(b) We can build bombs and pusher plates now ;)
Uranium melts at 1000 degrees centigrate, so thats your rough upper limit for how much your reactor can heat hydrogen. If your Uranium melts, it will be leaving the rocket with the hydrogen and spreading radiation everywhere. Basically like Orion but worse.
A normal chemical rocket reaches higher temperatures.
A nuclear bomb will heat anything to millions of degrees. Much more efficient, and any exhaust is moving so fast it will escape the solar system without bothering anyone, provided its pointed the right way.
Chemical rockets need oxygen, which is heavy and limits Isp. Nuclear rockets can use pure hydrogen and achieve higher Isp.
There are some high temperature nuclear rocket engine designs though, like nuclear gas core rockets. They're crazy reactor designs you would never try running down here inside a biosphere but have energy densities far higher than anything else save a bomb.
Explosions are cool. Orion is space-orc technology, extremely cool.
And while no nuclear thermal rocket engine has yet flown, there have been ground tests.