Start at the entrance near diode's workshop and check out the crane hill to the southwest, but watch your step since radiation's pretty nasty here. Finding the scaffolding path north of the hill keeps you safer, and following those kid-shaped signs past a wrecked white minibus leads you into some caves, which is good since they shield from radiation. Moving through the metal debris needs careful jumping and crouching, especially above those toxic spots but stay focused and you'll emerge near some broken crane scaffolding. Climb that ladder straight up to where richter's hanging out, and you'll find what you need. Might be interesting that taking med supplies helps if you slip into hot zones, and keeping an eye on those kid signs really makes the whole climb much easier. The flash drive up there points to varan's spot at the plant if you're curious about where to head next.