Presence of plutonium contamination in soils from Palomares (Spain)

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ Esta es la versión aceptada del artículo publicado en: This is an accepted manuscript of a paper published in: Environmental Pollution (2006): 24/01/2025 DOI: https://doi.org/10.1016/j.envpol.2005.10.030 Copyright: © 2005 Elsevier Ltd. All rights reserved. El acceso a la versión publicada del artículo puede requerir la suscripción de la revista. Access to the published version may require subscription. “This is an Accepted Manuscript of an article published by Elsevier in [Environmental Pollution] on [2006], available at https://doi.org/10.1016/j.envpol.2005.10.030” ON THE PRESENCE OF PLUTONIUM CONTAMINATION IN SOILS FROM PALOMARES (SPAIN) M.C. Jiménez-Ramos, R.García-Tenorio*, I.Vioque, G.Manjón and M.García-León. Applied Nuclear Physics Research Group. University of Sevilla, P.O.Box 1065, 41080-Sevilla, Spain. Abstract More than 30 years after the occurrence of an aircraft accident which involved the detonation of two nuclear weapons in the surrounding area of the village of Palomares (Spain), the affected terrestrial area has been investigated for remaining transuranic contamination. Evidence from the presence of this contamination was initially found through the analysis of the 241Am inventories in superficial soil samples collected in the region, and was confirmed through the analysis of the 239+240Pu inventories and their associated 238Pu/239+240Pu activity ratios in the same samples. However, it was also observed that a considerable fraction of the remaining contamination on the area was present in particulate form, i.e as “hot particles”. The work performed in our laboratory for identification, isolation and characterization of these “hot particles” as well as some conclusions obtained from these analyses are outlined in this paper. Keywords: Plutonium, Palomares, “hot particles” * To whom correspondence should be addressed. E-mail: [email protected] phone: +34-954556625 ; fax: +34-954557892 Postal address: Departamento Física Aplicada II, E.T.S. Arquitectura, Universidad de Sevilla, Avenida Reina Mercedes 2, 41012-Sevilla-Spain. 2 1. Introduction On 17th January 1966, due to an aircraft accident, two nuclear weapons went through their conventional chemical explosion, after having crushed with the earth, in the surrounding area of the village of Palomares (Spain). The result of these detonations was the fragmentation of the weapons, and the dispersion of their content. This dispersion produced a clear contamination of plutonium isotopes in the zone. Immediately after the accident, a monitoring programme of the affected zone was undertaken, and based on these results, different clean-up operations were performed in order to reduce/dilute the Pu contamination levels in the affected soils. In fact, in the most contaminated areas, the upper 10 cm layer of soil and the associated vegetation was collected, stored in drums and sent to USA for its disposal. The objective was to assure that no unacceptable radiation doses were received by the people either living in the affected area or farming the land (Iranzo et al., 1988). Since then, several radiological and radioecological studies have been performed in the affected area (i.e., Espinosa et al., 1999 & 2001). In autumn 2001, our research group carried out a sampling campaign by collecting superficial soils from several zones of the Palomares region. The first objective was to investigate the presence of weapon-grade Pu remaining from the accident. The results obtained in the performance of this study form the core of this paper. Some evidence supporting the existence of this contamination was investigated by determining the 241Am and 239+240Pu inventories in the collected soil samples, and by analysing their associated 238Pu/239+240Pu activity ratios. However, in addition, the screening of some of the soil samples with higher 241Am levels was carried out by 3 seeking the presence of a fraction of the contamination as “hot particles” whose origin was clearly from the accident. Our work on identification, isolation and characterisation of possible “hot particles” is also reported in this paper. 2.-Materials and Methods 2.1.- Site characteristics and sampling The village of Palomares is located in the south-east of Spain (37º 15’ N, 1º 50’ W) near the Mediterranean sea shore. It is on the right margin of the Almanzora river and surrounded by two hilly systems placed at its north and south-west. The terrestrial area initially affected by the accident surrounds the village of Palomares and covers an extension of about 230 hectares (Iranzo et al., 1987). The landscape of this region can be considered as semi-arid, with very little treecovering, scarce associated vegetation and temporal small streams that are dried most of the year. The soil in the area has a silica-carbonataceous nature, where SiO2, AlO2, CaO and (CO3)2- are its main components. In order to perform this work, several superficial soil samples were collected in October 2001 from a zone located to the west of the Palomares village (Figure 1), in the vicinity of the place where one of the bombs exploded (Iranzo et al., 1987). The sampling was carried out by marking a defined square area of 0.25 m2, and by collecting soil using hand tools to a depth of 5 cm. After collection, the samples were transported to the laboratory, and then air-dried and homogenised after the removal of stones bigger than 0.5 cm. No additional pre-treatment and/or conditioning steps were applied to the collected samples. 