NASA to test prototype Kilopower nuclear reactor
This is similar to the "Stirling radioisotope generator" [1] but it is an actual nuclear reactor (albeit a small one).  Both systems are a response to the same fundamental problem, the dwindling supply of Plutonium-238 [2].  The Stirling radioisotope generator uses Pu-238 more efficiently than thermoelectric RTGs, and the nuclear reactor from the article dispenses with Pu-238 altogether.[1] https://en.wikipedia.org/wiki/Stirling_radioisotope_generato...[2] https://en.wikipedia.org/wiki/Plutonium-238