The Lucky Dragon No. 5 was a singular incident that brought Nuclear concerns back to the forefront of Japanese consciousness, but according to David Kalat, Gojira was already under production when it happened (remember, the catalyst for Gojira was King Kong's re-release and the beast from 20,000 Fathoms).
Just the same, the Lucky Dragon No. 5 was not a disaster on par with the Tohoku quake or 9/11. Not even close. It was an international snafu that killed innocent civilians. It's more akin to the Korean Airs 007 flight being shot down, and implications that had towards escalating tension between the US and Soviet Union that were capitalized for the Return of Godzilla.
http://en.wikipedia.org/wiki/Korean_Air ... Flight_007