Stalker: Call of Pripyat → Quests → Variable Psi-Radiation
However, at the tunnel exit, you'll encounter a controller. If you can't kill it quickly, shoot your former teammates; they'll quickly turn against you under the controller's influence. If you can kill it quickly... well, that's even better. And the scientists will reward you handsomely.
Once you've got decent gear, at least better than standard, the scientists at the base will offer you the chance to study something strange and anomalous, located in a distant tunnel in the northern part of the area. You'll be sent there with a group of mercenaries. The "strange and anomalous" turns out to be a useless artifact guarded by a horde of zombies.
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