Environmental dosimetry

Transcription

1 radionuclide fact sheet Environmental dosimetry Laurent Zylberman/Graphix-Images/IRSN Contents Background 3 EDEN: IRSN's tool for calculating DCCs 5 Relative Biological Efficiency 10 Natural background 10 References 13

2 Written by: K. Beaugelin-Seiller Checked by: J. Garnier-Laplace Date created: September Illustrations: Laurent Stefano This document is the intellectual property of IRSN and may not be reproduced without its consent.

3 Background The effects of ionizing radiation are expressed not with respect to the concentration of radionuclides in environments (Bq.unit -1 of mass or volume), as is the case for conventional chemical substances (for which the molar or weight concentration of the substance in environments is referenced), but rather with respect to the quantity of energy received by the exposed organisms. The question is thus to determine, for a given organism and defined exposure conditions, the quantity of energy deposited into that organism due to its exposure to the radiation emitted by the radionuclide(s) in question. Ionizing radiation is also different in that it exposes the organisms not only upon contact or after internalization, but also at a distance. It is therefore necessary to take into account all of an organism s possible exposure pathways in order to determine the quantity of energy that it receives as a result. With regard to external exposure, it is possible to evaluate this quantity through measurement, which becomes practically unfeasible when the radionuclides have been incorporated. Additionally, the possible external exposure measurements do not completely integrate the geometric aspects of the exposed organisms. Determining the quantity of energy received by an organism exposed to ionizing radiation therefore requires models that can represent the situations to be studied and the corresponding hypotheses, which must be set forth clearly. As a result, both the human and environmental dosimetry calculations make use of common concepts, from dose coefficients to reference organisms. Concept of a dose coefficient Principles Developed for the human radiation protection system, the dose coefficients approach has been simplified for the environment. It consists of calculating a numerical multiplier coefficient which, applied to the radionuclide concentration of the source(s) of radiation, makes it possible to evaluate the corresponding energy received by the exposed organism. This energy constitutes a dose in the radiological sense, expressed in Grays (Gy). This notion is often used to define acute exposure situations, i.e., "flash" situations. Chronic exposure makes it necessary to additionally take into consideration the exposure time over which the dose is delivered, which is measured by the dose rate, expressed in Gy.unit -1 of time. Various notions of dose have been developed for human radiation protection, but at this point only the absorbed dose concept has achieved a consensus for environmental radiation protection. For this reason, the differences in the radiation sensitivity of tissues have not been integrated, and the weighting that relates to the relative biological efficiency of the various types of radiation is most commonly taken into account only for the purposes of studying sensitivity, given the lack of values that are accepted by the entire scientific community. The internal and external exposure pathways are distinguished by the determination of dose coefficients that are specific to each one. These coefficients are also specific to one radionuclide, generally taking into account its various types of radiation, or even sometimes its daughters in secular equilibrium. For environmental dosimetry, a choice has been made on the international level to term these coefficients DCCs (Dose Conversion Coefficients). Their unit is Gy.day -1 per Bq.unit -1 of mass or volume. Usually, it is a sub-multiple of this unit that is used, µgy.h -1 per Bq.unit -1 of mass or volume. It is assumed that the radionuclides all act in the same way, and that consequently the additivity hypothesis of risks characterized for each radionuclide (and each type of radiation) is valid. The total dose is then obtained by adding the received doses of all pathways and sources of irradiation. General assumptions The current approach to dose conversion coefficients rests on two major assumptions: any organism may be represented by a simple volume whose composition and density are each homogeneous; the radionuclide is uniformly distributed within the entire volume of the source, whether it is external or internal. The DCC approach is currently the most widespread worldwide. It particularly forms the basis for the most recent developments in assessing radiological risk to the environment that have been achieved in Europe 1, which have led to the production of a shared tool for that purpose (ERICA-tool, com/)

4 Concept of reference organism The diversity of the lifeforms observed in ecosystems makes it unfeasible to treat each one of them individually where dosimetry is concerned. The necessary simplification relies on the notion of reference organism, developed by analogy with that of the reference man, which was adopted for human radiation protection (CIPR, 1975). It leads to the definition of a standard dataset made up of a limited number of model organisms described in a simplified way and the corresponding information needed for a dosimetric calculation. This dataset then serves as a guide for any other organism. A reference organism is a virtual entity with fixed physiological and anatomical characteristics, whose lifecycle is known, intended to provide basic evidence for evaluating a dose for a set of typical or representative organisms from a contaminated environment. A reference organism is generally associated with a so-called model species, which is chosen based on various criteria, which include the species radioecological and ecological sensitivity and its contribution to the representativeness of the structure and workings of the ecosystem that hosts it. 4

