US Scientist Says He Saw Hundreds Of Centrifuges At Enrichment Facility David E Sanger Washington: North Korea showed a visiting American nuclear scientist earlier this month a vast new facility it secretly and rapidly built to enrich uranium, confronting the Obama administration with the prospect that the country is preparing to expand its nuclear arsenal or build a far more powerful type of atomic bomb. Whether the calculated revelation is a negotiating ploy by North Korea or a signal that it plans to accelerate its weapons program even as it goes through a perilous leadership change, it creates a new challenge for President Obama at a moment when his program for gradual, global nuclear disarmament appears imperiled at home and abroad. The administration hurriedly began to brief allies and lawmakers and braced for an international debate. The scientist, Siegfried S Hecker, a Stanford professor who previously directed the Los Alamos National Laboratory, said that he had been