Space is full of radiation.
Its probably one of the greatest dangers we face in space exploration that we do not have a good solution for .
In 1962 the US Detonated a nuke in space .
The radiation from this is just background noise.
While there are likely to be some highly radioactive particles as a result of the explosion these will disperse so quickly that there would be far greater damage from the explosion in any area where these would be of a concentration to be a concern.
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In space, you'd expect much the same effect from blast, radiation, and heat as in atmosphere.
Of course, with no atmosphere to "carry" the blast effect, it's going to be greatly limited.
There will be little or no effect from what you might call "shrapnel"...
The bomb itself is going to be vaporized and you're not going to get chunks of stuff flying around at high velocity.
Both we and the Russians experimented with the effects of nuclear weapons in space, if you get a chance there's a documentary on Netflix about the development of the hydrogen bomb.
It covers these space-based tests.
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Compared to everyday celestial events, the energy of an H-bomb is laughably tiny.
The sun is something like a billion H-bombs per second.
A gamma ray burst may be a billion-billion times that.
So anything humans can construct is like an single ant (not a colony) attacking mount Everest (or if you want to compare to a gammaray burst a single virus attacking the enture mountain range).
It just won't be noticable except in a small local area.
The one thing a nuke does that is different, is generate fission products, which for the most part aren't found in space (
although other radioactive isotops yes).
And even these isotopes, are probably generated in much greater volume by supernovas, but unless you are in the neighborhood of a recently exploded one, they have long since decayed away.
The earth based space tests, were significant, because they happened in the very rarified earths exosphere/magnetosphere, and generated high energy particles (mostly electrons), trapped in the earths radiation belts.
At the time it was feared we might have to wait decades for the activity to settle down to the undisturbed state.
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The biggest hazard would have to be any macroscopic particles left over.
Gamma rays, X-rays, and other ionizing radiation would spread out at the speed of light, or nearly so for electrons, alphas, etc., and within moments be less than the radiation that always present in space.
After an hour, any such radiation will be smeared out to a volume as large as the orbit of Jupiter.
I.e., it's effectively gone .
You'd have to worry more about minor solar flares than space nukes.
On the other hand, although most of what's in the immediate vicinity of the explosion would be vaporised to atoms (or parts thereof), it seems possible that you could end up with significant amounts of debris.
Perhaps the solar wind would blow the smaller ones away, much like the effect with a comet, but some of the larger chunks would present major navigational hazards.
NASA gets very concerned when millimeter sized bits float away in orbit.
Obviously anything larger will be more of a concern.
If you're moving around at planetary speeds, 10's of km/sec,
the kinetic energy in such space junk could easily put a hole your hull.
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Purely for informational purpose, there has/had been some consideration of using nuclear detonations as primary propulsion for a space-faring craft.
Wikipedia:
Project Orion (nuclear propulsion) Short term effects: propulsion (most likely)!
Long term effects: ?????
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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.
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Obviously intense EM rad close by but the intensity decreases as the root of distance from the point source so unless you were close, as others have said, normal space rad would be a greater threat.
No debris either as all matter would be vaporized and ionized.
Would prob be kind of like a ripple in the solar wind.
Really it's far worse exploding one at ground level and creating all those isotopic by products with nowhere to go.
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I think it looks close to the video linked in this article because what was filmed in Siberia on November 14, 2014, a very big flash, was probably a high-altitude nuclear test - perhaps 50 km to 80 kms above the ground.
Take a look at the video of the flash
: it expands very quickly then disappears.
The light is extremely bright.
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In the Honor Harrington universe, ship-to-ship combat takes two forms: missiles and direct energy weapons.  Missiles come in two forms - bomb-pumped lasers and contact nukes.  Both use multi-megaton nuclear initiations to damage enemy ships.  While this sort of event on a planetary surface will obviously have lasting effects, what long-term effects could be expected in a vacuum environment, aside from the destructive force associated with the explosions themselves, and the resulting EMP?  Especially, would there be lingering (or spreading) radiation?
Effects of nuclear explosions in space?
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In space, you would expect much the same effect from blast, radiation, and heat as in an atmosphere.  Of course, with no atmosphere to "carry" the blast effect, it's going to be greatly limited. The bomb itself would be vaporized and there would be no chunks flying around at high velocity. Although most of what's in the immediate vicinity of the explosion would be vaporized to atoms (or parts thereof), it seems possible that you could end up with significant amounts of debris. Basically, conventional nuclear weapons are far less destructive in space than they are in an atmosphere. Compared to everyday celestial events, the energy of an H-bomb is tiny. The sun is the equivalent of something like a billion H-bombs per second. The dangers from nuclear explosion in space are small end there power is dwarfed by normal celestial event. For example, the sun is like a billion H-bombs. The greatest danger after a nuclear explosion may come from any debris.
Nuclear weapons are far less harmful in space than they are on Earth. The greatest danger may come from any macroscopic particles left over. Normal celestial events are much more powerful than a nuclear explosion. The sun is like a billion H-bombs per second.