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Fukushima Radiation Levels High, But Leak Plugged
jmcvetta wrote in with a story about Fukushima radiation levels so high that monitoring devices have been rendered useless. Levels outside the buildings exceed 100 millisieverts in some places. But the good news is that the leak is patched using 1500 liters of sodium silicate.
Workers are pumping nitrogen into one of the reactors at Japan's damaged nuclear plant in an attempt to prevent an explosion caused by dangerously overheated fuel rods.
Officials at TEPCO, which operates the Fukushima plant, said a dangerous hydrogen buildup is taking place at its number-one reactor. Japan's NHK television quoted officials saying hydrogen is accumulating inside the reactor's containment vessel - an indication that the reactor's core has been damaged.
Crisis at Japan Nuclear Plant Shifts to New Blast Risk
Chemistry 201: Why Is Fukushima So Gassy?
But there are reasons...that Fukushima is particularly vulnerable.
One is its recent use of seawater to cool the reactors's fuel rods and cores. In addition to the oxygen in water molecules, cold seawater can hold a great deal of dissolved oxygen gas. But warm water cannot; so as the seawater was heated in the reactor, the dissolved oxygen emerged and gathered in the empty space above the water.
(Ordinary reactor cooling water has had the oxygen removed from it by plant operators to reduce the possibility of rust.)
In addition, gamma radiation from the nuclear fuel in the reactor would continuously produce small amounts of hydrogen and oxygen by breaking up water molecules --- and the normal method of recombining these elements into water at such plants in a controlled fashion is no longer available.
Plants of the Fukushima variety usually have catalytic converters that accomplish that at the point where steam has run through the turbine and is condensed back into water for another trip through the reactor. But that path has been closed since the plant lost power at the moment of the March 11 earthquake.
Hydrogen can also emerge from the zirconium metal used as fuel cladding. One of the lessons of the Three Mile Island accident in 1979 near Harrisburg, Pa., is that when the cladding comes into contact with steam rather than water, it goes through a reaction that is akin to rusting; it picks up oxygen from the water molecule and gives off hydrogen.
This only happens at high temperatures, but uncertainty reigns at the moment about temperatures in the Fukushima reactor cores. With some cooling channels blocked, they are likely to have hot spots.
By design, boiling water reactors like these have far more zirconium metal in them than pressurized water reactors do. They boil water directly in the core, covering the fuel assemblies with a water/steam mixture rather than keeping them immersed in water. The water has to be directed to each individual fuel assembly and therefore each sits in its own zirconium box.
All of that zirconium is available for an oxidation reaction with steam in which the metal absorbs oxygen from water and turns to a powdery rust, releasing hydrogen.