APPENDIX D - EPA WA...D-4 Radiation quantities and units There are two main types of measurement in radiation protection. The first concerns the amount of a radioactive substance,

APPENDIX D

Further information about Radiation issues

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This Appendix has been prepared to provide further information about radiation issues covered in

the ERMP and the Responses to Submissions.

It is in the following sections:

1. What is radiation and how can it affect public health?

2. Verification of radiation information used in the ERMP.

3. Occupational doses.

4. Dose estimates for administration workers.

5. Radioactivity Analysis Report for soil and vegetation.

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1. What is radiation and how can it affect public health?

Some submitters sought further information about the nature of radiation, how it is measured and

how it can affect public health.

Atoms, isotopes and radioactive decay

All matter is made of atoms. Atoms have a central code (nucleus) of positively charged protons and

neutral neutrons. The nucleus is surrounded by a cloud of negatively charged electrons. Normally,

the number of electrons equals the number of protons so that the charges balance out, leaving the

atom overall electrically neutral. The number of protons (and thus the number of electrons)

determines the chemical properties of the atom. Thus every atom with 1 proton is an atom of

hydrogen, and every atom with 92 protons is an atom of uranium. The number of neutrons in a

particular element is variable. Hydrogen usually has none, but can have one or two. Uranium most

commonly has 146 neutrons but can have from about 125 to 150. Atoms of an element with

different numbers of neutrons are called “isotopes” of that element: thus hydrogen has three

isotopes and uranium 25. An isotope is generally written with its normal chemical symbol and its

“mass the number” – the total number of protons and neutrons in its nucleus. Thus the commonest

isotope of uranium has 92 protons and 146 neutrons and is written 238U (pronounced and sometimes

written U-238).

Not all combinations of protons and neutrons in a nucleus are stable: some are unstable, and break

down, in the process emitting energy in the form of sub-atomic particles or electromagnetic

radiation, and forming a lighter nucleus. This process of breakdown is called radioactivity or

radioactive decay. Isotopes that undergo it are called radioactive (radioisotopes or radionuclides)

and the energy emitted is called radiation. Not all radioactive atoms decay at the same rate. Some

are extremely unstable and decay in minute fractions of a second, others may take billions of years

to decay. The time taken for one half of the atoms of a radioisotope to decay is called the half life,

and is always constant for that particular isotope.

Types of radiation

Knocking of electrons out of an atom is called ionisation. The remaining atom is called an ion and is

electrically charged. If the particles or energy emitted by radioactive decay have enough energy to

knock electrons out of other atoms, then that radiation is called “ionising radiation”.

There are three types of ionising radiation that are important in uranium mining:

Alpha radiation consists of relatively heavy particles (two protons and two neutrons bound together) travelling relatively slowly. They ionise heavily when they pass through matter, and so lose their energy rapidly, and have a short range (less than a sheet of paper, or a few cm in air)

Beta radiation consists of a stream of high energy electrons. They ionise moderately, and have a range of up to a few meters in air, and can pass through a centimetre or so of matter such as plastic.

Gamma radiation does not consist of particles, but bundles of intense electromagnetic energy. They are very similar to x-rays, but generally have more energy and greater power

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to penetrate matter. They can travel right through the human body, but are stopped by thick metal or concrete layers

Radiation that cannot ionise matter is called non-ionising radiation. Examples include light, lasers,

ultra-violet and infra-red, radio waves, microwaves etc. Non-ionising radiation is quite different to

ionising radiation and will not be considered here: “radiation” will mean “ionising radiation”

Uranium and its decay products

As noted above the commonest isotope of uranium is 238U, which comprises about 99.3% of

naturally occurring uranium. 238U has a long half- life of 4.2 billion years, and decays by emitting an

alpha particle, turning into an isotope of the element thorium, 234Th. But 234Th is itself radioactive,

and it decays by emitting a beta particle, and turning into an isotope of Protactinium 234Pa, which is

also radioactive. In total, there are 14 decay steps, before the original atom of uranium becomes an

atom of lead, 206Pb, which is stable, and does not decay.

Uranium ore will contain all of these 14 radioactive isotopes and they need to be considered in

determining the radiological effects of mining uranium, and the protection measures needed.

Radiation exposure pathways

A radioactive material is of no human health concern unless there is some pathway by which the

radiation it emits can reach a person. There are two general ways that radiation exposure can occur:

External exposure is exposure from radiation that is outside (external to) the body. Examples are exposure form a medical x-ray, or gamma dose from standing near a pile of ore

Internal exposure is exposure from radioactive material that is inside the body. Usually this is material that has been taken in by inhalation or in food or water that has been consumed.

