Chris Harris, former licensed Senior Reactor Operator and engineer, Nutrimedical Report, Nov. 7, 2013 (at 33:30 in): The borated rubber has been degraded. […] The better the shutdown margin, the better off you are. Now with the borated rubber — it has been admitted, Tepco said it’s been damaged — that was adding shutdown margin and now it’s reduced. And when you get down to no shutdown margin, you’ve got a criticality event.
Arnie Gundersen, chief engineer at Fairewinds Energy Education, KZYX, Sept. 9, 2013: 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.
Japan Times, Nov. 14, 2013: Another risk is dropping the assemblies and damaging them. “It is imperative not to drop the assemblies when removing and after removing them from the racks,” said Hisashi Ninokata, a nuclear expert and professor at Polytechnic University of Milan in Italy. “In the worst-case scenario, dropping a cask is conceivable. […]” said Masayuki Ono, a Tepco spokesman. […] Asked if it’s possible for the spent fuel to achieve recriticality, Zengo Aizawa, vice president of Tepco overseeing the Fukushima crisis, said this is highly improbable since the removal process basically deals with one assembly at a time, and the utility has confirmed that one assembly alone cannot cause a nuclear chain reaction.
Full Nov. 7 interview with Harris here