Appendix 4

INFORMATION ON DECAY CHAINS

NATURALLY OCCURRING RADIONUCLIDES

Tables A4.1, A4.2 and A4.3 list the radionuclides in each of the three radioactive decay chains of relevance to this study. Every radionuclide, when ingested, gives rise to a radiation dose to the individual. The amount of dose varies by orders of magnitude from one radionuclide to another, as illustrated by the tabulated values of annual dose per unit activity concentration in water. Details of the calculations involved are given in Appendix 8.

Notes on Tables A4.1, A4.2 and A4.3

The annual dose per unit activity in water is defined for the purposes of this study as the radiation dose, in millisieverts, received annually by an individual from the sole continuous use of drinking water at two litres per person per day containing 1 becquerel of activity of the radionuclide concerned per litre of water, averaged over a lifetime of 70 years.

(1 becquerel = 1 nuclear disintegration per second).

Radionuclides in bold italics are those measured in all or part of this study.

Radionuclides marked with an asterisk* are the radon isotopes and their short half-life daughters.

Dose conversion factors not given by the IAEA are left blank and are taken as negligible.

Uranium-238 and uranium-235 occur naturally in the approximate activity ratio 21 : 1.

Gross alpha activity, as measured in this study, is taken to be the sum of the activities (in becquerel) of all the alpha emitters (excluding radon and radon daughters), per litre of water.

Gross beta activity, as measured in this study, is taken to be the sum of the activities (in becquerel) of all the beta emitters (excluding radon daughters), per litre of water. The list of beta emitters includes potassium-40, a naturally occurring radionuclide which is found in water, but which does not form part of the uranium-238, uranium-235 or thorium-232 decay series. It is not of interest in dose calculations because its concentration in the body is essentially independent of intake.

Table A4.1: Uranium-238 series radionuclides

Table A4.2: Uranium-235 series radionuclides

Table A4.3: Thorium-232 series radionuclides