Date: 201701
Local time of day: 1201-1800
Location: S46.TRACON, WA
Light: Daylight
Aircraft: Large Transport; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E S46
ATC / advisory: TRACON S46
Second aircraft: Medium Large Transport
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness; Distraction; Communication Breakdown
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Separated Traffic; Air Traffic Control Issued New Clearance
Contributing factors: Human Factors; Procedure
Primary problem: Procedure

Narrative: I was working the arrival East and West sector combined at Seattle TRACON. Internally the TRACON had observed a VFR aircraft (Aircraft Z) squawking 1200 slowly climbing through the airspace. The aircraft appeared to have departed an airport to the north of SEA. Eventually the aircraft appeared to level off at 15;500 VFR. I tagged up the aircraft as 'TFC1' and showed them to the departure east sector (whose airspace the aircraft was transiting through) even though we only own up to 15;000 we; climb aircraft to 15;000 before shipping them to Seattle Center (ZSE).The aircraft was heading southbound directly for Mt. Rainer which is a common occurrence. Since I was working the arrival east sector the aircraft was on a course directly towards the CHINS TWO arrival into SEA. Sector 1 had two aircraft inbound on the CHINS arrival. I put a minimum separation on the VFR aircraft and the first arrival which was Aircraft X. Minimum separation had them at 3.0 miles; since Aircraft X was descending out of the flight levels I called over to sector 1 and made sure they were looking at the VFR aircraft north of Mt. Rainier at 15;500. They did in fact say they observe the aircraft.Shortly after that ZSE started the handoff on Aircraft X and behind them Aircraft Y. I took RADAR on both of the aircraft. ZSE shipped both the aircraft and immediately after communication transfer I amended Aircraft X to stop their descent and 17;000 and expedited Aircraft Y to 12;000. The VFR aircraft passed off Aircraft X's right side no less than 2 miles and passed directly overhead Aircraft Y. During all of this sector 1 was repeatedly calling over the shout line. I wasn't able to answer so Final controller answered the shout line. During this time the ZSE controller was giving me control for both aircraft to avoid the VFR aircraft.This was extremely lazy; unsafe; unprofessional; and a clear example of 'passing the buck' to another controller. IFR vs VFR is a top 5 now in the agency and it seems that ZSE is not on board with this concept. If no action would have been taking there would have been 2 TCAS RAs with two IFR arrivals. To make this worse I told ZSE about the aircraft 3 minutes prior to it being an issue! Unsatisfactory. Have ZSE review the top 5 in the FAA; especially IFR vs. VFR.




Synopsis: Seattle TRACON Controller reported an unsafe situation between two IFR and a VFR aircraft.