need good managers and organizational people to put the elements
together,'' Albright said. ''If you do a straight-line extrapolation,
terrorists will all get nuclear weapons. But they make mistakes.''

On the other hand, many experts underestimate the prospect of a
do-it-yourself bomb because they are thinking too professionally. All of our
experience with these weapons is that the people who make them (states, in
other words) want them to be safe, reliable, predictable and efficient.
Weapons for the American arsenal are designed to survive a trip around the
globe in a missile, to be accident-proof, to produce a precisely specified
blast.

But there are many corners you can cut if you are content with a big, ugly,
inefficient device that would make a spectacular impression. If your bomb
doesn't need to fit in a suitcase (and why should it?) or to endure the
stress of a missile launch; if you don't care whether the explosive power
realizes its full potential; if you're willing to accept some risk that the
thing might go off at the wrong time or might not go off at all, then the job
of building it is immeasurably simplified.

''As you get smarter, you realize you can get by with less,'' Albright said.
''You can do it in facilities that look like barns, garages, with simple
machine tools. You can do it with 10 to 15 people, not all Ph.D.'s, but some
engineers, technicians. Our judgment is that a gun-type device is well within
the capability of a terrorist organization.''

All the technological challenges are greatly simplified if terrorists are in
league with a country -- a place with an infrastructure. A state is much
better suited to hire expertise (like dispirited scientists from
decommissioned nuclear installations in the old Soviet Union) or to send its
own scientists for M.I.T. degrees.

Thus Tom Cochran said his greatest fear is what you might call a bespoke nuke
-- terrorists stealing a quantity of weapons-grade uranium and taking it to
Iraq or Iran or Libya, letting the scientists and engineers there fashion it
into an elementary weapon and then taking it away for a delivery that would
have no return address.

That leaves one big obstacle to the terrorist nuke-maker: the fissile
material itself.

To be reasonably sure of a nuclear explosion, allowing for some material
being lost in the manufacturing process, you need roughly 50 kilograms -- 110
pounds -- of highly enriched uranium. (For a weapon, more than 90 percent of
the material should consist of the very unstable uranium-235 isotope.) Tom
Cochran, the master of visual aids, has 15 pounds of depleted uranium that he
keeps in a Coke can; an eight-pack would be plenty to build a bomb.

The world is awash in the stuff. Frank von Hippel, a Princeton physicist and
arms-control advocate, has calculated that between 1,300 and 2,100 metric
tons of weapons-grade uranium exists -- at the low end, enough for 26,000
rough-hewed bombs. The largest stockpile is in Russia, which Senator Joseph
Biden calls ''the candy store of candy stores.''

Until a decade ago, Russian officials say, no one worried much about the
safety of this material. Viktor Mikhailov, who ran the atomic energy ministry
and now presides over an affiliated research institute, concedes there were
glaring lapses.

''The safety of nuclear materials was always on our minds, but the focus was
on intruders,'' he said. ''The system had never taken account of the
possibility that these carefully screened people in the nuclear sphere could
themselves represent a danger. The system was not designed to prevent a
danger from within.''

Then came the collapse of the Soviet Union and, in the early 90's, a few
frightening cases of nuclear materials popping up on the black market.

If you add up all the reported attempts to sell highly enriched uranium or
plutonium, even including those that have the scent of security-agency hype
and those where the material was of uncertain quality, the total amount of
material still falls short of what a bomb-maker would need to construct a
single explosive.

But Yuri G. Volodin, the chief of safeguards at Gosatomnadzor, the Russian
nuclear regulatory agency, told me his inspectors still discover one or two
instances of attempted theft a year, along with dozens of violations of the
regulations for storing and securing nuclear material. And as he readily
concedes: ''These are the detected cases. We can't talk about the cases we
don't know.'' Alexander Pikayev, a former aide to the Defense Committee of
the Russian Duma, said: ''The vast majority of installations now have fences.
But you know Russians. If you walk along the perimeter, you can see a hole in
the fence, because the employees want to come and go freely.''

