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Merry Christmas

Dear Reader,

I would like to wish you all a merry christmas and a happy new year, I have bought a turkey and a joint of ham. So I will be having both the traditional British and Swedish meats this christmas.

It has come to my attention that Norikazu Kinoshita, Keisuke Sueki, Kimikazu Sasa, Jun-ichi Kitagawa, Satoshi Ikarashi, Tomohiro Nishimura, Ying-Shee Wong, Yukihiko Satou, Koji Handa, Tsutomu Takahashi, Masanori Sato and Takeyasu Yamagata have written a paper on the subject of the Fukushima fallout. The paper is in Proceedings of the National Academy of Sciences of the United States of America, volume 108 (issue 49), pages 19526-19529.

If we look at page 19527 we will see a series of maps for different radioisotopes, the cesium isotope which is most easy to measure long after the accident is normally Cs-137. It is important to understand that bomb fallout contains Cs-137, so soils can contain this isotope from either an atomic bomb detonation or in some cases from a very much older accident.

But in the case of the soil contamination maps in Japan a map exists for Cs-134 which is a shorter lived radioisotope which is only formed when a system stays critical for a long time. Hence it is only seen normally in the fission product mixture from a power reactor and never in bomb fallout. As the half life is only two years any Cs-134 from the Chernobyl accident will have decayed away by now.

As the cesium maps for Cs-137, Cs-136 and Cs-134 all have the same I think that the cesium activity which has been observed recently in Japan is due to the recent accident and not something from yesteryear. This spread of cesium from the Fukushima plant should not be a big shock to anyone.

What is interesting is that the maps for I-131 and Te-132 are not quite the same shape, this suggests to me that the isotope signature of the accident changed with time. South of the plant the tellurium/cesium and iodine/cesium ratios in the soil are higher. This suggests that when the wind was blowing southwards that the temperature of the fuel have have been lower or some other factor may have reduced the amount of the less volatile cesium which was released into the air. This relates to fractional distillation which is both the method used for making hard liquor and the way that I sometimes separate things by distillation. Another day I might share my thoughts on distillation with you.

Right now as it has been impossible to do a detailed examination of the inside of the plants, and as the radiochemical examination of the soil contamination has not been finished yet. We need to wait a bit longer for these things to be done. But so far the lack of Ru-103/Ru-106 in the soil suggests to me that the hot fuel was not exposed to air during the worst phase of the accident. This is an important thing to know about.The lack of air also prevents the conversion of uranium dioxide into the higher oxidation state compounds of uranium, this lack of oxidation reduces the production of fuel powder which would have occurred before the core melt. Also while uranium dioxide is insoluble in water, uranium trioxide tends to dissolve nicely in water which either the acid or carbonate concentration is high.

While later on after the accident the surface of the uranium dioxide will be oxidized by the air to make uranium trioxide which in turn can dissolve in water, a rapid powdering or dissolution of the fuel will speed up the release of the fission products. One of the key things in nuclear reactor safety is to try to make sure that as many fission products are trapped as long as possible inside the fuel or the plant to enable these fission products to undergo radioactive decay into stable (harmless) nuclides.

What we are dealing with here is a corrosion issue.

In an aqueous corrosion process there will be an anodic and a cathodic reaction, the anodic reaction is the oxidation of the metal into a new (and oftein soluble form) while at the same time the cathodic reaction is a reduction reaction.

In our case the anodic reaction will be

UO2 + H2O → UO3 + 2H+ + 2e

While uranium trioxide (UO3) can dissolve in acid to form uranyl cations (UO22+), it is easy to dissolve the uranium trioxide in carbonate solutions because of the following reaction.

UO3 + 2HCO3 → [UO2(CO3)2]2- + H2O

This carbonate complex formation can prevent the formation of a solid layer of uranium trioxide. So now we have the anodic reaction for the corrosion of uranium dioxide. I suspect that it will be impossible to exclude carbonate / carbon dioxide from the water which was used to cool the striken reactors in the first days and weeks after the earthquake. Now lets get onto the cathodic reaction, I think that the cathodic reaction offers a means of controlling the rate of the uranium dioxide dissolution.

You will be glad to know that uranium dioxide is not sufficiently reactive to snatch the oxygen from water in the same way as iron can. This cathodic water reduction reactions are important when considering the corrosion of steel radiators in a central heating system where no oxygen is present.

