Top US nuke engineer slams US gov advice on evacuation
A single very brief spike of 400 millisievert/hr, recorded by an instrument near reactor No 3 following an explosion on Tuesday, is still being widely reported as if it were the current level at the site, probably not helped by a poorly translated and somewhat belated TEPCO press release issued yesterday, which mentions it. Nonetheless, levels even adjacent to the stricken reactors have seldom been above 4 millisievert/hr, and much lower elsewhere in the plant. Higher levels are detected by aircraft above the buildings because the steel-lined rooftop pools are shining short-ranged radiation straight up: this is why helicopters do not linger above them.
Overflights of the area by specially equipped US radiation-monitoring aircraft, conducted with Japanese permission, have confirmed the Japanese government's assessment that radiation levels beyond the powerplant site itself offer no cause for concern. The New York Times reports (NB – this article has already changed several times since it was published – quote correct as this is written):
While the findings were reassuring in the short term, the United States declined to back away from its warning to Americans there to stay at least 50 miles from the plant, setting up a far larger perimeter than the Japanese government had established. American officials did not release specific radiation readings.
[Consider] the Three Mile Island (TMI) incident, where 10 to 20 tons of the nuclear reactor melted down, slumped to the bottom of the reactor vessel, and initiated the dreaded China Syndrome, where the reactor core melts and burns its way into the earth ... In the real world, the molten mass froze when it hit the colder reactor vessel, and stopped its downward journey at five-eights of an inch through the five-inch thick vessel wall.
And there was no harm to people or the environment. None.
Yet in Japan, you have radiation zealots threatening to order people out of their homes, to wander, homeless and panic-stricken, through the battered countryside, to do what? All to avoid a radiation dose lower than what they would get from a ski trip.
Certainly the cores at Fukushima seem likely to come through in much better condition than the one at Three Mile Island did, offering even less chance of dangerous radioisotopes being emitted in significant quantities. As to the spent rods in the cooling pools, it seems to be far from established that they present any risk of melting down at all, or of major emissions to atmosphere if they did. The Japanese Nuclear Energy Institute says:
Even if the water level in the pools was to decrease sufficiently so that the fuel were exposed to air, the same level of overheating that can occur in a reactor accident would not occur in the used fuel pool because the used fuel assemblies in the pool are cooler than the assemblies in the reactor. It is highly unlikely that used fuel temperatures could reach the point where melting could occur, although some damage to the cladding cannot be ruled out.