Levels, distribution and bioavailability of transuranic elements released in the Palomares accident (Spain)

www.elsevier.com/locate/apradiso Author’s Accepted Manuscript Levels, distribution and bioavailability of transuranic elements released in the Palomares accident (Spain) M.C. Jiménez-Ramos, I.Vioque, R. García-Tenorio, M. García León PII: S0969-8043(08)00242-X DOI: doi:10.1016/j.apradiso.2007.12.015 Reference: ARI 4090 To appear in: Journal of Accounting and Economics Received date: 1 June 2007 Revised date: 15 October 2007 Accepted date: 28 December 2007 Cite this article as: M.C. Jiménez-Ramos, I. Vioque, R. García-Tenorio and M. García León, Levels, distribution and bioavailability of transuranic elements released in the Palomares accident (Spain), Applied Radiation and Isotopes (2008), doi:10.1016/j.apradiso.2007.12.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Accepted manuscript Levels, distribution and bioavailability of transuranic elements released in the Palomares accident (Spain) M.C. Jiménez-Ramos*, I. Vioque, R.García-Tenorio, M. García León Applied Nuclear Physics Research Group, University of Seville, Avd. Reina Mercedes, 2; 41012 Sevilla, Spain Received 1 June 2007; received in revised form 15 October 2007; accepted 28 December 2007 Abstract The current levels and distribution of the remaining transuranic contamination present in the terrestrial area affected by the nuclear Palomares accident have been evaluated through the determination of the Puisotopes and 241Am concentrations in soils collected 35 years after the accident. In addition, after confirming that most of the contamination is present in particulate form, some bioavailability laboratorybased experiments, based on the use of single extractants, were performed as an essential step in order to study the behaviour of the Pu contamination in the soils from the affected areas. Keywords: transuranic; bioavailability; Palomares 1. Introduction The US military aircraft accident that occurred in Palomares (SW of Spain) in 1966 caused the release and dispersion of the fissile material of two nuclear weapons into a terrestrial Mediterranean ecosystem. Undergrowth, farming and urban areas were contaminated with fissile material from the nuclear bombs. Although the contaminated areas were cleaned immediately after the accident (Iranzo et al., 1991), some contamination remained, as has been proved by subsequent studies which focused on the distribution, characterisation and behaviour of transuranics in the affected ecosystem. * Corresponding author. Tel: +34954555979. Fax: +34954557892. E-mail address: [email protected] (M.C. Jiménez-Ramos) Accepted manuscript In 2001, 35 years after the accident, a sampling campaign was undertaken of soils from different places of the area originally affected by the accident and of seaweeds collected in the neighbouring marine environment. The objectives, among others, were the following: a) to gain information about the actual spatial distribution of the remaining terrestrial transuranic contamination (241Am and 239+240Pu), b) to confirm the transfer of some of the contamination to the neighbouring marine environment, by the determination of the transuranic levels in the collected seaweeds, c) to make an empirical assessment of the Pu availability by plant uptake from Palomares soils (through the application of a laboratory-based single extraction technique to several Palomares contaminated soils), and d) to perform solubility studies on Palomares particle contaminated soils using an extraction scheme based on the leaching of the soils by a simulated gastrointestinal fluid. The main results obtained covering all these objectives are presented in this paper. 2. Sampling campaign The region covered by the sampling campaign was divided for practical reasons into three different zones: Zone A , where the chemical explosion of one of the two bombs which released their nuclear content to the environment took place, formed of mostly unperturbated soils, and Zones B and C, formed of mostly agricultural soils, where the chemical explosion of the other bomb occurred, (Figure 1). In total, 24 cylindrical soil cores, of 56 mm diameter and 10 cm thickness, were collected from the three zones. Furthermore, and in order to cover one of the proposed objectives, several alga samples (Fucus specie) were collected from the Palomares coast (Figure 1). 3. Experimental methods 3.1. Gamma Spectrometry The 241Am levels in the analysed samples were determined through gamma-ray spectrometry by using a Canberra n-type reverse electrode germanium (ReGe) detector, with a relative photo-peak efficiency of 30% and a thin Be window. A lead shield (10 cm-thick regular shield) lined with a thin copper layer (5 mm) surrounded the detector to protect it against environmental radiation interference. Several 50 g soil aliquots from each soil sample were measured by using Petri containers and a fixed sample-detector Accepted manuscript geometry. Only for the samples from zone C were the measurements restricted to one homogenised aliquot. The system was calibrated by measuring, in fixed sample-detector geometry, a calibration soil sample of the same characteristics as for the samples. 3.2. Alpha Spectrometry The Pu-isotopes alpha-emitters levels in the collected soils and alga samples were determined by alphaparticle spectrometry, after the application of a well-established radiochemical procedure for their isolation (Vioque et al., 2002). In the locations where it was previously observed that the 241Am contamination was higher, several soil aliquots from the same core were analysed while in the other locations only one aliquot was measured. The system used for the measurements was an Alpha Analyst alpha spectrometer (Canberra) with eight independent chambers. Each chamber was equipped with a 450 mm2 passivated implanted planar silicon (PIPS) detector. 3.3. Laboratory-based experiments applying extraction techniques In order to make an empirical assessment of the Palomares Pu availability by plant uptake, and from among the great variety of single chemical extractants which can be found in the literature which serve this purpose (Kennedy et al., 1997), we have selected 1M Cl2Mg pH 7 as the most suitable extractant. The extraction method (detailed in Figure 2) has been applied to Palomares soil aliquots with a different degree of Pu contamination. On other hand, other aliquots of the Palomares soils were subjected to leaching by a simulated gastrointestinal tract fluid, 0.16 M HCl (Lind et al., 2005), in order to gain information concerning the proportion of the transuranium contamination which is susceptible to be soluble after accidental soil ingestion. The simple extraction method is also detailed in Figure 2. 