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Uranium at the museum

Dear Reader

It has come to my attention that in the USA that some buckets of uranium ore were found in a museum site at the Grand Canyon.

Now it is claimed that where the buckets were that the dose rate was 13.9 milliREM per hour, this in european units translates into 139 microSv per hour. I find it interesting that at 5 feet from the buckets the dose rate was zero. A common error is to use a radiation meter which is senstive to beta particles to estimate the gamma dose rate without protecting the detector from the beta particles. This will cause the meter to over read.

Depending on the conditions and the meter in question the over estimate can be small or it can be large.

The problem with a bucket of uranium ore is that the self adsorption of gamma rays can be very large. The late John Pecket told me a funny story about this sort of thing. He had a kilo of plutonium in a glove box as the dioxide. This was in a bottle of dry powder, he pulled a small amount out and dissolved it in acid.

When he changed the geometry and spread it out by dissolving it the dose rate in the glove box went up.

As a result of the self adsorption problem it is hard to make an estimate of the dose rate. It will take me a lot of effort to calculate the dose rate in a room which has thick tiles of uranium dioxide on the walls. This will be a lot more effort than if the room had been painted with a uniform coating of something with a much greater radioactivity put unit volume.

I will have a go at making an estimate for a 5 gallon (25 litre) bucket. If we assume that bucket has a diammeter of 300 mm, it will be 350 mm high and it is a perfect cylinder then we can calculate some things.

If we consider the radiation dose at 1 meter from the end of the cylinder of uranium along the axis we can reduce the problem to a series of disks. Lets for example treat the bucket as a stack of 350 disks. We can consider first the top layer. If we assume that it is pure uranium dioxide which has reached a radioactive equilibrium with its daughters.

The volume of a 1 mm thick slice of the cylinder will be 70.7 cubic cm, as the relative density of uranium dioxide is 11, this means that in the 1 mm slide we will have 777.5 grams of uranium dioxide.

This is 2.88 moles of uranium atoms, this is 1.734E+24 atoms, we will have 8.53 MBq of activity of each nuclide in the decay chain in this slice. So the dose rate due to the top slice will be 1.29 microSv hr-1 at one meter from this slice. If we had no self adsorption then the dose rate at one meter from the cylinder would be about 450 microSv per hour which would be a jolly high dose rate. This is a dose rate which would under almost any circumstances cause me to back away from the source quickly. Well it is not the worst I have heard of, I have heard of a radioactivity worker who on arrival at a possible accident scene in Israel who encountered a radiation field of 500 mSv per hour, he very quickly retreated away from it. If I was to spend one minute in such a place then I would get a 8.33 mSv dose which is almost half my yearly limit as a nuclear worker in Sweden.

I think that when you encounter an unexpected radiation field of 500 mSv per hour the correct procedure is to turn and run (This could be a live saving act), if possible run around a corner in the hope that you can put some concrete between you and the source. So run around the corner and quickly check your dose rate meter before hissing some colourful language, getting to a place of safety, handing in your dosemeter for emergency rapid evaluation, going for a blood test (biological dosemetry), handing over any sugar cubes in your pockets to your radiation protection officer (For ESR dosemetry) and filling in an accident report. The swearing is optional but I will not have a problem with you if you let out some technicolour rant of dirty words which will turn the air blue.

I think that it is very hard to come up with a situation in which you could have a radiation field of 500 mSv per hour (500 mGy per hour). I estimate that you would need to have a point source of 36.5 curies of cobalt-60 at one meter. This is a very large source. Some radiographic sources might be in this range.

But as a result of self adsorption it will be much lower in our case of the uranium cylinder. If we use the NIST data for uranium, as I did and treat the uranium as a series of point sources along the axis of the cylinder. Then I got a value of 9.06 microSv per hour, this will work out at 0.9 milliREM per hour.

While the dose rate of 9 microSv per hour is higher than an area that the general public can legally have access to in Europe (2.5 microSv hr-1) it is not a dose rate which is superhigh. If we assume that a child goes to the museum each weekend and spends 1 hour 1 meter from the uranium bucket then they will get a dose of 453 microSv. This is half of what a member of the public (children are never allowed to be radiation workers so they will always be “members of the general public”).

So I do not think it is likely that any visitor has had a gamma ray dose which is anything to worry about.

 

 

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