"....unlikely case of a level nine earthquake and tsunami...."
The Japanese government also dismissed the possibility, not just TEPCO. A number of the designated safe refuges for those fleeing a tsunami were completely inundated and thousands died in them.

An article in the Santa Fe New Mexican about the nuclear test at the Trinity Site, near Alamogordo, White Sands, New Mexico, in July, 1945, in which a 19-kiloton plutonium device called "The Gadget" was detonated.
Tresa VanWinkle, director of the Alamogordo-based Cancer Awareness, Prevalence and Early Detection Center, points to cancer in her family and in others, and to spiking cancer rates among families in the wind-whipped Tulerosa Basin north of Alamogordo.
No compensation or recognition of cancer-caused deaths has come from the Federal government.

"The Gadget" carried about the same payload as the bomb dropped on Nagasaki.
Santa Fe New Mexican, April 16, 2011: "Six Decades after Trinity Site Blast, Area Residents Living with Fallout with No Help from Government," Dennis Carroll, The New Mexican.