The most compact nuclear power plants around (naval units used in submarines) weigh about 1000 tons. These use highly enriched uranium, so they would be seen as a security risk.
Containerizing this unit would mean at least 50 40-ft containers (with each container at its maximum weight), you probably need more because most containers won't reach this density. That would give something like 80 MW. Considering that a 20-ft container can hold at least a 1-MW diesel generator with its fuel supply, having a containerized nuclear reactor would seem to hold little advantage over diesel gensets.
There's also the problem that you really want the reactor vessel and the primary coolant loop as one unit, since you can't easily disconnect these once the reactor has been active and has irradiated the primary loop.
Now the reactor vessel alone is larger than a standard container. You'd end up with a very large and heavy undivisible central unit.
You'd be better off leaving the reactor on a ship and just running a cable ashore. For smaller power needs, existing containerized diesel gensets are a good solution.