NASA and DARPA have been funding studies for a new generation of fission-powered rockets.[1][2] They talk about launching a demo in 2027. BWXT (good old Babcock and Wilcox, makers of hundreds of submarine nuclear power plants) and Lockheed-Martin are the primary contractors.[3]

But the big bucks are still going into the Senate Launch System.

[1] https://www.nasa.gov/tdm/space-nuclear-propulsion/

[2] https://www.youtube.com/watch?v=nNnsnyIvzps

[3] https://www.bwxt.com/what-we-do/advanced-technologies/space-...