Sleepwalking With the Bomb (20 page)

     
It takes only 8 days to enrich 116 kilograms of 19.75 percent enriched fuel to make 15 kilograms (33 pounds) of 90 percent enriched uranium for weapons-grade fuel, enough to make a single Hiroshima-size bomb.

Other expert calculations assume a six-fold progression through the three stages, but let us assume ten-fold, to be conservative. In round figures apply two rules of thumb:

     
10-10-10
for the three tenfold stages of material shrinkage listed above, from uranium ore to medical-grade to weapons-grade.

     
11-1-1
for the three time periods: 11 months for commercial reactor fuel, then 1 month more for medical reactor fuel for research, then 1 week for weapons-grade fuel for a bomb.

Thankfully, putting together the vast, industrial-scale infrastructure needed to enrich uranium via these methods is extremely difficult; no terrorist is going to do this in a garage or on a back lawn with presently available methods.

To these 11-1-1 and 10-10-10 rounding rules noted above we can add one more number each, to complete the sequences. Adding another 1 to the first sequence tells us that once all components needed for a bomb are in place it takes about one day to assemble them into an operational bomb. Adding a final 10 to the 10-10-10 sequence captures the difference between the minimum amount needed for
a crude uranium bomb
a terrorist can use (roughly 60 kilograms—the amount used in the Hiroshima bomb), and the minimum amount needed for
a highly sophisticated plutonium bomb
that a first-rank nuclear state can use to optimize its nuclear arsenal (roughly 6 kilograms).

Some specialized reactors run on fuel enriched beyond commercial grade. Nuclear-powered submarines and surface ships actually run on weapons-grade fuel, because they must provide very high power in a very small space. Such fuel, if diverted, could make fuel for a nuclear weapon. (A submarine or surface-ship reactor, though running on weapons-grade fuel, cannot generate a nuclear explosion, for want of the necessary physical configuration and compression.)

Now the bad—
very
bad—news: You do not need a full U.S. weapons-grade enriched bomb to get a nuclear explosion.
Less than 20 percent enriched uranium suffices.
In 1962 the United States tested a uranium bomb at its Nevada underground test site, and obtained a nuclear explosion with fuel enriched somewhat short of 20 percent (the exact figure remains classified). It was, in the parlance, suboptimal. If detonated in a city, such a bomb would cause less devastation and kill fewer people than a full U.S.-grade enriched bomb. But its destructive power could still be immense. The 1,336-pound (two-thirds of a ton) conventional truck bomb that exploded in a garage of the World Trade Center in 1993, had it been more carefully placed a few of yards away, would have toppled one tower into the other, killing many tens of thousands. The much bigger 1995 Oklahoma City bomb, which destroyed a large federal building and killed 168 people, used two and a half tons of conventional explosive. A “puny” A-bomb (like that detonated in North Korea’s 2006 plutonium test, for example) could easily be equivalent to a few hundred tons of high explosive.

Plutonium, Fission, and Fusion

So much for uranium, the fuel of choice for proliferators. But what about plutonium? Plutonium barely exists naturally—the young American nuclear chemist, Glenn Seaborg, found it by making it from U-238,
²⁵
and every day more accumulates in the spent fuel collected from nuclear reactors. The U-238 in nuclear reactors will catch a neutron, and instead of fissioning, become an extremely unstable atom with 239 neutrons and protons. In a series of transmutations (changes in chemical composition), this U-239 naturally becomes fissile plutonium-239, the most common modern fuel for nuclear weapons.

How a reactor is designed and run determines how readily and conveniently it creates that plutonium-239. The reactor the Iraqis built in the late 1970s was to run on weapons-grade fuel and was made to maximize plutonium production. Israel understood this perfectly well, and hence destroyed it in 1981, before it was fueled, to avoid scattering radioactive material for miles upon bombing it. Proliferation expert Henry Sokolski writes that a light-water reactor rated at a tenth the size of a commercial plant can be run so as to produce dozens of pounds of plutonium in a year. This is more than enough to fuel several nuclear bombs.

Because a reactor can produce plutonium, a terrorist might think of stealing nuclear waste to obtain it. But plutonium is just one component of some forms of nuclear waste, and most plutonium in nuclear waste is not fissile. The longer the newly made Pu-239 sits in a reactor, the longer the neutron-capture process goes on, producing heavier, less controllable, forms of plutonium.
²⁶
These soon outnumber fissile Pu-239, and are hard to separate from it. This problem can be avoided by replacing fuel rods before they absorb too many neutrons.

