Gundersen: They’ve admitted that all the boron has disintegrated in between spent fuel at Fukushima — It can cause a nuclear chain reaction if rods get too close together in pool (AUDIO)
[link to enenews.com]

Arnie Gundersen, Chief Engineer at Fairewinds Energy Education:

Tokyo Electric has admitted that the boron between these fuel cells — there’s a boron wafer in between the fuel to prevent something called an inadvertent criticality, you can have a nuclear chain reaction in the fuel pool, and that’s not a good thing — but they’ve admitted that all the boron has disintegrated.

So the only thing preventing a chain reaction from occurring […] in the fuel racks themselves, is the fact they put all sorts of boron in the water.

But if the rods get too close to each other, they can still fire up again and create a chain reaction in the nuclear fuel pool.