Supervising Scientist Division

Supervising Scientist Annual Report 2003 - 2004: Radiological risk

Supervising Scientist, Darwin, 2004
ISBN 0 642 24391 3
ISSN 0 158-4030

3 Environmental research and monitoring (continued)


3.7 Radiological risk

This programme monitors and investigates radiological risk arising from present-day uranium mining operations in the Alligator Rivers Region and assists in planning for rehabilitation of former and present-day minesites from a radiological perspective. This work will provide important input into planning for rehabilitation of Ranger mine.

Key activities in 2003-04 included:

3.7.1 Development of a contaminant pathways conceptual model for Ranger mine

The general radiological exposure pathways for uranium mining are the inhalation of airborne radioactivity, the ingestion of radionuclides in aquatic and terrestrial foods, and the exposure via direct gamma radiation. Whereas the airborne and aquatic pathways are the main contributors during the operational phase of a uranium mine, terrestrial pathways become increasingly important after rehabilitation.

Nabarlek uranium mine is the first modern uranium mine rehabilitated in tropical Australia and provides a good opportunity to study the success of rehabilitation from a radiological perspective, assess radiological stability and risk associated with rehabilitation and ecosystem establishment and assist in the development of closure criteria for the rehabilitation of Ranger.

Inhalation of airborne radioactivity

This pathway includes the inhalation of radon and its progeny (radon decay products) and radioactivity trapped in or on dust. Radon is a radioactive noble gas emitted from natural soils and rocks. Its emission source strength is directly related to the radium-226 concentration and therefore, uranium mines, with large amounts of uranium and radium rich rocks, are a source of radon.

Figure 3.8

Figure 3.8 Staff from eriss and Queensland University of Technology measuring radon exhalation from the ore stockpiles on Ranger mine

A study conducted this year has addressed the Ranger radon exhalation source term. Radon exhalation fluxes were measured from the waste rock dumps, different grade ore stockpiles, the land application areas, and Pits 1 and 3. In addition, the mill source term was estimated. The data show that, apart from radium-226 concentration, radon exhalation is dependent on grain size, compactness and height of stockpiled material. The study will provide information on present day radon exhalation rates from the Ranger site before rehabilitation, help to optimise capping designs to minimise radon exhalation from the site and assess the overall success of rehabilitation measures.

Comprehensive data analysis of a radon exhalation study at the rehabilitated Nabarlek mine allowed an estimate to be made of the percentage area of the site and of individual areas that would exceed given radon exhalation fluxes. Examples are shown in Table 3.2.

Table 3.2 Percentage area of the surveyed and individual areas at Nabarlek exceeding given radon exhalation fluxes
Radon exhalation flux [mBq.m-2.s-1] 150 200 500 750 1000
Evaporation Pond 24 15 2 1 0
Waste rock dump + runoff pond 66 54 20 11 6
Total area 50 42 21 14 10

Although the Australian approach of radiological protection is based on a site radiological risk assessment methodology, which is based upon estimates of the total radiological dose, this study gives some indication of the success of rehabilitation and may provide a guideline on closure criteria for Ranger. For example, the US Environmental Protection Agency has requirements for the radon exhalation flux to be below 740 milli Becquerel per square metre per second after soil clean-up. Our study has shown that about 14% of the area surveyed would not comply with those criteria, whereas 99% of the evaporation pond areas, the most likely areas for habitation, would comply. The study also indicates that it is unlikely that post-mining radon exhalation rates are higher than pre-mining.

The site is hard to access during the wet season and a conservative estimate shows that during the dry season, assuming hunting activities on site for 50 days, a maximum of 10% of the public exposure limit of 1 milli Sievert would be received via inhalation of radon progeny due to past mining activities.

Dust monitoring data show that from a radiological viewpoint the airborne dust pathway is negligible. However, to gain a complete radiological picture, the geographical distribution of dust deposition around the Ranger mine has been mapped using stable lead isotopes. The data show that although a significant fraction of dust deposited in Jabiru East originates from Ranger, the radiological risk associated is negligible as overall dust levels are low. This method may provide an innovative tool to assess dust generation and deposition during and after rehabilitation activities at Ranger.

Bioaccumulation

Earlier studies have shown that radiological impact via the ingestion pathway during the operational phase of a uranium mine is dominated by aquatic foods, in particular by the concentration of radium-226 in mussels. Results of a study compiled on radium loads in mussels collected in 2000-02 and comparison with historical data have shown that radium loads in mussels of the same age from billabongs upstream and downstream from Ranger do not show a significant increase over the last two decades. In contrast, last year's evaluation of data from specimens collected in the South Alligator Valley suggest some influence on mussel radionuclide loads due to past mining activities. This small increase does not, however, represent any significant health risk.

With the rehabilitation of Ranger there will be radiological protection issues associated with the land use expectations by local Aboriginal people. Wild fruit and vegetables still play an important part in traditional Aboriginal diet, and radionuclide uptake by these foods will become increasingly important for dose assessment. In comparison with the situation for aquatic flora and fauna, there is little information available for radionuclide concentrations in local fruit and vegetables. Data being gathered as part of the bioaccumulation project is providing important information on the uptake of radionuclides and exposure pathways associated with ecosystem establishment and in particular post-rehabilitation situations for Ranger and other uranium mines in the region. Again, Nabarlek provides a test site for the study of uptake of radionuclides in terrestrial food items. Site-specific data are being obtained on the bioaccumulation of radionuclides in the small number of local species growing on the rehabilitated waste rock dumps and other areas on site.

Gamma radiation

Data from an airborne gamma survey (AGS) and subsequent ground truthing at Nabarlek were evaluated in order to gain a holistic view of radiological conditions of the site after rehabilitation. Structures associated with mining, such as the pit, waste rock dump and evaporation ponds, were distinguished and with ground-based calibrations, the AGS data provided area-averaged estimates of concentrations of radionuclides (potassium, thorium uranium) in surface soils. Consequently the data enabled an estimation of the average dose rate arising from terrestrial gamma radiation over large areas of the site. For the fenced rehabilitated minesite area this estimate was 0.31 micro Gray per hour. For 50 days continuous habitation, this gamma ray exposure would give rise to about 40% of the dose limit for members of the public.

This study highlighted the need for collection of data at various stages of mining operations for estimation of the dose increase that can result from a mining operation and to identify areas where attention can be focused during rehabilitation. Data captures during mining, such as for the Ranger minesite or for natural analogues such as for the Koongarra deposit, may also assist in assessment of the success of rehabilitation on its completion.

3.8 Communications and knowledge management

Communications and knowledge management works across all the themes to provide support to the research programmes and to develop communication programmes to inform and involve Aboriginal communities and our other research partners and stakeholders in the activities of the Supervising Scientist.

A full description of this programme can be found in Section 6 of this Annual Report.