What Keeps Nuclear Weapons from Proliferating: The hardest step in making a nuclear bomb
What Keeps Nuclear Weapons from Proliferating: The hardest step in making a nuclear bomb
Bill explains that the hardest step is making the proper type of uranium. Weapons and power plants require uranium that contains a greater amount of the isotope uranium-235 than found in natural uranium, which is mostly uranium-238. He outlines the key difficulty in separating the two isotope: They have nearly identical properties. He explains the two key methods for separation: Gas diffusion and centrifuges.
Content
5 -> What Keeps Nuclear Weapons from Proliferating: The hardest step in making a nuclear bomb
13 -> I have here a scale-model of the first atomic bomb ever used.
17 -> This bomb, which destroyed Hiroshima
19 -> contains about sixty kilograms of uranium-235
23 -> of which only about six-hundred grams underwent fission.
26 -> Enough though to generate an explosion equal
28 -> to more than thirteen kilotons of TNT.
31 -> The bomb’s designers divided the amount needed into two piece.
34 -> At the tip they placed about 40% of the necessary uranium.
38 -> They loaded the remaining 60% at the other end.
41 -> A conventional explosion drove the projectile
43 -> into the target initiating the nuclear explosion.
46 -> Now the exact details of this bomb remain classified
49 -> because they could still be used.
51 -> Although this design involved some brilliant
53 -> innovation by engineers in the twentith century
55 -> the really difficult part is preparing the uranium.
58 -> This lies at the heart of all efforts to
60 -> stop the spread of nuclear weapons.
62 -> The key problem.
63 -> Separating two nearly identical variants of the element uranium.
67 -> Natural uranium occurs as a metal ore
69 -> and it contains primarily two isotopes.
72 -> Most uranium is U-238.
74 -> U-235 however, can easily sustain a chain reaction
77 -> that releases tremendous energy, whereas the more common U-238 will not.
81 -> Most elements are stable so that when bombarded
84 -> with neutrons they simply absorb them
86 -> and decay later, or they require very high energy neutrons
89 -> but bombarding U235 with low energy neutrons causes its nucleus to split.
95 -> The emission of these extra neutrons allows
98 -> the initial fission to generate a chain reaction.
101 -> So how do we go about enriching the U-235 in natural uranium.
106 -> When we separate two items we make use of their differences.
109 -> The two major uranium isotopes have identical
112 -> magnetic and chemical properties
114 -> no magnets will tug on one more than the other
116 -> no solvent will wash away only one isotope
119 -> and neither will boil before the other.
121 -> So, to separate them engineers exploit the
123 -> one small difference between them
125 -> U235 weighs slightly less than U238.
129 -> Less than a two percent difference
130 -> just enough to make separation possible, but it’s not easy.
133 -> That tiny weight difference means that the two isotopes
136 -> will move at slightly different speeds when exposed to an equal force.
140 -> To enrich uranium for the first atomic bomb engineers
143 -> built immense gaseous diffusion plants that
146 -> capitalized on the differing speeds.
148 -> A gas containing uranium flows through miles of piping
151 -> in a kind of race, where the lighter U-235 wins out.
154 -> The gas flows through a tube encased in a chamber.
157 -> A pressure difference between the chamber
159 -> and the tube causes more of the U-235 to pass
161 -> through perforations in the tube’s wall.
164 -> To increase the separation the slightly enriched
166 -> stream in the chamber is passed through many more stages like this.
169 -> To enrich 3% U-235 to 90% takes nearly 4,000 stages.
175 -> Enriching uranium for the first atomic bomb
178 -> required a diffusion plant that covered over 40 acres.
181 -> It housed a maze of 100 miles of piping.
184 -> These diffusion plants use great amounts of energy to run
187 -> Compressors generating the pressures needed
189 -> and the energy to heat gas flowing throughout the miles of tubing.
193 -> Another method of separation exploits the small mass
195 -> difference by using a centrifuge.
198 -> A typical device consists of a stationary outer cylinder
201 -> and an inner rotor that spins.
203 -> A gaseous mixture of the two isotopes flows up a tube
206 -> along the central axis, filling the rotor.
208 -> As it spins rapidly more of the U238 is thrown out to the
212 -> edge than the lighter U235, which stays closer to the middle.
215 -> The enriched stream can be removed from the rotor
218 -> and sent to another centrifuge to be separated even more.
221 -> The amount of separation is exaggerated here
223 -> In an actual centrifuge the amount of enrichment
226 -> is a fraction of a percent so a typical plant might have
230 -> 60,000 centrifuges to enrich natural uranium to 30% U235.
235 -> Such a plant uses four percent the energy of a gaseous diffusion plant.
239 -> Even though this is a much more efficient process
241 -> the precision that the rotors need to be manufactured with
244 -> makes them very difficult to engineer.
247 -> The smallest defect and the rotor spins itself to pieces.
250 -> That’s lucky for us
251 -> otherwise we might have nuclear devices
253 -> right next to our microwave ovens.
255 -> I’m Bill Hammack, the Engineerguy.
257 -> This video is based on a chapter
259 -> in the book Eight Amazing Engineering Stories.
262 -> The chapters features more information about this subject.
265 -> Learn more about the book at the address below.
Source: https://www.youtube.com/watch?v=OcgKDSwINOA