When North Korea announced in 2009 that it would build light water reactors and start enriching uranium for their fuel, it was the beginning of what experts called a major transition in the North's nuclear weapons program from relying solely on the production of plutonium to adding enriched uranium as a new source of bomb fuel to augment its limited plutonium stockpile.
The sensitivity arises in part from another concern that is in the classified report but that Mr. Obama, Mr. Gates and Mrs. Clinton avoided discussing: that even as Pakistan's civilian government teeters on the edge, an insider could slip nuclear fuel out of its laboratories, which was the source of bomb technology a decade ago for Iran, North Korea and Libya.
A nuclear device with highly enriched uranium smuggled into the United States poses the greatest risk of mass destruction by a terrorist, but Professor Wein does not address the most important method for reducing the risk: eliminating sources of bomb-grade materials.
Israeli soldiers searched Nablus for explosives laboratories and suspected militants, retaliating against what it called a possible source of the bomb that killed seven people at Hebrew University.
The five-megawatt, graphite-moderated reactor, which experts say would require significant effort to bring back on line, had been the main source of plutonium bomb fuel until it was shut down under the deal with the United States.
Penning the old city of Nablus with scores of tanks, bulldozers and armored vehicles, Israel sent soldiers house to house today in pursuit of explosives laboratories and suspected militants, acting against what it called a possible source of the bomb that killed seven people, including five Americans, at Hebrew University on Wednesday.
A key question is whether the device detonated used some of the North's limited stockpile of plutonium or was a uranium bomb, which would be proof that the country has acquired a second, and more accessible, source of nuclear bomb-making materials.
Over the decades, scholars identified Klaus Fuchs as one possible source of H-bomb intelligence.
Readers said that, at a time of growing tensions between the United States and Iran, the article failed to offer persuasive evidence that Iran was the source of the bombs, known as explosively formed penetrators, E.F.P.'s, which can go through the armor of Humvees.
Initially there was confusion over the source of the bombing.
But investigators have no doubt that he was the source of the digitized bomb design.