4 2.2.-Determination of 239+240Pu inventories and 238Pu/239+240Pu activity ratios in soil Alpha-particle spectrometry In order to perform the determination of Pu-isotope alpha-emitters using alphaparticle spectrometry, a radiochemical procedure was applied to each soil sample to isolate and deposit these nuclides in thin layers. This radiochemical procedure is fully described in Vioque et al., 2002. The detection system used for the measurements was an alpha spectrometer Alpha Analyst (Canberra) consisting of eight independent chambers. Each chamber was equipped with a 450 mm2 passivated implanted planar silicon (PIPS) detector. Measurements were performed by placing the samples at 4 mm from the detector. Under these conditions the counting efficiency was 0.34. The radiochemical yield was controlled by adding known amounts of a 242Pu standard solution to each sample before the application of the radiochemical procedure. Yield values in the interval 50-80% were obtained. 2.3.- Isolation, identification and characterisation of the “hot particles” Gamma-ray spectrometry In order to screen some of the soil samples for the isolation of possible Pu “hot particles”, gamma-ray spectrometric measurements were performed by seeking the gamma signal of 241Am (59.54 keV). This radionuclide is a gamma-ray emitter which is present in the contaminated soils of Palomares as a daughter of the 241Pu which was present in the cores of the bombs involved in the accident. 5 The presence of Pu “hot particles” in one of the several aliquots corresponding to a previously homogenised superficial soil sample, should provoke the heterogeneity in the distribution of the 241Am contamination between the aliquots, with very high levels of this radionuclide in the aliquot containing the “hot particle”. Hence, once it is suspected from the 241Am determinations that one aliquot can contain a “hot particle”, a procedure based on its successive division and the measurement of each fraction by gamma-ray spectrometry is applied. The presence of a “hot particle” in the original aliquot must provoke clearly higher levels of 241Am in the fraction where it remains after the division. Therefore, through successive divisions of the sample and their measurements by gamma-ray spectrometry, it is possible to enormously reduce the mass of the sample in which the “hot particle” is present, thereby enabling its identification and characterisation. The gamma-spectrometric system used consists of 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) and an inner copper layer (5 mm) surround the detector to protect it against environmental radiation interference. Scanning Electron Microscopy (SEM) and X-ray microscopy analysis (XRMA) Once the amount of material containing a possible “hot particle” has been isolated as much as possible (to a few milligrams), it is placed onto an adhesive tape which is then fixed onto an aluminium specimen stub. The sample is coated with a carbon layer (thickness about 20 nm) to prevent artefacts from charging and finally examined at different magnifications using the scanning Electron Microscopy technique (SEM). To this end, a scanning electron microscope PHILIPS XL-30 with secondary and backscattering electron detectors was used 6 Specifically, for the identification of possible Pu “hot particles”, the backscattered secondary electron image mode (BSE) was used due to the high contrast found between elements with high and low atomic numbers. Elements with high Z produce more backscattered electrons than elements with low Z, thereby triggering a higher signal in the detector. Furthermore, when a “hot particle” has been identified, the use of the same technique in the secondary electron image mode (SE) gives information about its topography and size, while to obtain qualitative information about the composition of its superficial layers, an Energy dispersive X-ray Spectrometer (EDX) integrated with the Electron microscopy system, was used for pointing X-ray analyses. 3.- Results and Discussion 3.1.- 239+240Pu soil inventories and associated 238Pu/239+240Pu activity ratios. In Table 1 the 239+240Pu inventories (Bq/m2) determined in the superficial soil samples collected in the vicinity of the village of Palomares in October 2001 are presented. Sampling sites are indicated in Figure 1. The majority of these sites according to the indications given in Iranzo et al. (1987), were selected at the surroundings of the places where wreckage from the aircraft accident was found, and in the vicinity of the location where one of the bombs exploded. At some of the sampling points, replicate determinations in different aliquots were carried out so as to analyse the homogeneity in the Pu contamination. By observing the results it is quite clear to conclude that the most contaminated Pu spots are those located nearest to the village of Palomares (A1, A2, A3). The 7 239+240Pu levels