There are three main exposure pathways associated with uranium mining:

External gamma radiation. Uranium ore contains several isotopes that emit gamma radiation, and persons in the vicinity of ore or concentrates can receive doses as a result

Inhalation of radioactive dusts. Dusts from ore or concentrates contain radionuclides which if inhaled can lodge in the lung. They may remain in the lung, or be absorbed into the bloodstream and taken to other organs.

Inhalation of radon decay products. One of the radioactive isotopes in the uranium decay chain is a gas, radon. It can diffuse out of ore into the air, and be inhaled. Radon itself is not retained in the lung, but it decays fairly quickly into “radon decay products” (or radon progeny). These are metals, and if inhaled may lodge in the lung, where they may decay and release alpha radiation. In older poorly ventilated underground mines, this was the major source of radiation exposure to miners.

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Radiation quantities and units

There are two main types of measurement in radiation protection. The first concerns the amount of

a radioactive substance, and the second concerns the amount of radiation absorbed by an object.

They are quite different and there is generally no simple relationship between them.

Activity is the name given to the amount of radioactive material. It is measured by the number of

radioactive decays occurring per second. The unit is the becquerel (Bq) and is equivalent to an

activity of 1 decay per second. A becquerel is quite a small unit: 1 kg of typical soil contains a total of

approximately 1000 Bq. For large activities, units of kBq (kiloBequerel) and MBq (MegaBequerel) are

commonly used. Very large radioactive sources (for example those used in cancer treatment) can

have activities of many billions of Bq (GBq). Concentrations of radioactive material are typically

expressed as becquerels per kilogram (Bq/kg) in solids, Bq/L in liquids and Bq/m3 in air.

Dose is the name given to the amount of radiation absorbed by an object. As ionising radiation is

defined by its ability to ionise, “dose” is based on the amount of ionisation produced per unit mass.

There are a number of different types of dose but the most commonly used is called “effective

dose”. It is based on the amount of ionisation per unit mass, but includes corrections for the

different biological effects of different types of radiation (alpha, beta, gamma etc), and for the

different sensitivities of the various organs and tissues of the body to radiation. The unit of effective

dose is the seivert (Sv), but as this is a very large dose, practical doses are in millisieverts or

microsieverts (mSv or μSv). The “dose rate” is the amount of radiation absorbed in a unit time,

commonly in microsieverts per hour (μSv/h). When the term “dose” is used, it usually means

“effective dose”.

Health effects of radiation

The health effects of exposure to radiation are well known. At high doses (several thousand

millisieverts) significant numbers of cells may be killed, leading to the breakdown of sensitive tissues,

organ failure or death. Uranium mine workers generally receive doses hundreds of times lower than

the levels which would cause these kinds of effects.

At lower doses, health effects can arise from cells that are damaged by radiation but continue to

live. Such cells may develop the ability to proliferate without being under the body’s normal

controls, and this may be the initiating event in development of a cancer. However, the body has

mechanisms to repair damage, and the damaged cells may not survive. Studies have shown that the

increased cancer risk rises approximately proportionally with the radiation dose received; however

at low doses (below about 50 mSv), any increase in risk, if present, is too small to be detected. No

studies have been able to find genetic effects on humans, although such effects have been seen in

animal studies, and are presumed to also apply to humans.

These risks and potential risks, have been used in the setting of radiation standards. The

International Commission on Radiological Protection has stated that in setting standards, “it must be

presumed that even small radiation doses may produce some deleterious effects”. This is often

paraphrased as there being “no safe level of radiation”. This equates safety with “no risk at all”,

which is not the normal usage of “safe”. People generally consider that activities involving some

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level of risk may be considered safe if the level of risk is considered “acceptable”. An example is

commercial air travel, where people recognise that there is some element of risk, but still consider it

“safe”.

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2. Verification of Radiation information used in the ERMP.

There were several submissions that asked questions regarding the radiation data provided in the

ERMP and Appendices. The following table provides further information on the terms and

assumptions that were used in the preparation of the ERMP.

In general, the assumptions used in the radiation assessment are provided in Appendix E of

Appendix D of the ERMP.

(Note that where monitoring or background data is reported and is already referenced in the ERMP,

it has not been repeated here.)

Description of Radiation Term Verification/Source

Background radon concentration of 27 Bq/m3

Based on active sampling (using Rad7 monitors), passive sampling (using passive samplers) and comparison with historical monitoring results undertaken by the Australian Atomic Energy Commission (AAEC) in 1979. These results are summarised in Appendices F and G of Appendix D to the ERMP.

