NASA's Kilopower Reactor Using Stirling Technology (KRUSTY) prototype fission reactor for space travel. The reactor uses uranium-235 to generate heat that is carried to the Stirling converters with passive sodium heat pipes.
More:
https://www.nasa.gov/directorates/spacetech/kilopower
Previously:
Nuclear reactor lid bolt - photo
Forging a nuclear reactor - photo
Starting a nuclear reactor - GIF
Bookmarks