You don't need anything special. In space, there won't be any kind of shockwave one inch away from the exploding ship (you can be hit by the shrapnel, though), so the only thing you have to worry is about gamma radiation.
The bad news is that atmosphere is a great absorber of gamma rays, and in the near-vacuum of space the gamma rays will have virtually infinite range. But of course, the amount of energy generated by the explosion will be dispersed by the surface of the sphere whose radius is the distance you are away from the bomb. At one kilometer away, the amount of gamma rays that are going to hit you are not that much, and probably they are not the right energy to cause much harm to your body - they'll just past through. I would be more worried about secondhand radiation by irradiated metals on your ship but bodies are great to deal with radiation as long as it comes in low dosages over time.
If you want to add some credibility, just state that some 5% of the ship's crew gets ill, some of them dying, but your protagonist is not one of them. If you read the accounts of the two incidents of the infamous "demon's core" you'll see that in both cases there were people around, very close to the source, that went seemingly unscathed while other people died. That's just how particle physics work. Sure, the closer you are to the source, the higher the probabilities you are, but it's a game of statistics. Just state that some of your crew is affected and some are not so, and be done with it.