Or, forgoing the idea of a nation stealing the reactor/material, there's always the worry that terrorists will seize the shit and do terrorist things and make the reactor go NUCLEAR TERRORIST. Y"know, cause they're terrorists. Regardless of the actual danger, the public perception of danger is going to be (hell, is) a serious factor in keeping it from becoming widespread. Never mind what would happen in the media in the most vague instance of a radiation leak in the reactor or anything of the kind, which is the sort of thing that could put that company out of business, and set an example for other companies to avoid doing the same thing for the same risks.