Fukushima Daiichi - Efforts to Cool Reactor Cores in Units 1,2,3 Continue

WNN has reported that efforts continue to pump seawater into the reactor vessels of Units 1,2,3 which contain partially exposed fuel rods.  Because the fuel continues to generate heat, the water boils producing steam pressure, which must later be vented. Fuel assemblies are exposed by between one and two meters at the top.   The high thermal conductivity of the zirconium alloy that encases the fuel rods means that the exposed part of the rods are partially cooled, even with just the lower portion of the rods submerged. This process is set to continue until the heat produced by the core is reduced so that the entire core can be covered.

Radiation levels on the site are fluctuating,  Some of it comes from venting steam from units 1, 2 and 3.  But some emission may be the result of damage to Unit 2's  and Unit 3's toruses.  Levels of 400 millisieverts per hour have been recorded near Unit 3.  This means that in an hour you would get a dosage between the international maximum (500 millisieverts) and the Japanese maximum dosage (250 millisieverts).  Dosages of 1 sievert (1000 millisieverts) and above can result in radiation sickness.

Comments

[Joe Bogus]

I have been following the news about the Daiichi reactors, and the main stream reporting has been poor. Each time I read an update...it raises more questions that I hope you can clear up. For starters, what causes the hydrogen build up? Were does it build up? What exactly blows up? If they have the ability to pump water to the reactors, why are they still having trouble keeping them cool? Is it that they can not pump enough volume? Why is a retaining pond burning?

[Geoff]

All good questions.
1) The fuel is encased Zircaloy. At high temperature, there is a reaction between the Zircaloy and water (H2O) which produces hydrogen gas.
2) This is produced in the containment vessel and builds up together with steam from evaporating water and radioactive Cesium and Iodine from the fuel rods.
3) When the reactor vessel is vented to reduce pressure, this mixture is released into the top part of the building housing the reactor where there is air with oxygen, and as you know from the Hindenberg disaster, H2 and O2 react rapidly and violently. This happened at two of the units 1 and 3, and the cladding on the upper part of buildings was blown out. Fortunately the explosion was not violent enough to damage the reactor containment.
4) Fuel rods that are not installed in reactors are stored under water in spent fuel pools (SFP). The amount of heat generated by these rods depends on when they were last in a reactor. Because the heat that is generated is from the decay of products that were originally produced by uranium fission. When they are stored in the SFP they are kept separated from each other to prevent "criticality", or uranium fission.
5) If the water level in the SFP drops then the reaction between the Zircaloy casing of the fuel rods and water will produce hydrogen. If this is allowed to build up in a closed room with air (containing oxygen), the result is an explosion. This is what happened on the 4th floor of Unit 4.
6) As WNN explained, there are two things keeping the water levels from rising higher than about half way up the fuel rods in Units 1,3. As they pump water in, because of the heat being produced by the decay reactions in the fuel rods, the water is evaporating and it appears that they can't pump water in faster than it is evaporating, so you;re right it is related to volume. Secondly, the steam pressure buildup as a result of evaporation makes it increasingly harder to pump water in.
7) I don't believe the pool (SFP) is what is burning. I suspect that when the hydrogen explodes, it generates a lot of heat and materials, walls, floors, electrical equipment, etc in the SFP room may be burning.