First things first: What follows is essentially a thought experiment, nothing more.
Following a magnitude 7.1 earthquake off the northeastern coast of Japan (the latest of many aftershocks), tsunami warnings were issued for much of the same coastline devastated by the March 11th tsunami. Warnings were for a 1 meter tsunami in Fukushima. Those warnings have now been lifted, and there have been no new reports of damage to the crippled Fukushima Dai-ichi plant. So (further) disaster seems to have been averted. Here’s where the thought experiment comes into play.
My first thought when I saw the warnings was: “Oh god, a radioactive tsunami!” Alarmist? Maybe. I sent a few feelers around Twitter to see if anyone else was thinking along the same lines, then picked up the phone and called Dr. Ken Buesseler, an oceanographer right up the road at Woods Hole Oceanographic Institution who studies radioactivity in the ocean (and, consequently, has been a very busy man the past few weeks). I ran my nightmare scenario past him: were we looking at highly radioactive seawater washing over Fukushima prefecture?
While that has obviously not come to pass, the conversation yielded some surprising insights and perspective that seemed worth sharing, despite their purely speculative and theoretical nature.
Short answer: no. First off, a 1.5-3 foot rise in water levels wouldn’t go very far in that region. And high levels of radioactivity in seawater appear to be highly localized, falling by a factor of a thousand within just 1,000 feet. Levels 19 miles offshore are a million times lower than at the point of discharge. So Buesseler reasoned that the grounds of the Fukushima Dai-ichi plant itself would be the area most likely to be over-washed with radioactive seawater.
Given how contaminated the grounds of the Fukushima Dai-ichi plant are, and the fact that a tsunami would mix highly radioactive near-shore water with offshore waters where radioactivity has been greatly diluted, Buesseler further speculated that tsunami water would actually be less radioactive than the area it would wash over. He said he would actually be more concerned about the additional radioactivity the seawater would pick up and wash back into the sea – both from the ground, and from open pits holding the most highly radioactive waste water.
Above all else, though, Buesseler said the primary concern has to be the possibility of further structural damage and/or (re)loss of power due to another large earthquake or tsunami.