The fireball ... was seen 1,000 km [620 miles] away. The heat could have caused third degree burns at a distance of 100 km [62 miles]. ... The explosion could be seen and felt in Finland, even breaking windows there. ... The seismic shock created by the detonation was measurable even on its third passage around the earth.
The average power produced during the entire fission-fusion process, lasting around 39 nanoseconds, was ... equivalent to approximately 1% of the power output of the Sun.
-- Wikipedia article on the Tsar Bomba 50-megaton nuclear test, the largest manmade explosion in history
Got sidetracked from a Google search on instestinal bleeding (sigh ... yes, still dealing with some symptoms; not sure what I'll do if it starts getting worse again) by running into a fascinating page on how real and science-fiction astronauts have to deal with radiation. A fascinating read, from radiation effects to shielding mechanics to ship design.
Also did a little surfing around from there to discover that, billions of years ago, conditions were right for at least one of Earth's underground uranium deposits to turn into a natural fission reactor. (Don't worry; it can't happen today because natural decay has reduced uranium deposits' potency.)
Plus the aforementioned article on Tsar Bomba. That one bomb, alone, was equivalent to about 3,000 Hiroshimas. And each of those Hiroshimas is itself equivalent to 10,000 of the truck bombs used in the Oklahoma City bombing.
That we survived the dawn of the nuclear age is a testament to ... something. Given the known incidents that almost sparked off an Earth-sterilizing missile exchange, probably just pure dumb luck.