After the American deployment of nuclear weapons, the Swedish military leadership recognized nuclear weapons to be thoroughly investigated and researched to provide Sweden with the knowledge to defend itself from nuclear attack. At that time, Sweden knew virtually nothing about nuclear physics, as all information and research around were kept strictly confidential by the United States. With the mission to "make something with neutrons", the Swedish team, with scientists like Rolf Maximilian Sievert, set out to research the subject and eventually build a nuclear reactor for testing.
After a few years of basic research, they started building a 300 kW (later expanded to 1 MW) reactor, named "Reaktor 1", R1 for short, in a reactor hall 25 meters under the surface right underneath KTH. Today this might seem ill-considered, since approximately 40,000 people lived within a 1 km radius. It was risky, but were deemed tolerable since the reactor was an important research tool for scientists at the Royal Swedish Academy of Engineering Sciences (Ingenjörsvetenskapsakademien).
At 18:59, 13 July 1954, the reactor reached critical mass and sustained Sweden’s first nuclear reaction. R1 was to be the main site for almost all Swedish nuclear research until 1970 when the reactor was finally decommissioned, mostly due to the increased awareness of the risks associated with operating a reactor in a densely populated area of Stockholm. The reactor hall remains an amusement to many as once it was next door to what used to be Sweden’s first nuclear reactor. Close to the reactor hall is the restaurant Q.