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The Twisted Path to a Nuclear Weapon

by Jeremy Bernstein

How Iran Got the Bomb: The Twisted Path to a Nuclear Weapon


Nuclear fission of the uranium nucleus was discovered in Germany late in 1938. It was an unexpected discovery since it revealed a new source of energy. Einstein noted that it was the only source of terrestrial energy that did not directly or indirectly come from the Sun. It was also clear that this energy had military implications.

In February 1939 Niels Bohr published a brief letter in the Physical Review that changed all future considerations for making nuclear weapons. What Bohr showed was that the isotope Uranium-235 is “fissile.” This means that it can be split—fissioned—by neutrons of any energy. On the other hand, Uranium-238 is “fissionable.” This means that it takes a minimum of neutron energy to split the nucleus.

Bohr was pleased with this result since it seemed to rule out the possibility of nuclear weapons. You cannot make a weapon out of Uranium-238 because not enough neutrons of high energy are produced in the fission process. This makes an explosive chain reaction impossible. The chain reaction dies out since there are not enough high-energy neutrons to propagate it. The lower-energy neutrons will not split the Uranium-238 nucleus. Also, Bohr knew that Uranium-235 was a very rare isotope. Less that 1 percent of it is found in natural uranium, most of the rest being Uranium-238. To build a weapon, you would have to separate out enough Uranium-235 to make an explosive chain reaction.

Bohr knew that you cannot separate isotopes chemically. Both 235 and 238 have virtually the same chemistry, so you must use mechanical means that take advantage of the minuscule mass difference between the isotopes. To do this, he said, would require the resources of an entire country. He was wrong. It took the resources of three—the United States, Canada, and Great Britain.

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