An unnamed ROK intel official was quoted in a 19 March Yonhap News Service story about the status of North Korea’s Yongbyon nuclear reactor.

As of that day, the reactor was “operating normally,” according to “a ranking official at the National Intelligence Service.”

Even if this information is a little outdated, I think it’s worth noting that, according to the NIS official, “a constant stream of steam spotted from its cooling tower” showed that the reactor was still operational.

The official added that

“Whether the Yongbyon reactor is in operation or not can be determined by the existence of the cooling tower steam. Depending on some operational and weather conditions, however, such steam may not be visible“…

“Therefore, North Korea’s suspension of the Yongbyon reactor will be confirmed by the halt of the cooling tower steam or on-the-spot inspection by International Atomic Energy Agency (IAEA) inspectors,” he explained.

I will add that it’s more accurate to say that the IAEA will monitor the shutdown, per the 13 February agreement.

The cooling tower signature is, of course, well known. See, for example, this ISIS brieifng about North Korea’s plutonium production.