Senator John Kerry on Sunday accused the Bush administration of letting ‘‘a nuclear nightmare’’ develop by refusing to deal with North Korea when it assumed office.In an interview, he argued that President Bush’s preoccupation with Iraq let the North Korean crisis fester to the point that there were now indications that the country may be preparing to test a plutonium bomb. Secretary of State Colin L. Powell said on Sunday that the explosion in the region of Ryanggang last week was ‘‘not any kind of nuclear event’’, but he confirmed a report in The New York Times that ‘‘there is activity going on at a potential nuclear test site,’’ adding that ‘‘we’re monitoring this’’.While intelligence analysts are still debating whether the activity is a harbinger of a test, Kerry insisted that the fact that the North is threatening such an action is a sign of failed diplomacy. ‘‘I think that this is one of the most serious failures and challenges to the security of the US and it really underscores the way in which George Bush talks the game but doesn’t deliver,’’said Kerry.In the past, he has accused Bush of ignoring a far larger nuclear threat in North Korea because of his determination to oust Saddam Hussein.‘‘They have taken their eye off the real ball,’’ he said. ‘‘They took it off in Afghanistan and shifted it to Iraq. They took it off in North Korea and shifted it to Iraq. They took it off in Russia, and nuclear materials there, and shifted it to Iraq.’’ —(NYT)