A working nuclear research reactor operated by the Delft University of Technology, used for materials research, production of medical isotopes, and neutron studies. The Reactor Institute is not a conventional tourist attraction, but it occupies an interesting place in the history of science and in the oddities of Delft — a medieval canal city with a working reactor in its southern suburbs.
The institute organises public open days and tours, typically once or twice a year, where visitors can see the reactor hall (the characteristic blue Cherenkov radiation glow from the reactor core being the highlight). Check the TU Delft website for the schedule. For non-science audiences, the combination of the university campus architecture and the improbable presence of a nuclear facility makes for an interesting diversion from the canal and ceramic focus of the rest of the city.