Old news.
Great idea.
....and terrorist have way better sources of either radioactive material for dirty bombs; or true fissionables for a nuke.
The later is unlikely given, that it's pretty hard to put together a viable atomic bomb. (Viable in my book means, you, as a terrorist organization can effectively deploy it. There go all the easy to make 0,5 ton pure uranium bombs - which are "easy" to make if you can afford the astronomical cost of refining weapons grade uranium (90% U235). Plutonium bombs are very difficult to make as there are a million things that could go wrong with the implosion device).
This thing would be likely guarded, and the self-regulating; self-contained nature means, you can easily keep it under a very heavy security by denying access to anyone. Things like a 150 ton concrete lid that you can only lift with a special vehicle come to my mind.
You're UN inspectors? Sure, where is you goddamn monster crane to take off the lid? (I wonder how you'd talk yourself out of this one).
Actually all the movie-plot threats and the hype surrounding terrorism is just that - security theater. Read up on it and despair.