Background RnDP concentrations of 0.02 to 0.03 uJ/ m3

Based on active sampling (using RDS, RnDP real time monitor) and comparison with earlier monitoring undertaken by the AAEC in 1979. These results have been summarised in Appendices F and G of Appendix D of the ERMP.

Dust source terms used in air quality modelling

The air quality modelling provides Project impact dust concentration contours based on “source” terms. The “source” terms are used as inputs to the modelling as quantities of dust emitted from the various activities and these have been estimated using standard techniques described in Appendix B of the ERMP document. The output of the modelling is dust concentration contours in g/m3. Radionuclide concentration contours are calculated from the dust concentration contours. The air quality modelling considers all dust emitted, while the radiological assessment requires information on the radionuclides emitted and their concentration. To calculate radionuclide concentration from dust concentration is as follows:

It is assumed that the average uranium concentration in ore is 600 ppm, giving a U238 concentration of 7.4 Bq/g per radionuclide (note that the specific activity of pure U238 is 12,400 Bq/g. Therefore 600ppm (or 0.06%) gives 7.4 Bq/g)

However, not all dust that is emitted is ore dust. Some of the dust is road dust and some of the dust is overburden, both of which are non-mineralised.

It was assumed that approximately one third of the dust emitted would come from each source, giving an average uranium concentration of 200 ppm for all dust emitted.

200 ppm of U in dust was therefore used to calculate the environmental radionuclide concentrations in air contours.

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Radionuclide concentrations in dust at the key receptors sites

The radiation assessment identified nine sites near the proposed operations that were considered to be key receptor sites and these were mainly places where people lived. These locations were superimposed on the dust contour maps which came from the air quality modelling and the annual average dust (and radionuclide) in air concentrations were obtained from the modelled contours. The air quality modelling was conducted for years 4 and 8 of the project (because the mining changes from Lake Way to Centipede mine sites in this time), and concentrations were calculated for both cases. When assessing impacts, the highest concentration for either year 4 or year 8 was used.

Radon emanation rates To determine the impact of radon emissions from the Project, the air quality modelling was used to calculate the Project impact radon concentrations at the key receptor sites. This required determination of radon “source” terms to act as an input to the air quality modelling. Appendix C of Appendix D of the ERMP provides a summary of the estimated radon emanation rates from the various project components and is based on the AAEC 1979 work. Section 4.4.3.2 of the ERMP also describes radon emanation rates. More recent emanation work by Toro Energy (which was presented in the ERMP at table 27) indicates that the radon emanation rate is likely to be much lower than the earlier AAEC results showed. However, it was decided to continue to use the higher AAEC figures for impact assessment due to the fact that the newer figures were received after the air quality modelling was conducted and the results presented were therefore conservative.

Equilibrium Factor During the monitoring that was undertaken for preparation of the ERMP, Toro Energy undertook side by side monitoring of both radon and radon decay product concentrations, thereby enabling the calculation of actual real time equilibrium factors. This data was limited (being for a period of 3 weeks) and was presented for information purposes and as an additional indicator to assist in the characterisation of the background radiological environment. This was presented in Appendix G of Appendix D to the ERMP. The equilibrium factor is a measure of the degree of radionuclide equilibrium between radon and its short lived radioactive decay products measured as actual potential alpha energy concentration (PAEC) (UNSCEAR 2006 Annex E – Sources-to-effects assessment for radon in homes and workplaces) In Appendix G of Appendix D of the ERMP, a formula was provided to show the relationship between radon concentration in Bq/m3 and radon decay product (RnDP) concentration in uJ/m3, as follows;

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Equilibrium factor = 56.2 x RnDP / Rn This formula has subsequently been found to be inaccurate due to a transcription error. As a result of this, worker doses reported in the ERMP were overestimated and have subsequently been recalculated (as reported in the next section). The correct formula has been recalculated from first principles based on information stated in UNSCEAR 2006 (UNSCEAR 2006 Annex E – Sources-to-effects assessment for radon in homes and workplaces), which notes that 1 Working Level is equal to 100 pCi/l (assuming 100% equilibrium, or an equilibrium factor of 1). This relationship is in non SI units and to convert to SI units, the following conversion factors are used;

1 Working Level = 2.08 x 10-5 J/m3 1 pCi/I = 37 Bq/m3

For an equilibrium factor of 1, a radon concentration of 3,700 Bq/m3 produces a RnDP concentration of 2.08 x 10-5 J/m3. For an equilibrium factor of 0.5, 3,700 Bq/m3 of radon is then equivalent to 1.04 x 10-5 J/m3 of RnDP. This transcription error resulted in an overestimate of the RnDP doses for mine and plant workers presented in the ERMP. Doses to miners and plant workers were based on modelled radon concentrations being converted to RnDP concentrations using an equilibrium factor of 0.5 and have been recalculated. Equilibrium factors of 0.5 were used to calculate RnDP doses from modelled radon concentrations in the local communities. This resulted in all estimated annual public doses being less that 1/20th of the annual dose limit of 1 mSv/y.

