Radiation monitoring and ALARA
Ecosystem awareness13 February 2008
A draft report on using reference animals and plants for assessing the impact of radiation on the environment is currently out for consultation. By Corrina Thomson
An International Commission on Radiological Protection (ICRP) task group has released a draft report entitled Environmental Protection: The Concept and Use of Reference Animals and Plants (ICRP 52/401/07), which describes the basis for the use of non-human species in radiological protection.
The commission thinks its framework on this subject should serve as a foundation on which national and other bodies could develop more applied and specific numerical approaches to assessment and management of risks to non-human species under different circumstances and different exposure situations.
Due to the vast complexity of the living environment, and the limited radiobiological and radioecological databases relating to it, the commission thought that setting out data for a limited number of reference animals and plants would provide a vital component of the framework to gather and interpret data.
The commission is seeking comments and suggestions about the draft report by Friday 28 March.
Reference plants and animals are used in radiological protection to evaluate the impact of radiation on the environment, in environmental management and, particularly, pollution control and nature conservation, in the form of an environmental impact assessment. The purposes of these assessments can vary; some are to reassure the public or politicians, to demonstrate the ability to deal with the consequences of accidents, whereas others show compliance with pollution or habitat protection laws.
A requirement of the assessment process is a sound scientific base with the means of expressing and using relevant scientific information. In its draft report, ICRP says this has been the basis of success for radiological protection of humans, and therefore needs to be carefully considered for protection of the environment generally.
To evaluate the effect of radiation on the environment, several factors should be considered, including radionuclides of interest, sources, rates of introduction, and distribution. This information is also required in order to protect the general public.
The report states that for environmental protection, information is required about potential exposure of fauna and flora within the area of radionuclide distribution; plus the likely consequences for them in terms of radiation effects. Of these, addressing the former should not be too difficult, the nature of the problem having much in common with the environmental information needed for human radiation protection. The latter is more difficult, and the term ‘consequences’ is far more open-ended than it is for human protection – many other factors therefore need to be considered, not least the original objectives of the assessment.
The ICRP believes that its approach to environmental protection should be commensurate with the overall level of risk and compatible with other approaches being made to protect the environment from all other human impacts.
There are three aspects of extrapolation and interpolation to other animal and plant types that need to be considered: one is differences in biology – if the animals or plants are considerably different from those represented by the reference animals and plants (by definition generalised to the taxonomic level of family); the second is differences in dosimetry; and the third relates to differences in radiation effects.
In terms of biology, the report notes that it is not possible to cater for all types of flora and fauna, and there will be situations where the biotic objects of interest will be different from the reference animals and plants. “Such difference could be relatively small, such as differences in the time span of a particular stage in the life cycle, or in overall life span. In other cases, differences in biology could make large differences to estimates of exposure to certain radionuclides via different pathways,” it states.
The report contains an appendix on background information to the reference animals and plants to point to significant differences stemming from basic biology. One way in which differences from the set of twelve reference animals and plants would obviously make a difference is that of shape and size, and thus to estimates of received dose.
In terms of radiation dosimetry, the ICRP sees this as more easily addressed. There are several aspects to the extrapolation and interpolation of the basic dosimetry models used for the reference animals and plants to other biota, including shape, size, and location. Shape has been simplified by the use of solid spheres and ellipsoids, although it is recognised that such shapes may not readily extrapolate to some forms of organism. Nevertheless, some flexibility is possible and several variations on given shapes can be envisaged. In view of the uncertainties in estimating exposure to radionuclides, both internally and externally, it is first of all useful to have some form of approximate indication of the effects of size and shape on the absorption of alpha, beta, and gamma radiation.
For alpha particles, the range in tissue is 16–130µm for energies in the range of 3–10MeV. The dependence of the absorbed fraction of alpha particles on the radius of spherical organisms and the alpha energy is detailed in the report. The calculations are made with the EDEN model. For a radius of 1mm, which is the size of the flatfish fish egg (the smallest reference animal or plant), the absorbed fraction is about one for alpha energies around 5MeV. This energy is typical for many important alpha emitters as, for example, Pu-239, U-234 and U-238, and Ra-226. For all other reference animals and plants, the absorbed fraction for alpha emitters is one. Thus doses from external exposure to alpha particles are essentially negligible.
With regard to photons and electrons incorporated into the body, there are slight differences between spheres and other shapes. Absorbed fractions of electrons and photons in relation to mass and energy in spheres are also detailed in the report.
