I refer to the article by Kelvin Kemm in the Engineering News edition of May 27 and the subsequent responses by Sipho Kings and others in the June 10 edition concerning waste uranium rocks and other waste dumped near Beaufort West, in the Karoo. There is a need to put possible harmful effects in perspective.
It seems there is confusion in the units used and their interpretation in the correspondence. There is mention of a ‘Geiger counter’, a term that is often used generically, but let us assume that it is some radiation dose rate meter with units of microsieverts per hour. No hand-held meter can measure ‘Becquerels per mass’ unit. So, what does the quoted value of 0.2 microsievert per hour mean?
Firstly, it is a dose rate unit but not related to a public dose limit and was probably confused with radioactivity (Becquerels) limits per mass unit of solid radioactive waste, which has similar numerical value for uranium. Secondly, the general public dose limit internationally (and here) is 1 000 microsieverts per year above natural background and is not devolved down to an hourly rate limit. In fact, the yearly dose limit is calculated on a 50-year exposure duration for the low probability of cancer or other deleterious effects.
Government Gazette R388 specifies radiation dose limits to the public and the associated probabilistic risk limit. If a member of the public goes over the dose limit – and remember that must be averaged over a year’s exposure – that person is not immediately harmed but the probability of cancer increases. The associated average annual probabilistic risk limit for the population is one fatality for 100-million people, which is a rather conservative value that, I doubt, one would call dangerous.
An aspect of radiation measurements with a dose rate meter is that values detected are very sensitive to distance from the radioactive material. The articles did not give the details of the measurement process at the waste site but, from my long personal experience in such measurements, one can take a general reading from 0.5 m or 1 m away from suspect material and take other measurements at centimetre ranges to find hot spots. This can easily result in a factor thousand between values. Assuming the reported value of 0.2 microsieverts per hour is at 1 m, then a member of the general public would have to stay there day and night for over 200 days to reach the yearly limit, which is very unlikely. After several years, such continuous exposure could be considered risky, so the regulator was prudent to remove waste away from an inhabited house next to the original site.
We all live in a naturally radioactive world. Global average radiation background is 2 400 microsieverts per year from all natural sources, such as soil and rock and radon in the air we breathe and food we eat, as well as cosmic rays. The variation across the planet is from about 1 000 microsieverts to greater than 10 000 microsieverts per year. The public limit is another 1 000 added to local values. This means that millions of people are safely living with yearly exposures far exceeding the specified public limit. Uranium in rock is termed naturally occurring radioactive material, or NORM, which includes deposits of thorium and potassium.
What about the radon exposure from such uranium waste? The biggest fraction of natural background radiation exposure for the public is, in fact, due to the inhalation of radon gas and is about 50% of the total or about 1 200 microsieverts per year. Uranium is the parent of radon production and is found at trace levels in virtually all soils. Thus, radon is measurable anywhere. Radon is wind-borne from various directions, as it emanates from rocks and soils of up to several metres depth and from many kilometres distance.
Generally, outdoor concentrations are lower than indoor, where it is trapped by the house structure as it emanates from the ground below. Poor ventilation increases doses as a result of indoor radon. Contributions to the radon dose to members of the public from a small waste-rock area are considered very low. Contrary to the claim that no baseline studies have been done at gold mine dumps in the Johannesburg area, extensive studies on radon outdoors and indoors were done during the 1980s and the 1990s.
In summary, the impact of this isolated uranium waste site should not be seen as dangerous but as fitting into the range of natural fluctuations of radiation exposure. Any large-scale uranium mining should, of course, follow due process.
Dr Cairns Bain (PrSciNat)
Specialist: environmental radioactivity