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Appendix 2: Dose Calculation

  • Issued: July 2013
  • Content last reviewed: April 2013

The incremental doses received by individuals and populations can be determined by calculations resulting from the measurements.

The calculation used here considers the dose received by a Hypothetically Exposed Adult (HEA) positioned at specific sampling sites. It should be noted that this calculation is a first order approximation only and is used to make simple comparisons with existing dose limits and natural background radiation.

The HEA is defined as an adult member of the public who receives the greatest induced dose from all sources. This person is assumed to remain at this location 24 hours per day, 365 days of the year. They would be continually breathing air, drinking water and consuming a specified fraction food that is affected by releases of radioactivity measured at the site. The HEA dose is not received by an actual individual and is used as a health-conservative estimate (i.e. an overestimate) of the highest possible dose to any member of the public.

Based on the results of active sampling, tritium (3H) is found in larger concentrations than the other radionuclides associated with emissions from the nuclear stations in Ontario. Subsequently, tritium contributes to the largest portion of the dose received. Primarily, these emissions come in the form of tritium oxide or tritiated water vapour (HTO). HTO enters the body by ingestion, inhalation, and dermal absorption. Ingested HTO is distributed throughout the entire body and eliminated at the same rate as body water (apart from the small fraction metabolized). Inhaled HTO dissolves in the fluids of the lung and is absorbed.

Two principle pathways will be considered in determining the dose to the HEA. These are listed in Table A-1, together with the associated sampling programs and annual consumption rates.

Table A-1: Ingestion Pathways and Consumption Rates[ 1 ]
Pathway Sampling Program Annual Consumption
Ingestion of drinking water Drinking Water Tritium 700 L
Inhalation of air Tritium-in-Air 8000 m3

The annual consumption rates vary depending on which model is used for the intake of food and drink. For the calculation presented here the values that have been used are based on the ICRP reference man and are also recommended for use by the Canadian Standards Association (CSA) for dose pathway calculations.[R-6]

For the purpose of estimating the HEA dose, the tritium concentrations are taken from sampling data collected in the Pickering, Darlington and Bruce areas. The values used for each pathway are as follows:

  • Ingestion of drinking water – the median annual concentration at water supply plants sampled in the region minus the background concentration.
  • Inhalation of air – the median annual concentration based on active sampling in the community.

Calculating the annual whole-body dose rate from ingestion of a particular food or drink is expressible as a product of three factors:

  • the radionuclide concentration in the food or drink (e.g. Bq/kg or Bq/L)
  • the dose coefficient for the radionuclide (e.g. μSv/Bq)
  • the rate at which the food or drink is consumed (e.g. kg/y)

The dose contribution from inhalation is calculated in a similar way as the product of the radionuclide concentration in the air, dose coefficient, and the volume of air that is inhaled (i.e. m3/y × Bq/m3 × μSv/Bq).

In their Annual Environmental Report, Lawrence Livermore Nuclear Laboratories (LLNL) recommends using a factor of 1.5 for inhalation-plus-dermal-absorption. This is because the skin only absorbs 50% of the tritium the lung absorbs.[R-7]

The dose coefficient for tritium oxide for ingestion, inhalation and skin absorption has a value of 1.8 × 10-5 μSv/Bq. This value is from ICRP 96.[R-8]

Using these values, the tritium dose from the ingestion of water for HEA in the Bruce Region is then-

Dwater = 700 (L/y) × 1.8×10-5 (μSv/Bq) × (6.21 – 0.12) (Bq/L)

= 7.67 ×10-2 μSv/y

The inhalation/skin absorption dose (Dinh/sa ) is determined from-

Dinh/sa = 1.5 × 8000 (m3/y) × 1.8×10-5 (μSv/Bq) × 0.85 (Bq/m3)

= 1.84 ×10-1 μSv/y

The total incremental dose from tritium (Dtotal ) is the sum of the contribution via each pathway-

Dtotal = Dwater + Dinh/sa

= 7.67 ×10-2 + 1.84 ×10-1

= 2.60 ×10-1 μSv/y

Following the same procedure, the total incremental dose for a HEA has been calculated for the Pickering and Darlington Surveillance areas. These values are compared with Health Canada's guideline for the radiological characteristics of drinking water of 0.1 mSv and the typical level of background radiation for southern Ontario and are summarized in Table A-2.

Table A-2: Sampling Area & Total Incremental Dose Received

Table A-2: Sampling Area & Total Incremental Dose Received
Pathway Bruce Surveillance Area Pickering Surveillance Area Darlington Surveillance Area
Ingestion of drinking water 0.077 µSv/y 0.056 µSv/y 0.058 µSv/y
Inhalation of air 0.18 µSv/y 0.27 µSv/y 0.054 µSv/y
Total-Dose 0.26 µSv/y 0.33 µSv/y 0.11 µSv/y
% of Drinking Water Guideline[ 2 ] 0.26% 0.33% 0.11%
% of Background[ 3 ] 0.016% 0.020% 0.007%

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[ 1 ] [R-5]

[ 2 ] Drinking Water Guideline – 0.1 mSv = 100 µSv

[ 3 ] Southern Ontario Background Level – 1600 µSv [R-9]