According to this this story:

A large explosion occurred in the northern part of North Korea, sending a huge column of smoke into the air on an important anniversary of the communist regime, a South Korean news agency reported Sunday.

Also:

“We understand that a mushroom-shaped cloud about 3.5- to 4-kilometer (2.2 miles to 2.5 miles) in diameter was monitored during the explosion,” the source in Seoul told Yonhap. Yonhap described the source as “reliable.”