[pantone159]

Where do the neutrons come from??

(Hint:  From fission of U-235.)

[CGB_Spender]

What about if they use another source for neutrons as Be? Imagine a sphere reactor that in the center you put a small sphere of Be or Be mixed with Po. The fast neutrons become thermic with the neccessary heavy water. Outside you put U-238 dust. (Just imagine three concentric circles, the first small (Be), the other bigger enough (Heavy water) and the last one, bigger than the other two. (U-238 dust) The hole devise is sinked into a tank with common cold water as a cooler. After the neccessary time you pull the device, open it and collect the Pu-239. This device is called the ''Lepsia Sphere'' and it was desighned from NAZI scientists at the end of WW2. The big advantage is that it creates Pu-239 from U-238 AND ONLY. 

[constant thinker]

The statement you quoted Mark K, I left it broad purposely. I don't know everything that goes into a reactor, but I know that the U-235 is used as the main fissile material in a nuclear reactor.

The Be still requires an alpha radiation emmiter to give off neutrons I believe. Ra, Am, or Po would be good elements probably for the alpha radiation part. In the end though, you still require more than just U-238.

[Will]

The big advantage is that it creates Pu-239 from U-238 AND ONLY. 

Whats stopping Pu-240 from being produced?

The isotope Pu-240 undergoes spontaneous fission very readily, and is produced when Pu-239 is exposed to neutrons.

and

Fission activity is relatively rare, so even after significant exposure, the Pu-239 is still mixed with a great deal of U-238 (and possibly other isotopes of uranium), oxygen, other components of the original material, and fission products.

If I were a rich terrorist with resources to build the reactor you described, then it still wouldn't be worth it. It would be so much easier to do what a terrorist does with other non-radioactive stuff.

Governments should concentrate  on getting rid of terrorists, not elements like graphite. If you had a choice to vote for a governement that had effective policies to ban graphite and U-238 for citizens or one that had effective policies to exterminate terrorists (everything else equal), which one would you vote for?

[constant thinker]

I agree with you Will on both points. It's easier to terrorize people without bothering with radioactive material, and the government should go after the terrorists because if they don't have access to radioactive material they can still just go and use a conventional weapon.

[jdurg]

I think you are severely overlooking the fact that beryllium, while it does create neutrons, is HORRIFCALLY slow at generating neutrons and is pretty damned inefficient.  A clandestine lab would blow its budget in seconds in order to come up with the raw material needed to produce microscopic amounts of these materials.

When an "element dealer" sells elements, they are sold in very small amounts at a HUGE markup over the bulk price.  This is how they make money.  Terrorists and "bad people" aren't magical creatures that have unlimited resources.  Like everyone else on earth, if they want something they need to come up with the money to procure it.  The amount of money they would need to spend to get what they need would bankrupt them.  Do you honestly think Osama Bin Laden would be able to do the s#*$ he does if he wasn't loaded with money?  While terrorists are spineless cowards, they aren't stupid.

A great analogy is to say that you could ruin the economy by flooding it with gold and causing the price of gold to plummett to nothing.  Gold exists in the ocean dissolved as a salt.  You just need to extract that gold salt and then refine it.  There's millions of pounds of gold dissolved in the waters of the Earth, and if you purify it you can cause the price of gold on the market to drop to nothing and cripple the economy.  It may not be easy, but you can do it

[pantone159]

What about if they use another source for neutrons as Be? Imagine a sphere reactor that in the center you put a small sphere of Be or Be mixed with Po.

In another thread, I explained out how this can't get you *nearly* enough neutrons to make real trouble.

http://www.chemicalforums.com/index.php?topic=9519.0