Called "the Elephant's foot," it is a congealed slab of reactor core, called "corium." The Russians say it's pretty radioactive.
As for the 800,000 gallons of radioactive water, the solution is obvious but everyone will go nuts over it. Just dump it in the ocean. Seriously, 800,000 gallons is literally a "drop in the ocean." In fact, ocean water is a really good way of cleansing radioactivity. Look at the sunken warships at Bikini Atoll that were "expended" in the A-bomb tests. Scuba divers can go dive them now, the radioactivity has been "washed away" by the seawater. Of course, I know that the radioactive particles are still out there, but the point is that they have been so dispersed by the ocean that they are no longer noticeable.
The Russians have been dumping all kinds of radioactive waste into their oceans.
And think about the fact that a guy went scuba diving into the water filled chamber to take that picture.
Just ask Charlie Tuna! :-)