Fairewinds Chief Engineer Arnie Gundersen, Coast to Coast AM with John B. Wells, Nov. 9, 2013 (at 31:30 in):
Sources have told me — within Tokyo Electric — that they have no confidence that there’s any boron left between these fuel bundles. And they need boron to prevent the nuclear fuel from becoming a self-sustaining chain reaction, a criticality. So without boron in the plates — there are plates between these fuel bundles — but they got extraordinarily hot from not being cooled off the better part of a couple weeks, and they also were exposed to salt water. So that combination likely stripped out the boron. So the only thing Tokyo Electric can do is throw all sorts of boron into the water. Then pull the fuel. […]
I ran a division that built fuel racks, and these high density fuel racks like they have a Fuksuhima are very close to going critical anyway. […] Normally its .95, as high as .99, that means there’s a 1% margin before a self-sustaining chain reaction can occur. The problem there is that the fuel pool doesn’t have the ability to remove the heat if these nuclear fuel bundles turn back.— a criticality means they turn back on outside of the nuclear reactor. So they have to be extraordinarily careful that they don’t start a chain reaction in the fuel pool […] If they get close together you can cause a chain reaction, and what will happen then is the water will begin to boil violently. Hopefully Tokyo Electric is going to be monitoring this really closely and the first indication of water bubbling, they push the rods back in. The problem though is that the rack is distorted and as you pull it, you’re pulling way more friction than it was designed to handle. It’s a real problem.