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Supervising Scientist Environmental Monitoring Program
Background descriptions about the nature and sources of radon and radon progeny in the vicinity of the Ranger and Jabiluka mine sites may be found in the environmental monitoring ‘Background Paper’.
Average radon concentrations vary from year to year and with geographical location depending on a large number of factors, including: distance from the coastline; meteorological parameters such as wind direction and atmospheric stability; soil moisture and porosity; and average soil radium-226 concentration. Once released into the atmosphere, radon gas decays into a number of short lived radioactive heavy metals (Po-218, Pb-214, Bi-214, Po-214), the so called radon progeny. Radon progeny may attach to aerosols and be washed out by rain or deposit on the soil. The concentration of radon progeny in the air therefore depends on how long the radon has been decaying for (the ‘age’ of the radon in the air) and the airborne particle concentration.
Results of the Supervising Scientist's radon progeny measurements are shown in the Mudginberri radon chart and the Jabiru radon chart. As for radon gas, the variations in radon progeny concentration reflect the change from wet to dry seasons, as well as day-to-day variability in meteorological parameters.
PAEC is the Potential Alpha Energy Concentration and gives a measure for the potential energy originating from the alpha decays of radon progeny in air. The following calculation can be performed to estimate the effective dose resulting from exposure to radon progeny:
| ERDP = hRDP CRDP t | |
| With: | |
| ERDP: | effective dose due to the inhalation of radon decay products [µSv] |
| hRDP: | dose conversion factor |
| CRDP: | radon progeny PAEC [µJ/m3] |
| t: | inhalation time. |
The dose conversion factor hRDP recommended in ICRP 65 (1993) is 1.1 μSv/(μJh/m³) (for radon in equilibrium with progeny).
Last year's (2008) average total PAEC amounted to approximately 0.030 µJ/m³ and 0.039 µJ/m³ at Mudginberri and Jabiru respectively. With the equations given above an effective dose rate for full-time occupancy at Mudginberri would be approximately 0.29 mSv and Jabiru approximately 0.38 mSv for 2008. This dose is primarily natural and hence not subject to dose limits. The difference in dose levels in the locations is caused by weather conditions inhibiting effective mixing of air and closeness to the Magela floodplain.
Estimating the contribution from the Ranger minesite to radon and radon progeny concentrations at Mudginberri is difficult because it is much lower than the natural concentrations. An estimate for the period March 1989 to February 1990 using an atmospheric dispersion model gave the contributions to be approximately 0.8 Bq/m3 for radon and 0.8 nJ/m3 for radon progeny PAEC at Mudginberri and 3.1 Bq/m3 for radon and 2.6 nJ/m3 for radon progeny PAEC at Jabiru (Martin 2000).
For full-time occupancy (365 days per year) at Mudginberri and Jabiru, radon progeny PAECs of 0.8 nJ/m3 and 2.6 nJ/m3, respectively, convert to 0.0077 mSv and 0.025mSv. These doses are less than 1% and 3%, respectively, of the dose limit of 1 mSv per year for a member of the public. The dose limit applies to total dose including all pathways; nevertheless it is plain that the contribution of radon progeny from Ranger to effective dose at Mudginberri and Jabiru is extremely small.
Akber R, Pfitzner J and Whittlestone S 1991. A mobile station for radon measurements. In: Internal Report 33. Proceedings: Workshop on environmental radiochemistry and radionuclide measurement, Supervising Scientist, Canberra. Unpublished paper.
ICRP65 1993. ICRP Publication 65, Protection against Radon-222 at home and at work.
Nero AV (1988). Radon and its decay products in indoor air: an overview. Chapter 1 in: Radon and its decay products in indoor air, ed. WW Nazaroff and AV Nero, Jr. John Wiley and Sons, New York.
Martin P 2000. Radiological impact assessment of uranium mining and milling. Section 3.3: Radon and radon progeny. PhD thesis, Queensland University of Technology.
UNSCEAR (2000) Sources and effects of ionising radiation, United Nations Scientific Committee on the Effects of Atomic Radiation, UNSECAR 2000 Report to the General Assembly, United Nations, New York.