The bulk of American investment in nuclear safety goes to lock the stuff up
at the source. That is clearly the right priority. Other programs are devoted
to blending down the highly enriched uranium to a diluted product unsuitable
for weapons but good as reactor fuel. The Nuclear Threat Initiative, financed
by Ted Turner and led by Nunn, is studying ways to double the rate of this
diluting process.

Still, after 10 years of American subsidies, only 41 percent of Russia's
weapon-usable material has been secured, according to the United States
Department of Energy. Russian officials said they can't even be sure how much
exists, in part because the managers of nuclear facilities, like everyone
else in the Soviet industrial complex, learned to cook their books. So the
barn door is still pretty seriously ajar. We don't know whether any horses
have gotten out.

And it is not the only barn. William C. Potter, director of the Center for
Nonproliferation Studies at the Monterey Institute of International Studies
and an expert in nuclear security in the former Soviet states, said the
American focus on Russia has neglected other locations that could be tempting
targets for a terrorist seeking bomb-making material. There is, for example,
a bomb's worth of weapons-grade uranium at a site in Belarus, a country with
an erratic president and an anti-American orientation. There is enough
weapons-grade uranium for a bomb or two in Kharkiv, in Ukraine. Outside of
Belgrade, in a research reactor at Vinca, sits sufficient material for a bomb
-- and there it sat while NATO was bombarding the area.

''We need to avoid the notion that because the most material is in Russia,
that's where we should direct all of our effort,'' Potter said. ''It's like
assuming the bank robber will target Fort Knox because that's where the most
gold is. The bank robber goes where the gold is most accessible.''

Weapons of Mass Disruption The first and, so far, only consummated act of
nuclear terrorism took place in Moscow in 1995, and it was scarcely
memorable. Chechen rebels obtained a canister of cesium, possibly from a
hospital they had commandeered a few months before. They hid it in a Moscow
park famed for its weekend flea market and called the press. No one was hurt.
Authorities treated the incident discreetly, and a surge of panic quickly
passed.

The story came up in virtually every conversation I had in Russia about
nuclear terror, usually to illustrate that even without splitting atoms and
making mushroom clouds a terrorist could use radioactivity -- and the fear of
it -- as a potent weapon.

The idea that you could make a fantastic weapon out of radioactive material
without actually producing a nuclear bang has been around since the infancy
of nuclear weaponry. During World War II, American scientists in the
Manhattan Project worried that the Germans would rain radioactive material on
our troops storming the beaches on D-Day. Robert S. Norris, the biographer of
the Manhattan Project director, Gen. Leslie R. Groves, told me that the
United States took this threat seriously enough to outfit some of the D-Day
soldiers with Geiger counters.

No country today includes radiological weapons in its armories. But
radiation's limitations as a military tool -- its tendency to drift afield
with unplanned consequences, its long-term rather than short-term lethality
-- would not necessarily count against it in the mind of a terrorist. If your
aim is to instill fear, radiation is anthrax-plus. And unlike the fabrication
of a nuclear explosive, this is terror within the means of a soloist.

That is why, if you polled the universe of people paid to worry about weapons
of mass destruction (W.M.D., in the jargon), you would find a general
agreement that this is probably the first thing we'll see. ''If there is a
W.M.D. attack in the next year, it's likely to be a radiological attack,''
said Rose Gottemoeller, who handled Russian nuclear safety in the Clinton
administration and now follows the subject for the Carnegie Endowment. The
radioactive heart of a dirty bomb could be spent fuel from a nuclear reactor
or isotopes separated out in the process of refining nuclear fuel. These
materials are many times more abundant and much, much less protected than the
high-grade stuff suitable for bombs. Since Sept.11, Russian officials have
begun lobbying hard to expand the program of American aid to include
protection of these lower-grade materials, and the Bush administration has
earmarked a few million dollars to study the problem. But the fact is that
radioactive material suitable for terrorist attacks is so widely available
that there is little hope of controlling it all.