found in these sampling points are at least two orders of magnitude higher than those which would have been expected at the Palomares latitude if the only source of Plutonium had been the atmospheric fallout, which are in the range 30-50 Bq/m2, (Hardy et al., 1973, UNSCEAR, 1982). This gives a clear indication of the presence of some traces of the destroyed weapons more than 30 years after the accident and confirms previous evidence based in 241Am determinations by gamma-ray spectrometry in the same soil samples. The levels of Pu soil contamination decrease when the distance from the village increases. In fact, taking into account that only the uppermost 5 cm layer at each sampling point has been collected, the inventories in samples A8, A9 and A10 are compatible with the expected and previously given values when the Pu origin is weapon test fallout. Moreover, the fingerprint of the Palomares accident in the soil samples with higher Pu inventories was additionally investigated by analysing 238Pu/239+240Pu activity ratios. The obtained values, compiled in Table 2, are clearly compatible with the possible weapon-grade origin of the Pu found in them, and are clearly lower than would be expected if the origin of the Pu contamination were the weapon-test fallout and the destruction in the stratosphere of the SNAP-9A satellite (in this case, 238Pu/239+240Pu activity ratio values in the range 0.03-0.04 corrected to the date of our measurements, would be expected (Hardy et al., 1973; Bunzl and Kracke, 1988)). This fact, together with the associated inventory levels, reinforces the idea that a vast proportion of the Pu associated to the most contaminated samples originates from the Pu dispersed in the Palomares accident. 8 On the other hand, the inhomogeneity of Pu concentrations among the aliquots of samples A2 and A3 made us consider the presence of “hot particles”. This fact was first confirmed by taking each one of the most contaminated soil samples (A1, A2 and A3), and by dividing them after homogenisation into six aliquots for 241Am determination by Ge gamma-ray spectrometry. In spite of the homogenisation steps, quite different concentration levels of 241Am were found in some cases in several of the aliquots corresponding to the same superficial soil sample. However, definite evidence of the presence of “hot particles” was obtained by performing 241Am measurements of successive divisions of a given aliquot from sample A2 which presents high levels of this radionuclide. The final result of this exercise was that about 90% of the 241Am originally present in the initial aliquot analysed (about 50 grams) was confined in a mass amount lower than one milligram, obviously a “hot particle”. The unambiguous verification of the presence of a considerable fraction of the Pu in the Palomares area as “hot particles” is extremely important for the establishment of an appropriate strategy for a proper and accurate estimation of the remaining Pu inventory. Furthermore, knowledge about the structural and physical characteristics of the Pu source-term in the area is essential to assess the short- and long-term consequences of the contamination, since it can have a great influence on the mobility and biological uptake of the transuranic elements in the affected ecosystem with subsequent implications for dose and environmental impact assessment (Salbu et al., 1998). 15 Table captions Table 1.- 239+240Pu inventories (Bq/m2), determined by alpha-particle spectrometry, in aliquots of the superficial soil samples collected in the Palomares analysed region. Table 2.- 238Pu/239+240Pu activity ratios found in the aliquots of the Palomares soil samples which show higher levels of contamination. 16 Figure Captions Figure 1.- Palomares sampling site. In the bottom right-hand corner of the Figure, the location of the Palomares village is highlighted. The exact location of the sampling stations are marked. Figure 2.- a) BSE (Back-scattered secondary electrons) image of one isolated hotparticle from Palomares. b) The same as in a), but under higher magnification. Figure 3.- a) X-ray spectrum obtained by impinging electrons onto a point of the surface of the hot-particle identified in Figure 2. To maximize the ionisation of U and Pu L-lines, an acceleration voltage of 30 kV was applied. b) X-ray spectrum obtained by impinging the electrons onto the surface of a background soil particle. Figure 4.- SE (Secondary electron) image corresponding to the hot particle identified in Figure 3. Tilt angle = 0º. 17 FIGURE 1 18 FIGURE 2a FIGURE 2b 19 FIGURE 3a FIGURE 3b 20 FIGURE 4 21 Table 1.- 239+240Pu inventories (Bq/m2), determined by alpha-particle spectrometry, in aliquots of the superficial soil samples collected in the Palomares analysed region. Sampling point 239+240Pu (Bq/m2) A1 57 900  3500 A2 6450  350 - 8210  240 - 56 800  1900 A3 2070  130 - 4320  300 A6 250 20 - 310  30 A7 110  10 A8 20  4 A9 8  2 A10 25  3 22 Table 2.- 238Pu/239+240Pu activity ratios found in the aliquots of the Palomares soil samples which show higher levels of contamination. SAMPLE 239+240Pu (Bq/m2) 238Pu/239+240Pu A1 57 900  3500 0.017  0.002 A2a 8210  240 0.015  0.002 A2b 56 800  1900 0.016  0.001 A2c 6450  350 0.021  0.004 A3a 4320  300 0.020  0.005