I decided to see if I could find that movie clip, and I found this:
A nice website not only describing the experiment, but how to set it up with a minimum amount of fuss should you decide to try it yourself. And yes, there are several videos.
has the top view in real-time.
There are some others, if you Google "mousetrap reactor".
In the dream, I was remembering the difficulty with actually achieving critical mass. Let's say that you have two pieces of uranium that are 2/3 of critical mass each. In theory, if you slam them together hard enough, you'd have one piece that was greater than critical mass, and you'd have a really big boom. In practice, it's pretty damn hard to slam them together with enough force to fuse them into one mass. It's kind of like welding with explosives.
With plutonium, it's even harder. The chain reaction has to get started with a catalyst like polonium. And when you're talking about really large yield bombs, you're talking about being able to fuse together a lot of chunks of radioactive metals simultaneously and with great precision. The most efficient design is actually a spherical implosion of material that is less dense into a sphere of supercritical mass density.
The hard part is getting a controlled implosion that maintains radial symmetry. Aside from needing highly refined and precise high explosive material, it requires precisely coordinated detonations, and by precise, I mean at sub-nanosecond timings. That's faster (and with higher energies) than can be reliably controlled by transistors, so specialized vaccuum tubes called Sprytrons are used.
There is a whole section in wikipedia on nuclear weapons design that's pretty awesome.