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Copyright © 2014 ALLIANCE Noble gas dosimetry for non-human biota International Conference on Radioecology and Environmental Radioactivity, Barcelona, 7 – 12 Sept. 2014 www.er-alliance.org Jordi Vives i Batlle COMET and STAR participant email@example.com
Copyright © 2014 ALLIANCE 85 Kr and 41 Ar account for approximately 75% and 10% of airborne releases for nuclear reactors Smaller amounts of 131m Xe, 133 Xe and 88 Kr also released. 41 Ar and 85 Kr contribute 80% from next-generation AP1000 reactors 85 Kr the main radionuclide discharged from Sellafield (~ 40 PBq y -1 ). For inert gases Ar and Kr plume immersion is the only concern. For Rn there is the problem of internal dose by the daughters. General drive to ensure that the environment is protected Ongoing need to prove limited impact in new reactor designs Compliance and public perception (birds roosting on stacks?) The US NRC now includes the plume dose from noble gas emissions in assessing dose to biota for new reactor licensing Reasons for noble gas dosimetry
Copyright © 2014 ALLIANCE Noble gases have a small but finite solubility in water and body fluids. 41 Ar and 85 Kr are inert gases and internal incorporation in animals can be neglected compared with cloud immersion. Noble gases are not deposited to soil (so no plant uptake, etc.). Will be exchanged within the air pore volume of surface soil (but small component). So we assume pore air concentration = ground level air concentration Points to note
Copyright © 2014 ALLIANCE Argon and krypton methodology
Copyright © 2014 ALLIANCE Calculate DCC values using the EA R&D 128 ellipsoid-based Monte Carlo approach. Compare with human dose conv. factors (DCFs) from ICRP 72. Study dependence of DCC with area/volume ratio Incorporate methodology into R&D 128 terrestrial model. Update with ERICA geometries Approach
Copyright © 2014 ALLIANCE Inputs: Ar and Kr in air (measured or calculated with simple semi- infinite cloud model) Outputs: Convert DCC in Gy h -1 /Bq m -3 instead of kg -1 using 1.2 kg m -3 air density. Usual approximations: equilibrium transfer, ellipsoid geometries, uniform density between organism and media, uniform distribution in organism and doses averaged for whole body. Species considered: all terrestrial R&D 128 + “reference man” ellipsoid defined for comparison with ICRP-72. Method has been recently revamped to include: DCCs for the ERICA reference organisms DCCs for 131m Xe, 133 Xe and 88 Kr DCC calculation
Copyright © 2014 ALLIANCE DCCs FOR Ar, Kr and Xe (ERICA organisms) 131m
Copyright © 2014 ALLIANCE Implementation in EA R&D 128 Internal dose negligible: default Ar and Kr CRs for all organisms set to 0. Although no deposition, some migration into soil pores possible leading to a transfer factor. Assume pore air is at the same concentration as ground level air Soil free air space =10 -4 m 3 kg -1 =>Bq m -3 (air) × 10 -4 = Bq kg -1 (soil). A TF of 10 -4 is therefore specified as a default Plants and fungi occupancy factors set to 1 soil, 0.5 air (not 0!). Soil, bacteria and earthworms assumed exposed only to 41 Ar and 85 Kr in air in soil pore spaces. External DCCs for fungi same as bacteria (i.e. infinite medium DCCs). Internal DCCs same as for fruiting body.
Copyright © 2014 ALLIANCE Dose calculation – humans vs. biota This is a modified semi-infinite cloud model for close distances from source For biota we adapt tis to use a = 1, b = 0, f i ind = 0, and f i occ = 1.
Copyright © 2014 ALLIANCE Dose calculation formulae The R&D 128 spreadsheet uses the following formulae for all radionuclides whose concentration is referenced to air: 3 H, 14 C, 32 P, 35 S, 41 Ar and 85 Kr. Where the reduction factor is the modifier for dose to biota in air as received from exposure to soil: 0 for and low-energy 0.25 for high energy + .
Copyright © 2014 ALLIANCE Conceptual differences human vs. biota Human DCFs are calculated for a plume emitting "from above", i.e., over a 2 (semi-infinite) geometry. Biota DCCs, are calculated assuming that the medium envelops the organism (infinite) geometry. This is not an inconsistency in so far as it is recognised that the human DCFs and biota DCCs refer to two different geometry definitions. The DCC method averages the external dose over the whole volume, whereas the DCF method averages over an outer layer.
