Großwelzheim Nuclear Power Plant, (German: Kernkraftwerk Großwelzheim, HDR), was an experimental nuclear power plant consisting of one 25 MW reactor in Großwelzheim, a district of Karlstein am Main.

The prototype boiling water reactor, designed to produce superheated steam was under construction from 1965 to 1969, and was first connected to the power grid on October 14, 1969.

The reactor could not be run at full capacity due to structural defects in the fuel elements.

[2] After the decommissioning in 1983, the plant was used for reactor safety tests until dismantling was completed in 1998.

When the community of Karlstein am Main was created in 1975, an atomic symbol was included in the municipal coat of arms, due to the two reactors present in the newly formed town.