Nuclear bombs produce 3 effects on Earth
- Thermal flash
- Blast (caused by conversion of X-rays into heat)
In space, you need only concern yourself with the neutrons and X-rays.
Radiation Enhanced Bombs
According to Atomic Rockets: Radiation Flux:
A one megaton Enhanced-Radiation warhead (AKA "neutron bomb") will
deliver a threshold fatal neutron dose to an unshielded human at 300
Normal Nuclear Weapons
A non-radiation enhanced bomb produces much less neutron radiation but more X-ray radiation. A 1 kton nuclear bomb is borderline survivable at a range of 30 km due to the X-ray flux.
The survivability range of nuclear bombs scales (roughly) linearly with bomb yield and as the inverse square of distance between bomb and victim. Meaning a 1 mton bomb would be borderline survivable at a range of ~900 km due to X-ray flux.
All numbers are for unshielded humans. Shielding can significantly alter these numbers.
The effectiveness of X-Ray shielding is primarily determined by the amount of mass it contains (high atomic mass materials work slightly better than low atomic mass ones).
The effectiveness of neutron shielding is dependent upon the number of low atomic mass nuclei between the bomb and the victim. High atomic mass nuclei in your radiation shielding can make neutron radiation more difficult to manage.
I concur with Thucydides, anyone interested in this topic should go to Atomic Rockets and read all relevant sections. It includes a description of what a nuclear detonation would look like, what effects it'd have on spacecraft, etc.
As for survivability, my answer only considers a person wearing a minimal spacesuit for protection. The actual physical damage a 1 kton weapon would inflict on a body (human or otherwise) at a range of 30 km would be minimal. A person at that range would be hit with a lethal dose of radiation. Without medical care it might take them days or longer to die in an extremely unpleasant manner.
You should also realize that being outside the "deadly" zone listed above does not necessarily mean you will live. Radiation sickness is nasty and you'll require intense medical treatment in order to survive a large dose. Atomic Rockets has a Acute Radiation Syndrome Chart which tells you what symptoms you can expect from a given dosage. The chart gives you a probability of surviving any given dosage.
My numbers were for ~2.0+ Gray dosage - this gives you a survival probability of 35-40%.