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Latest speculation is that more of the fuel in #2 melted down, probably due to the plumbing changes. Reactor #2 was only reported to be about 50-60 % melted. They cannot rely on temperature guages alone. Those guages have been off by 20 C historically in this situation. There is a lot of smoke coming out of the reactor, which leads many to believe the temperature is probably closer to 85 C.

The reason they injected boric acid as a precaution was due to very low traces of fission products.

Here is the latest I have on the NEI newsfeed:

"Tepco was able to discount recriticality as a potential cause of the temperature rise after conducting an analysis of charcoal filters in the containment gas control system. These showed very low traces of fission products that were below the threshold that would indicate criticality. Nevertheless Tepco this morning injected boric acid into the reactor vessel as a precaution and increased the core spray injection rate by three cubic metres per hour."

The overall core temperature remains within the limits specified for cold shutdown (less than 100 deg. C.).