He said all the technical questions concerning the project had been solved apart from finding a solution on how to cool the nuclear reactor.
So, they’ve solved everything, except one of the biggest problems with sending systems into space. Not to minimize all the other incredibly difficult parts of spaceflight, but cooling is one of the biggest problems there is. And cooling a nuclear reactor is difficult enough on Earth where you can just pump a bunch of water around the reactor and then dump said water back out into the ecosystem. In space, there is nothing to convect or conduct the heat away. So, you either have to find a way to radiate away excess heat, which is really hard to do fast enough; or, you have to boil off a gas and dump that into space. Which just changes the material you have to worry about running out of. It’s not that the problem can’t be solved, but that’s still a pretty high bar to get over.