"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.).
Here is how the nuclear industry is reporting the situation.
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.
Stabilisation after Fukushima cooling change
So it was not no detectable fission products. Again, you always need to know the limits of detection for every Tepco reported value. It was measured fission products that were determined to be below the thresholds that would indicate fission. Now how they determine the thresholds for an unknown situation is beyond me. And that is probably why they added the boric acid.