Calculation of the dose caused by internal radiation, 13.6.2014ST 7.3

This Guide is valid as of 1 September 2014 until further notice.It replaces Guide ST 7.3, Calculation of the dose caused by internal radiation, issued on 23 September 2007.

Helsinki 2014
ISSN 1456-8160
ISBN 978-952-309-187-0 ( pdf)
ISBN 978-952-309-188-7 (html)

Authorization

The Radiation Act stipulates that the party running a radiation practice is responsible for the safety of the operations. The responsible party is obliged to ensure that the level of safety specified in the ST Guides is attained and maintained.

Under section 70, paragraph 2, of the Radiation Act (592/1991), STUK – Radiation and Nuclear Safety Authority (Finland) issues general instructions, known as Radiation Safety Guides ( ST Guides), concerning the use of radiation and operations involving radiation.

Translation. In the event of any differences in interpretation of this guide, the Finnish and Swedish versions shall take precedence over this translation.

This Guide includes the requirements relating to the implementation of Council Directive 96/29/Euratom; OJ No. L 159, 29.6.1996, p. 1.

1 General

For the purposes of monitoring radiation exposure, it is necessary to determine or to estimate the dose caused by both external and internal radiation. When comparing the value of exposure to the dose limits, account must be taken of the total dose incurred from different sources.

This Guide explains how to calculate the committed effective dose caused by internal radiation and gives the conversion factors required for the calculation.

Dose limits (maximum limits of radiation exposure) are laid down in chapter 2 of the Radiation Decree (1512/1991). Radiation and Nuclear Safety Authority (STUK) is authorized to issue guides on the calculation of radiation dose as provided in section 7 of the Radiation Decree. Application of the dose limits is described in the Guide ST 7.2, which also sets out the definitions of the quantities and concepts most commonly used in the monitoring of radiation exposure. The monitoring of exposure and the reporting of the results of dose monitoring to the dose register of STUK is described in Guide ST 7.1 and ST 7.4.

2 How to calculate the committed effective dose

The committed effective dose E(τ) is defined as:

(1)

where

w T is the tissue weighting factor for tissue T

H T(τ) is the committed equivalent dose in tissue T over the time τ.

The time τ, over which the accumulation is determined, is 50 years for adults and (70-n) years for children (where n is the child’s age). The unit of the committed effective dose is the sievert (Sv).

In practice, the committed effective dose is calculated using the activity of the radioactive substances that have entered a person’s body, i.e. intake. Intake means the activity of the ingested or inhaled radioactive substance irrespective of the fraction of the activity that is exhaled. The committed effective dose E(τ) for a person in the age group g can thus be calculated using the following formula:

(2)

where

h(g) j,s and h(g) j,h are the dose conversion factors for the radionuclide j entering the body by ingestion (s) or inhalation (h), respectively, for a person in the age group g

J j,s and J j,h are the activities (intakes) of the radionuclide j entering the body by ingestion or inhalation.

The dose conversion factor *) h(g) means the committed effective dose per unit of activity (intake). The unit of the conversion factor is Sv·Bq -1.

When using a dose conversion factor, the intake of the radioactive substance must be known. In some cases, the intake can be calculated or estimated by measuring directly from the body the activity of a radioactive substance that has entered the body or measuring it indirectly from the excretions or other biological samples obtained from the body.

Intake is discussed in Guide ST 1.9 and the determining of intake based on measurement results in, for example, ICRP Publication 78.

3 Dose conversion factors needed in the calculation of the committed effective dose

The values of dose conversion factors for different nuclides are given in tabulated form in the Appendix to this Guide. However, the values of the conversion factors for the decay products of 222Rn (radon) and 220Rn (thoron) are presented separately in item 4.1, since the principles applied in calculating the dose caused by radon and thoron differ from those used for calculating the dose caused by other nuclides.

The values of the dose conversion factors depend not only on the mode of intake, but also on the age of the person into whose body the radioactive substance has entered. The groups of age g for which the factors are given are: under 1, 1–2, 2–7, 7–12, 12–17 and over 17 years of age.

When calculating the committed effective dose caused by an inhaled radioactive substance, account must also be taken of the particle size of the substance and the rate at which the substance is absorbed from the lungs. The values of the dose conversion factors are given for four lung absorption types. The values indicate the rate at which the compound or substance is absorbed from the lungs into bodily fluids.

  • V = very fast absorption
  • F = fast absorption
  • M = moderate absorption
  • S = slow absorption.

The particle size of the inhaled substance is expressed by the concept AMAD (Activity Median Aerodynamic Diameter). The values of the dose conversion factors are given for two AMAD values: 1 µm and 5 µm. If the particle size is not known when calculating the dose of exposed workers, the default value of 5 µm should be used for the AMAD.

The fraction of the element or compound that passes into the body fluids from the gastrointestinal tract is described by the transfer factor f 1 . The values of the factors are given in the tables of the Appendix **).

Table A of the Appendix gives the values of the dose conversion factors for ingested radionuclides for members of the public according to age group. Table B of the Appendix gives the values of dose conversion factors for inhaled nuclides for members of the public in different age groups and for different lung absorption types. The values in Tables A and B are also applied to students and apprentices who are over 16 but under 18 years of age, and who participate in the use of radiation sources for training purposes. Tables A and B also give the values of the f 1 transfer factors for ingested nuclides and for inhaled nuclides for the fraction of the intake that has passed into the gastrointestinal tract.

