# XKCD What If References SDMB! **URL:** \ **Category:** About This Message Board\ **Created:** [August 13, 2014, 11:00pm UTC](https://boards.straightdope.com/t/xkcd-what-if-references-sdmb/695471 "2014-08-13T23:00:23Z")\ **Posts on this page:** 12\ **Page:** 3
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [March 24, 2015, 9:04pm UTC](https://boards.straightdope.com/t/xkcd-what-if-references-sdmb/695471/41 "2015-03-24T21:04:24Z")
> [@Frylock](#): > > Found it–it’s in footnote 3, invisible to ctrl-f ctrl-f? How do I see these footnote, using a Mac? ---
**Author:** ![FXMastermind](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fxmastermind/32/7289_2.png) [@FXMastermind](https://boards.straightdope.com/u/FXMastermind)\ **Post date:** [March 24, 2015, 9:42pm UTC](https://boards.straightdope.com/t/xkcd-what-if-references-sdmb/695471/42 "2015-03-24T21:42:46Z")
> [@DrDeth](#): > > ctrl-f? How do I see these footnote, using a Mac? Same as everyone else, you put your cursor on them. ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [March 24, 2015, 9:56pm UTC](https://boards.straightdope.com/t/xkcd-what-if-references-sdmb/695471/43 "2015-03-24T21:56:36Z")
> [@FXMastermind](#): > > Same as everyone else, you put your cursor on them. No, you click them. ---
**Author:** ![FXMastermind](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fxmastermind/32/7289_2.png) [@FXMastermind](https://boards.straightdope.com/u/FXMastermind)\ **Post date:** [March 24, 2015, 9:59pm UTC](https://boards.straightdope.com/t/xkcd-what-if-references-sdmb/695471/44 "2015-03-24T21:59:26Z")
> [@BigT](#): > > No, you click them. I have to explain everything? ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [March 24, 2015, 10:00pm UTC](https://boards.straightdope.com/t/xkcd-what-if-references-sdmb/695471/45 "2015-03-24T22:00:12Z")
> [@FXMastermind](#): > > I already let him know. I’m sure he will fix it soon. All the more reason to let us know, before he Lucases it away. ---
**Author:** ![FXMastermind](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fxmastermind/32/7289_2.png) [@FXMastermind](https://boards.straightdope.com/u/FXMastermind)\ **Post date:** [March 24, 2015, 10:08pm UTC](https://boards.straightdope.com/t/xkcd-what-if-references-sdmb/695471/46 "2015-03-24T22:08:03Z")
You can’t ever, in any possible way, get 300 kilos of plutonium anywhere close to shoebox size. Hell, you can’t even get 20 kilos together in a shoebox. Since Pu-239 doesn’t need anything besides itself to go critical, 10 kilos is enough to cause it to turn into molten metal or gas, along with anything else near it. You couldn’t have 300 kilos in the same room with out serious trouble. So it’s absolutely impossible to put 300 kilos anywhere close to a shoebox, in any possible way. ---
**Author:** ![The\_Joker\_and\_the\_Thief](https://avatars.discourse-cdn.com/v4/letter/t/e79b87/32.png) [@The\_Joker\_and\_the\_Thief](https://boards.straightdope.com/u/The_Joker_and_the_Thief)\ **Post date:** [March 24, 2015, 11:02pm UTC](https://boards.straightdope.com/t/xkcd-what-if-references-sdmb/695471/47 "2015-03-24T23:02:29Z")
> [@FXMastermind](#): > > You can’t ever, in any possible way, get 300 kilos of plutonium anywhere close to shoebox size. Hell, you can’t even get 20 kilos together in a shoebox. Since Pu-239 doesn’t need anything besides itself to go critical, 10 kilos is enough to cause it to turn into molten metal or gas, along with anything else near it. > > You couldn’t have 300 kilos in the same room with out serious trouble. So it’s absolutely impossible to put 300 kilos anywhere close to a shoebox, in any possible way. You use explosives to rapidly accelerate a number of disparate subcritical masses of plutonium into a rough shoebox shape before it went critical, which is what Randall meant when he said " So while you could fit 300 kilograms of it in a shoebox, you could only do so briefly." ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [March 25, 2015, 12:28am UTC](https://boards.straightdope.com/t/xkcd-what-if-references-sdmb/695471/48 "2015-03-25T00:28:30Z")
> [@FXMastermind](#): > > You can’t ever, in any possible way, get 300 kilos of plutonium anywhere close to shoebox size. That there’s a joke, son. ---
