For a simple bomb you want Uranium, Plutonium emits too many neutrons which are likely to cause a small chain reaction while your critical mass is in the process of forming and this predetonation can then blow everything apart before a proper critical mass has been formed. A gun type design where you just smash two subcritical lumps of metal into each other is, as I remember, nearly impossible to achieve with Plutonium you can realistically produce.

EDIT: Interestingly another commenter says that this actually was a Plutonium design.

No, making a true atomic bomb is extremely difficult science and engineering.

Now, a dirty bomb, which is just a conventional explosive together with radioactive material - designed to spread radioactive material instead of releasing the fission energy into one giant explosion - is easy to create, but not that much more effective than just a conventional bomb. It would trigger a massive cleanup effort, and it would surely give some people cancer later down the line, but it wouldn't be that spectacular.

No, it is not that difficult, at least if you are fine with not having the most efficient weapon. You take a metal pipe, put a lump of Uranium and some explosive at each end and to a first approximation you are done. The USA did not even test their gun type design before dropping it onto Hiroshima because it was so simple that they were confident that it will just work.

Extracting u238 from 235 is a hard process. Using flourine gas, it requires you to turn an ore, to a metal, then to gas and then back to metal. If terrorists can get that done they should quit terrorism and just open a chemical plant

But they would not, as terrorism isn't about making money.

Terrorism isn't about solving extremely hard chemical engineering scaled process problem either.

It's not about becoming a commercial pilot or truck driver either but those are a means to an end.

I'm not aware of any terrorist who actually became a commercial pilot, merely those who took a few lessons (1). And I think that's at the upper end of the lengths that terrorists will go to: Past a certain point the low-tech approaches are just way more "bang for the buck", so to speak. And past a certain specialisation of skills, it's way more effective to get that high-paying job and donate to political causes.

1) https://www.sandiegouniontribune.com/lifestyle/people/sdut-g...