2H2O + 2e → H2 + 2OH

H2O + 2e → H2 + O2-

For the corrosion of uranium dioxide the water reduction cathodic reactions are thermodynamically unfavorable, while we are at it I think that thermodynamics is a misnomer. While modern thermodynamics is based on the idea of many molecules zipping around and doing things, classical thermodynamics is all about static systems changing into other static systems so I think that thermostatics might be a better name for thermodynamics.

For the corrosion of uranium dioxide I think that oxygen and hydrogen peroxide will be important. The hydrogen peroxide is formed by the action of radiation on water. The chemistry which occurs when pure water is exposed to radiation is fiendishly complex, but it does roughly does the following.

2H2O → H2O2 + H2

The hydrogen gas can vanish out of the mixture leaving the hydrogen peroxide behind. This water radiolysis reaction can generate both hydrogen and also the hydrogen peroxide. The hydrogen peroxide can then act as an oxidant, inside the reactors will be a range of different materials which can reduce hydrogen peroxide back to water.

H2O2 + 2e → 2HO

The surfaces inside the reactor can also act as catalysts for the conversion of hydrogen peroxide to water and oxygen gas, it is well known that metal surfaces will cause hydrogen peroxide to decompose into oxygen and water. For example a British submarine (HMS Sidon) sank after an accident during which hydrogen peroxide came into contact with metal surfaces, in a similar way it is possible that some of the metal surfaces inside the reactor would cause hydrogen peroxide to break down to oxygen.

The fact that the zirconium / water reaction generates hydrogen already will make the water chemistry a bit different, lets start by thinking again about the pure water case. The chemistry starts with

H2O → e + H2O+

Then the solvated electron (e) and the positively charged water (H2O+) can react further. A vast number of reactions will occur all at once in the water, I am only going to show you two of them for a moment. This one makes hydrogen peroxide.

H2O+ → H+ + HO.

2HO. → H2O2

When lots of oxygen is present then a new reaction can happen.

e + O2 → O2-.  then O2 + H+ → HOO.

This is important as the reducing radicals such as the solvated electron have been converted into oxidizing radicals. By adding an excess of a reducing agent we can convert the oxidising radicals into reducing ones, for example Max S. Matheson and Joseph Rabani, Journal of Physical Chemistry, 1965, volume 69, issue 4, pages 1324-1335 explains how hydrogen gas can alter the chemistry of the water.

They were able to observe the following reaction

H2 + O-. → HO + H.

I think that this reaction will be most important when the pH of the system is higher as the hydroxyl radicals (HO.) could be converted by base into the oxygen radical anions (O-.), if we add the base to the reaction it will become.

HO + H2 + HO. → H2O + HO + H.

We can make it more simple by removing the hydroxyl anion (HO) from both sides of the equation, which then gives us a more simple equation.

H2 + HO. → H2O + H.

So here we can see how the hydrogen gas from the zirconium corrosion may be able to make the radiation induced reactive species in the water more reducing, this is a good thing as it will reduce the corrosion rate in the reactor of the fuel and the other metal work. It is important to understand that every extra substance which is added to the water will make the radiation chemistry much more complex. We all know that salt water from the sea was added to the reactors during the first days of the accident, the salt in the water will make the chemistry a bit more complex. Some people in Germany (M. Kelm, V. Metz, E. Bohnert, E. Janata and C. Bube) published a paper (Radiation Physics and Chemistry, 2011, volume 80, issue 3, pages 426-434) in which they report some results for experiments done using solutions of common salt in water. Their work suggests that the yield of the corrosive elemental chlorine (Cl2) and other corrosive species (Cl3) will be lower when hydrogen gas is added to the salt water. Another paper by M. Kelm and E. Bohnert (Journal of Nuclear Materials, 2005, volume 346, issue 1, pages 1 to 4) does suggest that a small amount of bromide does greatly increase the yield of both oxygen and hydrogen formed when salt water is irradiated inside an autoclave (thing like a pressure cooker). But on the other hand experiments done at 90 oC generate far less gas pressure than those done at only 35 oC. We all know that sea water is not perfectly pure sodium chloride (it will have some traces of bromide in it) which would have increased the radiolytic yield of oxygen gas during the accident but as the reactor water was very hot the temperature effect may have lowered the yield of gases. I am sure in due time we will find out more about what has happened inside the damaged plants.

Have a merry christmas, and a happy new year (God Jul och gott ny år)


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