Jane's Defence Weekly, examining images taken on August 6 by the GeoEye-1 satellite, said the dome, which had been lying on the ground at the Yongbyon complex, has now been put in place.
"North Korea has taken a major step in its mission to build a modern, indigenous nuclear reactor with a 21-metre diameter dome now having been emplaced over the reactor building," said image analyst Allison Puccioni.
"The emplacement of the dome is a significant development, although it may take several more years for the facility to be completed and brought into full operation."
The secretive, communist state shut down its plutonium-producing reactor at Yongbyon, some 90 kilometres (56 miles) north of the capital Pyongyang, in 2007 as part of an international disarmament deal which it later abandoned.
North Korea first disclosed in 2010 to visiting US scientists that it was working on a new light-water reactor at Yongbyon, ostensibly for civilian purposes.
The impoverished nation desperately needs energy, but the reactor could also be run to produce weapons-grade plutonium.
The reclusive regime has based its nuclear weapons programme on plutonium and has tested two nuclear bombs since 2006, triggering repeated international crises.
Pyongyang has also said that it is building a uranium enrichment plant to produce low-enriched fuel for the new reactor.
Scientists believe the accompanying site could be converted to produce highly enriched uranium, giving North Korea a second way to make nuclear weapons.
The steel-reinforced concrete reactor dome was placed on top of the reactor's containment structure "at some point between June 21 and August 6," Puccioni said.
"The cloud cover from weather patterns that caused extensive flooding in the region during late June and July precluded imagery analysis over the territory at Yongbyon during that time.
"Indeed, the river levels around the reactor building on August 6 were significantly higher than previously observed during any season over the past four years, extending to within 100 metres of the reactor building itself."