Russia, facing a mounting outcry to keep its weapons-grade plutonium under tighter control, said it had arrested smugglers on its own soil. Police said they had detained three men Aug.12 in Kaliningrad, a western outpost on the Polish border, after they tried to sell a 132-pound container of the radioactive material for $1 million. The would-be buyers included Poles, Germans and Russians. The disclosure of the six-day-old arrests gave Moscow brief cover as officials from Germany, the U.S. and other Western countries demanded cooperation on tracking smugglers and securing nuclear power plants. Meanwhile, persistent Russian denials that any nuclear material came from the motherland sounded hollower. Euratom, the agency that keeps tabs on the 300 tons of plutonium officially in the European Union, traced a cache of plutonium seized in Germany to Russian military sites.