Explanatory notes on radon and radon progeny

Background

Radon-222 (radon) is a naturally occurring radioactive gas (half-life 3.8 days) formed through the decay of radium-226 in the uranium-238 decay series.

Because uranium-238 and radium-226 occur naturally at low concentrations in most rocks and soils, radon emanates from these materials, permeating through the soil pore space before being exhaled from the ground surface to the atmosphere where it is dispersed by diffusion and wind currents.

When radon in air decays, it forms a number of short-lived radioactive decay products ('radon progeny'), which include polonium-218, lead-214, bismuth-214 and polonium-214. All are radioactive isotopes of heavy metal elements and all have half-lives that are much less than that of radon.

Radon itself does not contribute much to dose since it is immediately exhaled from the lung before decaying. The main contribution to dose is from the inhalation of radon progeny in air. Some of the inhaled radon progeny are retained in the lung, with the subsequent alpha decays delivering a radiation dose.

The concentration of radon progeny in air depends on several factors:

  • soil properties (eg moisture content, porosity, radium-226 content), which affect radon exhalation from the ground surface to the atmosphere;
  • meteorological conditions (eg rainfall, temperature, pressure), which can also affect radon exhalation from the ground surface to the atmosphere;
  • wind speed, which affects dispersion (dilution) of radon and radon progeny in the atmosphere through air mixing;
  • wind direction, which determines the regional source term of radon in air;
  • how long the radon in air has been decaying for (the 'age' of radon in air), since radon progeny grow-in from the decay of radon.

The typical daily (diurnal) trend of radon progeny in air is for concentrations to peak in the early morning when atmospheric conditions tend to be most stable and then reduce during the day when air mixing increases through thermal convection and advection by wind.

Radon progeny concentrations in air may also vary seasonally. In the Top End of the Northern Territory the typical seasonal trend is for concentrations to be lower in the wet season due to rainfall suppression of radon exhalation from the ground surface and washout of radon progeny from the atmosphere. Higher concentrations generally occur in the dry season due to dry soils allowing greater permeation and exhalation of radon gas from the ground surface to the atmosphere.

Total annual effective dose from radon progeny in air

The total annual dose to the public from radon progeny in air includes contributions from both natural background and mine-related sources and is calculated as:

ERP-TOTAL = PAECRP-TOTAL × DCCRP-PUBLIC × tOCC

where:

ERP-TOTAL is the total annual effective dose from the inhalation of radon progeny in air;

PAECRP-TOTAL is the annual average radon progeny potential alpha energy concentration;

DCCRP-PUBLIC is the current ICRP recommended dose conversion coefficient for radon progeny for the public of 0.0011 mSv per µJh/m3 (ICRP 1993); and

tOCC is the occupancy time, which is assumed to be 8760 hours.

Annual effective dose from mine-related radon progeny in air

The annual effective dose to the public dose from mine-related radon progeny in air is determined using a simple wind correlation model. It is assumed that the measured radon progeny PAEC at Jabiru town includes both a mine-related and natural background component when the wind is from the 90°–110° sector. When the wind is from other directions or when there is no wind blowing (ie still conditions), it is assumed that the measured radon progeny in air is due to natural background sources only. The same assumptions are made for Mudginberri when the wind is from the 140°–160° sector. Wind direction data is acquired from the Bureau of Meteorology weather station at Jabiru airport. The data is analysed to determine the number of hours per year that the wind is from the direction of the mine at both Jabiru town and Mudginberri. By correlating the hourly wind direction data with the hourly radon progeny data, the average PAEC when the wind is from the direction of the mine and when the wind is not from the direction of the mine is determined. The mine-related dose to the public from radon progeny is then calculated as:

ERP-MINE = (PAECRP-MINE – PAECRP-OTHER) × DCCRP-PUBLIC × tMINE

where:

ERP-MINE is the annual effective dose from the inhalation of mine-related radon progeny in air;

PAECRP-MINE is the annual average radon progeny potential alpha energy concentration when the wind is from the direction of the mine;

PAECRP-OTHER is the annual average radon progeny potential alpha energy concentration when the wind is from other directions;

DCCRP-PUBLIC is the current ICRP recommended dose conversion coefficient for radon progeny for the public of 0.0011 mSv per µJh/m3 (ICRP 1993); and

tMINE is the number of hours that the wind is from the direction of the mine.

The annual effective dose to the public from mine-related radon progeny in air at both Jabiru town and Mudginberri is typically a few percent or less of the annual dose limit of 1 mSv for the public. Although the dose limit applies to the sum of doses received from all above background sources and exposure pathways, it is clear that there is currently no unacceptable radiation risk to the public at Jabiru town or Mudginberri community from radon progeny in air that may originate from the Ranger mine.

Anticipated change in radon progeny dose coefficient

The ICRP has recently published a report on lung cancer risk from the inhalation of radon and radon progeny (ICRP 2010). The report recommends changes to the dose conversion convention for estimating lung cancer risk from radon and radon progeny. These changes will result in new dose coefficients for inhalation of radon progeny, which are expected to be larger by a factor of two or more than the existing dose coefficient. However, until such time as the ICRP publishes new dose coefficients for the inhalation of radon progeny, the existing dose coefficient of 0.0011 mSv per µJh/m3 for members of the public remains valid.

References

ICRP 1993. Protection against radon-222 at home and at work. ICRP Publication 65, Annals of the ICRP 23(2).

ICRP 2010. Lung cancer risk from radon and progeny and statement on radon. ICRP Publication 115, Annals of the ICRP 40(1).