Soil-to-plant transfer factors of radioactive Ca, Sm and Pd isotopes: critical assessment of the use of analogies to derive best-estimates from existing non-specific data Pascale Henner a, * , Pierre Hurtevent a , Yves Thiry b a Institute for Radioprotection and Nuclear Safety (IRSN/PRP-ENV/SERIS), Laboratory of Bioavailability, Biogeochemistry and Transfer of Radionuclides (L2BT), Cadarache, Bat. 183, BP 3, 13115 Saint Paul-lez-Durance, France b French Agency for Radioactive Waste Management (Andra), Parc de la Croix Blanche, 1-7, Rue Jean Monnet, 92298 Ch^ atenay-Malabry Cedex, France article info Article history: Received 4 November 2013 Received in revised form 23 April 2014 Accepted 3 June 2014 Available online Keywords: Radionuclides Soil-to-plant transfer factor Use of analogues Calcium Samarium Palladium abstract 45 Ca, 151 Sm and 107 Pd are three radionuclides present in low to intermediate in activity radioactive wastes for which no soil-to-plant Transfer Factors (TF) values are available to be used in biosphere models for Ecological Risk Assessment. In the absence of specific radioecological studies, this work reviews and analyzes the existing literature for stable isotopes of Pd, Sm and Ca in order to derive best estimates for TF values that could be used as Transfer Factors. Alternative methods of extrapolation are also critically assessed. The values have been classified according to climatic zone, plant class and soil type for each element. The overall geometric mean TF values (for all plants and conditions) was calculated as 8.4E-02 for Pd, for which the value of radioRu in TRS-472 is also available. The mean TF for Sm was 4.2E-04. This value was lower than the TF values for radioactive Ce that are proposed as alternative values for Sm in TRS-472. The former may be relevant for long term assessments and the latter could possibly used to describe the short term 151 Sm post-release behaviour. The mean value for Ca is 2.3E-01 but varies considerably among plants of a given class due to the variety of plant Ca uptake behaviors. Alternatively, to limit this variability, Ca data content for dry plant matter, as analyzed using the phylogenetic method, could be used to derive TF values if the conservation of isotopic ratio of 45 Ca to stable Ca in soils and in plants hypothesis is taken into account. The TF for Ca in sub-tropical zones is 10-fold lower than in temperate zones. There is a lot of data available about exchangeable Ca in soil, which mean that we could calculate an available TF. The analysis shows that Ca bioavailability is also a key factor within transfer. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction The transfer of both natural or artificial radionuclides has been widely investigated during the past 40 years. The assessment of the potential impact of releases from nuclear facilities during normal operation or following an accident has driven the need to deter- mine best-estimates for a number of aggregated parameters used to describe the behavior of radionuclides within the biosphere. Among these, the soil-to-plant Transfer Factor (TF), which describes the transfer of radionuclides from soil to plant, is of particular in- terest because soil is a long-term reservoir for plant contamination and the ingestion of contaminated plants contributes to a large extent to the dose to human beings. The soil-to-plant Transfer Factor (TF) is defined as the ratio of the activity of a given radio- nuclide in a given plant organ to its activity in soil (IAEA, 2010). It is the radioecological equivalent of the Concentration Ratio, which is defined as the ratio of the concentration of a given element in a given plant organ to its concentration in soil and is widely used in the study of stable isotopes. The robustness and reliability of the predictions of any model using TF parameters relies primarily on the quality of the generic values used widely used reference data- bases, such as the one produced by the IAEA (2010). Despite a long history of determining these values, there is still a need to fill in the gaps within these databases. Some of these gaps, in particular values for naturally-occurring radionuclides, have been recently assessed (U, Th, Ra, Pb and Po, Vandenhove et al., 2008; iodine, Sheppard et al., 2010). Some of the derived values have already been included in the latest release of the IAEA database (IAEA, 2010). Others still need assessment, which is the case for the three radionuclides investigated in this study ( 45 Ca, 151 Sm and 107 Pd). These are part of the inventory of radionuclides present in low-to-intermediate activity radioactive wastes. Their behaviours are studied within a prospective risk assessement of the scenario in * Corresponding author. Tel.: þ33 (0)4 42 19 95 61; fax: þ33 (0)4 42 19 9151. E-mail address: pascale.henner@irsn.fr (P. Henner). Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad http://dx.doi.org/10.1016/j.jenvrad.2014.06.003 0265-931X/© 2014 Elsevier Ltd. All rights reserved. Journal of Environmental Radioactivity 136 (2014) 152e161