That's the question. Large power plants like today's are well protected(in most cases). Here, the idea is to put a encapsulated nuclear reactor underground where it's needed (factories, data center, and whatnot). If those reactors become widespread, some dumb suicidal person might access one. As it would work on nuclear waste, there would be no potential bomb material(as i understand, but not exactly sure) the only threat would be a voluntary radiation pollution. In french, it's "bombe sale", it's an explosive device containing radioactive material.