I am right in the middle of a blogging hiatus, immersed as I am in the herculean task of getting a start-up off the ground. For reasons of personal survival, I hope to be able to report soon.
In this situation, my blogging is purely reactive, as in the case of the questions received by my friend Marco, the author of a cool divulgative page “La Fisica che non ti aspetti” (=Unexpected Physics).
For those willing to understand completely the answers, it may be a good idea to first read this other post which is not specific to the BWR (boiling water) reactor technology used at Fukushima Dai-ichi
Q1. News reports said the cores of one or more reactors melted and contaminated the underlying aquifer. Why wasn’t it contained by the concrete vessel encasing the reactor vessel?
All three reactors at Fukushima experienced core meltdown (while 4, 5 and 6 were on scheduled maintenance), and all three for the same reason: the 13 meter tsunami wave overcame the 10-meter seawall and struck the generators, cutting power; when the batteries ran out of energy, the coolant ceased to circulate and temperature rose.
The resulting corium breached the reactor vessel but not the Primary Containment (concrete) Vessel. It’s all still there, as is the Secondary Vessel encasing the Primary.
That is not to say that all the radioactivity was contained as neutrons have the bad habit of traveling around. Additionally, several controlled and uncontrolled events lead to the dispersion of mainly Ce-137 in ocean waters: given the very strong currents in the area, this is believed to have dispersed in a very large area, causing little or no damage.
Q2. What contaminated people if there was no radioactive cloud?
Fuel rods are encased in a Zirconium allow which is inert at the normal functioning temperature of the core (300 °C); when overheated to 1200 °C however, they react with the surrounding water and create gaseous hydrogen; when the percentage of hydrogen in the air reaches the explosive saturation, chemical explosions occur, as it was the case in all three reactors, dispersing radioactive material.