Public Gamma Dose Estimate The public gamma dose estimate was undertaken with a Wise dose calculator (http://www.wise-uranium.org/index.html) using a stockpile of 600 ppm ore from a nominal surface area of 500 m2 (with no self attenuation).

Gamma from transport of uranium container

BHP Billiton 2009 quotes a measured gamma dose rate of 5 uSv/h at 1 m and 0.2 uSv/h from 10 m and this was used as an estimate of the gamma doses from Toro’s final product.

Gamma dose rates in open cut mines

The gamma dose rate for the open cut mine was estimated using two methods. The first was a theoretical calculation using the formula of Thompson and Wilson (1980). This provided an absolute upper maximum gamma level, based on ore grade, of 3.8 mSv/y. This approach does not take into account any shielding provided by mining equipment or attenuation afforded by non

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mineralised material. The second method was a more practical method and based on estimating miners’ gamma radiation levels from levels measured in other open pit mines. This method is likely to be the most accurate indicator of average dose. The survey of other similar mines indicated that the average annual dose would be about 1 mSv/y.

Dust concentration in air in the pit

The assumed ambient dust concentration in the open pit during mining was estimated to be 1 mg/m3. Published data on average dust concentrations in mines is sparse. BHP Billiton (EIS) referred to an average dust concentration of 3 mg/m3 which was based on measurements from open cut coal mines from the UK. However, 3 mg/m3 is relatively dusty and experience suggests that active controls (such as watering) would be implemented in the event that dust approached such a level. The assessment noted that if the dust levels were to be higher than this, then the dose would increase in direct proportion. However, the assessment also showed that at 1 mg/m3, the potential dose from inhalation of radioactive dust would be approximately 0.3 mSv/y. The ERMP also noted that Toro would be implementing active dust controls, such as watering etc. This would lead to the conclusion that the long term dust concentration averages would be low and well controlled and that 1 mg/m3 is a reasonable figure to use.

Dust Inhalation Doses The mine inhalation dose was based on a long term average dust concentration of 1 mg/m3, together with an assumed long term average mined uranium grade of 300ppm (which is made up of about half the material being uranium ore at 600ppm and half the material mined being overburden containing essentially no uranium). 1 gram of 300 ppm uranium, gives a radionuclide concentration of 3.7 Bq/g for each of the radionuclides in U238 decay chain. Therefore a dust cloud of 1 mg/m3 gives a radionuclide concentration of 3.7 mBq/m3. Dust dose is usually associated with the long lived radionuclides in the decay chain of which 5 are alpha emitting. The Code of Practice for Radiation Protection and Radioactive Waste Management in Mining and Mineral Processing

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(2005) (ARPANSA 2005) provides dose conversion factors to convert radionuclide in dust concentration into a committed dose. For uranium ore, this is 7.2 uSv/alpha dps. This is also combined with the estimated exposure hours (assumed to be 2,000 hours per year) to give an estimated dose. The annual dose is therefore; Annual Dose (uSv) =7.2 μSv/∝dps x 0.0037 Bq/m3 x 5∝dps/Bq x 2,400 m3/y, giving, 320 μSv/y The process was described in Appendix D of the ERMP Radiation Technical report (which was Appendix D of the ERMP). As part of the re-assessment of workers RnDP doses, dust doses were also re-assessed using the dose conversion factors provided by the IAEA and reported in the next section.

Emissions of Radon from the Pit Estimated emissions of radon from the Project are provided in table 72 of the ERMP. As noted in text supporting this table in the ERMP, the emission rates are based on the work conducted by the AAEC in 1979. The ERMP noted that additional emission work had been undertaken by Toro indicating that the emission rates were lower than previously thought. However, the final results were not available for the air quality modelling. The ERMP reports that total radon emissions from the two pits are as follows;

- 5.7 MBq/s (Centipede) - 7.4 MBq/s (Lake Way)

Calculation of the Miners’ Doses including calculation of radon concentrations in the pit

See next section.