The guts of a dirty bomb could be cobalt-60, which is readily available in
hospitals for use in radiation therapy and in food processing to kill the
bacteria in fruits and vegetables. It could be cesium-137, commonly used in
medical gauges and radiotherapy machines. It could be americium, an isotope
that behaves a lot like plutonium and is used in smoke detectors and in oil
prospecting. It could be plutonium, which exists in many research
laboratories in America. If you trust the security of those American labs,
pause and reflect that the investigation into the great anthrax scare seems
to be focused on disaffected American scientists.

Back in 1974, Theodore Taylor and Mason Willrich, in a book on the dangers of
nuclear theft, examined things a terrorist might do if he got his hands on
100 grams of plutonium -- a thimble-size amount. They calculated that a
killer who dissolved it, made an aerosol and introduced it into the
ventilation system of an office building could deliver a lethal dose to the
entire floor area of a large skyscraper. But plutonium dispersed outdoors in
the open air, they estimated, would be far less effective. It would blow away
in a gentle wind.

The Federation of American Scientists recently mapped out for a Congressional
hearing the consequences of various homemade dirty bombs detonated in New
York or Washington. For example, a bomb made with a single footlong pencil of
cobalt from a food irradiation plant and just 10 pounds of TNT and detonated
at Union Square in a light wind would send a plume of radiation drifting
across three states. Much of Manhattan would be as contaminated as the
permanently closed area around the Chernobyl nuclear plant. Anyone living in
Manhattan would have at least a 1-in-100 chance of dying from cancer caused
by the radiation. An area reaching deep into the Hudson Valley would, under
current Environmental Protection Agency standards, have to be decontaminated
or destroyed.

Frank von Hippel, the Princeton physicist, has reviewed the data, and he
pointed out that this is a bit less alarming than it sounds. ''Your
probability of dying of cancer in your lifetime is already about 20
percent,'' he said. ''This would increase it to 20.1 percent. Would you
abandon a city for that? I doubt it.''

Indeed, some large portion of our fear of radiation is irrational. And yet
the fact that it's all in your mind is little consolation if it's also in the
minds of a large, panicky population. If the actual effect of a radiation
bomb is that people clog the bridges out of town, swarm the hospitals and
refuse to return to live and work in a contaminated place, then the impact is
a good deal more than psychological. To this day, there is bitter debate
about the actual health toll from the Chernobyl nuclear accident. There are
researchers who claim that the people who evacuated are actually in worse
health over all from the trauma of relocation, than those who stayed put and
marinated in the residual radiation. But the fact is, large swaths of
developed land around the Chernobyl site still lie abandoned, much of it
bulldozed down to the subsoil. The Hart Senate Office Building was closed for
three months by what was, in hindsight, our society's inclination to err on
the side of alarm.

There are measures the government can take to diminish the dangers of a
radiological weapon, and many of them are getting more serious consideration.
The Bush administration has taken a lively new interest in
radiation-detection devices that might catch dirty-bomb materials in transit.
A White House official told me the administration's judgment is that
protecting the raw materials of radiological terror is worth doing, but not
at the expense of more catastrophic threats.

''It's all over,'' he said. ''It's not a winning proposition to say you can
just lock all that up. And then, a bomb is pretty darn easy to make. You
don't have to be a rocket scientist to figure about fertilizer and diesel
fuel.'' A big fertilizer bomb of the type Timothy McVeigh used to kill 168
people in Oklahoma City, spiced with a dose of cobalt or cesium, would not
tax the skills of a determined terrorist.

''It's likely to happen, I think, in our lifetime,'' the official said. ''And
it'll be like Oklahoma City plus the Hart Office Building. Which is real bad,
but it ain't the World Trade Center.''

