Tuesday, March 15, 2011

Understanding the Fukushima news coverage

News coverage about the problems at the Fukushima nuclear reactors has been somewhat confused.

Radiation and radioactive are different terms. Radioactive substances emit radiation. A release of radiation is dangerous to those nearby. A release of radioactive substances is a wider problem, because those substances can then travel and emit radiation wherever they end up. For those of us who aren't near the Fukushima reactors, the key question is the volume and type of radioactive substances that are released into the environment, not the radiation levels near the plant.

A fuel rod within the reactor being exposed to the air does not mean that the fuel rod is exposed to the environment. There is air inside the containment structure. The concern with fuel rods at the Fukushima reactors being exposed to the air is that they then overheat, damage the cladding around the fuel rods, and make a core meltdown more likely.

A core meltdown is not, by itself, a widespread disaster. A meltdown that happens within an intact containment structure, as at Three Mile Island, doesn't mean that radioactive substances will escape. The key question with meltdowns at the Fukushima reactors is whether the containment structures stay intact.

Spent fuel rods are not safe. And the spent fuel pools at the Fukushima reactors are not inside containment structures the way that the reactors are (or used to be).

Using sea water for cooling is not just as good as using the purified water that is usually used for cooling. Purified water that becomes activated (turned radioactive) has a very short half-life. Sea water contains a lot of other substances that will stay radioactive a lot longer. Sea water can also damage pumps that are designed for fresh water, and the salt left behind may physically block continued access for the water needed to cool the fuel rods.

When radioactive substances escape into the outside air, it doesn't make a lot of difference whether they escape in a hydrogen explosion, in a steam explosion, in a fire, or in a steam release. What matters is the volume and type of radioactive substances that are then in the outside air, because they will gradually fall down to the ocean or the ground. The other key question that is unknown is how far the fallout will travel.

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