I think that if you think Fukoshima is the worst that can possibly happen you are replicating the exact fallacy that produces black swan events in the first place. Of course a worse outcome then what happened is possible. You could have more radioactive material leaked, you could have the radioactive materials spread before the evacuation go wrong. These things might seem unlikely but people underestimate the likelihood of unlikely events enormously.
I don't think anyone said Fukushima was the worst that could possibly happen. I'm not even sure people said that Chernobyl was the worst that can possibly happen, and that was a darn sight worse than Fukushima. The closest statement to that was that Chernobyl was the worst that has actually happened, which is accurate according to most criteria. That said, I could drop dead tomorrow should half the atoms in my body spontaneously quantum-tunnel into my bedroom wall (along with making a rather unsightly mess, I should imagine). I could have a meteor strike me from orbit (and that's actually something that has happened to
a person EDIT: multiple people, actually). I think that it's worth considering the worst-case, but it's not worth it to be ruled by it. It doesn't seem to be far from the suggestion that we can't use oil because any movement of oil could result in a second Exxon Valdez, any coal since all coal mines run the risk of collapse or seam fires, or hydroelectric due to the risk of a second Banqiao dam/Shimantan Reservoir failure.
Oh, by the bye, I forgot who stated that there were probably no more reactors of the type used at Chernobyl, but there are still 11 RBMK-1000s operating in Russia in Kursk, Smolensk, and St. Petersburg. Outside of that, the last Chernobyl reactor shut down in 2000, and the last Lithuanian reactor (technically RBMK-1500s, but same family) shut down in 2009. All of them have seen significant safety improvements, however, including shortening the SCRAM sequence, lowering the void coefficients, and additional absorbers and manual control rods. I won't pretend they're the safest reactors in the world, but how many RBMK's have had catastrophic failures in the three decades since 1986?