(This video is from EngineerGuy series #4)
Would you like to translate the captions for this video? Visit http://www.engineerguy.com/translate
I have here a scale-model of the first atomic bomb ever used. This bomb, which destroyed Hiroshima, contains about sixty kilograms of uranium-235, of which only about six-hundred grams underwent fission. Enough though to generate an explosion equal to more than thirteen kilotons of TNT. The bomb’s designers divided the amount needed into two piece. At the tip they placed about 40% of the necessary uranium. They loaded the remaining 60% at the other end. A conventional explosion drove the projectile into the target initiating the nuclear explosion.
Now the exact details of this bomb remain classified because they could still be used. Although this design involved some brilliant innovation by engineers in the twentith century; the really difficult part is preparing the uranium. This lies at the heart of all efforts to stop the spread of nuclear weapons. The key problem? Separating two nearly identical variants of the element uranium.
Natural uranium occurs as a metal ore, and it contains primarily two isotopes. Most uranium is U-238. U-235 however, can easily sustain a chain reaction that releases tremendous energy, whereas the more common U-238 will not.
Most elements are stable so that when bombarded with neutrons they simply absorb them and decay later, or they require very high energy neutrons, but bombarding U235 with low energy neutrons causes its nucleus to split. The emission of these extra neutrons allows the initial fission to generate a chain reaction.
So how do we go about enriching the U-235 in natural uranium? When we separate two items we make use of their differences. The two major uranium isotopes have identical magnetic and chemical properties: no magnets will tug on one more than the other, no solvent will wash away only one isotope, and neither will boil before the other.
So, to separate them engineers exploit the one small difference between them: U235 weighs slightly less than U238. Less than a two percent difference, just enough to make separation possible, but not easy.
That tiny weight difference means that the two isotopes will move at slightly different speeds when exposed to an equal force. To enrich uranium for the first atomic bomb engineers built immense gaseous diffusion plants that capitalized on the differing speeds.
A gas containing uranium flows through miles of piping in a kind of race, where the lighter U-235 wins out. The gas flows through a tube encased in a chamber. A pressure difference between the chamber and the tube causes more of the U-235 to pass through perforations in the tube’s wall. To increase the separation the slightly enriched stream in the chamber is passed through many more stages like this. To enrich 3% U-235 to 90% takes nearly 4,000 stages.
Enriching uranium for the first atomic bomb required a diffusion plant that covered over 40 acres. It housed a maze of 100 miles of piping. These diffusion plants use great amounts of energy to run: Compressors generating the pressures needed and the energy to heat gas flowing throughout the miles of tubing. Another method of separation exploits the small mass difference by using a centrifuge.
A typical device consists of a stationary outer cylinder and an inner rotor that spins. A gaseous mixture of the two isotopes flows up a tube along the central axis, filling the rotor. As it spins rapidly more of the U238 is thrown out to the edge than the lighter U235, which stays closer to the middle. The enriched stream can be removed from the rotor and sent to another centrifuge to be separated even more. The amount of separation is exaggerated here: In an actual centrifuge the amount of enrichment is a fraction of a percent so a typical plant might have 60,000 centrifuges to enrich natural uranium to 30% U235.
Such a plant uses four percent the energy of a gaseous diffusion plant. Even though this is a much more efficient process, the precision that the rotors need to be manufactured with makes them very difficult to engineer. The smallest defect and the rotor spins itself to pieces. That’s lucky for us - otherwise we might have nuclear devices right next to our microwave ovens! I’m Bill Hammack, the Engineerguy!