The Seattle law firm that served as special counsel to the Washington Public Power Supply System agreed to a $7.25-million settlement with buyers of WPPSS bonds issued to build two abandoned nuclear power plants. The agreement settles the class action against the firm of Houghton Cluck Coughlin & Riley. An official in the case said the award, which must be approved by the court, would be paid by the law firm's insurance carriers. The firm was a defendant in the case along with WPPSS, engineers on the project and the 88 utilities who participated. WPPSS, headquartered in Richland, Wash., attempted to build five nuclear power plants in the 1970s. Of those, only one is operating. Two have been mothballed; the others were never built. The default on the bonds used to start construction on No. 4 and No. 5 was the biggest in the history of the municipal bond market.