PDX Controller described an issued go around event when traffic on final for PDX Runway 10R was in conflict with an unknown suspected Pearson Field VFR target; the reporter advocating the closure of Pearson Airport.

Narrative

PIREPS at the time indicated cloud tops 2;000 FT and bases 700 FT on the FAC. Aircraft X was on the ILS10R approach to PDX. I noticed a 1;200 (VFR) target in the vicinity of Pearson Field; turning eastbound as if to land on Runway 8 at VUO. The unverified Mode-C was indicating approximately 1;500 FT. The DH8 was nearing the VFR target and at 3 miles west of PDX the [Dash 8] would be immediately adjacent to Pearson. At this point the [Dash 8] flight was also indicating descending through 1;500 FT. I became concerned because the two targets were appearing to merge; with less than 500 FT vertical separation. I issued at least two Traffic Alerts to [the Dash 8]. The pilot reported no visual contact with the traffic. Somewhat surprisingly; the pilot never advised he had received a RA from his TCAS. As the traffic was still closing; I canceled Aircraft X's approach clearance when the VFR target was about 1/2 mile north of the DH8; same indicated altitude; and issued a climb to 2;000 FT and vectored the aircraft right heading 210. Coordination for this go-around was accomplished with the Hood and Final P80 sectors. Aircraft X's second approach was uneventful. Recommendation: shut down Pearson Field. That airport is notorious for generating Traffic Alerts; TCAS RAs and concerns from pilots flying into PDX. This airport being 3 miles west of PDX underneath two ILS approach courses is an accident waiting to happen.