Appendix 1

Dose Calculation Methods

CALCULATION OF THE ANNUAL RADIATION DOSE 

Introduction

Water resources that are either category A or C are usually evaluated by calculating the lifetime average annual dose.  This calculation method is explained in the following section.

In some cases it may be necessary to evaluate a water resource by considering the most sensitive age group.  This is the age group for which the lifetime dose committed during a specific year is the highest.  The following approach is followed for determining water quality:

·        Calculate dose to individual age groups.

·        If the difference between the minimum and maximum of these doses is less than a factor of five, calculate the weighted life-time average dose and use it for classification purposes.

·        If the difference between the minimum and maximum of these doses is more than a factor of five, use the maximum dose age specific dose for initial classification purposes.

The dose calculation methods are illustrated in the tables that follow.

Lifetime average annual dose [1]

The lifetime average annual dose associated with a water resource is calculated from the expression:

where:

D is the lifetime average annual dose (mSv/a)

A_i is the activity concentration of nuclide i (Bq/L)

F_i is a proportionality constant for nuclide i with units of (mSv/a) per (Bq/L).

The determination of the parameters A_i and F_i is described in Sections A8.1 and A8.2 below.

Determination of Proportionality Constant F_i

The proportionality constant F_i for nuclide i was determined from the following relationship:

where:

C_x is the annual water consumption for an age group x (L/a)

(DCF)_ix is the dose conversion factor (dose coefficient) for nuclide i and age group x (mSv/Bq)

W_x is a weighting factor for age group x

Table A1        Annual Water Consumption Values

The dose conversion factors for the various nuclides and age groups were taken from the IAEA Basic Safety Standards [18].

The weighting factor for each age group was determined by dividing the number of years in the age group by the average life expectancy, taken to be 70 years.  For example, the weighting factor for the 7 - 12 years age group was:

and for the > 17 years age group:

The lifetime average annual dose using Method 1 – a screening method

Table A2 illustrates the calculation of dose. 

A default activity concentration equal to 1 Bq/L has been inserted for the analysed nuclides in the tables with the exception U-235 that is equal to U-238 divided by 21.7.  The actual activity values for a water resource have to be determined through radioanalyses and equilibrium assumptions as described for Method 1 in the main text.  The dose elements reported in the last column are therefore meaningless until real values are inserted.

Table A2: Lifetime Average Dose Calculation using Method 1 Analysis