5 EDEN: IRSN's tool for calculating DCCs Environmental dosimetry gained real traction in the early 2000s. At the time, available methods include pure Monte Carlo simulations on one hand, and empirical analytical equations on the other. The first technique uses physical calculation codes that simulate particles transport and energy deposition, which both take a great deal of time and skill to implement. The second approach is limited to a very restricted range of uses, as long as the user wants to guarantee a significant degree of accuracy with respect to Monte Carlo calculations. IRSN has chosen to explore a middle path by developing a new approach, intended to evaluate the DCCs for any user-defined organism irradiated by internal contamination and/or by external exposure, by a given radionuclide, regardless of the type of radiation, alpha, beta, or gamma. Formalized by the computer application EDEN (Elementary Dose Evaluation for Natural environments; Beaugelin-Seiller et al., 2006), this approach makes it possible to determine the dose (or dose rate) that an organism would receive based on scenarios with defined characteristics such as the living environment, the elements that compose the source and target, their dimensions, etc. (Figure 1). System definition Elementary parameters SHAPE COMPOSITION SOURCE OPTIONS Organism Medium 6 C N O H Co 238 U 54 Mn 110m Ag α β γ Monte Carlo Ponctual deposit Weighting Decay SCENARIO organism Medium + medium medium 1 medium 3 organism medium 2 organism medium 1 medium 2 organism CALCULATION RESULTS Figure 1: principles of performing a DCC calculation with EDEN. 5

6 Radionuclide fact sheet ENVIRONMENTAL DOSIMETRY Implementation Determining DCCs with a tool like EDEN requires knowing the application s input data. By default, a set of reference organisms has been identified for each type of ecosystem (freshwater, seawater, terrestrial). They are associated in a conceptual ecosystem model, illustrated by Figure 2 for the terrestrial ecosystem, Figure 3 for the continental aquatic ecosystem, and Figure 4 for the marine ecosystem. Chickadee Bee AIR 2.5 m Fescue 0.6 m Rabbit Snail 0.2 m Earthworm 1m SOIL Figure 2: conceptual model of the terrestrial ecosystem. The corresponding information that is needed is listed in the tables given below: Tables 1 to 3 give composition and density of organisms and their life environments; Tables 4 to 6 give the dimensions of the reference organisms and their position in their environment. 6

7 Radionuclide fact sheet ENVIRONMENTAL DOSIMETRY AIR WATER Roach Algae Daphnia 1m Water milfoil Carp Chironomid 1m SEDIMENT Figure 3: conceptual model of the continental aquatic ecosystem. AIR WATER Mackerel Phytoplankton Zooplankton Rockweed Plaice 1m Lobster 1m SEDIMENT Figure 4: conceptual model of the marine ecosystem. 7

8 Table 1: elementary chemical composition (weight) of the reference organisms Simplified elementary chemical composition (% in weight) Organism C H O N P S Ca Na K Mg Al Si Fe Plant Animal Table 2: density of reference organisms Organism Density (kg.m -3 ) Terrestrial plant (grass) 8* Terrestrial plant (woody) 900** Aquatic plant 1,000 Animal 1,070 * yield of perpetually grass-covered surfaces on the order of 5,000 kg.ha -1 by dry weight, i.e. about 2.5 kg.m -2 by fresh weight (water content 80%). Based on the dimensions recorded for the fescue plant, the equivalent ellipsoid is combined with a m 2 horizontal projection, for a volume of m 3 (as about 6/8 of the total volume is above ground), i.e. a volume of m 3 for 1 m 2. The estimated density of the plant life as a whole is therefore approximately 8 kg.m -3. ** the density of softwood (resinous) at 50% humidity is on the order of 900 kg.m -3. Table 3: elementary chemical composition in weight of the exposure environment Density (kg.m -3 ) Simplified elementary chemical composition (% in weight) Environment C H O N P S Ca Na K Mg Al Si Fe Ar Cl Air Freshwater 1, Seawater 1, Ground 1,700* Sediment 1,300* Sediment from seas 1, * TGD EC (2003) Technical guidance document in support of Commission Directive 93/67/EEC on risk assessment for new notified substances and Commission Regulation (EC) n 1488/94 on risk assessment for existing substances, Directive 98/8/EC of the European Parliament and of the Council concerning the placing of biocidal products on the market. Part II., Office for Official Publication of the European Communities, Luxembourg. 8

