Can a nuclear reactor be used as a space-based weapon?

Imagine you have a ship, running on nuclear reactor. We will assume this ship uses some kind of advanced technology that allows nuclear reactors to function in gravity-less environment.

This ship is in critical shape, having fought 3 alien ships at the same time. The nuclear reactor is going critical, so the captain wants to eject it and have it crash against an alien ship. Crashing the ship itself is against his good judgement, as he wishes to give his crew a chance to survive.

We shall assume that in such a split second decision, it is impossible to calculate detonation timer and it is too late to fit a proximity or remote detonator onto the nuclear reactor.

Now my question,

1) If the critical nuclear reactor crash against an alien ship, will it achieve the same devastating effect as a nuclear weapon as seen in Battlestar Galactica for example?

2) If this happens in the orbit of Earth, will it have any effect on those on the surface, or will it simply be turn into harmless microwave?