Your hunch is in line with my hunch, mostly. I doubt it has to do with it being of Russian origin - but more-or-less the unknown level of maintenance the ship

Your hunch is in line with my hunch, mostly. I doubt it has to do with it being of Russian origin - but more-or-less the unknown level of maintenance the ship and it's reactor have had since the fall of the USSR. The reactors on these ships cannot be readily weaponized; there's practically no risk there. However, the reactors require a full crew to operate, even when just sitting in port being babysat (there is no "turning off" a rector). During the collapse of the USSR, regular maintenance and full crew staffing would have likely been challenging, putting the condition of the reactor in uncertain territory.