Copyright © 2014 ALLIANCE Using 85 Kr as an analogue for other Kr and Xe Previous statement that other Xe and Kr isotopes can be modelled using 85 Kr as a surrogate (Copplestone et al., 2010). We tested this assumption by calculating DCCs for 131m Xe, 133 Xe and 88 Kr(+ 88 Rb) explicitly. The DCCs for 88 Kr can vary significantly with respect to 85 Kr: by a factor between 10 and 300 ( -radiation > 10 keV) and between 1200 and 1400 ( -radiation). The corresponding factor ranges of variation for 131m Xe and 133 Xe are 0.2 - 3800 and 4 – 20, respectively; similar for both radionuclides. The main sources of difference are therefore (a) the larger amount of -emissions of 88 Kr (52 vs. 1 for 85 Kr and 10 for the Xe isotopes), and (b) the stronger high-energy -component of the Xe isotopes. The 85 Kr analogue approximation is therefore not valid for 88 Kr. For Xe, only valid for small biota – not for birds, mammals and trees.
Copyright © 2014 ALLIANCE Radon – allometric model
Copyright © 2014 ALLIANCE Basis of the approach A model based on allometrically derived respiration rates and target tissue masses, designed for calculating 222 Rn daughter dose rates to sensitive tissues and the whole body of terrestrial animals and plants.
Copyright © 2014 ALLIANCE Problem formulation Model the input of a constant flow of atoms into a compartment with continuous decay, with these two fluxes in equilibrium. Assume that the compartment is 100% efficient at trapping the material - no particles escape by exhalation and decay is the only source of removal. The input flow I 0 i equals the specific activity breathing rate / decay constant (in order to convert disintegrations per unit time to particles). From here the DCC is: Where D p is the potential -energy per Bq activity of the short-lived radon daughters in secular equilibrium i: Index labelling the radionuclide: 1 to 5 for 222 Rn, 218 Po, 214 Pb, 214 Bi and 214 Po; A i : Activity of radionuclide i [Bq m -3 ] = A 1 (secular equilibrium) BR: Breathing rate [m 3 s -1 ] = tidal volume (V T ) breathing frequency ( R ) i : Decay constant of radionuclide i [s -1 ].
Copyright © 2014 ALLIANCE Allometric scaling Many biological parameters relating to organism structure relate to metabolism and scale according to the Brody-Kleiber law: Other parameters scale on the basis of surface exchange, like radiation flux and heat transfer: For this study we use the following relationships: M is the mass in kg and B R is the ventilation rate in m 3 h -1.
Copyright © 2014 ALLIANCE F U : Unit conversion factor (3.6 10 9 Gy h -1 per Gy s -1 ) BR: Gross extrapolation to the bronchial epithelium (airway generations 1 - 8) TB: Full tracheobronchial epithelium; L: Full lung; WB: Whole body A BR (A LM ), B BR (B LM ): Base and exponent of the allometric formula for breathing rate & lung mass; S TB RM and S B RM : surface area of the tracheobronchial tree or the bronchial epithelium; R wf : Radiation weighting factor for -energy (default = 20). Mammal DCCs Simple power functions for DPCCs in Gy h -1 per Bq m -3 : This approach is only recommended for mammals. Applicability to other animals with structurally simpler respiratory systems (birds, reptiles, amphibians and insects) is conjectural and likely over-conservative.
Copyright © 2014 ALLIANCE Plant DCCs Assume that the whole plant is a surface passively exchanging gases with the atmosphere. Simple power functions for DPCCs in Gy h -1 per Bq m -3 : D P a : Potential -energy factor 5.54 10 -9 J Bq -1 ; A PL : Allometric base for breathing rate in plants, 1.95 10 -4 m 3 s -1 calculated by Vives i Batlle et al. (2012) based on previous data (Reich et al., 2005) and based on net CO 2 efflux data; a: Minor axis of the ellipsoid representing the plant in m (if the two minor axes of the geometry are dissimilar then the average is taken); h T : Depth of sensitive tissue = 5.5 10 -5 m; F U : Unit conversion factor (3.6 10 9 Gy h -1 per Gy s -1 ); R wf a : Radiation weighting factor for -energy (default = 10).
