Answer to Question #9574 Submitted to "Ask the Experts"

Category: Environmental and Background Radiation — Building and Construction Material

The following question was answered by an expert in the appropriate field:

I am a geologist working on permitting a mine to produce concrete. The material is volcanic tuff that would be used to replace a portion (about 20 to 50 percent) of the cement powder in concrete. The rock contains an average of about 10 parts per million (ppm) uranium. I am concerned with both radon emission as well as radiation from the uranium. The rocks were erupted 19 million years ago, so not too much degradation has occurred. Is this material safe for use in foundations/slabs in residential or commercial buildings?

There is no substantial risk associated with the use of this volcanic tuff that you describe.

As a rough rule of thumb, the activity concentration for 238U is about 1 pCi g-1 for every 3 ppm. So 10 ppm of uranium gives about 3.3 pCi g-1. 235U is present also, but in lower concentrations; 238U and its decay series produce most of the dose.

The uranium concentration you mention is actually a little lower than what is typically found in rocks and soil: the typical range (from United Nations Science Committee on the Effects of Atomic Radiation [UNSCEAR] reports and from Eisenbud and Gesell, Environmental Radioactivity, 1997) is about 7-60 pCi g-1. As a rule of thumb, the concentrations of uranium and thorium are highest in felsic rocks and lowest in mafic and ultramafic rocks. The uranium concentration you mention suggests this is mafic rock, but there are felsic rocks that just happen to have lower uranium concentrations. It all depends on the age, the amount of geochemical processing, and the uranium concentration of the original magma.

238U in equilibrium with its decay series gives a radiation dose of about 14 mrem y-1 for every pCi g-1 of activity (also from Eisenbud and Gesell). So, 3.3 pCi g-1 would give an annual dose of 45 mrem y-1 for a person spending all of his or her time on (say) the ground floor of a house built on a slab made of 100 percent tuff. When you dilute the tuff with the rest of the cement you have even lower concentrations of uranium and lower radon emanations. As the concentration drops, so does the dose, so 20 percent tuff would give about 9 mrem y-1. This is a trivial dose.

As far as radon goes, 19 million years is long enough for the decay series to grow in to equilibrium, so we can expect that the tuff will contain radon in about the same concentrations as the parent 238U. If you crush the tuff, then the radon will be more likely to diffuse out of the small particles, but these particles will be encased in the matrix. The bottom line is that the majority of the radon diffusion should be from the surface of the concrete. And, since the activity concentrations are lower than those of average geologic materials, the radon emissions should also be relatively low.

One good reference is the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) report; if you check on the Web, you’ll find all of UNSCEAR's reports as free PDF downloads. Another good reference is the Eisenbud and Gesell text mentioned earlier. There are no relevant regulations for this because you are not artificially enhancing the uranium concentrations in the tuff.

Andrew Karam, CHP, PhD

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