Uranium and carbonate

Dear Reader,

It has come to my attention that some statements have been made in public which suggest that uranium sticks to DNA and thus is more dangerous than a radioactive metal which does not stick to DNA. This statement has been recently repeated by Paul Langley who is an antinuclear blogger.

I would like to point out something, that while uranium(VI) does bind to DNA it also binds to carbonate. As the concentration of carbonate in the human body is so much higher than that of DNA, the carbonate will by competing with the DNA (and all the other molecules which can bind uranium) for the uranium. As a result the amount of uranium which will bind to the DNA may be much lower than would be predicted if only the binding of uranium to DNA was considered. A paper on this subject can be seen here. I think that those who like to talk of uranium binding to DNA in living cells should repeat their calculations after having taken into account the effect of carbonate anions.

If you want to see what a uranium carbonate complex looks like then please see here.

I would also like to point out that an alpha emitter does not need to bind to DNA in order to have a baneful effect. I think that the uranium binding to DNA is a bit of a red herring, one which will confuse the general public.

The length of an alpha track in water is sufficiently long (at least 50 micrometers) that the alpha particle can travel some distance through a cell. You can see a graph of alpha range as a function of energy here. I will warn you that the example of the range (in meters) of the beta particles of Sr-90 used on this latter page is misleading as the Y-90 daughter has a much higher beta energy.

Uranium in cells

Dear Reader,

I was reading recently about the biochemistry of uranium inside living cells, I found the work of Diane M. Stearns, she works on the effects of uranium and other metals on the DNA inside living cells. Now before I get going I would like to warn you that a lot of odd things have been written about uranium.

1. Uranium is not man made, it is in almost all soils as a result of the actions of nature and not the “hand of man” (or even the master/slave grabber thing of man). Thus as a result it could be very hard to prove if man has (or has not) added uranium to the environment through his actions.

2. Uranium is not intensely radioactive, I always use the following test in my mind for “intense radioactivity”. The test is to ask myself the question of ”would I be willing to pick up a pencil sized rod of the material ?” with only a thin surgical glove on my hand. In the case of natural uranium I would say yes, the radiation dose to my hand would be very low. In the case of used uranium dioxide fuel from a nuclear reactor the answer is a big no. But the reason why I would never dream of going near a pencil length of used fuel is not the uranium content. The reason is that the high concentration of fission products such as Cs-137 would make the radiation level near the fuel very high. So we can rest assured that U-234, U-235 and U-238 are not gram for gram such potent radioactive menaces as Ir-192, Ra-226, Sr-90, P-32, Co-60 or Cs-137.

3. The long half life of uranium does not make it dangerous, a long half life means that the chance of a given atom decaying during a second is low. Both things with supershort and superlong half lives tend to pose lower threats to the public than things with medium term half lives. For example the nitrogen-16 formed in a water cooled nuclear reactor by the np reaction from oxygen-16 is not a credible threat to the public as it has a half life of 7.13 seconds. While a BWR or PWR might have nitrogen-16 activity inside it during normal operation this activity is not a threat to the general public as within two minutes the activity has dropped by a factor of 118451. So as a result if the reactor shuts down during a loss of cooling accident then the nitrogen-16 will rapidly vanish from the coolant.

On the other hand bismuth which is used for making Pepto-Bismol which is a bright pink over the counter medicine, I think it is about the same colour as a breast cancer ribbon. It has been found that bismuth-209 (natural bismuth) undergoes alpha decay with a half life of years 2 x 10¹⁹ years. This compared with uranium-238 which has a half life of 4.5 x 10⁹ years. This does not mean that your jar of Pepto-Bismol is a radioactive monster which will endanger your health. According to the Pepto-Bismol site a 15 ml tea spoon of the medicine contains 262 mg (0.262 grams) of bismuth subsalicylate. A bottle of the pink medicine contains 354 ml (12 fluid ounces). Lets make an estimate of the bismuth radioactivity in the bottle.

The formula weight of bismuth subsalicylate is 361 grams per mole, so the 15 ml tea spoon contained 726 μmol of bismuth. This means that the whole bottle contains 17.13 mmol of bismuth. This is 1.03145 x 10²²atoms of bismuth, for those of you without a science background who were not paying attention in their maths lessons this is 10314468254847600000000 atoms of bismuth. Now I want to tell you do not ever let anyone try to frighten you with a large number of digits.

