I would say the Sr-90 was there from the initial release. The article didn't say where or when the samples were taken and at what levels the Sr-90 is at. The severity of the accident was classified separately for each unit. When they combined the curie content of the releases of units 1,2, and 3 (each given a 5 rating previously) they totaled a 7. Perhaps this was done to trigger certain relief and aid available. Unit 4 still has a separte classification of 3. Depending on the source, I have seen the release been stated as 10%, 40% and >100% of Chernobyl. Certainly with fuel from 3 reactors and over a 1,000 spent rods in a fuel pool, the potential release is greater than Chernobyl.
In comparing accidents, it is like comparing apples and oranges. Yes people died at Chernobyl, but how much of the countryside and nation's econmy did it effect? The Soviet military came in, shot all the pets and herded people into buses. Stay away. Area isolated.
In Japan they have contaminated part of their "bread basket." Farm lands can not be used. Factories in the US have been shut down. Fishing waters can not be used. Massive amounts of radioactivity is being dumped into the ocean. 400,000 gallons of waste water is generated per day and they don't have anywhere to put it all. The units are in continuous release. They are still trying to get the accident under control after a month. Japan was "fortunate" because people left due to the tsunami.
Nuclear power: We are safe for all the accidents we plan for.