Image published by embassy in Japan shows Fukushima melted fuel deep underground…
A “critical mass” doesn’t have a “half life”. It’s not getting cooler, it’s getting hotter. It would have a rate of accretion, meaning a rate at which it’s converting it’s environment into more and more uranium and plutonium. As it grows in mass, it grows in density and gets hotter. When it first melted down it was about 230 tons of nuclear fuel at about 9000 degrees Fahrenheit. It melted about 100 feet down in 5 months. By now it’s got to be about 1000 feet down if not further, I don’t know exactly what it’s rate of accretion is. But they apparently drilled a channel from the ocean to it and installed a pipe and since August 20th 2013, they’ve been boiling the ocean with it. There’s a report available on ENE news, find it with Fukushima boiling ocean. Just about the whole ocean is dead now. If there’s anything left alive it’s dying fast.