Subsequent commentary knocks down that speculation and comes to the (reassuring!) conclusion that it is very likely just a big industrial plant. Eg, Mark Hibbs of Nuclear Fuel, quoted on Arms Control Wonk, says this:
And from Arms Control Verification.org, via The Interpreter in Sydney, extra photos and commentary supporting the same "less than meetings the eye" conclusion. Eg:
According to some information that sources said has been made available to Western governments and the IAEA, the "box" in the photos is likely not a reactor but a nonnuclear industrial workshop or machinery center.
That determination, the sources said, follows from the absence of certain "overhead signatures" for a reactor in the photos and from specific information derived from firsthand knowledge of the site and its activities, deemed to be highly reliable.
'We can conclude that it's not a reactor with near certainty," the Western analyst said.
We learned from two sources, independent from each other, that the box-like building has been under scrutiny by the IAEA's [International Atomic Energy Agency] Department of Safeguards for quite some time, and that the department is nearly certain that the building does not serve any nuclear programme. An official, associated with a Western intelligence agency, later told us that, "we've been looking at that site for years, since construction started. You cannot hide a reactor in a low building without a basement level". A relatively recent visit to the facility has reportedly confirmed with '99 per cent confidence' that it is a machine shop..We'll take our reassuring news where we can find it.