Now we have a nice and useful equation for predicting the number of radioactive decays per time period it is

A (activity) = N (number of atoms) x λ (decay constant)

Now lets work out the value for the decay constant (λ),

λ = ln (2) / half life

As the half life for the bismuth is 6,3072 x 10²⁶seconds, the decay constant will be 1,09898 x 10²⁶ s^-1. Which means that the number of radioactive decays per second in our medicine bottle will be 0.00001134 decays per second (or 11.36 μBq). This means that if we had a perfect ability to detect the alpha emission of the bismuth in the bottle we would have to wait on average 24.5 hours between the radioactive decay events. This is a far lower rate of events as you will get in a typical place on earth due to the background of cosmic rays and other events. To observe this low level of radioactivity would require a super clean lab with very low background detectors.

So as a result do not get nervous just when you see a long half life, what you should ask yourself is how much activity is present. The half life will tell you how long the radioactivity will last but a long half life does not make a radioisotope dangerous to your health.

Now back to uranium, it is important to understand that some shorter lived isotopes of uranium such as uranium-232 (half life 69 years) do emit so many alpha particles per second per gram that they may well pose a threat on the same scale as Pu-238 to your health, but for normal uranium we need to think about the chemical effects.

It has been shown that uranium increases the oxidative stress in living things, oxidative stress has been blamed for inducing cancer and causing the body to age. I always thought that Micheal Jackson’s reported habit of sleeping in an oxygen tent might have increased the rate at which his body aged. I think that prolonged exposure to high pressure oxygen for a prolonged time is likely to speed up aging or make something else go wrong inside a human.

It has been shown that uranium and other metals are able to react with things like hydrogen peroxide to create free radicals such as the hydroxyl radical. The hydroxyl radical is like a molecular scale drunken yob, it has little respect for its surroundings and it will attack anything which has the bad luck to cross its path. In the same way as the drunken yob is a threat to nuns, old ladies, policemen and bar men the hydroxyl radical will attack close to any organic molecule. It can attack fats, proteins and DNA with equal wild abandon. The good news is that after attacking a molecule the hydroxyl radical is normally converted into a molecule of ordinary simple harmless water, so unlike the yob it will not beat up a series of molecules in your body.

One classic way to make hydroxyl radicals is to react iron(II) sulphate with hydrogen peroxide, I used to use this reaction to generate hydroxyl radicals and use them to make chemical reactions happen for me. In the same way it is possible to generate free radicals using uranium(VI) inside cells. The free radicals will then damage the molecules inside the cell which include the DNA. A paper by A.C. Miller, M. Stewart, K. Brooks, L. Shi and N. Page, Journal of Inorganic Biochemistry, 2002, 91, 246-252 states that the majority of the carcinogenic activity of depleted uranium is due to the chemical effects of the metal. The paper points out that the concentration of 8-oxo-7-hydro-2-deoxyguanosine (a marker for oxidative stress related damage to DNA) is increased when either nickel, iron or uranium is added to the cells. This is good evidence that the uranium is able to damage the DNA.

One last point, it is important that when considering the question of ”is uranium bad for your health” to be very careful about using health / disease data from humans. Most uranium miners from the bad olde daze have been exposed to radon, radon daughters and dust/smoke. While V.H. Coryell and D.M. Stearns include in the introduction of one paper on chemically induced mutations caused by uranium a mention of uranium miners, they do quite rightly include a caveat that these miners were exposed to higher than normal levels of radon.

My own view of radon is that it is a much nastier threat to the lungs when it is combined with smoke from either diesel engines or cigarettes. I was taught years ago that the most important health protection action which relates to lung cancer in a uranium mine is to enforce a smoking ban inside the mine. A strong synagism exists between radon and smoking, either is bad for your health but when taken together the effect is very dire.

Becuase of the radon exposure to uranium miners I think it is important to be careful when trying to apply the lessons from uranium mining to people who have been exposed to uranium only.

Mononuclear uranium carbonate complexes

Dear Reader,

As I promised I am now going to show you some mononuclear uranium carbonate complexes.