9 Table 4: dimensions and position of the reference organisms for the terrestrial ecosystem Organism a (horizontal) Axes of the ellipsoid (m) b (horizontal) c (vertical) Volume (m 3 ) V Mass (kg) m Burrowing depth (m) Distance to ground (m) Earthworm Not Snail Not Bee Not Chickadee Not Rabbit Not Fescue plant Not Table 5: dimensions and position of the reference organisms for the freshwater ecosystem Organism Algae (Chl. reinhardtii) a (horizontal) Axes of the ellipsoid (m) b (horizontal) c (vertical) Volume (m 3 ) V Mass (kg) m Burrowing depth (m) Not Daphnia Not Distance to sediment (m) Chironomid Not Roach fish Not Common carp Not Watermilfoil Not Table 6: dimensions and position of the reference organisms for the marine ecosystem Organism a (horizontal) Axes of the ellipsoid (m) b (horizontal) c (vertical) Volume (m 3 ) V Mass (kg) m Distance to sediment (m) Phytoplankton* Rockweed (tussock) Zooplankton* Lobster* Plaice* Mackerel* * Reference organisms derived from FASSET (Pröhl et al., 2003) 9

10 Relative Biological Efficiency The different types of radiation (a, b, g, neutrons), given the same deposited energy (or absorbed dose), have different effects on the exposed organisms. Although the effect of radiation is measured basically in physical terms by the quantity of energy deposited in a given mass of material (absorbed dose), this is only one of the aspects that shows the corresponding potential biological damage. As a result, the damage will be greater per unit of energy when that energy is deposited across a shorter distance. This way, an alpha particle that transfers all of its energy over a very short distance will cause biological damage much greater than farther-travelling gamma radiation. The intensity of the effects is also influenced by other parameters, such as the mass of biological matter that receives the energy, or the sensitivity of the exposed tissue. The concept of relative biological efficiency (RBE) has been created to express these differences. The RBE is defined as the ratio between the dose resulting from exposure to a baseline radiation and the dose resulting from exposure to the tested radiation needed to achieve the same level of effect. Normally, the reference radiation used is X-ray radiation with low linear energy transfer (e.g., X-rays, g radiation of 60 Co or 137 Cs, etc.). The RBE varies based on the sensitivity of the exposed tissue, the duration of exposure, etc. It involves using a radiological weighting factor denoted W R, which increases with the radiation's efficiency and is representative of the RBE for the low-dose stochastic effects. The product of the absorbed dose by this factor, called the equivalent dose, incorporates the radiation's dangerousness. From a regulatory standpoint, the adopted simplification consists of using a single factor (a quality factor) for each type of radiation, a derivative of the radiological weighting factor W R. The concept of the equivalent dose has been defined for human radiation protection. The values of the radiological weighting factor W R are thereby set for the human species; they vary between 1 (for g radiation) and 20 (for a radiation). With respect to environmental radiation protection, there is not currently any consensus with respect to the values of the radiological weighting factor to be used. Whenever a value is applied, it is generally for the purposes of a sensitivity analysis. The ERICA tool (Beresford et al., 2007) thereby proposes using 1 by default for b and g radiation, 3 for low-energy b radiation and 10 for a radiation. These values apply to internal exposure alone. When weighting by the radiological weighting factor W R is used, the term used for environmental radiation protection is weighted dose (rate). Natural background The natural environments and living organisms are naturally radioactive, owing to the presence of radioisotopes like potassium 40 among the elements that make them up. The flora and fauna are therefore naturally exposed to ionizing radiation, and characterizing this exposure is necessary to evaluating the radioecological risk. The protection criteria currently proposed with respect to effects of exposure to ionizing radiation apply to exposure from human activities added to natural exposure; the evaluated dose rates come in increments of dose rates or doses associated with the natural background. Various studies show background values for natural exposure. For terrestrial organisms, Gomez-Ros et al. (2004) estimated an external natural exposure that varies from 0.01 to 0.09 µgy.h -1. The main contributor to these dose rates is potassium 40, the same as for internal exposure. The dose rates associated with internal exposure are, according to those authors, more variable. The ERICA tool (Beresford et al., 2007) also proposes ranges of variation for total natural dose rates for terrestrial organisms, which encompasses the previous one, because it ranges from 0.01 to 1.1 µgy.h -1 (Figure 5). The data used to fill out the ERICA tool were re-assessed by Beresford et al. (2008) for wildlife in England and Wales. The authors thereby determined the absorbed dose rates for eight reference organisms (grass, pine, earthworm, bee, frog, duck, deer, and rat). By applying the weighting with respect to RBE used in the ERICA tool (see previous paragraph), they obtain a range of variation from 0.07 to 0.61 µgy.h -1, which is logically consistent with the previously obtained values. Brown et al. (2004) assessed the doses received by the aquatic organisms exposed to natural radionuclides and present their results as a qualitative approach based on the available information, completed by various assumptions. According to these authors, the marine organisms are thereby exposed to weighted dose rates (incorporating RBE) varying from 0.08 (fish) to 27 µgy.h -1 (crustacean) when the dose rates received by the freshwater organisms range from 0.12 (pelagic fish) to 57 µgy.h -1 (macrophyte). The major contributions come from polonium 210 and radium 226. These values are the ones reported in the ERICA tool for marine (Figure 6) and continental aquatic (Figure 7) environments. The most recent compilation (Hosseini et al., 2010) indicates average dose rates that vary from 0.37 to 1.9 µgy.h -1, the highest values that relate to marine organisms. One noteworthy exception to this observation relates 10

