The danger of “spent nuclear fuel rods” at Fukushima

These rods, which will have become laced with radioactive by-products of the main reaction, including plutonium and radioactive isotopes of strontium, iodine and other elements, are taken out of reactors and put in pools of water for storage

The Fukushima crisis: A new danger at Fukushima | The Economist, 18 March 11, “…….the Japanese Atomic Industrial Forum, a trade body, acknowledged that there were worries about the water levels in the ponds at units 3 and 4.The past 24 hours have seen several attempts to put more water into the ponds, though efforts seem to have focused more on unit 3’s pool rather than unit 4’s. First military helicopters were deployed to dump water on the site from the air, although fears over radiation levels meant they had to give up after about an hour. Those same radiation levels prevented police water cannon (originally designed for riot control) from getting within range. Eventually, around 30 tonnes of water was sprayed on the pondsby vehicles from the Japanese army.

All this is necessary because, despite its name, “spent” nuclear fuel still has something of a kick. It is made up of old fuel rods that have become sufficiently depleted of uranium-235, the isotope that drives a reactor’s main chain reaction, to be no longer useful. These rods, which will have become laced with radioactive by-products of the main reaction, including plutonium and radioactive isotopes of strontium, iodine and other elements, are taken out of reactors and put in pools of water for storage until beng taken offsite to something more permanent (though at some sites they may hang around for a long time). The water keeps them cool, and shields workers from their residual radiation. The particles the fuel gives off, which travel faster than the speed of light in water, can as a result produce a particular type of light, harmless in itself, called Cerenkov radiation. Pools bathed in the distinctive eerie blue glow of Cerenkov light provide some of the most recognisable images of nuclear plants.

Left to their own devices, spent-fuel rods would eventually boil their pools dry. They would then react with the air and melt, at least in part. Under normal circumstances, this is easy to stop by keeping the ponds at room temperature with regular supplies of cool, fresh water. As a result the elaborate containment facilities around reactors designed to keep any nuclear fuel that melts down from harming the outside world are not applied to spent fuel. But in at least three of the ponds at the Fukushima site, that routine cooling no longer seems to be happening. The International Atomic Energy Agency published the following chart showing temperatures in three of the ponds on its website yesterday, using information from Japanese sources. (There are no data for the ponds at units 1-3):

The “no data” given for the pond at Unit 4 on March 16th is particularly ominous, and could mean that the NRC is right—the reading may be showing “no data” because there is no water left to measure. On the other hand, it could mean that measuring systems are offline because of the fires.
ven if the fuel doesn’t melt, there are other dangers. The fuel rods could still get hot enough to damage their cladding and release radioactive particles into the air. And a completely dry pond would expose the rods to the open sky. If this happens gamma rays, which can pass through air much more easily than other radiation does, could pose a threat to anyone in the line of sight. That may explain why the army’s helicopters seemed reluctant to linger over the site for long, and are reported to have been fitted with extra shielding. Gamma rays scattered off particles in the air—a phenomenon known as “skyshine”—could contribute to the raised radiation levels around the plant….

The Fukushima crisis: A new danger at Fukushima | The Economist