The Peril of Power Plants Every eight years or so the security guards at each
of the country's 103 nuclear power stations and at national weapons labs can
expect to be attacked by federal agents armed with laser-tag rifles. These
mock terror exercises are played according to elaborate rules, called the
''design basis threat,'' that in the view of skeptics favor the defense. The
attack teams can include no more than three commandos. The largest vehicle
they are permitted is an S.U.V. They are allowed to have an accomplice inside
the plant, but only one. They are not allowed to improvise. (The mock
assailants at one Department of Energy lab were ruled out of order because
they commandeered a wheelbarrow to cart off a load of dummy plutonium.) The
mock attacks are actually announced in advance. Even playing by these rules,
the attackers manage with some regularity to penetrate to the heart of a
nuclear plant and damage the core. Representative Edward J. Markey, a
Massachusetts Democrat and something of a scourge of the nuclear power
industry, has recently identified a number of shortcomings in the safeguards,
including, apparently, lax standards for clearing workers hired at power
plants.

One of the most glaring lapses, which nuclear regulators concede and have
promised to fix, is that the design basis threat does not contemplate the
possibility of a hijacker commandeering an airplane and diving it into a
reactor. In fact, the protections currently in place don't consider the
possibility that the terrorist might be willing, even eager, to die in the
act. The government assumes the culprits would be caught while trying to get
away.

A nuclear power plant is essentially a great inferno of decaying radioactive
material, kept under control by coolant. Turning this device into a terrorist
weapon would require cutting off the coolant so the atomic furnace rages out
of control and, equally important, getting the radioactive matter to disperse
by an explosion or fire. (At Three Mile Island, the coolant was cut off and
the reactor core melted down, generating vast quantities of radiation. But
the thick walls of the containment building kept the contaminant from being
released, so no one died.)

One way to accomplish both goals might be to fly a large jetliner into the
fortified building that holds the reactor. Some experts say a jet engine
would stand a good chance of bursting the containment vessel, and the sheer
force of the crash might disable the cooling system -- rupturing the pipes
and cutting off electricity that pumps the water through the core. Before
nearby residents had begun to evacuate, you could have a meltdown that would
spew a volcano of radioactive isotopes into the air, causing fatal radiation
sickness for those exposed to high doses and raising lifetime cancer rates
for miles around.

This sort of attack is not as easy, by a long shot, as hitting the World
Trade Center. The reactor is a small, low-lying target, often nestled near
the conspicuous cooling towers, which could be destroyed without great harm.
The reactor is encased in reinforced concrete several feet thick, probably
enough, the industry contends, to withstand a crash. The pilot would have to
be quite a marksman, and somewhat lucky. A high wind would disperse the fumes
before they did great damage.

Invading a plant to produce a meltdown, even given the record of those mock
attacks, would be more complicated, because law enforcement from many miles
around would be on the place quickly, and because breaching the containment
vessel is harder from within. Either invaders or a kamikaze attacker could
instead target the more poorly protected cooling ponds, where used plutonium
sits, encased in great rods of zirconium alloy. This kind of sabotage would
take longer to generate radiation and would be far less lethal.

Discussion of this kind of potential radiological terrorism is colored by
passionate disagreements over nuclear power itself. Thus the nuclear industry
and its rather tame regulators sometimes sound dismissive about the
vulnerability of the plants (although less so since Sept.11), while those who
regard nuclear power as inherently evil tend to overstate the risks. It is
hard to sort fact from fear-mongering.

Nuclear regulators and the industry grumpily concede that Sept. 11 requires a
new estimate of their defenses, and under prodding from Congress they are
redrafting the so-called design basis threat, the one plants are required to
defend against. A few members of Congress have proposed installing
ground-to-air missiles at nuclear plants, which most experts think is a
recipe for a disastrous mishap.

