BWR schematic courtesy NRC/NPR & Wikipedia
Information flow on the crisis at the Fukushima nuclear plant appears to have marginally improved, we now have a clearer picture of what may be going on inside the reactor buildings and it's not pretty. The image above, taken from this excellent NPR piece, shows the general architecture of a boiling water reactor and the three major components of concern: the spent fuel rods at the bottom of their containment water pool, the reactor vessel itself containing active fuel rods, and the suppression pool used as a thermal sink ringing the base of the reactor vessel.
Last night, Rachel Maddow did a great job of explaining that nuclear fuel rods are tubes of corrosion, heat resistant alloy filled with pellets of radioactive material. Each tube is about 12 feet tall and an inch or so thick. These individual rods are hot to the touch. What makes nuclear power work is that when the rods are bundled together, the entire bundle becomes much, much hotter.
Those bundles of active or spent fuel rods left to themselves, sitting in the open at room temperature, would become so hot that the alloy case and the fuel pellets would melt. Inside a damaged reactor or storage pool, deprived of circulating water, the hot fuel rods can boil standing water and then break down some of the the resulting steam into hydrogen and oxygen -- an explosive mix. Much of the work done at Fukushima, almost all undertaken by a few dozen workers at enormous personal risk, has been a battle to keep enough water flowing through the reactor vessel and the spent fuel pool to cool the rods and keep them submerged.
Explosions from steam buildup or hydrogen ignition, red-hot melting fuel rods, and the corrosive effect of air on fuel rod casing that get exposed can all weaken the various systems, causing leaks, and creating short and long term risk of introducing more radiation and radioactive material into the environment. A sufficiently large radiation leak would force remaining workers away from the site leaving the other units unattended. Which puts this statement in grim context:
Gregory Jaczko, the chairman of the commission, said in Congressional testimony that the commission believed that all the water in the spent fuel pool at the No. 4 reactor of the Fukushima Daiichi Nuclear Power Station had boiled dry, leaving fuel rods stored there completely exposed.
Latest developments are mixed at best
. An emergency powerline to the Fukushima complex is said to be near completion and may help relieve the crisis. If so it can't arrive soon enough: at last report helicopters were dropping water on the ailing reactors in what looks like an effort to buy critical time and water cannon trucks are reportedly speeding to the site to assist.
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