In an essay by that title, Akio Matsumura has warned about the risks of Fukushima Daiichi Unit 4's high-level radioactive waste storage pool collapsing:

"In the last weeks, I have been speaking constantly with Japanese government and party leaders on this urgent issue. Surprisingly, most of them were not aware of the dangerous situation. I, along with many eminent scientists, are emphasizing the precarious situation of the fourth reactor that contains 1,535 nuclear fuel rods in the pool and is balanced on the second floor [sic*], outside of the reactor containment vessel. If the fuel rods spill onto the ground, disaster will ensue and force Tokyo and Yokohama to close, creating a gigantic evacuation zone. All scientists I have talked with say that if the structure collapses we will be in a situation well beyond where science has ever gone. The destiny of Japan will be changed and the disaster will certainly compromise the security of neighboring countries and the rest of the world in terms of health, migration and geopolitics.  The Japanese government should immediately create an independent assessment team to determine the structural integrity of the spent fuel pool and its supporting structure. This is of the highest importance: the structure’s security is critical to the country’s future."

Dr. Gordon Edwards, Montreal based President of the Canadian Coalition for Nuclear Responsibility (CCNR), responded to Matsumura's essay in whole hearted agreement about the risks, and calling for international assistance to prevent a worsening of the catastrophe.

Edwards provided Matsumura more technical detail in a letter on January 13th.

[*General Electric Boiling Water Reactors of the Mark 1 design actually have high-level radioactive waste storage pools located several stories up in the air]