Weapons-grade plutonium is a more efficient bomb fuel than weapons-grade uranium, and thus offers more explosive power per pound. The actual amount of plutonium converted into energy released by plutonium-239 nuclei that fissioned inside the core of the Nagasaki bomb was about one gram—one-third the weight of a penny. Einstein’s E = mc
²
equation explains this. The released mass (m) is infinitesimally small—less than a thousandth of the mass that fissioned, as most of what fissioned careened around in search of other nuclei to split; the remainder was converted into and released as kinetic, thermal, and radiation energy. But the “c
²
” represents the square of the free-space speed of light in kilometers per second, a huge multiplier that explains the vast energy liberated from an infinitesimally tiny nucleus. Applying this to every atom whose nucleus is split in a nuclear detonation yields a vast release of energy in various forms.

But Pu-239 is much harder to make into a nuclear bomb. It must be placed in a special configuration, far more complex than that for a uranium bomb. The Manhattan Project scientists were so certain a gun-trigger design would work with uranium that they did not even test it—uranium was in short supply and they needed it to create plutonium for the Trinity test and then the Nagasaki bomb.

A plutonium detonation occurs in about a nanosecond (a billionth of a second), a thousand times faster than a uranium detonation. To make sure as much of the plutonium as possible fissioned, the Trinity and Nagasaki bombs were “implosion” devices. A complicated arrangement of 32 symmetrically spaced conventional explosives surrounded those bombs’ plutonium cores. Thirty-two lenses converted the shock waves from convex to concave, to compress the plutonium core extremely rapidly and symmetrically. A timing discrepancy among the implosion lenses of one-millionth of a second reduces symmetry and can create a dud; a timing discrepancy of 10 microseconds—10 millionths of a second—is enough to create a partial dud. In essence, plutonium bombs require super-speed, super-symmetry, and super-small compression.

For a nuclear weapons state seeking to use missiles to carry nuclear warheads, plutonium is the fuel of choice, because it provides more yield per pound, and thus is more suitable for small warheads. It is very unlikely that terrorists would be able to build a plutonium fission device on their own, due to the extreme sophistication involved.

And it is even harder to master the deep subtleties of a hydrogen bomb. This requires a conventional explosive to trigger an atomic bomb, whose radiated thermal energy then compresses the plutonium core so rapidly and compactly as to fuse hydrogen atoms and generate a thermonuclear explosion.

Terrorist Bombs and Military Bombs

In the parlance of nuclear proliferation there are three significant nouns commonly added after the adjective nuclear: “capability,” “device,” and “weapon.” A
nuclear capability
means the ability to make a nuclear device or weapon. A
nuclear device
is the kind of weapon we have been worrying about since 9/11, a bomb too large to be delivered by traditional military means, but which can be put into a van, truck, or shipping container. A
nuclear weapon
denotes a bomb compact and light enough to fit into a missile warhead, or the business end of a bomb or artillery shell.

A nuclear device is the kind of crude weapon a terrorist would use. Once the necessary amount of enriched uranium is in hand, a crude terror device can be easily assembled. How much is needed is design dependent: for the simplest device more is needed than for a sophisticated bomb. A nuclear weapon is the kind of bomb we worried about during the Cold War and is what proliferators like North Korea and Iran are working on. What remains for them is to achieve the requisite miniaturization required to place a nuclear bomb inside a missile nose cone.

But a nuclear state need not have a weapon to aid terrorists. A device will suffice. For terrorists, a uranium-fueled atom bomb with a gun trigger is the preferred route. But even this is not duck soup for an individual, and not just any design suffices.
²⁷
The damage a crude nuclear device—in proliferation parlance, an “Improvised Nuclear Device” or IND—can inflict if detonated in the nation’s capital was recently assessed by the Federal Emergency Management Agency. A 10-kiloton blast—about 70 percent of the explosive yield of the Hiroshima bomb—would extinguish nearly all human life and obliterate most structures within a half-mile. Glass would be shattered out to 10 miles, radioactive dust would spread at least 20 miles. Severe structural damage would reach 1.5 miles from ground zero, with many casualties due to blast shockwave and thermal effects. Out to nearly 5 miles there would be light structural damage, plus numerous casualties from radioactive particulate fallout. In all, the study estimated 45,000 fatalities and 323,000 injured. This would overwhelm local medical facilities. (The 9/11 jetliner explosions yielded about 1/10 of a kiloton.)

To go nuclear using currently available methods, a terrorist organization needs the help of a state.
²⁸
This is especially true of the so-called “suitcase nuke” attack scenario. A truly man-portable nuclear weapon requires highly advanced miniaturization of components, to make it compact and light enough to be carried. Russia developed an atomic demolition munition (ADM) the size of a footlocker, not designed to be carried by one person. Tales of Russian “loose nukes” floating around are dubious; and they likely wouldn’t work now anyway, as their core elements decay over time and eventually are no longer fissile.

With this understanding of what is required to build a nuclear weapon, and a sense of the thin line between development of commercial nuclear power and the development of weapons, we turn to the Indian subcontinent.

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23.
As explained below, a “bomb” is either a military “weapon” or a terrorist “device.”