Reprocessing the remaining spent fuel rods at Yongbyon would give it one more bomb.
North Korea also said on Tuesday the Yongbyon reactor would provide electricity for the energy-starved country.
Reports suggest that the North Koreans may allow the delegation into the Yongbyon nuclear facility.
CENTERFORSECURITYPOLICY: Deja vu all over again in North Korea
Last week, U.S. officials say, North Korea apparently began steps to restart the Yongbyon reactor.
The destroyed installation was a virtual clone of North Korea's Yongbyon plutonium production facility.
In response to the uranium dispute, North Korea reactivated the Yongbyon plant and expelled IAEA inspectors.
Yet all the attention on Yongbyon may be an intentional distraction by North Korea.
The US has urged China and Russia to apply pressure on Pyongyang to change its Yongbyon plans.
BBC: North Korea 'crisis gone too far' says UN's Ban Ki-moon
North Korea also said on Tuesday the Yongbyon reactor would provide electricity for the energy starved country.
The BBC's Lucy Williamson in Seoul says Yongbyon has provided the North with successful leverage in the past.
BBC: North Korea 'crisis gone too far' says UN's Ban Ki-moon
The work is taking place near North Korea's old nuclear site in Yongbyon, just north of the capital Pyongyang.
In the Spring of that year, the Clinton administration seriously considered air strikes on North Korea's small nuclear reactor at Yongbyon.
The uranium-enrichment facility at Yongbyon is North Korea's only declared such facility, but experts suspect Pyongyang may have other enrichment centers.
One big potential stumbling block after the initial shutting down of the Yongbyon plant will be agreeing what to tackle next.
We were not taken to the Yongbyon nuclear facility or their side of the DMZ even though we and Richardson repeatedly asked.
In Washington, a State Department spokesman didn't elaborate Wednesday on the reports that North Korea had resumed operating its Yongbyon nuclear facility.
CNN: Clinton slams N. Korea's rhetoric, 'belligerent' actions
Harrison also said the North demanded the completion of the light-water reactors as compensation for the dismantling of the Yongbyon nuclear reactor.
Also, concurrently, North Korea has been systematically disabling the Yongbyon nuclear reactor.
North Korea had shut down its nuclear reactor in Yongbyon in July.
White House spokesman Jay Carney said the decision to reopen the Yongbyon reactor was "another indication" of North Korea "violating its international obligations".
BBC: North Korea 'crisis gone too far' says UN's Ban Ki-moon
On Saturday, the New York Times reported that North Korea showed an American scientist a recently built uranium enrichment facility at Yongbyon.
As a statement and a commitment of goodwill and intent, North Korea is going to blow up its cooling tower at Yongbyon ...
He is likely to be just as canny over disabling his Yongbyon facilities, says Mark Fitzpatrick of the London-based International Institute for Strategic Studies.
Recall that the North promised to close Yongbyon in 2007 after the U.S. and South Korea caved to an earlier series of North Korean threats.
Satellite imagery in recent months has shown construction work at the Yongbyon plant but it's not clear how much progress North Korea has already made.
In 1994, when North Korea put the Yongbyon reactor on the negotiating table, it also began talks to get HEU technology from Pakistan via A.Q.
Diplomats say the U.S. and North Korea's neighbors are pushing for North Korea to shut down its main nuclear reactor at Yongbyon and submit to international inspections.
Now the North is going to restart Yongbyon too, and this week its rubber-stamp legislature passed laws emphasizing that its nuclear programs can never be negotiated away.
In exchange for energy and financial assistance, North Korea last year agreed to shut down its Yongbyon nuclear reactor and declare the scope of its nuclear program.
Tests to the power system of a reprocessing plant at Yongbyon suggest that North Korea may soon start extracting plutonium from a stock of 8, 000 spent fuel-rods.
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