Sleepwalking With the Bomb (19 page)

At the root of miscalculation in a Yom Kippur type of confrontation is the seductive trap of mirror imaging: believing an adversary assesses the nuclear balance the same way, and regards nuclear arsenals as only for last-ditch deterrence purposes. We can avoid such mistakes if we keep in mind that assessments of the nuclear balance are not intellectual exercises in philosophical pure reason. They turn rather on prosaic everyday factors—specifically, what adversaries think, which can be quite different from what America thinks.

This is especially true as to revolutionary and rogue powers, but also can apply to rivals. It forms the basis for the Fifth Lesson of nuclear-age history: T
HE NUCLEAR BALANCE MATTERS IF ANY PARTY TO A CONFLICT THINKS IT MATTERS, AND THUS ALTERS ITS BEHAVIOR
.

T
HE
S
EPTEMBER
11 attacks brought to global public consciousness the fear that rogue nations might use or transfer nuclear weapons to terrorist groups, and that terrorist groups could themselves make a nuclear bomb. The first fear has far more foundation than does the second.

The good news is that it is very hard to make bombs; the bad news is that it is not impossible. Let’s look at uranium and plutonium and see why this is so.

Uranium and Fission

All uranium atoms have 92 positively charged protons at their nucleus (with 92 almost weightless negatively charged electrons orbiting that nucleus). But all uranium atoms are not the same. Though there is no way to tell one from another chemically, different isotopes of uranium have different numbers of neutrons, the proton’s neutrally-charged companion. Most atoms in a vein of natural uranium ore have 146 neutrons (for a total mass, protons plus neutrons, of 238). But a very few of them—less than 1 percent—have three fewer neutrons, and this “U-235” is extremely important for our story.

U-238 is fissionable, but not readily so. U-235, on the other hand, is fissile—its nucleus is easily split, creating two smaller nuclei, but more importantly, releasing energy plus two or three free neutrons. In a small enough space, those neutrons can each enter other uranium nuclei, splitting them to release more energy and neutrons, and so on, in a chain reaction. A critical mass of uranium-235—roughly 100–115 pounds in metal form and smaller than a soccer ball—will start a self-sustaining chain reaction on its own. If the same object is sufficiently compressed, it can become supercritical, dangerously increasing the rate of the chain reaction.

But the vast majority of uranium ore is U-238 and cannot emit neutrons rapidly enough to support a chain reaction. The solution for anyone seeking that reaction is for high-speed centrifuges to spin uranium that has been processed from ore to a powdered form called yellowcake, so that the marginally heavier U-238 molecules move to the bottom of the spinning cylinder, separating out from the precious, infinitesimally lighter U-235 that stays on top of the centrifuge. This process—of removing U-238 from U-235—is called uranium enrichment.

In order to generate an uncontrolled, supercritical chain reaction in uranium (a nuclear explosion), a would-be bomb maker must: 1. sufficiently enrich the uranium, 2. compress it ultra-rapidly into a supercritical mass, and 3. set it in an explosion-friendly physical shape.

For example, the Hiroshima bomb used uranium enriched to 80 percent U-235. Within the bomb, half the uranium was fired—by a miniature version of a World War II warship’s naval gun—into the other half, causing a supercritical mass to form and detonate in microseconds (millionths of a second).

Supercritical chain reactions in uranium typically at least double with each fission. Think of the parable about the king who offers a peasant serial doublings of wheat stalks on a chessboard—one stalk of wheat on square one, two on square two, etc. Before reaching 64 doublings the kingdom goes broke; the final squares are never covered, as there is no wheat left with which to do so. The difference in the nuclear case is that doublings go past the 64th square—to the 84th. Exponential progressions look like the famed “hockey stick” curve, one that accelerates at an ever-increasing rate with each doubling.

In the 84-doubling sequence not uncommon in a fission weapon, after 70 doublings only 1 percent of the energy will have been released. After 80 doublings only 5 percent will have been released, and after 83 doublings only 50 percent. North Korea’s early tests fell far short of the Hiroshima bomb in yield. A primitive weapon releases far less energy than a well-engineered one.

Commercial fuel is not sufficiently enriched to attain supercriticality, but failure to control the reaction or a failure in the cooling system can lead to an uncontrolled chain reaction and a “meltdown” in which the reactor fuel in the core overheats and melts into the floor. This is a highly radioactive event, and highly dangerous to those exposed to the intense doses of radiation (few in number, if the containment vessel protecting the reactor remains intact). A runaway chain reaction cannot generate a nuclear explosion but in water-cooled designs can cause a hydrogen explosion from the reaction of steam with core-surrounding cladding, as happened in the 1986 Chernobyl nuclear accident in Ukraine and at several reactors in the March 2011 nuclear meltdowns in Japan.

The Simple Arithmetic of Nuclear Proliferation

At first glance it seems a huge leap for a nuclear proliferator state to get from 3.5 percent, low-enriched, commercial uranium fuel for a power reactor all the way up to 93 percent, highly enriched, weapons-grade uranium fuel for a bomb. But simple arithmetic gives a counterintuitive result:
commercial-grade fuel is perilously close to weapons-grade fuel.

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.