Nuclear powered rockets
or Swords into ploughshares
Some modifications to the design have been suggested that would include running the reactor so hot that it became liquid or even gaseous. This would result in problems containing the core but the result would be a nuclear rocket that could achieve a specific impulse of up to around 5000 s combined with a high thrust level, sufficient for fast interplanetary travel.
If you are really not worried about radioactive exhaust then why not try the Orion Drive? It’s incredibly 'dirty' but it has the added advantage that it uses up 'urplus nuclear bombs', just what is needed in these post cold war times. Its operation couldn't be simpler; just build a huge steel shield underneath your spacecraft and then explode a nuclear bomb directly underneath it, this pushes the shield and craft forwards. When you’re ready push out another bomb and let that off. The specific impulse could reach 100,000 s and the thrust is so high that you could even use it to take off from a planet’s surface so long as one could push the bombs out fast enough (and didn't mind leaving the surface a radioactive slag heap).
Not surprisingly the Orion Drive was a product of Cold War thinking along with unshielded nuclear ramjet cruise missiles and 'Mutually Assured Destruction' (MAD) and no sane person today would even contemplate such a device. Any nuclear power plant that is to considered for future spacecraft will have to be clean, sealed and so heavily armoured that it would be undamaged by either re-entry into the Earth's atmosphere or the accidental detonation of the entire launch vehicle. Humanity may be infinitely inventive but we also have a strong sense of self preservation.