''Probably the only way to protect against someone flying an aircraft into a
nuclear power plant,'' said Steve Fetter of the University of Maryland, ''is
to keep hijackers out of cockpits.''

Being Afraid For those who were absorbed by the subject of nuclear terror
before it became fashionable, the months since the terror attacks have been,
paradoxically, a time of vindication. President Bush, whose first budget cut
$100 million from the programs to protect Russian weapons and material (never
a popular program among conservative Republicans), has become a convert. The
administration has made nuclear terror a priority, and it is getting plenty
of goading to keep it one. You can argue with their priorities and their
budgets, but it's hard to accuse anyone of indifference. And resistance --
from scientists who don't want security measures to impede their access to
nuclear research materials, from generals and counterintelligence officials
uneasy about having their bunkers inspected, from nuclear regulators who
worry about the cost of nuclear power, from conservatives who don't want to
subsidize the Russians to do much of anything -- has become harder to
sustain. Intelligence gathering on nuclear material has been abysmal, but it
is now being upgraded; it is a hot topic at meetings between American and
foreign intelligence services, and we can expect more numerous and more
sophisticated sting operations aimed at disrupting the black market for
nuclear materials. Putin, too, has taken notice. Just before leaving to meet
Bush in Crawford, Tex., in November, he summoned the head of the atomic
energy ministry to the Kremlin on a Saturday to discuss nuclear security. The
subject is now on the regular agenda when Bush and Putin talk.

These efforts can reduce the danger but they cannot neutralize the fear,
particularly after we have been so vividly reminded of the hostility some of
the world feels for us, and of our vulnerability.

Fear is personal. My own -- in part, because it's the one I grew up with, the
one that made me shiver through the Cuban missile crisis and ''On the Beach''
-- is the horrible magic of nuclear fission. A dirty bomb or an assault on a
nuclear power station, ghastly as that would be, feels to me within the range
of what we have survived. As the White House official I spoke with said, it's
basically Oklahoma City plus the Hart Office Building. A nuclear explosion is
in a different realm of fears and would test the country in ways we can
scarcely imagine.

As I neared the end of this assignment, I asked Matthew McKinzie, a staff
scientist at the Natural Resources Defense Council, to run a computer model
of a one-kiloton nuclear explosion in Times Square, half a block from my
office, on a nice spring workday. By the standards of serious nuclear
weaponry, one kiloton is a junk bomb, hardly worthy of respect, a fifteenth
the power of the bomb over Hiroshima.

A couple of days later he e-mailed me the results, which I combined with
estimates of office workers and tourist traffic in the area. The blast and
searing heat would gut buildings for a block in every direction, incinerating
pedestrians and crushing people at their desks. Let's say 20,000 dead in a
matter of seconds. Beyond this, to a distance of more than a quarter mile,
anyone directly exposed to the fireball would die a gruesome death from
radiation sickness within a day -- anyone, that is, who survived the
third-degree burns. This larger circle would be populated by about a quarter
million people on a workday. Half a mile from the explosion, up at
Rockefeller Center and down at Macy's, unshielded onlookers would expect a
slower death from radiation. A mushroom cloud of irradiated debris would
blossom more than two miles into the air, and then, 40 minutes later, highly
lethal fallout would begin drifting back to earth, showering injured
survivors and dooming rescue workers. The poison would ride for 5 or 10 miles
on the prevailing winds, deep into the Bronx or Queens or New Jersey.

A terrorist who pulls off even such a small-bore nuclear explosion will take
us to a whole different territory of dread from Sept. 11. It is the event
that preoccupies those who think about this for a living, a category I seem
to have joined.

''I think they're going to try,'' said the physicist David Albright. ''I'm an
optimist at heart. I think we can catch them in time. If one goes off, I
think we will survive. But we won't be the same. It will affect us in a
fundamental way. And not for the better.''



Bill Keller is a Times columnist and a senior writer for the magazine.

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Wirt Atmar

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