A 31-year old man in Sweden had no idea, but was at least in doubt, which is why he called local authorities to make sure. The man, whose name was withheld, constructed an apparently functional reactor in the kitchen of his apartment, kept track of radiation levels using his own Geiger counter, blogged about his project, and eventually inquired about possible legal issues.
The authorities had an issue with the project and arrested the nuclear enthusiast on the spot. According to media reports, he succeeded in building a functional reactor, but was far away from producing electricity, which would have required a turbine and generator. However, he did have access to nuclear materials, which he acquired "overseas" and by disassembling a fire alarm. His teenage dream of operating his own nuclear reactor became a reality for the expense of about $950.
He was released from jail when he promised not to build another reactor. His equipment was confiscated.