Imagery captured on August 22 indicates high level of vehicle activity at the 5MWe reactor and radiochemical laboratory complex of the Yongbyon Nuclear Scientific Research Centre which was not observed previously, US-Korea Institute of Johns Hopkins University wrote on its blog "38 North" yesterday.
The two areas at the facility produce plutonium for building nuclear weapons.
Vehicle tracks at the reactor extended into ground level of the building and out to a road where large trucks can easily manoeuvre, Kyodo news agency reported, citing the blog.
At the complex, where plutonium is chemically separated from uranium and other radioactive materials, heavy vehicle tracks enter the facility, circle a support building and then extend to the spent fuel receiving building, they added.
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The institute also said that while determining the reason for such an activity was impossible, possible explanations include maintenance or renovation activities as well as the moving of contaminated from the reactor to the reprocessing complex.
The reactor was scheduled to start in April, 2013 but many problems have apparently been encountered in running the aged facility, according to the blog.