Gamma doses to processing plant workers

The estimated gamma dose rates to processing plant workers were based on gamma dose rates actually recorded in other similar plants. The main reference was the work completed by BHP Billiton for its EIS, which showed gamma dose rates for plant workers being 1 to 2 mSv/y.

Dust doses to processing plant workers

The plant inhalation dose was determined in a similar manner to that for miners. The assessment was based on a long term average dust concentration of 1 mg/m3, together with an assumed long term average mined uranium grade of 600ppm (since all of the material being processed would be uranium ore). A dust cloud of 600 ppm uranium, gives a radionuclide concentration of approximately 7.4 Bq/g for each of the radionuclides in U238 decay chain.

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ARPANSA 2005 (Code of Practice) provides a number of dust dose conversion factors, including uranium ore and this is 7.2 uSv/alpha dps. This is also combined with the estimated exposure hours (assumed to be 2,000 hours per year) to give an estimated dose.

Radon Concentrations in the processing plant.

The radon concentrations were estimated from the air quality modelling which showed that at the site of the processing plant, the maximum Project impact radon concentration would be 5 Bq/m3.

Doses During product Packing Toro’s product packers would pack uranium on a batch basis (rather than as a full time occupation). At other facilities (such as Beverley), doses to product packers are generally less than 2 mSv/y and it is expected that this will be the case in the processing facility as the Toro product packing facility would be similar to the Beverley facility.

Doses To Truck Drivers The doses to truck drivers were based solely on exposure to gamma radiation as the product would be contained in sealed drums in sealed containers, and therefore not result in an inhalation exposure pathway. A dose rate of 1 uSv/h (based on measurements of uranium product from other mines) was used. It was also assumed that the trip from Wiluna to Port Adelaide would take 36 hours and that a single truck driver would make 12 trips per year. This gives an annual dose of between 0.4 and 0.5 mSv/y.

Exposure to other workgroups (admin and construction workers)

See later section.

Doses to key receptors (average annual doses)

The factors used to calculate doses at the key receptor locations were as follows;

- 8670 exposure hours in a year at each receptor location - Dust and radon concentrations based on air quality

modelling results

Reference

Thomson, J.E., & Wilson, O.J., 1980. Calculation of Gamma Ray Exposure Rates from Uranium Ore

Bodies, Australian Radiation Laboratory, Yallambie, Victoria.

UNSCEAR 2006. UNSCEAR 2006 Report, ‘Effects of Ionizing Radiation’.

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ARPANSA 2005. Radiation Protection and Radioactive Waste Management in Mining and Mineral

Processing, Radiation Protection Series, vol.9, Australian Radiation Protection and Nuclear Safety

Agency.

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3. Occupational Dose Estimates

As part of the ERMP consultation process, a number of questions were raised regarding the

estimates of doses to workers in the mine and in the processing plant.

The following information standardizes the assumptions made and clarifies the approach to

occupational dose assessment.

The assessment of potential doses for the Project was undertaken by considering the three main

exposure pathways being;

- Irradiation by gamma radiation;

- Inhalation of radioactive dusts; and

- Inhalation of the decay products of radon.

Irradiation By Gamma Radiation

Gamma doses for miners were based on two main sources of information:

- Gamma doses to miners observed in other similar operations; and

- Theoretical maximum probable dose which was calculated using the method provided by

Thompson and Wilson 1980.

Gamma doses for pit miners at Rossing Uranium Mine and McLean Lake mine gave approximately 1

mSv/y. The uranium grades of these mines are similar to the grades at Lake Way and Centipede.

Gamma doses for Ranger uranium mine were also reviewed and even though the average uranium

grade is higher than that for Lake Way and Centipede, the average doses are similar at 1 mSv/y. It

was also noted that the Ranger miners only spend about half of their time in the open pit.

Using the Thompson and Wilson formula gives a theoretical maximum of 3.9 mSv/y. (Also, see

section 6.2.5.2 of the ERMP).

For plant workers, the estimated annual average gamma doses are between 1 and 2 mSv/y. This

assumption is based on recently published actual dose data workers in similar work environments in

the Olympic Dam processing facility (BHP Billiton 2009).

Inhalation of Radioactive Dusts

Dust doses were determined by considering the type of dust that the miners or processing plant

workers would be exposed to.

Miners and pit workers would be exposed to a mixture of dust containing ore (which, for the

purposes of assessment was considered to contain 600 ppm uranium) and non mineralised waste

material giving an average uranium grade of all material being mined and handled of 300ppm. Based

on the specific activity of uranium 238 (being 12,400 Bq/g), the quantity of uranium 238 in mine dust

was calculated to be 3.7 Bq/g (12,400 x 300 x 10-6).

