During the post World War II race among countries to arm themselves with nuclear weapons, some risky decisions were made.
Among them was the building of Sweden’s first nuclear reactor, Reaktor 1, or R1, buried 25 meters directly underneath the KTH Royal Institute of Technology, and within a 1 km of about 40,000 people. Nonetheless, in the heady days of the Cold War and nuclear armament it was deemed a tolerable risk for the scientists at KTH to be able to study reactions.
Attempting to “make something with neutrons,” scientists brought the reactor online and on July 13, 1954, the reactor produced Sweden’s first nuclear reaction.
R1 continued to be used for most of Sweden’s nuclear research until the 1970s, when the concern over the reactor proximity to Stockholm led to its closure.
The reactor hall has since been opened to visitors and as of April 2007 the reactor hall can be visited from the KTH Museum of Science and Technology. The reactor is now a popular venue for modern art and dance performances.
Update April 2017: The reactor is generally only open to visitors a couple of times per year, in conjunction with events, and is currently not open to the public at all while working with infrastructure.
Update October 2018: The reactor is open once more! It can be visited during special events like the Stockholm Open House.