**Author:** ![FXMastermind](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fxmastermind/32/7289_2.png) [@FXMastermind](https://boards.straightdope.com/u/FXMastermind)\ **Post date:** [March 25, 2015, 1:50am UTC](https://boards.straightdope.com/t/xkcd-what-if-references-sdmb/695471/49 "2015-03-25T01:50:37Z")
> [@The\_Joker\_and\_the\_Thief](#): > > You use explosives to rapidly accelerate a number of disparate subcritical masses of plutonium into a rough shoebox shape before it went critical, Can’t be done. Even 40 kilos of Pu-239 can’t be put together into a shoebox shape, it goes critical long before that. The misunderstanding is how fast nuclear reactions happen. It’s the speed at which a critical mass of Pu-239 goes critical that makes it impossible to use a gun type mechanism to create an explosion. Pu-239 will turn everything to plasma long before you got it together, and that’s talking about a small amount. Bombs use from 3 to 5 kilos of Pu-239, [but it can be as little as 1 kilo](http://ieer.org/resource/nuclear-power/plutonium-factsheet/). Putting over 10 kilos of Pu-239 in the same small area is a disaster. It’s impossible to get 300 kilos anywhere close to “together”. the entire mass will turn everything, including the Plutonium, into a white hot molten and gas mess, just as soon as it is anywhere close to being in the same space. Close being a matter of many meters, but the exact information would have to be calculated by a nuclear physicist. The thing is, even with explosives, you can’t get Plutonium to slam together and make a bomb. It reacts far too fast. This is the exact problem they had to solve when making the first Plutonium bomb. Note: It’s a very good thing you can’t make a bomb using Plutonium by slamming it together really fast. ---
**Author:** ![Shalmanese](https://avatars.discourse-cdn.com/v4/letter/s/45deac/32.png) [@Shalmanese](https://boards.straightdope.com/u/Shalmanese)\ **Post date:** [March 25, 2015, 2:10am UTC](https://boards.straightdope.com/t/xkcd-what-if-references-sdmb/695471/50 "2015-03-25T02:10:04Z")
> [@FXMastermind](#): > > Can’t be done. Even 40 kilos of Pu-239 can’t be put together into a shoebox shape, it goes critical long before that. > > The misunderstanding is how fast nuclear reactions happen. It’s the speed at which a critical mass of Pu-239 goes critical that makes it impossible to use a gun type mechanism to create an explosion. Pu-239 will turn everything to plasma long before you got it together, and that’s talking about a small amount. Bombs use from 3 to 5 kilos of Pu-239, [but it can be as little as 1 kilo](http://ieer.org/resource/nuclear-power/plutonium-factsheet/). > > Putting over 10 kilos of Pu-239 in the same small area is a disaster. It’s impossible to get 300 kilos anywhere close to “together”. the entire mass will turn everything, including the Plutonium, into a white hot molten and gas mess, just as soon as it is anywhere close to being in the same space. Close being a matter of many meters, but the exact information would have to be calculated by a nuclear physicist. > > The thing is, even with explosives, you can’t get Plutonium to slam together and make a bomb. It reacts far too fast. This is the exact problem they had to solve when making the first Plutonium bomb. > > Note: It’s a very good thing you can’t make a bomb using Plutonium by slamming it together really fast. What if it were mixed with a strong neutron poison like Xenon-135? Even small amounts of neutron poison are enough to stop a reactor from going critical and might delay a chain reaction just long enough to get 300kg of Pu to remain shoebox sized for a shake or two. ---
**Author:** ![FXMastermind](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fxmastermind/32/7289_2.png) [@FXMastermind](https://boards.straightdope.com/u/FXMastermind)\ **Post date:** [March 25, 2015, 2:34am UTC](https://boards.straightdope.com/t/xkcd-what-if-references-sdmb/695471/51 "2015-03-25T02:34:58Z")
> [@Shalmanese](#): > > What if it were mixed with a strong neutron poison like Xenon-135? Even small amounts of neutron poison are enough to stop a reactor from going critical and might delay a chain reaction just long enough to get 300kg of Pu to remain shoebox sized for a shake or two. Let’s try it! ---
**Author:** ![FXMastermind](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fxmastermind/32/7289_2.png) [@FXMastermind](https://boards.straightdope.com/u/FXMastermind)\ **Post date:** [April 11, 2015, 7:54pm UTC](https://boards.straightdope.com/t/xkcd-what-if-references-sdmb/695471/52 "2015-04-11T19:54:20Z")
After a lengthy discussion elsewhere, the issue of “is it possible” still remains a scientific mystery. However, in the discussion the knowledge of just how expensive pure Pu-239 actually is was brought up, and it seems that a shoebox full of pure Pu-239 (not the weapons grade stuff) would be the most expensive shoebox in the Universe. If it was ever possible to actually make that much pure Pu-239. The issue of how to cram 300 kilos into a shoebox size was never answered, and probably never will be. [Previous page](https://boards.straightdope.com/t/xkcd-what-if-references-sdmb/695471.md?page=2)