Louis Slotin would probably have something to say
in response to our UK friend's fantasy.
As would the downwinders, the people of Japan, Ukraine and the
former residents of Bikini..

For the more pragmatic... here is a link to a comprehensive study of the geology of Fukushima..

[link to fukushima.over-blog.fr]

more on corium...

[link to docs.google.com (secure)]

[link to www.simplyinfo.org]