Absorbed fractions for non-spherical shapes are summarised as a function of the initial electron or photon energy. For electron energies below 100keV, the absorbed fraction is nearly one, even for very small organisms. The range of electrons in living tissue increases from 160µm for 100keV electrons to 5mm for 1MeV electrons. The absorbed fraction is thus close to unity if the diameter of the target is well above the range of the electron. The absorbed fraction of electrons is considerably smaller than 0.5 only in cases where the targets are very small and the energies are high.
The relative position of the source and the organism also play an important role. This is particularly significant for organisms within or on soil or sediment contaminated to different depths. With regard to the location of the radiation source, dose conversion coefficients for low-energy photons to animals living on the soil are low, because small soil layers are sufficient to attenuate the photons completely. Relationships with the thickness of the contaminated soil are more complicated.
According to the report, for photons, the size of the organism itself is not of great significance within contaminated soil, but the relative location of the source is.
An earthworm-shaped example is given, showing the exposure rate as a function of the photon energy and soil depth. The upper 50cm of the soil is assumed to be homogeneously contaminated. The maximum exposure rate is for an organism at a depth of 25cm and the lowest is derived for organisms on the interface separating contaminated and uncontaminated layers (depths of 0 and 50cm). At these locations, the dose conversion coefficient is a factor of two lower compared to the centre of the contaminated layer because the organisms are exposed to a two-pi geometry compared with a four-pi geometry within the contaminated layer.
The difference in the exposure rate at a depth of 25cm compared with a depth of 5cm is only about 20%. This small difference is due to the relatively short mean free path of photons in soil, which is about 0.2, 2 and 10cm for 20keV, 100keV and 3MeV photons, respectively.
The report states that the relationship between energy and external exposures from photons to a planar source on the top of soil for on-soil organisms is more significant. The dose conversion factor decreases from 10–100keV by a factor of about five for small animals and by a factor of two for big animals. In this energy range, the mean free path of photons is much shorter, and once an interaction with matter occurs, a significant fraction of the energy is transferred. From 0.1–3MeV, the absorbed dose per photon increases by approximately two orders of magnitude. The exposure decreases with increasing mass due to self-shielding, which is more pronounced for low energies.
The relationship between kerma in air at a height of 1m and the thickness of a contaminated layer with a constant activity concentration is given in the report. For low-energy photons, radiation sources below a depth of 5cm do not contribute to total kerma due to the self-shielding, whereas for 1MeV photons about 50% of kerma is due to activities below a depth of 5cm.
A note of caution is sounded by the ICRP when it comes to extrapolating radiation effects. It states: “In contrast to dosimetry, it is not currently possible to provide recommendations as to how to perform extrapolations that have general applicability in relation to radiation effects, and thus each case has to be carefully considered on its own merits.”
Due to the “relative paucity” of information, the main cases for extrapolations, and challenges for methodological development include the following factors.
It says there are clearly issues with regard to extrapolating from high acute doses and dose rates of low linear energy transfer gamma- and X-rays to lower doses accumulated at lower dose rates.
In the radiobiological and radioecological literature, the qualifiers “low-level”, “chronic”, “higher”, “acute” and so on are often used without any definition. But a radiation exposure lasting several days may be effectively “chronic” for a short-lived organism, and yet effectively “acute” for a long-lived organism.
“Unfortunately, there are very few data that relate directly to the chronic, low-level irradiation conditions of relevance for animals and plants in the wild, that is, exposures at dose rates of 100–1000µGy/day over the life span of the organisms, and the response end points most commonly assessed after acute, high-dose, irradiation are not those that are relevant in such situations.”
A further issue exists in the extrapolation from one organism to another. The ICRP says that although information is limited, there is “clear evidence” that there are substantial variations in the radiosensitivity of organisms both within, and between taxonomic groups – this differential sensitivity also extends to different stages of the life cycle for any given organism. The report suggests it is possible that extrapolation becomes easier the more closely related organisms are, and the more similar the effects end points considered for the relevant stage in the life cycle.
Extrapolation from individual effects to those on a population or community also poses a problem. The report points out this will also, in many cases, involve the extrapolation from lab conditions (where most experimental information originates) to field conditions (where populations interact with the environment as well as with other organisms).
Interactions at community and ecosystem level can be particularly complex but it is necessary to start somewhere. Developing an understanding of radiation effects on a limited number of animals and plants at individual level, and exploring consequences at population levels and amongst different populations, will build a broader understanding that can be used in assessments, it says.
Corrina Thomson is deputy editor of Nuclear Engineering International