There is NO secondary containment dome, just a primary reactor containment vessel that could not withstand a steam explosion. A catastrophic reactor explosion would expose reactor core to atmospheric conditions. The plant was built in the early 60’s and is a 1st generation 1950’s design. It is among the oldest plants in use in Japan.
For whatever it’s worth Fukushima I - 1 went critical March 26, 1971, while Fukushima I - 2 was activated July 18, 1974. So, yes, that does make them 40 years old.