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If it is conservatively assumed that the ore is in approximate secular equilibrium, then it can be

assumed that the concentration of each of the radionuclides in the uranium 238 decay chain is also

3.7Bq/g.

To assess the potential doses as a result of inhalation of the dust, it was assumed that the dust

concentration in air would be 1 mg/m3. [Note: The assumed ambient dust concentration in the open

pit during mining was estimated to be 1 mg/m3.]

Published data on average dust concentrations in mines is sparse. The Olympic Dam EIS (BHP Billiton

2009) referred to an average dust concentration of 3 mg/m3 which was based on measurements

from open cut coal mines from the UK.

However, 3 mg/m3 could be considered to be relatively dusty and experience suggests that active

controls (such as watering) would be implemented in the event that it became dusty. In addition, the

damp nature of the ore body would act to prevent dust generation.

Since 1 gram of 300 ppm uranium gives a radionuclide concentration of 3.7 Bq/g for each of the

radionuclides in U238 decay chain, a dust cloud of 1 mg/m3 gives a radionuclide concentration of 3.7

mBq/m3 (0.0037 Bq/m3) for each of the radionuclides.

In the ERMP, the radionuclide dose conversion factor for dust from the Code of Practice for

Radiation Protection and Radioactive Waste Management in Mining and Mineral Processing (2005)

(ARPANSA 2005) was used which provides a dose conversion factor for uranium ore of 7.2 μSv/alpha

disintegration per second (∝dps).

Another method of calculation is to use the specific individual radionuclide conversion factors for

inhalation provided in the IAEA Basic Safety Standards (International Atomic Energy Agency (1996a) -

International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of

Radiation Sources - Safety Series No.115).

The IAEA dose conversion factors give an inhalation dose conversion factor for radionuclides in dust

(for an AMAD of 5 um) of 2.92x10-5 Sv/Bq (for head of chain concentration).

To calculate the annual dose also requires an estimate of how much dust is inhaled. This is

determined by assuming that the number of hours worked in a year is 2,000 and that a person

breathes in air at a rate of 1.5 m3/h. Therefore, in one working year, it is assumed that a person

would breathe in 3,000 m3 of air.

The annual dose is therefore;

Annual Dose (uSv) =2.95x10-5 Sv/Bq x 3.7mBq/m3 x 3,000 m3/y

= 330 μSv/y

For processing plant workers, it was assumed that the dust would be all ore, therefore the quantity

of radionuclides in dust in the processing plant would be 7.4 Bq/g for each of the radionuclides in

the U238 decay chain. The ERMP determined the dose using dose conversion factors from the

Mining Code. To enhance accuracy, the dose has been re-assessed using the IAEA BSS figures. If it is

assumed that the ambient dust concentration is 1 mg/m3 for a full year, and using the same method

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as for miners, the estimated annual dose to plant workers (using the BSS conversion factors) from

inhalation of radioactive dusts is 640 μSv/y.

Inhalation of the Decay Products of Radon

The assessment methods provided in the ERMP were based on differing assumptions and methods,

with the result that estimates of doses from this pathway were overestimated in the ERMP. The

dose assessment method has been simplified and re-assessed and is described as follows:

RnDP Dose Estimates for Miners

The method of assessment for estimating doses from the inhalation of Project originated radon

decay product (RnDP) for miners in the open cut mine considers two different potential exposure

conditions, being; exposure under normal ventilation conditions (where there is wind blowing and

naturally ventilating the pit) and exposure under stable atmospheric conditions (where the still

atmospheric conditions prevent natural dispersion causing a build up of radon in the pit).

This latter condition was observed in the background monitoring and is reported in Appendix G of

Appendix D of the ERMP.

Calculating the RnDP dose for miners depends upon the following factors;

- Determining the radon emissions into the pit;

- Calculating radon concentration in the pit under the two exposure conditions;

- Converting the radon concentrations to radon decay product concentrations; and

- Applying standard dose conversion factors.

Radon emissions into the pit were calculated in the early work of the Australian Atomic Energy

Commission (AAEC) (see Appendix C of Appendix D of the ERMP) which reported emission rates of

3.6 Bq/m2.s for the pit. (Note that more recent work by Toro has indicated that radon emissions

rates could be significantly lower than the AAEC reported figures. However the newer emission

rates have not been used in this assessment as Toro is conducting additional work to confirm these

findings).

