Is it true that a Chernobyl type disaster is not possible in Japan?

I read that officials and professionals say that a catastrophe similar to Chernobyl is impossible in Japan, so what is the worst possible scenario if there is a meltdown at Fukushima? The same person that made a statement about a Chernobyl repeat not being possible, also said that there may be a “little” radiation leakage and equipment damage – but no explosion. However, latest news stories are reporting that an explosion has already occurred. So what really is the worst case outcome at the nuclear power plants?