It was leap day Feb. 29, 1940, and University physics professor Alfred Nier had an important errand to run.

After leaving his Tate Hall laboratory that Friday, Nier went straight to the Minneapolis Post Office and mailed overnight a handwritten letter to a colleague at Columbia University.

More important than the actual letter was what was pasted in the margins: tiny samples of isolated uranium-235, a rare isotope that would serve as an essential ingredient in the atomic bomb dropped by the United States on Hiroshima in World War II.

Nier was the first person to isolate uranium-235 from the much more common uranium-238 isotope.

Ever since the U.S. government used Nier’s separation method to make its first atomic weapon, the country has spent billions on perfecting and protecting the craft.

Several countries have strived to become nuclear powers, but few have actually obtained atomic weapons. What makes the weapons so elusive?

University physics and astronomy professor Marvin Marshak said the difficulties come not only from building or obtaining them, but also in keeping it a secret from the rest of the world.

Marshak said the nuclear powers’ intelligence agencies track the equipment needed to manufacture atomic weapons.

“They know who makes it, and they watch who buys it. If you start buying a lot, they’re going to take an interest in you,” he said.

There are two types of atomic bombs: uranium and plutonium. To build a uranium bomb, Marshak said the first step is getting the uranium, which is not too rare and can be mined in several regions.

The problem is that most of the Earth’s uranium – more than 99 percent – is useless for atomic bomb-making. The only type that can be used is uranium-235, which has three fewer neutrons than uranium-238, Marshak said.

“So you need to enrich the uranium,” he said. “You have to somehow separate the 235 from the 238.”

The task is complicated because isotopes are chemically identical. They only vary slightly in mass.

Marshak said Nier isolated micrograms of uranium-235 using a mass spectrometer, a device using a combination of electric and magnetic fields to separate the uranium atom by atom.

To get the kilograms of uranium-235 used in the Hiroshima bomb, the government took the technology that sat on Nier’s basement bench in Tate Hall and scaled it up to an industrial-sized separation plant in Oak Ridge, Tenn.

“There are very few separation plants in the world, and basically, they are controlled by the big powers,” Marshak said.

Countries that have uranium separation technology are not interested in sharing it, so any aspiring nuclear power would have to build its own plant.

“These separation plants use enormous amounts of energy and they are enormously large,” he said. “It would be hard to build a plant without people knowing it.”

Plutonium bombs, like the one dropped on Nagasaki, Japan, are easier to make and are preferred today, Marshak said. In the plutonium method, uranium-238 is placed into a reactor and bombarded with neutrons. The neutrons stick to the uranium, changing its chemical properties until it becomes plutonium. The plutonium can then be chemically separated from the remaining uranium.

“This is not trivial because plutonium is highly toxic. It’s chemically toxic, not only radioactive,” he said. “You can’t just go over to Smith Hall and make some of this stuff up. You’d kill yourself doing it.”

Plutonium bombs also require highly precise and powerful detonation devices. Marshak said without proper detonation, the bomb will “predetonate, in which the bomb blows itself up, but never blows up much else.”

That’s why dirty bombs might be appealing to terrorist organizations today. Dirty bombs are essentially predetonating plutonium bombs.

“Basically, all you’ve got to do is take a bunch of plutonium, grind it into powder and put some explosive that will disperse it,” Marshak said. “I’m not telling you any great secret here.”

A dirty bomb wouldn’t create the kind of mushroom cloud explosion associated with nuclear weapons but it could scatter radioactive and toxic debris across a large area.

Still, Marshak said the best bet for aspiring nuclear powers is “to go to one of the former Soviet republics … . You can’t just buy all this stuff at Radio Shack.”