H3 Area is where 1,800 mSv/Hr beta radiation (70 micrometer dose equivalent) was detected on August 31, 2013, setting off a worldwide silly season (here, here and here).
Looking at the photos, it would be a wonder if the water was not leaking, although TEPCO actually claims the water didn't leak.
Who spec'd this, and who built the tanks? They should be held accountable.
From TEPCO's Photos and Videos Library, 9/3/2013:
(Click to enlarge)
(I believe this is a new tank, to show how the flange at the bottom is treated. The white substance look like putty, sloppily applied.)
(I believe this is also an interior of a new tank.)
(Actual tank in H3 Area, with sealing material coming out.)
In the accompanying report in PDF, TEPCO seems to be saying it was not leaked water that gave off high beta radiation but packing that extruded as seen in the above photo. Their reasoning is that they didn't find a sign of current and past water leak there, and the concrete floor didn't give off high radiation, therefore it must be the packing.
TEPCO also has several design diagrams of the flange at the bottom of the tank:
The bottom edge of the tank:
Inside the tank:
So, inside the tank, they are supposed to completely bury the seams and rivets in 1:2 mortar and put the asphalt coating on top. ("1:2 mortar" means mixture with 1 part cement and 2 parts sand.)
Here are the photos of the bottom of the inside of No.5 tank in H4 Area that leaked and was subsequently emptied. The rivets and seams are exposed instead of buried in mortar, although it does seem the bottom is coated with something (epoxy coating, most likely):
I've heard that the No.5 tank in H4 area is constructed differently from the tanks in other areas, so it is possible that newer tanks have bottom seams and rivets completely covered with water-tight mortar and asphalt. Looking at the third photo above, though, I am not very hopeful.