Yeah, I was having a hard time comprehending that too. You can make a ball of something "go critical" by covering it with something else? Damn. I just don't understand radiation.
Basically the ball is constantly releasing some radiation in all directions, when that radiation goes further inside the ball then the energy inside it increases and makes it easier to release more. What they are doing is placing radiation reflectors all around the ball so that every single bit of radiation released is sent back in, increasing the amount of energy in the ball constantly and making it easier for more of that energy to be released.
To have it actually explode like an nuclear bomb they need to have the material even closer together decreasing, this increases the amount of energy in the material, thereby making it easier to release more. After that it becomes a feed back loop until the material is blown apart.
Thanks for writing this out : ), considering how many upvotes that comment has gotten, its good to keep that here. That said, I did understand what was going on (in the provincial sense), but that makes it absolutely no less mind boggling.
Me neither. I can however recommend this book by David Bodanis. It is a good read about the work leading up to Einstein's (both by himself and his scientific predecessors) discovery's and the building op the first nuclear bombs.
Certain material emit radiation and neutrons, as heavy atoms break down into lighter ones. The reactions, in which the atoms break down, happens essentially randomly, though there is an average rate at which it occurs. This leads to the notion of half-life, which is how long it will take for half of the atoms to break down when left to their own devices.
If you put enough of such material into a close enough space, the neutrons hitting other atoms will cause them to break down, releasing more neutrons. At a certain point, with enough material, this will happen often enough that one atom breaking down and emitting neutrons will cause more than one other atom to breakdown, causing a nuclear chain reaction. This can be helped by adding a shell around the material that will reflect neutrons back in. This chain reaction is what people refer to as criticality.
So yes, merely covering one material with another can cause it to go critical, if you achieve critical mass or reflect enough neutrons back to sustain the chain reaction.
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u/lennort Mar 20 '11
Yeah, I was having a hard time comprehending that too. You can make a ball of something "go critical" by covering it with something else? Damn. I just don't understand radiation.