A joint Department of Energy and NASA team has demonstrated a simple, robust fission reactor prototype [note: see Comments for more accurate and complete description] intended for development for future space exploration missions. The DUFF (Demonstration Using Flattop Fissions) experiment represents the first demonstration in the United State-since 1965-of a space nuclear reactor system to produce electricity.
The uranium-powered reactor is the first use of a "heat pipe" to cool a small nuclear reactor (measuring one foot!) and power a Stirling engine. The following short video from Los Alamos National Laboratory explains the hows and whys:
Many future space missions will only be feasible with the use of reliable and safe nuclear energy, and this proof-of-concept is a steppingstone toward that future.