"They were saying that they are constructing a small experimental light-water reactor (at Yongbyon), eventually of the size of about 25 to 30 megawatts," Siegfried Hecker said in footage carried by Japanese public broadcaster NHK.
Light-water reactors are generally used for civilian nuclear purposes. Experts say it is relatively difficult to extract plutonium from them to make atomic weapons.
Some Seoul analysts say the North may be stressing its overall atomic expertise in hopes of prodding the United States into resuming stalled six-nation nuclear disarmament talks.
Foreign ministry spokesman Kim Young-Sun said the South has no information on its neighbour's capabilities in light-water reactors.
However, Kim said the
, if confirmed, "would be going contrary to expectations from members of the six-party talks and the international community."
The North promised to give up all nuclear weapons and nuclear-related programmes in a 2005 six-party agreement, Kim said.
"It is important for the North to sincerely carry out the promise and international obligations for peace and stability on the Korean peninsula."
The North quit the talks in April 2009 and staged its second nuclear test a month later.
In recent months it has expressed conditional willingness to resume talks, but in return it wants a lifting of sanctions and a US commitment to hold separate talks on a peace treaty.
The United States says the North must show willingness to denuclearise and mend ties with South Korea before the six-party talks can resume.
In 1994 the United States reached an agreement with the North Korea under which several countries were to build it two light-water reactors to generate electricity.
In return, the North was to shut down its plutonium-producing reactor at Yongbyon.
The deal broke down in 2002. The light-water reactors were never completed and the North restarted its original reactor.