Q) What do you do with the reactor when you have used up the water?
A) It is in space. Doesn’t really matter much but toss it into the sun if it bothers you and it is out of fuel or send it back out for another asteroid. Not a complicated setup: see: Direct Energy Conversion in Fusion Reactors, by Ralph W. Moir, Lawrence Livermore Laboratories, Livermore, CA
Q) Who is going to operate and maintain the reactor?
Robots. The RF round trip when it is way out there is too long for remote controls.
Q) What happens if the reactor has an accident?
A) Not much. It is in space. Not near space. Way way out there. Heck even if it went supercritical where k > 1 as in kaboom it would be a little flash in the sky. The scary thing would be if the asteroid hit the earth hence going for a lunar orbit. Not a big deal if we accidentally hit the moon.
We haven’t quite got a working fusion reactor yet. One that is autonomous that we can put is space I think is a few decades off.
If we are talking about fission reactors (all that is available today) we are talking about an expensive piece of hardware and if you include autonomous robot reactor, we’re talking big money.
The difficult thing about fission reactors is that after a fairly short time at power the reactor will continue to produce a prodigious amount of heat even when shutdown.
If that heat can not be removed from the reactor core the core will destroy itself.
The robot reactor you’re talking about sounds like a multi-billion dollar machine. I don’t think we want that bit of tech out there on its own with no one to take care of it.