Here is a question for you nuclear scientists. How much U-235 do you need to make a one kiloton bomb (fission

Question

I calculated approximately eight pounds. I am correct?

How much would the whole bomb weigh and how large would it be? Would it be the size of a suitcase?

Answer 1

I'm not allowed to share the nuclear secrets with you for fear of loosing my life, but I can share what is available on the internet and let you fill in the pieces.  

So let's start from a purely theoretical yield here.  According to all-knowing Wikipedia, U-235 fission releases 73 terajoules of usable fission energy per kilogram, and there are 4.1 terajoules of energy per kiloton of TNT, so isn't it just 4.1/73=.056kg or 56 grams of U-235 per kiloton TNT.  Of course, if you don't get a perfect fission yield, then you'll get less of an explosion.  

There's also the issue of critical mass.  The critical mass of u235 is 52 kilograms, if it's in a sphere.  So unless you have a good neutron source and some access to some complicated explosive compression and neutron triggering mechanisms, your smallest simple bomb is going to be over a hundred pounds.  The detonation methods for a sub-critical mass would certainly be bigger than a suitcase, even though the uranium isn't.  Luckily uranium is very dense and you only need a ball that's about 6" in diameter for a critical mass.  Just don't keep it all in one ball until you are ready to blow it up.  Better to keep it in two different suitcases.