It seems cautious optimism may have been out of order. Reactor 2 just had an explosion described as being significantly larger than the other two, this suggests that it was a steam explosion within the containment vessel. They have been having problems with the pressure relief valves on this plant, and such an explosion would be consistent with this. Early reports suggest that there is some dispersion of radioactive material across the site as a result.
This raises the possibility that the site becomes not merely unsafe, but unworkable. Not a pleasant thought with a significant number of other reactors in need of supervision, as well as fuel storage pools requiring cooling etc.
There may be some very significant technical challenges ahead.