So the instrument tube penetration failure may be a possible pathway for release, according to this analysis. From what I know of BWR instrument tubes, those do not have a very large diameter. So we may be talking about several small holes rather than bulk failure of the pressure vessel. That could very well be the case. The point of those early vapor releases was to keep internal pressures done owing to lack of residual heat removal and subsequent coolant vaporization. We know that was the case, so they were following the guidelines in the BWR Owner’s Group specifications. Until they can get some up-close inspection of the lower vessel structure, it is somewhat of a guess.
Too bad the fuel storage pools aren't better contained. This could be a problem if the radiation levels increase and the entire site has to be abandoned.