11 to freshwater plants, whose dose rates naturally vary between 0.7 and 4.2 µgy.h -1, which is likely to be connected with a greater uptake of the radionuclides 226 Ra. 210 Po and 232 Th. In addition, there is the effect of weighting the contribution of a emitters. 1,2 1 0,8 Dose rate (µgy.h -1 ) 0,6 0,4 0,2 0 Reptile Amphibian Bird Flying insect Herb & grass Mammal (deer) Mammal (rat) Soil invertebrate Tree Figure 5: total natural dose rates for terrestrial organisms according to ERICA ( : range of variation; p: typical value associated with average-exposure concentrations) Dose rate (µgy.h -1 ) Benthic fish Benthic mollusc Crustacean Macroalgae Mammal Pelagic fish Phytoplankton Zooplankton Figure 6: total natural dose rates for marine organisms according to ERICA ( : range of variation; : typical value derived from average-exposure concentrations). 11

12 Dose rate (µgy.h -1 ) Benthic fish Bivalve mollusc Crustacean Pelagic fish Phytoplankton Figure 7: total natural dose rates for freshwater organisms according to ERICA ( : range of variation; : typical value) 12

13 References Beaugelin-Seiller K, Jasserand F, Garnier-Laplace J, Gariel JC (2006). Modelling the radiological dose in nonhuman species : principles, computerization and application. Health Phys, 90 : Beresford N, Brown J, Copplestone D, Garnier-Laplace J, Howard B, Larsson CM, Oughton D, Pröhl G, Zinger I (2007). D-ERICA: An Integrated Approach to the Assessment and Management of Environmental Risks from Ionising Radiation. Final Deliverable to the project ERICA, Environmental Risks from Ionising Contaminants: Assessment and Management. Contract N FI6R-CT , Swedish Radiation Protection Authority. Beresford NA, Barnett CL, Jones DG, Wood Md, Appleton JD, Breward N, Copplestone D (2008). Background exposure rates of terrestrial wildlife in England and Wales. J. Environ. Radioactiv, 99(9) : Brown JE, Jones SR, Saxen R, Thorring H, Vives I Battle J (2004). Radiation doses to aquatic organisms from natural radionuclides. J. Radiol. Prot. 24 : A63-A77. CIPR (1975). Report of the Task Group on Reference Man. Annals of the ICRP, 23 : Gomez-Ros JM, Pröhl G, Taranenko V (2004). Estimation of internal and external exposures of terrestrial reference organisms to natural radionuclides in the environment. J. Radiol.Prot. 24 : A79-A88. Hosseini A, Beresford NA, Brown JE, Jones DG, Phaneuf M, Thørring H and Yankovich T (2010). Background dose-rates to reference animals and plants arising from exposure to naturally occurring radionuclides in aquatic environments. J. Radiol. Prot. 30 : Pröhl G, Brown J, Gomez-Ros J-M, Jones S, Woodhead D, Vives J, Taranenko V and Thorring H (2003). Dosimetric models and data for assessing radiation exposure to biota. Deliverable 3 to the project FASSET, Framework for the Assessment of Environmental Impact, Contract N FIGE-CT Swedish Radiation Protection Authority. 13