R. Graziani, G. Bombieri and E. Forsellini, Dalton Transactions, 1972, 2059-2061 report the structure of tetramethyl ammonium salt of the [UO₂(CO₃)₃]^4- anion. This is a mononuclear uranyl carbonate complex. A similar complex was observed in the crystal structure of hydrated calcium uranyl carbonate (Liebigite) by K. Mereiter. Below I am showing two pictures of the uranyl carbonate complex from the Dalton paper.

A mononuclear uranium carbonate complex

Uranium and carbonate

Dear Reader,

I have commented before about uranium and its affinity for carbonate. The reaction of uranium(VI) cations with water can form uranyl dications (O=U=O)²⁺ which are able to bind to carbonate. Both mononuclear and polynuclear complexes are possible, the term polynuclear means a metal complex where more than one metal atom is present in it.

In a paper by P.G. Allen, J.J. Bucher, D.L. Clark, N.M. Edelstein, S.A. Ekberg, J.W. Gohdes, E.A. Hudson, N. Kaltsoyannis, W.W. Lukens, M.P. Neu, P.D. Palmer, T. Reich, D.K. Shuh, C.D. Tait and B.D. Zwick (Inorganic Chemistry, 1995, 34, 4797) a complex containing three uranium atoms has been reported. Below I am showing the structure of the [(UO₂)₃(CO₃)₆]^6- anion first here is a ball and stick view.

The complex of three uranyl cations and six carbonate dianions

Now here is the space filling view of the same complex.

Space filling view of the complex of three uranyl cations and six carbonates

I hope to be able to write more on the subject of uranium and carbonate later.

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 + O^2-

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 (Cl_3^-) 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)

Uranium and DNA

Dear Reader,

I have become aware that within some parts of the ”Green” community that uranium is being regarded as public enemy number one. I think that this attention to uranium started around about the time of the first gulf war when a series of friendly fire events (fraternicide) where allied personnel were shot using depleted uranium antiarmour ammunition.

One of the ideas which is circulating is that uranium has a strong ability to bind to DNA and it thus causes DNA damage. I am well aware that for years that uranium has been used as a stain in electron microscopy work to allow DNA to be seen with greater ease. But this fact alone does not mean that uranium will stick to DNA and thus cause the DNA damage which leads to cancer, birth defects and the rest of the horror show which some people claim is caused by the use of “DU”.

I would like to apply some common sense and critical thinking to the question of uranium and health. Firstly there is this question of “does uranium exert a harmful effect through binding to DNA ?”, this is an important question as some metals such as chromium and platinum can bind directly to the nitrogen DNA bases and thus damage the DNA.

Now I will not pretend to be all knowing, but I do know something about uranium chemistry. If we were to assume that the human body has two compartments for uranium. That is the “free” uranium and the uranium bonded to DNA then using a binding constant for uranium to DNA we could get an answer using the following simple equation. (Langmuir isotherm equation)

Θ = k [U][DNA] / (1 + k [U][DNA]) = k [U] / (1 + k [U])

This weird looking thing Θ is the fraction of the DNA which has uranium bonded to it.

However in real life a range of molecules other than DNA can bind to uranium, a classic example is carbonate. As all living humans are making carbon dioxide inside them then I think it is safe to assume that we should consider the binding of carbonate to uranium in any model of uranium in humans.

The new equations are going to be a lot more complex.

I am sure that our new value of Θ which takes into account the carbonate binding of DNA will now be lower. I have to say that this carbonate binding does make uranium look much less scary from the DNA binding point of view but we are not done with uranium yet. I may well write another blog post on uranium soon.

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.

David’s fright

Dear reader,

Some time ago recently I commented on the fright that David Hahn gave himself, this fright lead to his arrest and the uncovering of his nuclear experiments. What I think may have happened was that when David did the chemical conversion of thorium dioxide into thorium metal that he removed the radium from the thorium.

Some of you will know the equation for simple radioactive decay.

A = A_o e ^-λt

Where λ is the radioactive decay constant, which is equal to ln 2 / the half life = ln 2 / t½

For some thing simple like a cobalt-60 source in a cancer treatment machine this simple equation is all you need to predict the radioactivity level at a future date.

