As an engineer fag, this "report" is suspect. There would have had to have been a complete failure of the cooling system for the stated to have happened. The scenario which really has the Russians and anyone else who cares about preventing a looming disaster worried, is s missile strike on the unprotected spent fuel warehousing. Under the right circumstances, this would turn the resulting explosion(s) into huge dirty bombs like Fukushima.
The fuel inside the reactors is protected such that a direct hit with a fully loaded fueled 747 or airbus would not even dent the primary protective reinforced concrete. Again, this fuel is only susceptible to damage from lack of cooling water like ehat happened at Fukushima. The Russians have taken steps to shut down the reactor if and wheh the cooling system is compromised.
The real deal here will be the large blackouts caused by shutting the plant.