It may also be a sign of frustration from Pyongyang that weeks of posturing and threats haven't driven U.S. and South Korean negotiators back to nuclear disarmament-for-aid talks.
"What they really want is a safety blanket and a blackmail tool," Ralph Cossa, president of the Pacific Forum CSIS think tank in Hawaii, said in an email.
The announcement "is primarily political, designed to signal strength and intimidate. It should not necessarily be seen as a revelation about North Korea's capabilities and true intent," Greg Thielmann, a senior fellow at the Arms Control Association in Washington, said in an email.
A reactor at the main Nyongbyon nuclear complex could eventually make, in one year, enough plutonium to power one bomb. It was shuttered as part of international disarmament talks in 2007, its cooling tower blown up in a dramatic show of commitment to a now-scrapped nuclear deal. North Korea shocked many when in 2010 it unveiled an industrial-scale uranium enrichment facility, which gives it an alternative route to create bombs.
Estimates on restarting the vital facilities at the plutonium reactor vary from three months to a year, depending on the expert.
North Korea has already begun construction at the reactor and it could be back in operation sooner than expected, according to a U.S. research institute that analyzed recent commercial satellite imagery of Nyongbyon. Rebuilding the cooling tower would take six months, but a March 27 photo shows building work may have started for an alternative cooling system that could take just weeks, the U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies said Wednesday.
But even if the reactor is now up and running, Cossa estimates it would be two to three years before scientists could obtain more plutonium for bombs.