An American nuclear scientist says he was shown a vast nuclear facility when he visited North Korea last week. The scientist Siegfried Hecker told the New York Times that he had been stunned by how sophisticated the new plant was.He was shown what he described as hundreds and hundreds of centrifuges were already installed and running in the plant which can be used to make nuclear weapons.American officials say that in April 2009 when international weapons inspectors were expelled from North Korea this plant did not exist. In recent months North Korea has expressed conditional willingness to return to dialogue.