Uranium 235 is the magic stuff that makes nuclear weapons possible. It’s not the most common isotope of Uranium though, not by a long-shot. You can order a chunk of Uranium ore on Amazon. Generally speaking, when we’re talking about Uranium ore we’re talking about pitchblende — Uranium Dioxide.

Assuming you have a literal ton of pure pitchblende — so no contaminants of any other sort — that means it’s 88.14% uranium by weight.

Your ton is down to 1762.99 pounds.

But now you need to get the U-235 out. The problem is that U-235 is chemically identical to U-238 and the 238 doesn’t do you much good at this stage. Separating one set of Uranium atoms from another is thus a physical process. To give you an idea of the scope of this process, this is the K-25 plant at Oak Ridge — one of the steps in the American Uranium enrichment program towards the end of WWII. Note, not the only step.

Note the size of the cars at the extreme left of the photo.

So how much U-235 do you get out of your ton of Uranium ore? Well, U-235 has an abundance of 0.720% So we multiply our 1762.99 pounds by 0.00720 and get… 12.69 pounds.

12 pounds is not a lot of Uranium and you don’t even get that. The process is never perfect; some will be lost along the way. Now the good news is that you don’t need this stuff at 100% purity, just 90% purity to make a 1940s era bomb so your 12.69 pounds of U-235 gives you 14.1 pounds of weapons grade uranium.

Great news. Now do that nine more times. The core of the Little Boy bomb was 120 pounds of U-235.

That’s why it’s so difficult. Even though the way this is done is well understood, the actual doing of it is very hard.

We put a man on the moon with 1960s technology; that doesn’t mean your local boyscout troop could manage it today.