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Uranium at the Grand Canyon II

Dear Reader,

The story continues, now I am not going to pretend that uranium ore is harmless but at the same time we should not exaggerate how bad it is. One of the problems is that there is a shortage of information on the subject of what was present in the museum building, and also I have no idea of the shape, size and other properties of the museum.

Now I hold the view that the main threat posed by uranium ore in a bucket is the release of radon from the ore. I would like to point out that uranium is not a very toxic substance. Uranium if it is inhaled by a human will be able to dissolve, form a substance that can be excreted with ease via the urine.

So I think that if a person had inhaled some uranium radioactivity then in terms of damage done per Bq inhaled it is less than what something which forms a much less soluble oxide (such as plutonium) would be able to do.

I imagine that the uranium ore was being stored dry, while dry solids tend to form more dust than wet solids in some ways this may have reduced the radiological risk posed by the uranium. The main risk of uranium ore is from the radon which is generated by the radium in the ore.

What will happen is that the uranium (238U) will decay according to the following decay chain.

238U → 234Th → 234Pa → 234U → 230Th → 226Ra → 222Rn → 218Po → 214Pb → 214Bi → 214Po → 210Pb → 210Bi → 210Po → 206Pb

I have chosen to ignore the small amount of 235U as this decays by a pathway which goes via a very short lived radon (219Rn) which due to its very short half life is unable to escape as a gas from the rocks and then deliver an alpha dose to the lungs of the people. In the following diagram I am showing the decay chain of the minor isotope of uranium, I have chosen to ignore any branch which is less than 1 % of the decay chain. The diagonal arrows are alpha decays while the vertical arrows are beta decays.

u235 chain

When a radium containing mineral sample is stored dry the emission of radon from the solid is less than when it is stored wet. A good example of this can be seen in the paper by A. Sakoda, Y. Ishimori, K.Hanamotoa, T. Kataokaa, A. Kawabea and K. Yamaoka (Radiation Measurements, Volume 45, Issue 2, February 2010, Pages 204-210). In this paper the effect of changing the size of the particles was not very clear.

What has to happen for radon to be released from a mineral grain is for the radon atom to be close to the surface of the grain or in the air space between two grains. The radium starts off in the solid grains, when it undergoes an alpha decay the recoil from the emission of the alpha particle will make the atom jump backwards. This recoil can help to liberate the radon atom from the solid.

If the recoil does not bring the radon to the top layers of the grain (a) then it will be trapped so long inside the grain that it will decay. If it recoils into the air gap (b) and the air gap is nice and large then it will slow down in the air gap and find itself floating about in the air gap. However if the air gap is very thin and the recoiling atom strikes the surface of another grain then it can hop between grains. It is possible if it strikes the other grain with sufficient energy that it will bury itself (c) in the other grain where it will decay harmlessly without being able to fly off into the air.

radon recoils

When the ore is wet the gaps between the grains are filled with water, this will reduce the distance the recoils taking path c can take. This can prevent the radon atoms hitting the other grain as they fly along path c. This is because the recoiling nuclei will lose more energy per unit distance of travel when they are flying through water instead of air. The radon can then transfer from the water into the air with ease.

I imagine that the ore was in the form of large lumps, this is a good thing. The reason is that the radon is being generated at an equal rate throughout the whole of the volume of the ore. After being generated it has to diffuse out of the lumps before it can enter the air. The radon has a half life of about 3 days.

To escape from the rock lump it must first be free from the grains and in the air spaces. It then has to diffuse out through the cracks and large pores in the lump to the surface of the rock. One method of reducing the emission of radon from the rock lump would be to paint the outer surface of the rock. But I doubt if they will do that in the museum as it would spoil the appearance of the rock lumps. Another method is to put the rock lumps in a sealed container, if the sealed container delays the escape of the radon by a few weeks then it will make a large difference as the radon will decay inside the container rather than in the air of the room.

If the ore had been crushed to a fine solid then the distance that the radon must migrate is smaller than if the ore is left in large lumps.

Now the next thing which needs to happen for the radon to deliver a dose to people is it has to get into the lungs and stay there. Now if you inhale radon gas then if it is 222Rn from the decay of natural uranium then it is unlikely to decay inside your lungs. You are more likely to exhale the radon before it has had a chance to decay. Even if the radon is adsorbed into your blood in your lungs it still has a good chance of being rereleased again from your body.

The bigger problem is if the radon decays in the air to form a radon daughter such as 218Po, this short-lived polonium can absorb onto dust and smoke particles. These are much more able to lodge in the lungs than the radon gas. One method of greatly reducing the alpha dose to lungs due to radon is to wear a dust mask. This will stop the dust and smoke particles from getting to the lungs. It is well known that uranium miners who are smokers are more susceptible to the induction of lung cancer by radon than non smoking miners. This is thought to be due to the smoke effect.

I am of the view that as smoking is banned in US goverment buildings, as long as the building is clean and free of smoke the radon is less dangerous than it would be in a smoky place. The escape of radon from the building as a result of ventillation will lower the lung dose caused by the radon. I am unable to make a dose estimate due to the fact that I am not privy to the full facts of the case. For my british readers who are able to read between the lines the meaning of the phrase “I am not privy to the full facts of the case” will be very clear. For those of my readers who do not understand british understatement, this means I am missing so many important details about he situation in the museum that it is clearly impossible to make a meaningful dose estimate or prediction of what will happen next.

But at least I can explain some of the things which are important in this case.

I have seen that the US goverment will be investigating what was going on in the museum, this will include dose reconstruction. Rather than jumping to any conclusions I think it will be better to wait for the report from the experts which should appear in about 90 day.+åps0

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