If the reactor is not active, then it is not likely to have any explosion. If there really were a nuclear explosion, like a bomdb, whoever is close to it is pretty much screwed. The further you get from it, the better off you are.

If there is a nuclear blast, the first thing you'd want to do is shield yourself, but you better be able to do it immediately, because things move fast. If you cannot do it right now, then just get down and stay down. With a nuclear blast, you can expect to waves of pressure, one out and one back in, so let them pass over you.

After that occurs or in case of release, you want three things: distance, shielding and washing. The more distance between you and a source of radiation, the better. So, if you are close, you may want to just grab your BOB (bug out bag) and get out of there. The problem there is that you might be taking radiation with you, that's where the washing comes in. You need to wash it off, and just dump anything that might have radiation on it. Shielding of course is a barrier between you and the radiation, that keeps it off of you. It would be good to have a mask that filtered particles, but that also means the mask would be collecting particles in the filter. It would be good to have a suit that covered everything. These things need to get dumped and/or washed.

Additionally, you could try to make a safe room, and let clean up happen while you stay in it. It's very often a better idea to bug-in and not bug out. Those bugging out might to be taking radiation with them. If you want to have a safe room, then you want it to be as sturdy as possible, think fall out shelter, and you also want it to have a filtered air supply. That means you need a fan and power for that fan to draw air through a filter, push it into the room, and then let the air leak out of the room, keeping other unfiltered air from entering the space. You'd also need to have that room stocked with food, water, something to use for hygeine, etc. You might want to also do this in case of pandemic.

Look at the articles here for some more thoughts on this: http://www.alpharubicon.com/basicnbc/basicnbc.htm

Finally, I see you are new this week. Welcome and stick around.