[swiley]

Meh, we've put plenty of various kinds of nuclear reactors in space.

[m4rtink]

Also if you don't activate the reactor until after launch the fuel is still effectively just heavy metal.

[ianai]

Or even build it out there. Ie launch mostly pre built components. A ship that never has to worry about first leaving earths gravity well can have vastly different characteristics than something we’d launch from here.