Corium-concrete interactions

"The fast erosion phase of the concrete basemat lasts for about an hour and progresses into about one meter depth, then slows to several centimeters per hour, and stops completely when the melt cools below the decomposition temperature of concrete (about 1100 °C). Complete melt-through can occur in several days even through several meters of concrete; the corium then penetrates several meters into the underlying soil, spreads around, cools and solidifies." Wikipedia

"Fukushima Dai-ichi

At an estimated eighty minutes after the March 11, 2011 tsunami strike (which caused various nuclear accidents, the worst of which being the Fukushima Daiichi nuclear disaster), the temperatures inside Unit 1 of the Fukushima Daiichi Nuclear Power Plant reached 2 300 ˚C to 2 500 ˚C, causing the fuel assembly structures, control rods and nuclear fuel to melt and form corium. The reactor core isolation cooling system (RCIC) was successfully activated for Unit 3, however the Unit 3 RCIC subsequently failed and at about 08:00 on March 13 the nuclear fuel had melted into corium. Unit 2 retained RCIC functions slightly longer and corium is not believed to have started to pool on the reactor floor until around 18:00 on March 14^{[49]"   Wikipedia}