Why is uranium used in nuclear bombs?

In 1934, a team led by Italian scientist Enrico Fermi began bombarding uranium atoms with neutrons, producing much smaller atoms, such as barium, and some neutrons. This process came to be known as nuclear fission.

Fission typically occurs when a neutron and a target nucleus collide. Two nuclei are produced, each with roughly half the mass of the initial target nucleus. Two or three neutrons are produced as well, but the sum of the products’ masses is less than the initial mass; 0.1% of the initial mass is converted to energy, according to the equation E = mc2. The neutrons produced then bombard other target nuclei, creating a chain reaction and producing energy.

U-235 and U-238 are the two most commonly occurring isotopes of uranium. U-235 has only a 0.720% natural abundance but is much more efficient in fission reactions. “Uranium enrichment” actually refers to the purification of naturally occurring uranium to remove U-238; 90% U-235 is considered to be weapons-grade. As more U-238 is removed from a sample, the percentage of U-235 present increases exponentially. Harvested U-238 is used to produce Pu-239, another isotope that can fuel warheads.

Iran’s nuclear program has come under fire in recent years because of the nation’s supposed desire to acquire nuclear weapons. Before February 2010, Iran was enriching uranium to 3.5%, a figure below the 5% most nuclear power plants demand; it began enriching uranium to 20% U-235, however, in February of 2010. Since enrichment speed increases exponentially, going from 0.720% to 20% is much more difficult than going from 20% to 90%, allowing Iran to make the leap to nuclear-grade U-235 in the near future.