I didn't notice the "slew" of comments here until now. Haven't heard much more about the gravel, but it seems it all stayed in Fukushima prefecture. My wife is from western Fukushima, and it has quite a few refugees there put up in new housing, and it is almost guaranteed they used some/a lot of it for the buildings.

Kate, my thoughts are, FWIW, that there is very little radioactivity, b/c the tsunami came first and it was days before the plant reached critical and that stuff was already below the surface and heading out to sea. I don't have time to look at your link but I will try later.
We are what we repeatedly do - Aristotle