Construction underway at North Korea's Yongbyon nuclear facility

SEOUL, July 27 (UPI) -- North Korea's nuclear facilities at Yongbyon may be undergoing maintenance or functioning at low-power levels, according to a satellite image analysis.

"The reactor appears to have been operating only sporadically since fall 2014 perhaps because the facility is aging," said William Mugford.

Mugford, a retired U.S. government satellite imagery analyst, published his findings Friday on 38 North, a Johns Hopkins University website dedicated to North Korea issues.

Despite diminished activity at the nuclear reactors, satellite images indicate North Korea is working on other projects at Yongbyon.

Construction appears to be underway at Yongbyon's incomplete Experimental Light Water Reactor that began in late spring 2015. The most recent imagery from July 2 indicated a transformer yard was nearing completion, Mugford said.

The yard would connect the electricity-producing reactor to the grid, but not all the needed equipment has been installed, the report stated.

A building on the site indicated Pyongyang had installed five cells that may be used to store explosive components of a nuclear weapon.

"The wall's five evenly spaced panels that are a different tone and texture than the rest of the wall are probably a decorative or imagery anomaly," the report stated.

"However, if there is a difference in the construction of the outside walls of the cells, they could be blow-out panels."

South Korean outlet Newsis reported the image includes evidence North Korea was transporting carbon dioxide by truck – most likely to be used in the reactor-cooling system.

Despite the signs of a downtime at the facilities, Mugford said other work continued at a "rapid pace" at the Uranium Enrichment complex at Yongbyon.

North Koreans have completed more than 75 percent of a roof on a large new building at the site, the report stated.