Missing is hyperbolic headline writing. What's happened is the site had gone off-line to international review. Has anything "gone missing" or not? We don't know.
Nobody thinks Russia is short of neutrons. These things are valuable to the oil industry and manufacturing. Sure, They can make dirty bombs but they can make money too.
Do people realise insecticide factories can make CBW? Do people reaize ammonia factories can make explosives? Sure! Or, they can sell ammonia fertiliser for desperately needed foreign currency.
As the very fine article (/s) does point out, almost any FSU economy has disastrously bad chain of custody over radiological materials.
Look, I'm scared too. Can we stop with hyperbole and stick to facts please. The international atomic regulatory body is concerned its not allowed to talk openly to people in Chernobyl and cannot formally account for a lot of radioactive materials in wartime. That's the story. Not "dirty bombs". Dirty bombs is a risk, sure.