The Atomic Rockets website has a whole section devoted to describing the effects of nuclear explosions in space.
Basically, conventional nuclear weapons are far less destructive in space than they are in an atmosphere. However, there are some neat things you can do to make them more deadly.
Chief among these ideas are:
- Form a nuclear shaped charge (called Casaba Howitzer).
- Use enhanced radiation (i.e. neutron) bombs.
Here's an interesting description of how a nuclear explosion attack might look in space:
First off, the weapon itself. A nuclear explosion in space, will look
pretty much like a Very Very Bright flashbulb going off. The effects
are instantaneous or nearly so. There is no fireball. The gaseous
remains of the weapon may be incandescent, but they are also expanding
at about a thousand kilometers per second, so one frame after
detonation they will have dissipated to the point of invisibility.
Just a flash.
The effects on the ship itself, those are a bit more visible. If
you're getting impulsive shock damage, you will by definition see hot
gas boiling off from the surface. Again, the effect is instantaneous,
but this time the vapor will expand at maybe one kilometer per second,
so depending on the scale you might be able to see some of this
action. But don't blink; it will be quick.
Next is spallation - shocks will bounce back and forth through the
skin of the target, probably tearing chunks off both sides. Some of
these may come off at mere hundreds of meters per second. And they
will be hot, red- or maybe even white-hot depending on the material.
There's a lot more interesting material at the link. Including discussion of radiation survival and other aspects.