if you mean the bunker you can find in the irradiated zone, the code is found inside the house you may explore before reaching the city with said irradiated zone, there you can get a code with you will have to write down somewhere IRL to keep track of it, then you can use it to break inside, the loot isn't too worth it if you ask me, but it's there for the taking
also bring something to raise your HP with you if you do go there