MadSci Network: Physics

Re: Howmany atoms does it take to get an atom bomb?

Date: Mon May 14 10:24:11 2007
Posted By: Donald Howard, Nuclear Engineering, Retired
Area of science: Physics
ID: 1178667735.Ph

Well, Sarah, you have a nuclear power plant just south of town – Vermont 
Yankee, a GE BWR that I visited many times when I worked for GE.  

As for building a bomb, to quote Dr. Edward Teller, the “Father” of the 
Hydrogen bomb, “It’s tough to build a bomb.  You don’t start out to build 
a power reactor and wind up with a bomb.”

First, to build a fission bomb, you must have a “critical mass” of fissile 
material, either U-235 or Pu239, in a pure, or almost pure form.  For U-
235, the Hiroshima bomb material, the mass was a sphere about the size of 
a soft ball. That puts the minimum critical mass at about 50 Kilograms.  
Below that mass, any fission chain reaction would die out, resulting in no 

Assuming you have a minimum critical mass and a little more, the next 
problem is holding the mass together as long as possible.  As the fission 
rate and power released by that fissioning increases, the Uranium tries to 
blow itself apart.  Without any constraint, it would blow itself into sub-
critical pieces at a very low total power output, called the “yield” in 
the bomb business. 

So, to get back to your question, if the “softball” answer isn’t enough of 
an answer for you, to figure the exact number of atoms in 50 Kg of U-235, 
use Avogadro’s law, which is basic stuff in any High School Chemistry or 
Physics class.  If you haven’t heard of it, ask either of those classes 
instructors.  They might be impressed that you know enough to ask. 

As a side light, or as they say on the Internet, BTW, they started 
rebuilding the city of Hiroshima five years (not 10,000 years as they say 
in the movies) after the bomb obliterated it.  And, the “Kiloton/Megaton” 
rating system for those bombs was first mentioned in a Buck Rogers science 
fiction story as "Kilotons of TNT" written by Philip Francis Nowlan in the 
late 1920’s.. (or, at least, that's the story I've been told???)

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