From Reuters quoting experts, nobody knows how bad it can get.
http://www.reuters.com/...
No one knows how bad it can get, but independent consultants Mycle Schneider and Antony Froggatt said recently in their World Nuclear Industry Status Report 2013: "Full release from the Unit-4 spent fuel pool, without any containment or control, could cause by far the most serious radiological disaster to date."
So what happens if Fukushima goes critical, catches fire and burns for years? Why can't this happen?
It seems to me that positing the worst case might be a good idea because then maybe people will decide to do more to avoid a catastrophe at Fukushima.