Recall that significantly less than 1 percent of mined uranium is fissile—the less-desirable isotope makes up 139 out of every 140 uranium atoms. Commercial-grade fuel requires a minimum of 3.5 percent U-235, which means that 1 out of every 28 atoms must be U-235. Thus, for every 139 U-238 atoms, 112 of them must be removed. Now you can run a commercial nuclear reactor. To appreciate how close you already are to having nuclear weapons fuel, at this stage you have done 80 percent of the isotopic separation needed to build a full weapons-grade bomb of the kind in the U.S. arsenal.
The next important step is 20 percent enriched fuel—four atoms of U-238 for every U-235 atom—that can run a medical research reactor. Reaching this step requires taking away a total of 135 out of the 139 U-238 atoms that originally accompanied each atom of U-235. At this stage, you have done 97 percent of the isotopic separation work needed to make a full weapons-grade nuclear bomb.
Put another way:
• Natural Uranium Ore. In nature, there are 139 atoms of nonfissile U-238 for every one atom of fissile U-235.
• Commercial Nuclear Reactor Fuel. Remove 112 U-238 atoms, leaving 27 U-238 atoms and 1 U-235 atom. This makes 3.5 percent enriched uranium.
• Medical Research Reactor Fuel. Remove 23 U-238 atoms, leaving 4 U-238 atoms and 1 U-235 atom. This makes 20 percent enriched uranium.
• Weapons Grade Uranium. Remove 4 U-238 atoms, leaving 1 U-235 atom. This leaves 1 U-235 atom, 100 percent enriched uranium. U.S. weapons grade uranium fuel is 93.5 percent enriched uranium.
Such separation work is by far the hardest part of the total work needed to assemble a nuclear device. One conservative estimate for Iran in early 2012 showed how enrichment times accelerate with higher levels of enrichment:
• Start with 14,000 kilograms (15 tons) of natural unenriched uranium ore.
• It takes 331 days to enrich to 1,400 kilograms of 3.5 percent commercial-grade fuel.
• It takes 37 days to enrich the 1,400 kilograms of commercial fuel to make 116 kilograms of 19.75 percent medical research– grade fuel.
• 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.
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 = mc2 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 “c2” 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.
25
This was in 1941, 11 years after the discovery of Pluto, and over a century and a half after a German apothecary and chemist, Martin Klaproth, discovered uranium in 1786 and named it after the planet Uranus, newly discovered that same year. U-238 decays—transmutes itself by releasing energy—in 23 minutes to neptunium-239, named after the planet Neptune; Np-239 then decays in 2.3 days to plutonium, P-239, named after Pluto.
26
These include plutonium isotopes Pu-240, Pu-241, and Pu-242. The complex physics and chemistry of how they interact with Pu-239 are beyond this book’s scope.