But for the more complex system where one thing decays into another thing which decays into another thing …….. ad nauseum something else is needed. For the next step up in complexity (two radioisotopes)

Here you need to use

A_y = A_xo . (λy / λy –λx) . (e ^–λxt – e ^{– λyt})

Which is a bit more maths, but I shudder at three radioisotopes in a series. If any of you feel up to it then please mail me your equation.

For natural thorium we have the following decay series.

Isotope	Half life	Mode
232Th	1.405 x 1010 years	α
228Ra	6 years	β
228Ac	6 hours	β
228Th	1.9 years	α
224Ra	3.7 days	α
220Rn	1 minute	α
216Po	0.2 seconds	α
212Pb	10.6 hours	β
212Bi	61 minutes	β
212Po	Short	α
208Pb	Stable

Looking at the decay series I think it was the ingrowth of the radium isotopes which was responsible for giving David his fright, all the other isotopes seem too short lived to be important in the kinetics. But I am sure that someone out there wants to be clever by using a more mathematically rigorous method.

If we assume that David started with a thorium which reached a secular radioactive equilibrium between the two thorium isotopes then we can start. Lets assume that David has 1 kBq of each of the thorium isotopes in his stash of gas mantles and at time zero he does the separation. Then if we consider only the thorium-228, radium-224, radon-220 and polonium-216 then we can make a graph. We will assume that none of the radon-220 is lost from the thorium “fuel” then if we just consider the four alpha emitters which I have mentioned in the last sentence then the graph of the total alpha activity of the thorium-228, radium-224, radon-220 and polonium-216 against time will look like this.

The change in alpha activity for Th-228, Ra-224, Rn-220 and Po-216 after the chemical separation of 1 kBq of Th-228 with time.

I am sure that we can get a even better set of graphs by adding more of the radioactive isotopes to it, but while the better graphs might be more useful for radiation protection purposes I think that this simple graph can show you the general idea of what can happen if you purify thorium and then leave it to stand. It is possible that David’s neutron source may have induced some radioactivity by activation but I suspect that his neutron source was quite weak. To know for sure we would need to have obtained his neutron source and then tested it with a series of target materials to see how much activity it could induce. On the top of my hit list of targets for the test would be sulphur, silver, rhodium and iridium. I would use a range of solid targets to allow an estimate to be made of the neutron flux for a series of different energies.

Crystals and the home made nuclear reactor

Dear Reader,

I feel that nature has not taken its course yet with the Swedish home made nuclear reactor but it is high time I wrote again on the subject of crystals. So lets do both at once !

I commented on how I thought that it was a bad idea to try to use sulphuric acid to dissolve up the radium which is in solid form. I suspect that the radium in a radioactive source or on the surface of ye olde glow in the dark clock will be as the insoluble radium sulphate.

Much of radium chemistry is shrouded in darkness when compared with other metals, for example only four crystal structures have ever been published which contain radium. One of the key gaps in our knowledge is radium sulphate; we will assume for a moment that radium sulphate is isostructural to barium sulphate. The word isostructural is a big technical sounding word which means that the basic structure is the same, but the exact distances between the atoms in the unit cell might differ.

For example calcium fluoride and uranium dioxide are isostructural, the fractional coordinates of the uranium / oxygen atoms match those for calcium and fluorine atoms. But the size of the cubic unit cells are different. But lets get back to our barium and radium chemistry.

I think that the radium will have a lower solubility in sulphuric acid than it will in tap water. Tap water is normally low in sulphates; this lack of sulphate will become clear in a moment.

For many poorly soluble metal salts a thing called a solubility product is known.

This is often written as Ksp.

For barium sulphate Ksp = [Ba²⁺][SO₄^2-]

[Ba²⁺] and [SO₄^2-] are the concentrations of barium and sulphate in the solution.

Now those of you who paid attention in your GCSE maths lessons should understand that when barium sulphate is placed in pure water and stirred (until it reaches equilibrium) that

[Ba²⁺] = (Ksp)^½

But when the barium sulphate is placed in 0.01 sulphuric acid, then the concentration of the barium will be given by a new equation.

[Ba²⁺] = Ksp / [SO₄^2-]

It should be clear to you that by increasing the sulphate concentration that the equilibrium concentration of the barium will go down. It is very likely that the radium will behave the same way as the barium; Marie Curie isolated the radium from uranium ore together with the barium fraction. As I said yesterday for public safety reasons I will not tell you how she converted the barium / radium fraction into a water soluble form. If you are keen to know, please do not ask me about that chemical step as refusal often offends! If you want to know about other bits of chemistry then feel free to ask.

