It is true that during the 1960s and 1970s, scientists designed a spacecraft based on nuclear propulsion, but it was never built.
Some of the examples of spacecrafts based on nuclear propulsion that were proposed are the following (in no particular order):
Project Daedalus
Project Orion
AIMStar
Project Longshot
External Pulsed Plasma Propulsion
ICAN-II
