The description he makes of how a plant works is nice, but being "a PhD" in an unrelated field doesn't mean much to me... if he had been a nuclear physicist with nice publications or nuclear engineer working at the xxx plant AND if he had access to additional information on what it's really happening/ed i would have cared more for his conclusions...
Looking at the comments [1] (guy from Japanese Atomic Energy Agency) it even seems that he is describing a different configuration of nuclear plant (newer,safer).
Frankly I don't entirely credit the conclusions of any "authority" right now -- everyone outside the plant management loop is making educated guesses, and many of them have powerful institutional biases shaping their thinking.
And if we set aside conclusions, this is the best orientation to the problem that I've seen so far. I'm not worried about his credentials, as this is internally consistent, and externally consistent with what I know of physics. Maybe the plant lacks a third containment, or is using a different fuel, but I can adjust my understanding of the system for those facts as they come along.
[1] https://morgsatlarge.wordpress.com/2011/03/13/why-i-am-not-w...
Check out the other comments from Daniel on that post for additional info on procedures being put in action by the jp government.