^ As said before, it largely depends on the type of reactor.
CANDUs, which were first built in the 1950s, are among the best, along with Pebble beds. Both are meltdown proof. Terrorists are not really a concern, because even well-funded groups like Al-Queda cannot figure out to do anything with waste material and even if they did get their hands on it the waste would be so hot it would kill all involved before they could do much with it.
Doing nuclear reactors right is as pointed out important. But most nations who have nukes handle it alright. It requires investment and a lot of it, and would require a very large number of highly trained, intelligent personnel. In the Western World this can be done, in the third world not a chance.