As far as I know it really does just take two large enough lumps or uranium bein held to gether for a few seconds to create a nuclear explosion, and the size required isn't all that big.
Therefore, it really isn't complex to create a nuclear explosion if you don't mind dying in it, it's only if you want to detonate it remotely that it actually becomes complex to make, because then you have to have conventional explosives to blast the two pieces together, and make sure they touch for long enough, and of course you also need the detonation mechanism.