While those USSR generators are indeed nuclear-powered, they use a completely different method from nuclear plants. They simply convert the decay heat of a radioactive material into energy, and as such aren't subject to any drastic reactions such as meltdowns.
The neglect was largely associated with the fall of the USSR and their economy. While this should be a scenario we should be concerned about, I don't think it is particularly relevant to the conversation and certainly not a coup d'etat
More info: https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...