"TEPCO spokesman Taichi Okazaki said that (an) explosion at reactor No. 4 was likely to have taken place because of a gas leak, and assured that it did not cause any additional radiation leak."

Donuel-

If you had provided a link or a quote from a news source, your post above might have appeared less alarming, or maybe it was a "gigantic explosion." However, any gas explosion (what kind of gas?) has to be alarming in that it will disrupt attempts to stabilize the damaged reactor units.

There is still no update on the 4 spent fuel pools.

Charley Noble