My colleague at MIIS, Ferenc Dalnoki-Veress has written a paper, What Was The Cause Of The High Cl-38 Radioactivity In The
Fukushima Daiichi Reactor #1
.  Ferenc  concludes that we cannot exclude the possibility that little pockets of melted fuel have gone critical, even if for a short period of time.  The possibilty of localized criticalities could severely hamper on-site operations.

Comments are welcome.