But now we have thought about solubilities lets look at the solid.

The unit cell of barium sulphate is 8.884 by 5.458 by 7.153 Å and it has atoms with the following fractional coordinates.

Ba	0.1846	0.2500	0.1581
Ba	0.6846	0.2500	0.3419
Ba	0.3154	0.7500	0.6581
Ba	0.8154	0.7500	0.8419
S	0.0630	0.2500	0.6914
S	0.5630	0.2500	0.8086
S	0.4370	0.7500	0.1914
S	0.9370	0.7500	0.3086
O	0.0814	0.0298	0.8190
O	0.1808	0.2500	0.5515
O	0.0814	0.4702	0.8190
O	0.9122	0.2500	0.6062
O	0.4122	0.2500	0.8938
O	0.5814	0.4702	0.6810
O	0.5814	0.0298	0.6810
O	0.6808	0.2500	0.9485
O	0.9186	0.9702	0.1810
O	0.8192	0.7500	0.4485
O	0.9186	0.5298	0.1810
O	0.0878	0.7500	0.3938
O	0.5878	0.75	0.1062
O	0.4186	0.5298	0.319
O	0.4186	0.9702	0.319
O	0.3192	0.75	0.0515

If you build a unit cell with these atoms then I think you need a prize from your teacher! I am not sure how it will apply to those of us who either left school twenty years ago or used a copy of ORTEP or some other computational aid.

For those of you who are not motivated to draw or build a unit cell here is a unit cell for BaSO₄.

A unit cell of barium sulphate, barium is in green, sulphur in orange and oxygen in red

Now the unit cell for strontium sulphate is a 8.377 by 5.350 by 6.873 Å box, all the atoms have the same fractional coordinates except the bariums are now strontiums. I suspect that radium sulphate has the same structure as barium sulphate and that the cell will be slightly bigger than that of barium sulphate. The fluorides of calcium, strontium, barium and radium all have the same fluorite structure, but the unit cells differ in size. Here is a table of the lengths of the sides of the unit cells of the fluorides.

Element	Length of unit cell (Å)
Ca	5.450
Sr	5.800
Ba	6.196
Ra	6.381

Sadly magnesium and beryllium has a different structure so we can not compare it to these other alkaline earth fluorides. Well I suspect that I have given you something to think about for a while.

Home made nuclear reactor part II

Dear Reader,

I was going to tell you some more about crystals and crystallography but that will have to wait for a while. Instead I am going to tell you some of what I think about garden shed nuclear experiments. In case you have come here hoping to read how to make your own reactor, then I have to apologise and tell you that for a series of reasons (including security reasons and a lack of time/space here) it is not possible for me to give you a how to guide to build a reactor in the space we have today.

The way that David Hahn and the Swede were discovered were very different. David gave himself a fright and then in his panic he attracted the attention of a random policeman, while the Swedish man wrote a letter to SSM asking if it is legal to build nuclear gadgets at home. Before we go any further I would like to stress that almost all nuclear and radioactive activities and equipment are regulated by criminal law in all the countries I have heard of. The IAEA have said publicly that they want the penalty for the illegal possession of some nuclear materials to be as high as murder, so we are dealing with “serious stuff” here!

David Hahn wanted to breed uranium-233 which is fissile from natural thorium-232 which is not fissile but just fertile. Fertile means that an isotope can be converted by a simple nuclear reaction into an isotope which either is (or decays into) a fissile isotope.

What David did was to burn up a large number of gas mantles to obtain thorium oxide; he then cut up a large number of lithium batteries to get the lithium. Then using the lithium he converted the thorium into the metal. I think that this was never needed, many reactors using metal oxides as fuels or as targets for radioisotope production. While neutrons can react with oxygen to form radioisotopes such as nitrogen-16 (by a np reaction) this is normally not a major problem.

David then tried to bombard his thorium with neutrons which were from a homemade neutron source. He used a combination of radium-226 and beryllium. This is a horrible mixture; the radium is properly one of the most radiotoxic isotopes in the world while the beryllium is the most toxic non-radioactive element. It causes a series of horrible diseases including a horrible lung rotting disease!

