The PhD from MIT failed to mention in his article, when he talks about the 3rd containment being concrete poured around the steel casing, that the reactor which caused an explosion was built prior to this 3rd containment layer being mandatory in BWR design. So, it would seem that his confidence in the 3rd containment layer preventing any melted radioactive materials from getting out should be taken with a huge grain of salt.

Updating with more information: Please see this list of BWR reactors throughout the world: http://en.wikipedia.org/wiki/List_of_boiling_water_reactors

You will notice that Fukushima Daiichi No. 1 (which caused the hydrogen explosion to the building walls) is a BWR/3, MK-1 model reactor. From this page, http://en.wikipedia.org/wiki/BWR#Evolution_of_the_BWR, you will see the following:

The first [General Electric] series of production BWRs evolved through 6 iterative design phases, each termed BWR/1 through BWR/6. (BWR/4s, BWR/5s, and BWR/6s are the most common types in service today.) The vast majority of BWRs in service throughout the world belong to one of these design phases.

1st generation BWR : BWR/1 with Mark-1 containment.

2nd generation BWRs : BWR/2, BWR/3 and some BWR/4 with Mark-I containment. Other BWR/4, and BWR/5 with Mark-II containment.

3rd generation BWRs : BWR/6 with Mark-III containment.

So, it would seem to me, that this confirms, it was not until at least BWR/4 or above that Mark-II containment was used. I'm pretty sure this confirms there is no 3rd containment layer.