Table C1 of the Appendix gives the values of the dose conversion factors for ingested or inhaled radionuclides for exposed workers. The values of the conversion factors for inhaled intake are given for two AMAD values (the symbols of the factors being h(g) 1µm and h(g) 5µm ) and for different lung absorption types. Table C1 also gives the values of the f 1 transfer factors. Table C2 of the Appendix gives the values of the dose conversion factors for inhaled soluble or reactive gases and vapours for exposed workers.

Table D of the Appendix gives, by element and compound, the values of the f 1 transfer factors for ingested substances for exposed workers. The values in Table D may also be applied to members of the public.

Table E of the Appendix gives, also by element and compound, the lung absorption types and the values of the f 1 transfer factors for inhaled substances for exposed workers. References to the literature in which the corresponding data for members of the public can be found are given in Table F.

Table G of the Appendix gives the values of the dose conversion factors for inhaled soluble or reactive gases and vapours for members of the public in different age groups. The values for adults (over 17 years of age) also apply to exposed workers. Table G supplements the information given in Table C2.

Lung absorption classes are presented in publications ICRP Publication 72 and 78.

4 Exposure due to noble gases must be determined separately

4.1 Radon and thoron

The effective dose caused by exposure to radon and thoron stems from the short-lived decay products of these nuclides, which are deposited in the lungs following inhalation. The effective dose E is calculated from the formula:

E = h · W (3)

where

h is the conversion factor for the decay products

W is the potential alpha energy exposure.

The potential alpha energy generated in the decay chains of radon and thoron means the total alpha energy that is ultimately emitted during the decay of the radon and thoron progenies through the decay chain:

  • up to but not including 210Pb for the progeny of radon
  • up to the stable 208Pb for the progeny of thoron.

The unit of the potential alpha energy is the joule (J).

The alpha energy concentration is the potential alpha energy per unit volume. The potential alpha energy exposure W is the product of the alpha energy concentration and the exposure time, and its unit is J·h·m -3.

The unit of the decay product conversion factors h is Sv·J -1·h -1·m 3. The following numeric values are used for the factors ***):

  • for radon in dwellings 1.1
  • for radon in the workplace 1.4
  • for thoron in the workplace 0.5.

4.2 Other noble gases

When a person is surrounded by a noble gas, the effective dose caused by the inhaled gas is generally insignificant compared to the dose caused by external radiation. The effective dose E caused by external radiation can be calculated from the formula:

(4)

where

h j is the conversion factor for the nuclide j

c j is the activity concentration of the nuclide j or the mean of the concentration during the time of exposure

t j is the time of exposure for the nuclide j.

Table H of the Appendix gives the values of the conversion factors h j for isotopes of argon, krypton and xenon. The unit of the conversion factors is Sv·d -1·Bq -1·m 3.


Bibliography

  1. International Commission on Radiological Protection. Limits for intakes of radionuclides by workers. ICRP Publication 30 (Part 1). Annals of the ICRP 1979; 2 (3–4).
  2. International Commission on Radiological Protection. Human respiratory tract model for radiological protection. ICRP Publication 66. Annals of the ICRP 1994; 24 (1–3).
  3. International Commission on Radiological Protection. Dose coefficients for intakes of radionuclides by workers. ICRP Pub­lication 68. Annals of the ICRP 1994; 24 (4).
  4. International Commission on Radiological Protection. Age-dependent doses to members of the public from intake of radionuclides – Part 5 Compilation of ingestion and inhalation dose coefficients. ICRP Publication 72. Annals of the ICRP 1995; 26 (1).
  5. International Commission on Radiological Protection. Individual monitoring for internal exposure of workers, (Replacement of ICRP Publication 54). ICRP Publication 78. Annals of the ICRP 1998; 27 (3–4).
  6. International Atomic Energy Agency. Radiation protection and safety of radiation sources: International Basic Safety Standards. IAEA Safety Standards No. GSR Part 3 (Interim). Vienna; IAEA: 2011.
  7. Delacroix, D. et al, Radionuclide and Radiation Protection Data Handbook. Radiation Protection Dosimetry 2002; 98 (1): 9–18 .

* ) Subscripts have been omitted if the particular conversion factor is obvious from the context or if the conversion factor is referred to in a generic sense.

** ) The f 1 factors are not needed in the dose calculations described in this Guide. They are given, however, in order to denote the fraction of the substances that passes into the bodily fluids from the gastrointestinal tract. The f 1 factors are needed to calculate the h(g) factors.

*** ) Source: EU directive 96/29/Euratom, Appendix III, item C.

Appendix

Conversion factors and parameters used in calculation of the committed effective dose

TABLE A Values of dose conversion factors h(g) for ingested radionuclides for members of the public.

TABLE B Values of dose conversion factors h(g) for inhaled radionuclides for members of the public.

TABLE C1 Values of dose conversion factors h(g) for ingested or inhaled radionuclides for exposed workers.

TABLE C2 Values of dose conversion factors h(g) for inhaled soluble or reactive gases and vapours for exposed workers.

TABLE D Values of f 1 transfer factors by element and compound, for ingested substances for exposed workers. The factors may also be applied to members of the public.

TABLE E The lung absorption types and the values of f 1 transfer factors by element and compound, for inhaled substances for exposed workers.

TABLE F Literature references to lung absorption types for inhaled elements and compounds for members of the public.

TABLE G Values of dose conversion factors h(g) for inhaled soluble or reactive gases and vapours for members of the public.

TABLE H Values of h j conversion factors for the effective dose caused by the noble gases argon, krypton and xenon for adults.