The Deadly Arithmetic of Nuclear Proliferation

 

 

 

 

 

 

By John C. Wohlstetter from the October 2012 issue

How hard is it to build a bomb? Mr. Biden might want to pay attention.

THE SEPTEMBER 11 ATTACKS brought to global public consciousness the fear that rogue nations might use nuclear weapons or transfer them to terrorist groups, or that terrorists might 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.
The magnitude of the nuclear threat posed by a terrorist state or a terrorist group sponsored by such a state can be shown via certain metrics—call them metrics of mass destruction. But let’s begin with baseline concepts.
In the parlance of proliferation, there are three significant nouns that the adjective “nuclear” commonly modifies: “weapon,” “device,” and “capability.” A nuclear weapon is 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 bomb we have been worrying about since 9/11: one too large to be delivered by traditional military means, but which can be put into a van, truck, or shipping container.
And a nuclear capability is the wherewithal to make either of the above.

Uranium Enrichment Math

AT FIRST GLANCE IT SEEMS a huge leap for a potential proliferator state to get from low-enriched, commercial uranium Fuel for a power reactor all the way up to highly enriched, weapons-grade uranium fuel for a bomb. But simple arithmetic gives a counter-intutive result: commercial-grade fuel is perilously close to weapons-grade fuel.
Recall that significantly less than 1 percent of mined uranium is fissile U-235; the less desirable, non-fissile isotope U-238 makes up 139 out of every 140 uranium atoms. In order to run a reactor or build a nuclear weapon, the ratio of U-235 to U-238 must be much higher.
Commercial-grade fuel is at minimum 3.5 percent enriched—it has one atom of U-235 for every 27 of U-238. The next important step is 20 percent enriched fuel—one atom of U-235 for every four of U-238—that can run a medical research reactor. Weapons-grade uranium is about 90 percent enriched.
The key is that the process is not linear. To illustrate, start with raw uranium, which contains one atom of U-235 for every 139 of U-238. Enriching to 3.5 percent requires removing 112 of the U-238 atoms. At this stage you have done 80 percent of the isotopic separation needed to build a full weaponsgrade bomb of the kind in the U.S. arsenal.
Moving from 3.5 percent to 20 percent enriched requires removing only an additional 23 of the U-238 atoms. At this stage, you have done 97 percent of the isotopic separation work needed to make a full weapons- grade nuclear bomb.
One conservative estimate for Iran in early 2012, made by scholar Maseh Zarif of the American Enterprise Institute, showed how enrichment times accelerate:
• Start with 14,000 kilograms (15 tons) of natural unenriched uranium ore.
• It takes 331 days to enrich that to 1,400 kilograms of 3.5 percent commercial-grade fuel.
• It takes 37 days to enrich the commercial fuel to make 116 kilograms of 20 percent medical research-grade fuel. Cont. Reading