The quantity of radon entering each pit is calculated from the emission rate and the pit surface areas

and is as follows:

- Centipede 5.7 MBq/s; and

- Lake Way 7.4 MBq/s.

The radon concentrations in the pit depend upon the ventilation rate of the pit and under the

normal ventilation conditions this can be expressed as the number of air changes that occur each

hour. Using the formula of Thompson (1994) this can be calculated as follows;

T = 33.8(V/UrLW) x (0.7 cos(x) + 0.3) where;

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- T is residence time of the air in the pit,

- Ur is the wind velocity in meters per hour,

- L is the length or the pit, and

- W is the width of the pit,

- x is the angle of the wind to the long axis (however, in this calculation, cos(x) is assumed to

be one as the pits are being modelled as circles).

- Ur (the average windspeed) is determined from the air quality study which gives an average

of 2 m/s.

T therefore calculates to 0.07 hr for both pits (which is approximately 14 air changes in the pits per

hour).

The steady state equilibrium radon concentration is calculated using the following equation;

Radon Concentration (Bq/m3) = ER/(PV x VR), where

- ER is the radon generation rate for the pit in Bq/hr,

- PV is the pit volume and

- VR is the ventilation rate or number of air changes per hour.

Under normal ventilation conditions, the calculated long term average radon concentration, as a

result of the mining operation in the pit is 60 Bq/m3.

For the stable atmospheric conditions, a simple method was used to estimate the potential radon

concentration in the pit based on the natural radon concentration ratio between the two

atmospheric conditions observed in the background monitoring (as shown in Appendix G of

Appendix D of the ERMP). The monitoring showed that under stable conditions, on average, the

radon concentrations were four times higher than the concentration that occurred during normal

ventilation conditions.

The next step in estimating RnDP doses depends upon converting the calculated radon

concentrations into RnDP concentrations. The method used in the ERMP involved using an

equilibrium factor which is essentially a measure of the ratio of radon to its decay products in air. Air

that is “new” (which means that radon has only recently entered the air) is characterized by a low

equilibrium factor. Older air (where the decay products have had time to grow in, such as in

enclosed spaces) is characterized by a higher equilibrium factor.

To provide certainty about the estimated doses reported in the ERMP, the following clarification is

provided:

The relationship for determining equilibrium factors (E) is;

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- For an E of 1, 3,700 Bq/m3 of radon is equal to 20.8 uJ/m3 of RnDP,

When E drops, the concentration of RnDP drops proportionally.

UNSCEAR 2006 summarises information on equilibrium factors, and notes that the range is usually

between 0.1 and 10 and is dependent upon the particular situation. For the assessment of potential

doses to workers at Lake Way and Centipede, the equilibrium factors measured during background

side by side radon and RnDP monitoring (giving an average of 0.25) were considered to be the best

indicator of equilibrium factors and it was decided that a conservative factor of 0.5 would be used. F

Therefore, under normal ventilation conditions in the mine pit the radon concentration is 60 Bq/m3

giving an average RnDP concentration of 0.173 uJ/m3.

The calculated doses from these concentrations can be determined using the following formula;

RnDP Dose (mSv) = DCF (mSv/mJ.h.m-3) x RnDP Concentration (mJ/ m3), x number of working hours

in a year (2,000 h/y)

[DCF is the dose conversion factor and is quoted as 1.4 mSv/mJ.h.m-3 (ARPANSA 2005)]

For miners, the dose from RnDP for a full year under normal ventilation conditions is therefore 0.48

mSv/y (say 0.5 mSv/y).

During stable atmospheric conditions, radon concentrations were estimated to increase by a factor

of 4. This increase is the same as the increase observed during the background monitoring of the

natural levels and should be applicable during mining. The equilibrium factor used during the stable

conditions was 0.5.

Therefore the dose from RnDP for a full year under stable ventilation conditions is approximately 2

mSv/y.

Based on an assumed full year, 12 hour roster, in which 50% of the shifts would be at night (under

the potentially stable conditions) and 50% of the shifts during the day (under normal conditions), the

estimated average doses for miners for inhalation of RnDP is calculated as follows;

- 50% of the year at 0.5 mSv/y + 50% of the year at 4 x 0.5 mSv/y = 1.25 mSv/y.

[It is noted that even under the most conservative situation where the equilibrium factor is 1, then

the calculated potential dose to miners from RnDP would be 2.5 mSv/y]

RnDP Dose Estimates for Plant Workers

The method for calculating the plant workers doses is to use the outputs of the quality modelling

which provided an annual average radon concentration plot shown in Figure 71 of ERMP. The air

quality modelling takes into account both the normal and stable ventilation conditions to provide

contours of average radon concentration at various locations, giving a radon concentration of 5

Bq/m3 at the location of the processing plant.

