Answer to Question #9222 Submitted to "Ask the Experts"
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This question relates to the collecting of radioactive minerals and, in particular, the mineral uraninite and radon gas. Given that the sample is pure uraninite and weighs about 1.1 kg, how much radon can we expect to be emitted each day? Also do we need only worry about emission of radon from the surface of the specimen or does radon diffuse easily through such a mineral? If created deep within the mineral does it diffuse through before decaying to polonium?
A 1.1 kilogram (kg) sample of pure uraninite (uranium dioxide, UO2) contains slightly less than 1 kg (0.97 kg) of natural uranium. Natural uranium consists of three isotopes, uranium-234 (234U), uranium-235 (235U), and uranium-238 (238U). Uranium-238 is the very long-lived parent of a radioactive decay chain containing, among other decay products, 234U, radium-226 (226Ra), and radon-222 (222Rn). The radioactivity in 1 kg of natural uranium includes 12 megabecquerels (MBq) of 238U. Assuming radioactive equilibrium, which is reasonable in natural rocks, there will also be 12 MBq each of 234U and 226Ra. Radium-226 is the immediate parent of 222Rn and continuously produces 222Rn by its decay. Because some of the 222Rn escapes from the rock, there would likely be a little less than 12 MBq of 222Rn in the 1.1 kg sample of pure uraninite. Although unrelated to the 222Rn question, natural uranium also contains 0.57 MBq of 235U plus its radioactive decay products.
The only radon isotope of concern in this situation is 222Rn, with a half-life of 3.8 days. The half-life of an isotope is the time required for half of a large number of atoms to transform (decay) to the next daughter product. The mean (average) life of a 222Rn atom is 5.4 days; that is the average time it has to migrate as a gas within the crystalline structure before decaying to a solid atom.
The potential distance that an atom of radon can migrate is short in solid rock or any solid crystal. The geometry of the sample is very important, since the amount of radon escaping will be proportional to the surface area. A lump approximating a spherical shape will allow much less radon release than a thin sheet of the same material.
The ratio of the number of 222Rn atoms escaping from a material to the number produced in the material by the decay of 226Ra is known as the emanation fraction. This fraction varies considerably; for example, in a public document of the Health Physics Society (HPS), emanation fractions in granites between 0.03 and 0.28 are given. Because of this degree of variation, it is impossible to accurately determine how much radon can be expected to be emitted each day from your particular sample of uraninite. While the best way to make the determination would be a measurement of the sample itself, we can use the approach for estimating radon emission from granite given in the document of the HPS mentioned above.
In the HPS document, a 50,000 square centimeter (cm2) by 3 cm thick granite countertop with a density of 2.75 (mass = 413,000 grams [g]) was assumed to have a 226Ra concentration of 0.30 becquerels per gram (Bq g-1), resulting in a total 226Ra activity of 120 kBq. The emanation fraction was estimated to be 0.1. This assumed countertop was calculated to emit 93 becquerels per liter (Bq L-1) per hour of 222Rn. A conservative estimate of the contribution of the countertop to indoor radon concentration in the kitchen was given as 0.005 Bq L-1, approximately one-eighth of the average radon gas concentration in U.S. homes (0.04 Bq L-1) and well below the Environmental Protection Agency guideline of 4 pCi L-1 (150 Bq m-3).*
In the 1.1 kg uraninite sample there is 12 MBq of 226Ra, one-tenth of the amount in the assumed countertop. A simple scaling calculation indicates that the uraninite sample would emit about 9.2 Bq L-1 per hour of 222Rn, or 24 x 9.2 = 220 Bq per day. The contribution to the radon concentration in the room containing the sample could be similarly estimated as one-tenth of the countertop situation, or 0.0005 Bq L-1 (0.5 MBq L-1).
I have not measured radon emission from uraninite, but I have had experience with several uranium ore samples in a variety of sizes. The only case I found in which radon in the air was a problem was in a geology department that had a large number of specimens. In that case, we placed the specimens in a display case with its own exhaust system to prevent radon from entering the room.
Keith Schiager, CHP, PhD
*The radon concentration units are given here in picocuries per liter (pCi L-1, called traditional units) because those are the units used by the Environmental Protection Agency. However, the Health Physics Society has adopted SI (International System) units and these are given in parentheses.