( CNN ) -- North Korea has more uranium enrichment facilities than it has admitted to previously , a U.S. scientist charged Thursday . Prof. Siegfried Hecker of Stanford University told CNN his conclusion is based on his study of recent satellite images and other research , and what he saw when he was invited by North Korea to visit its Yongbyon nuclear power plant in 2010 to see its secret uranium enrichment program . `` When I saw the sophistication and scale of that uranium enrichment facility at Yongbyon in a building that I had been in before that housed something totally different , it was clear that they started the program long before the time that they had said , which was April 2009 . So my conclusion was , they had to have another site someplace else , '' said Hecker . The Stanford University scientist visited Yongbyon in November , 2010 , and reported that he saw a facility that housed 2,000 centrifuges and was producing low-enriched uranium . It could , however , he wrote then , `` be readily converted to produce highly-enriched uranium ( HEU ) bomb fuel . '' Hecker , an emeritus director of Los Alamos National Laboratory , also estimated that North Korea has between four and eight plutonium bombs , each of which could do damage similiar to that of the atomic bomb dropped on Nagasaki in Japan at the end of World War II . He is in South Korea attending international nuclear conferences ahead of the Seoul Nuclear Security Summit next week . North Korea , although not officially on the agenda , will be discussed in bilateral meetings at the summit , which will be attended by more than 50 heads of state , including U.S. President Barack Obama . Hecker 's assessment comes just weeks after the United States agreed to a deal with North Korea in which Pyongyang would allow United Nations weapons inspectors back into the country in exchange for food aid . North Korea ejected inspectors from the U.N. 's International Atomic Energy Agency from its Yongbyon nuclear complex in April 2009 , about a month before it conducted its second nuclear weapons test . However , per the new agreement , the inspectors will only have access to the sites to which North Korea has admitted having , not the ones Hecker suspects are still undisclosed . As part of the deal , Pyongyang also agreed to refrain from nuclear tests and long-range missile tests , but just last week announced it would launch a rocket with a satellite attached in mid-April . `` I was n't so surprised that they would do a space launch , '' said Hecker , who is also the co-director of the Center for International Security and Cooperation . `` What I was surprised at is , first of all they even agreed to a missile moratorium but then immediately turned around to say , 'We 're doing this space launch ' and quite frankly , that makes a mockery of the agreement . '' North Korea says it has a right to a peaceful space program and invited international space experts and journalists to witness the launch , the state-run Korean Central News Agency said last week . The satellite would be sent into orbit in mid-April to mark the 100th anniversary of the birth of Kim Il Sung , North Korea 's founder and current leader Kim Jong Un 's grandfather .
Kim Il Sung visited the Yongbyon nuclear plant in 2010 . The scientists estimates Pyongyang has 2,000 plutonium bombs . North Korea recently agreed to allow inspectors back in in exchange for food aid . It also agreed to halt nuclear and missile testing .