Hardly anyone paid attention back then, either to the high beta radiation or to the tank that was assembled instead of welded.
Here's from my post on February 3, 2012 (Original Nikkei article is in Japanese, which is in my original post):
From Nikkei Shinbun (2/3/2012):
TEPCO announced on February 3 that the water leaked from one of the contaminated water storage tanks at Fukushima I Nuclear Power Plant. 2,000 millisieverts/hour beta radiation was detected. The amount of the leak was small, and there was no leak into the ocean. The leak stopped when the bolt was further tightened. The radiation was then shielded with acrylic plates, and the beta radiation dropped to 15 millisieverts/hour.
The bolt may have gotten loose at the joint of a storage tank that stores the contaminated water that was condensed by the desalination apparatus (Reverse Osmosis), letting the water leak. TEPCO said a large amount of radioactive strontium might be in the water. On the concrete where the leaked water was, 22 millisieverts/hour gamma ray was also detected in addition to the beta radiation. It dropped to 1 millisievert/hour after shielding.
In TEPCO's photo from February 3, 2012, you can see the packing in between the sheets up close.
(Click to enlarge)