Based on an equilibrium factor of 0.5, the average annual RnDP concentration at the processing

plant location is 0.014 uJ/m3.

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This gives an estimated annual average RnDP dose for processing plant workers of 0.04 mSv/y.

Summary of Occupational Dose Estimates

The estimated total doses are in the following table:

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Dose (mSv/y)

Group Gamma Dust RnDP Total

Miners 1 0.32 1.30 2.6

Plant Workers 1 – 2 0.64 0.04 1.7 – 2.7

References

Thompson, R.S., 1994. Residence Time of Contaminant Release in Surface Coal Mines – a Wind

Tunnel Study. Proceedings of the 8th Air Pollution and Meteorology Conference, American

Meteorological Society.

International Basic Safety Standards for Protection Against Ionizing Radiation and for the Safety of

Radiation Sources, Series No. 115, 1996, IAEA.

ARPANSA 2005. Radiation Protection and Radioactive Waste Management in Mining and Mineral

Processing, Radiation Protection Series, vol.9, Australian Radiation Protection and Nuclear Safety

Agency.

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4. Dose estimates for administration workers.

The following provides information on the calculation of dose estimates for administration workers

in the proposed Wiluna Uranium Project.

The assessment was undertaken by considering three pathways;

- Irradiation by gamma radiation,

- Inhalation of radioactive dusts and

- Inhalation of the decay products of radon.

Irradiation By Gamma Radiation

There is no exposure pathway for administration workers for gamma radiation.

Inhalation of Radioactive Dusts

For administration workers, it was assumed that the dust would be all mineralized and in secular

equilibrium, therefore the concentration of radionuclides in dust in the processing plant was

assumed to be 7.4 Bq/g for each of the radionuclides in the U238 decay chain.

The air quality modeling (see appendix B of the ERMP), shows that annual average dust

concentrations in the location of the administration area is 3 ug/m3 (based on the average of the

year 4 and year 8 TSP dust contours), giving a U238 concentration of 22 uBq/m3.

The dose conversion factor (DCF) for U238 in secular equilibrium can be seen as follows (IAEA 1996);

- 2.92 x 10-5 Sv/Bq (for a worker exposed to a dust cloud of AMAD = 5)

When assessing occupational radiation doses, it is usual to assume that the breathing rate of an

individual is 1.2m3/h and that they have worked for 2,000 hours in a year. For this assessment, “total

dust” has been used, with an assumed AMAD (activity median aerodynamic diameter – a measure of

the particle size and the depth of penetration into the lung) of 5 µm.

Using the conversion factor above, the annual dose from dust from the various materials can be

calculated as follows;

Annual dose (µSv/y) = Exposure hours (h) x breathing rate (m3/h) x Activity concentration (Bq/m3) x

DCF (µSv/y)

The annual dose from inhalation of radionuclides in dust is therefore;

Annual Dose (uSv) =29.2 μSv/Bq x 22μBq/m3 x 2,400 m3/y

= 1.5 μSv/y

Inhalation of the Decay Products of Radon

To calculate the dose to a administration workers, the average radon concentration at the

administration area is determined from the air quality modeling (see appendix B of the ERMP),

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which shows that shows that annual average radon concentration in the location of the

administration area is 3 Bq/m3 (based on the average of the year 4 and year 8 TSP dust contours).

Assuming an equilibrium factor is 0.5, the conversion factor between radon concentration and RnDP

concentration is;

1 Bq/m3 of Radon = 2.28 x 10-6 mJ/m3

This gives an annual average RnDP concentration from the project of approximately 7 x 10-6 mJ/m3.

The calculated doses from these concentrations can be determined using the following formula;

RnDP Dose (mSv) = DCF (mSv/mJ.h.m-3) x RnDP Concentration (mJ/m3), x number of working

hours in a year (2,000 h/y)

DCF is the dose conversion factor and is quoted as 1.4 mSv/mJ.h.m-3 (ARPANSA 2005).

This gives a RnDP dose for administration workers of 20 uSv/y.

References

International Basic Safety Standards for Protection Against Ionizing Radiation and for the Safety of

Radiation Sources, Series No. 115, 1996, IAEA.

ARPANSA 2005. Radiation Protection and Radioactive Waste Management in Mining and Mineral

Processing, Radiation Protection Series, vol. 9, Australian Radiation Protection and Nuclear Safety

Agency.

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5. Radioactivity Analysis Report for soil and vegetation.

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