[schlauerfox]

In the USA, it's likely the Nuclear Regulatory Commission. Their site says "The Commission as a collegial body formulates policies, develops regulations governing nuclear reactor and nuclear material safety, issues orders to licensees, and adjudicates legal matters." https://www.cnbc.com/2023/04/20/nuclear-fusion-will-not-be-r... Depending on how early in development it is, it would likely be grid tied at some point because of the huge power needs to drive the power generating reactor before it gets going and the fuel generation reactor which is not energy producing.