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oxidation of uranium dioxide (voloxidation)

Dear Reader,

So far it we have had eight months since the earthquake and tidal wave wrecked the nuclear power plant at Fukushima.
I have discussed the question of when we will have the final answers as to what happened, one of my coworkers thinks it may take ten years before the exact story is known. He holds the view that either humans or cameras must explore the reactor buildings to allow an inspection of a series of key parts of the plant.

I agree with him that a detailed examination of the plant will yield up a vast number of details, but as a chemist I can tell from the radiochemical fingerprint of the accident some facts about what has happened. The fact that very little radioactive ruthenium has been released suggests to me that little if any air came into contact with the hot fuel. It is important to understand that the hydrogen formed by the horrible exothermic reaction between steam and zirconium would have kept the fuel in a reducing environment as long as hydrogen was in an excess.

This maintenance of a reducing environment is a good thing as it prevents the oxidation of the uranium dioxide. Uranium dioxide has a density of 10.97, this means a 1 meter cube of uranium dioxide will have a mass of 10970 kg. This is a rather big mass so lets recalculate for a 10 cm cube which will have a mass of 10.97 kg. As the molar mass of uranium dioxide is 270 grams per mole, and uranium has an atomic mass of 238 this means that one litre of uranium dioxide contains 40.63 moles of uranium atoms.

If we now do some calculations for U3O8 we will find as it has several forms with densities of 8.38, 8.32 and 8.42. If we assume that the density of the solid is 8.35 then out 10 cm cube will now have a mass of 8.35 kg.

As the formula mass of U3O8 is 842, then the block will have 9.917 moles of U3O8 in it. As each unit of U3O8 has three uraniums then the block will have 29.75 moles of uranium atoms in it. This is a large difference in the uranium density, it will result in a 37 % increase of the volume when the uranium dioxide (UO2) is converted into triuranium octaoxide (U3O8). During the oxidation reaction is it likely that the volume change of the solid will induce mechanical stresses in the small crystals which will convert the nice hard lump into a pile of powder.

This powdering due to a volume change is the basis of the modern head end process which is known as voloxidation. In voloxidation the fuel is heated under oxidising and reducing conditions to reduce it to a powder and to drive out the tritium from the fuel before it is dissolved in acid for PUREX processing.

I have recently read an article by Dr Salomon Levy on what happened and how the Fukushima accident was managed. The article is in Nuclear Engineering International on page 14 of the November 2011 issue. It is interesting reading.


One Response

  1. […] oxidation of uranium dioxide (voloxidation) « Mark Foreman's Blog […]

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