Wasn't something like that poured under Chernobyl after the accident which prevented the radioactive slurry from entering deep into the ground?
Regarding the radiation, I think you've fallen victim to ignorance of basic facts about the nature of radiation and the oceans. Pretty much anywhere in the Pacific, radiation from Fukushima quickly dropped far below environmental background levels [1].
1 - http://www.whoi.edu/page.do?pid=127297
Here's a good quote from the source above: "The highest levels of cesium (10 Bq/m3) attributable to Fukushima that we have measured were found 1,500 miles north of Hawaii. Swimming every day in the ocean there would still result in a dose 1,000 time smaller than the radiation we receive with a single dental x-ray."