http://www.bbc.co.uk...ironment-12762608
If you are in any doubt as to what this means, it is that in the company's view, it is possible that enough fissile uranium is present in the cooling pond in enough density to form a critical mass - meaning that a nuclear fission chain reaction could start.
Apparently, the pools are overstuffed and the boron disappeared with the water. Now they're concerned that adding water may moderate neutron speed enough to restart fission.