Radium-226; Properly the nastiest radioisotope in the world!

When David was doing his experiment he noticed that the radioactivity level of the thorium target was increasing, this increase in radioactivity prompted him to get into a bit of a panic. What was likely to be happening was during the pyrochemical processing of the thorium dioxide he would have separated the radium-228 and radium-224 from the thorium. These two radioisotopes would have then started to reappear because of the alpha decay of the thorium-232 and thorium-228.

When I get around to it I will plot some graphs of the radioactivity levels as a function of time in a sample of thorium which has been purified. But that will have to wait for another day.

The Swede was spotted in a different way, he wrote a letter to SSM asking for legal advice regarding his home made reactor. I have read the reactor builder’s blog and he does seem to be having some trouble with some concepts.

I am doubtful that even if he had been left to get on with his experiments that he would ever have managed to get his reactor to work. From the little information I have obtained from his blog it looks like he was trying to build a similar gadget to David’s one. It looks like he wanted to build a subcritical reactor which would be driven by an external neutron source.

He describes how he wanted to dissolve radium in 96% sulphuric acid, I think that this was a bad idea for several reasons. Firstly radium is horrible and radiotoxic, while secondly radium sulphate is very insoluble. One of the classic ways to extract uranium is to boil up uranium ore in sulphuric acid, the uranium dissolves while the radium together with the barium will form an insoluble sulphate. For those of you reading in America sulphate = sulfate, and sulphuric acid = sulfuric acid.

I have written some more about barium and radium sulphate, if you want to read about how it applies to this case then go here.

Then the mixed barium/radium sulphate is dissolved, I think in the interests of public safety I will not tell you how to do it here!

So the choice of reagents for the dissolution of the radium was a poor choice, I have also noticed that an ash tray was close to the cooker. I know that you all know that smoking is a dirty habit but smoking anywhere near radium is a very dirty habit. The problem is that in a radium contaminated environment such as an old uranium mine that the air contains radon-222. This ‘evil creeping death gas’ might be able to diffuse through rocks into caves and houses. But the real villain is the polonium-218 and the other radon daughters. These tend to stick to dust and smoke particles.

If you were to breathe radon contaminated but totally dust free air then it would not be good for you but compared with smoke mixed radon daughters the dust free radon air is positively health giving! What happens with tobacco smoke is that the particles get coated with the radon daughters and then they stick in the lungs. The smoke thus acts like glue to stick these nasty alpha emitters into the lungs. As a result a combination of smoke and radon has a far greater baneful effect on the lungs than the sum total of the two if they are done separately in time and space.

Before you are inclined to feel sorry for the Swede bear in mind that while he did stop to ask if home made reactors were legal, he did not think to ask until he had already started his experiments. I suspect that if a random person in Sweden was to write a letter to SSM asking if it was legal to build a nuclear reactor in the basement, then they would get investigated. But if the person had neither acquired nuclear/radioactive materials nor had started to try to build a reactor then they might at worst get their home searched, but I think that after a stern warning that SSM would send him on their merry way.

I do not quite know what the text of the warning would be but maybe the following might be a good one

“The unauthorised construction and use of nuclear reactors in the home is dangerous and may result in a large fine or lengthy imprisonment. I strongly suggest you take up some alternative recreational activity such as …………..”

I once had my lab inspected by a team of UN inspectors who wanted to check that I was not doing some undisclosed nuclear activity in my lab. I found the UN inspectors to be a courteous and professional group of men. What they did was to look in my lab at what sort of equipment I had, they wanted to ask me what sort of chemistry I was doing and to take a gamma spectrum to check what sort of radioisotopes were present in my lab (They found nothing as I have next to no radioactivity in the lab).

While I have no experience of being investigated for illegal back street nuclear reactor operation, I suspect that SSM might use similar tools such as the portable gamma spectrometer to check what radioisotopes were present in the man’s flat. I would also expect them to take some samples in the form of wipe samples to allow for different tests to be done later. One option would be to use an alpha/beta scintillation type contamination meter to search the surfaces in the man’s flat for hot spots of activity. When a hot spot is found it would be logical to sample it and then use a more sensitive and selective counting method to evaluate the sample.