Perhaps only a vlave has burst. They suspect that air from the containment vessel is coming out. They can not be sure about the bottom of the containment vessel but they mentioned that the vessel has lost its water.

100 tons of 95% uranium and 5% Plutonium shoudld not be allowed to collect into one giant ball.

Despite the danger boron and water should still be poured on this mess despite the risk of steam explosions and drastic radiation release. ITs better than the small chance of criticality growing into a runaway chain reaction. Yes I know they say this is impossible but at temps over 5000F we have no experiments to refer to.