Large Transport; Low Wing; 2 Turbojet Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 6
Situational Awareness
ATC Issue All Types; Conflict NMAC
Air Traffic Control Separated Traffic; Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert
Airspace Structure; Human Factors
Took a handoff on [Aircraft X] from S46. Traffic volume was steady but normal. In my scan I saw conflicting VFR traffic for [Aircraft X] at 11;500 feet (unverified). I expected S46 to stop [Aircraft X] under the VFR traffic and wait for them to pass before shipping them to me; they did not. [Aircraft X] checked on around 10;000 and my first transmission was to stop their climb immediately and I gave them traffic. I did not use proper phraseology in fear of time constraints. I gave [Aircraft X] traffic again and he reported them in sight. I cleared [Aircraft X] to climb at pilot discretion once I saw targets pass. In my career this is the closest I've seen two airplanes come nose to nose in an uncontrolled manner. The likelihood of collision in my opinion was severe.Raise the top of the bravo airspace to 150. There is no reason a VFR [aircraft] should fly uncontrolled through the top of highly congested airspace and not talk to a controller.
ZSE ARTCC Controller reported Approach handed off an aircraft into a traffic conflict with a nonparticipant aircraft within Center's airspace.