It’s probably the content of the spent fuel pool. Spent fuel that is withdrawn from the reactor is screaming hot - both thermally and radiation-wise. It is kept underwater in the reactor containment until the heat and the radioactivity die down and it can be put into dry storage.
The spent fuel pool is like an olympic-sized pool, only deeper.
I’m not even vaguely educated as to what the degree and halflife of what you describe is, but I would guess that in terms of particulate and curies or roentgens, it must be pretty high......and that it would be a whole lot safer to reduce it to solids as soon as is practicable, esp in such a quake-prone place as Japan. It’s difficult to get a picture with such hedged comments almost concealed within an article like this one, with the comments of very reluctant officials. Since it mentions both “slosh” and barrels that fell over and “lost lids” one must wonder if there aren’t at least 2 releases of liquids. I think there probably were 2....