4. Results and conclusions Accepted manuscript The 241Am and 239+240Pu deposition activities (Bq/m2) determined in aliquots of soil samples collected in the three zones analysed are compiled in Table 1. As transuranic determinations were performed in different aliquots of each soil sample, the averages and the extreme values found for theses nuclides at each sample point are reflected in the table. The first conclusion which can be obtained through the evaluation of these results is the quite different and heterogeneous 241Am and 239+240Pu values which were obtained for different aliquots of the same sampling point. This fact should be explained by the presence of a considerable fraction of the remaining contamination in the form of discrete particles, known as hot particles. This possibility has been confirmed in other works performed by us, which identify and characterize hot particles from these soil samples. Particles from the Palomares soils were isolated by sub-sampling and non-destructive gamma spectrometry and analysed using electron microscope techniques and X-ray microanalysis (Jiménez- Ramos et al., 2006), while their isotopic composition was determined by ICP-MS. The ICP-MS results indicated the major presence of uranium in enriched form and of weapon-grade plutonium in the hot particles (Jiménez-Ramos et al., 2007). It is important to point out that the size of these hot particles ranges from 20 to 100 µm, and that these large hot particles are not easily resuspended (García-Olivares and Iranzo, 1997). The typical size of the inhalation particle fraction is lower than 10 µm. By means of the Pu-isotopes determinations performed in this work on the soil samples it was also possible to determine the origin of the remaining Pu contamination in the analysed soils by measuring their 238Pu/239+240Pu activity ratios. The average value for this isotopic ratio within the region covered was 0.018 ± 0.005, which is clearly below the typical fallout value (0.03-0.04) (Hardy et al., 1973) and is totally in agreement with the ratio for weapon-grade plutonium. Furthermore, and due to the stronger effect of the contamination, lower values for the 238Pu/239+240Pu ratios were determined in zone A than in zones B and C. Indeed, the results of Table 1 reflect that the highest levels of transuranic contamination remain in some points of Zone A, associated to undisturbed soils, while the values obtained in zones B and C are lower than could be expected, because these zones are agricultural areas where some dilution of the contamination has occurred due to ploughing. In the most contaminated sampling points the 241Am and Accepted manuscript 239+240Pu deposition values are at least two orders of magnitude higher than the values expected for fallout (25 Bq/m2 for 241Am, Breban et al., 2003, and 50-60 Bq/m2 for 239+240Pu, Hardy et al., 1973). The transuranic contamination associated to the nuclear Palomares accident has not been confined only to the terrestrial environment. Some traces of weapon-grade Pu originating from the accident were also detected in the neighbouring coastal marine environment (Romero et al., 1992). These traces reached the sea mostly associated with soil material which was transported by overflowing that sporadically affects the region after heavy rain storms. The presence of these traces 41 years after the accident in the marine environment has been confirmed by the analysis of two different seaweed samples (Fucus species) collected from the beach of Palomares (see Figure 1). The 239+240Pu activity concentrations in these two Palomares seaweed samples were around 1 Bq kg-1 (0.95 ± 0.02 and 0.82 ± 0.03 Bq kg-1 dry weight, respectively), one order of magnitude higher than the concentrations determined at other coastal points in the South of Spain for the same species (Manjón et al., 1995). Once the levels and spatial distribution of the remaining transuranic contamination in the terrestrial affected area is defined, it is essential to assess their long-term environmental, radioecological and radiological impact. In the Palomares area, although soils can act as a sink for the radioactive particles, they can also act as a potential diffuse source, depending on source-term characteristics and processes which influence particle weathering and mobilisation of associated radionuclides (Salbu, 2005). By performing solubility studies on Palomares contaminated soils using relevant extraction schemes, information on weathering and dissolution kinetics as well as on bioavailabity can be obtained and linked to site-specific particle characteristics (Lind et al., 2005). Papers have recently been published (Lind et al., 2007; Espinosa et al., 2005) about experiments that highlight the importance of periodically performing solubility studies to evaluate their evolution over time. To this end, two different laboratory experiments based on the application of selected extractants were performed in an effort to estimate a) the current amount of Palomares Pu that can be incorporated from the contaminated soils into vegetables through their roots (root uptake), and b) the current amount of Palomares Pu susceptible to solubility in the gastrointestinal tract if some Palomares soil is accidentally ingested. Accepted manuscript The estimation of the Palomares Pu fraction which can pass from the contaminated soils to the plants was carried out by applying 1 M Cl2Mg as extractant to a set of different soil aliquots with different activity concentration levels. The obtained results (Table 2) indicate that the bioavailability fraction was relatively variable but low and ranges from 0.4% to 6%. In addition, when 0.16M HCl was used as extractant to simulate a gastrointestinal tract fluid, the soluble fraction was also low and variable. This last extraction method was also applied to an isolated hot particle with a 239+240Pu activity of 12 Bq and the result obtained for the availability fraction was 10%. Neither of the Pu availability fractions in the two experiments was high. This indicates that the Pu continues to be present in the Palomares soils in a mostly refractory form, which explains why the hot particles released in Palomares over 40 years ago still persist in the soils. 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