Copyright © 2014 ALLIANCE Applicable to all radionuclides whose concentration is referenced to air - that is, 3 H, 14 C, 32 P, 35 S, 41 Ar, 85 Kr and 222 Rn: Where the reduction factor is the modifier for dose to organisms in air is received from exposure to soil: 0 for and low-energy radiation and 0.25 for high energy + radiation. Dose calculation formulae (slide 12 again!)
Copyright © 2014 ALLIANCE Internal DCCs for Rn (ERICA organisms)
Copyright © 2014 ALLIANCE External DCCs for Rn (ERICA organisms) Directly calculated from concentration in air by the Monte Carlo method
Copyright © 2014 ALLIANCE Example of application: Rn in burrowing mammals
Copyright © 2014 ALLIANCE Radon field studies Available dose rate estimates for 222 Rn: One study in area of ‘Rn rich soils’ in Canada Whole body dose rate >100 mGy y -1 for small burrowing animals (c. 10µGy h -1 ) So Dose rate similar to predicted no effect dose So Beresford et al. (2012) delivered an estimation of 222 Rn dose rates to burrowing mammals at sites in the United Kingdom Dose rates calculated from measured field soil gas concentration, using the allometric methodology described previously Assuming an equilibrium factor F = 0.8 Assuming an α-radiation weighting factor of 10 7 woodland, scrub and pasture sites selected to have range in potential Rn soil gas concentrations
Copyright © 2014 ALLIANCE Approach Make artificial burrows Use passive detectors developed by NRPB and SSI to measure soil gas 222 Rn activity concentration Sites across gradient of expected 222 Rn concentrations Detector placed in approximately 10-cm diameter perforated land-drainage tubing = ‘artificial burrow’ Tube (c.1.2 m) open ended on surface detector 50 cm below soil surface (surface length c.1 m). 3 per site Detector changed every 4-6 weeks (summers of 2009 - 2010)
Copyright © 2014 ALLIANCE Range: <0.1 to 14.5 kBq m -3 Measured 222 Rn concentrations in soil gas
Copyright © 2014 ALLIANCE Dose rate from 222 Rn to burrowing mammals likely to be at least 10 times higher than previously considered natural exposure sour- ces ( 40 K, Th/U series). In many areas likely to considerably exceed predicted no-effect dose rate benchmarks. Weighted dose rates
Copyright © 2014 ALLIANCE Conclusions
Copyright © 2014 ALLIANCE Conclusions Argon and krypton (xenon) Ar and Kr dosimetry codified into EA R&D 128 terrestrial model. Methodology for biota is consistent with that for humans, except: External DCCs calculated for infinite geometry; DCFs for semi-infinite DCC method averages doses over whole volume Radon (allometric model) Radon dosimetry codified into DCCs for internal -irradiation Radon exposures in mammals Levels in burrows exceeding background levels and no-effects benchmarks for non-human biota. Advised benchmark dose rates need to be better put into context with background dose rates Context determined by benchmark purpose & assessment level.
Copyright © 2014 ALLIANCE Published papers Vives i Batlle, J., Jones, S.R. and Copplestone, D. (2014). A methodology for the assessment of doses to terrestrial biota arising from external exposure to 41 Ar, 83,88 Kr and 1321,133 Xe. Science of the Total Environment (submitted). Beresford, N.A., Barnett, C.L., Vives i Batlle, J., Potter, E.D., Ibrahimi, Z.-F., Barlow, T.S., Schieb, C., Jones, D.G. and Copplestone, D. (2012). Exposure of burrowing mammals to 222 Rn. The Science of the Total Environment 431: 252-261. Vives i Batlle, J., Copplestone, D. and Jones, S.R. (2012). Allometric methodology for the assessment of radon exposures to wildlife. Science of the Total Environment. 427-428: 50–59 Vives i Batlle, J., Smith, A., Vives-Lynch, S., Copplestone, D., Strand, T., Proehl, G. and Brown, J. (2011) Model-derived dose rates per unit concentration of radon in air in a generic plant geometry. Radiation and Environmental Biophysics 50(4): 513-529. Vives i Batlle, J., Jones, S.R. and Copplestone, D. (2008) Dosimetric Model for Biota Exposure to Inhaled Radon Daughters. Environment Agency Science Report – SC060080, 34 pp.
Copyright © 2014 ALLIANCE Models are freely available from: www.ceh.ac.uk/PROTECT/www.ceh.ac.uk/PROTECT/
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