Japan’s Nuclear Crisis: forensic scientific analysis. What happened.

“Clearly we’re witnessing one of the greatest disasters in modern time.”

“Clearly, there’s no access to the core,” the official said. “The Japanese are honestly blind.”

water levels at the reactor cores dropping by as much as three-quarters, temperatures in those cores soaring to nearly 5,000 degrees Fahrenheit, hot enough to burn and melt the zirconium casings that protect the fuel rods.

evaluations also show that the reactors at Fukushima Daiichi escaped the deadliest outcomes — a complete meltdown of the plant.

about 70 percent of the core of one reactor had been damaged, and that another reactor had undergone a 33 percent meltdown — came from forensic modeling.

The atomic age has seen the construction of nearly 600 civilian power plants, but according to the World Nuclear Association, only three have undergone serious accidents in which their fuel cores melted down.

now three of the reactors at the Fukushima Daiichi complex in some stage of meltdown.

By definition, a meltdown is the severe overheating of the core of a nuclear reactor that results in either the partial or full liquefaction of its uranium fuel and supporting metal lattice, at times with the atmospheric release of deadly radiation.

Partial meltdowns usually strike a core’s middle regions instead of the edge, where temperatures are typically lower.

The main meltdowns of the past at civilian plants were Three Mile Island in 1979, the St.-Laurent reactor in France in 1980, and Chernobyl in Ukraine in 1986.

a first sign of major trouble at any reactor was the release of hydrogen — a highly flammable gas that has fueled several large explosions at Fukushima Daiichi.

The gas indicated that cooling water had fallen low, exposing the hot fuel rods.

First “as the core gets hotter and hotter,” easily evaporated products of atomic fission — like iodine 131 and cesium 137 — fly out.

If temperatures rise higher, threatening to melt the core entirely, he added, less volatile products such as strontium 90 and plutonium 239 join the rising plume.

The lofting of the latter particles in large quantities points to “substantial fuel melting"

a blow-by-blow of the accident’s early hours and days.
It said drops in cooling water exposed up to three-quarters of the reactor cores, and that peak temperatures hit 2,700 degrees Celsius, or more than 4,800 degrees Fahrenheit.
That’s hot enough to melt steel and zirconium — the main ingredient in the metallic outer shell of a fuel rod, known as the cladding.

A slide with a cutaway illustration of a reactor featured a glowing hot mass of melted fuel rods in the middle of the core and noted “release of fission products” during meltdown.
The products are radioactive fragments of split atoms that can result in cancer and other serious illnesses.

Stanford, where Dr. Hansen is a visiting scholar, posted the slides online after the March presentation.
At that time, each of the roughly 30 slides was marked with the Areva symbol or name, and each also gave the name of their author, Matthias Braun.

The posted document was later changed to remove all references to Areva.

more at the original New York Times article:

Japan’s Nuclear Crisis Is Seen Clearly From Afar - NYTimes.com

from www.nytimes.com - www.nyt.com