Environmental Pollution 142 (2006) 487e492www.elsevier.com/locate/envpol

Presence of plutonium contamination in soils from Palomares (Spain)

M.C. Jimenez-Ramos, R. Garcıa-Tenorio*, I. Vioque, G. Manjon, M. Garcıa-Leon

Applied Nuclear Physics Research Group, University of Sevilla, P.O. Box 1065, 41080 Sevilla, Spain

Received 18 February 2005; accepted 7 October 2005

The remaining transuranic contamination in Palomares soils is mainly present in particulate form.

Abstract

More than 30 years after the occurrence of an aircraft accident which involved the detonation of two nuclear weapons in the surrounding areaof the village of Palomares (Spain), the affected terrestrial area has been investigated for remaining transuranic contamination. Evidence fromthe presence of this contamination was initially found through the analysis of the 241Am inventories in superficial soil samples collected in theregion, and was confirmed through the analysis of the 239 þ 240Pu inventories and their associated 238Pu/239 þ 240Pu activity ratios in the samesamples. However, it was also observed that a considerable fraction of the remaining contamination in the area was present in particulateform, i.e. as ‘‘hot particles’’. The work performed in our laboratory for identification, isolation and characterisation of these ‘‘hot particles’’as well as some conclusions obtained from these analyses are outlined in this paper.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Plutonium; Palomares; Hot particles

1. Introduction

On 17 January 1966, due to an aircraft accident, two nuclearweapons went through their conventional chemical explosion,after having crushed with the earth, in the area surrounding thevillage of Palomares (Spain). The result of these detonationswas the fragmentation of the weapons, and the dispersion oftheir content. This dispersion produced a clear contaminationof plutonium isotopes in the zone.

Immediately after the accident, a monitoring programmeof the affected zone was undertaken, and based on these re-sults, different clean-up operations were performed in orderto reduce/dilute the Pu contamination levels in the affectedsoils. In fact, in the most contaminated areas, the upper10 cm layer of soil and the associated vegetation was col-lected, stored in drums and sent to the USA for disposal.

* Corresponding author. Departamento de Fısica Aplicada II, E.T.S. Arqui-

tectura, Universidad de Sevilla, Avenida Reina Mercedes 2, 41012 Sevilla,

Spain. Tel.: þ34 954556625; fax: þ34 954557892.

E-mail address: gtenorio@us.es (R. Garcıa-Tenorio).

0269-7491/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.envpol.2005.10.030

The objective was to ensure that no unacceptable radiationdoses were received by the people either living in the affectedarea or farming the land (Iranzo et al., 1988). Since then,several radiological and radioecological studies have beenperformed in the affected area (i.e., Espinosa et al., 1999,2001).

In autumn 2001, our research group carried out a samplingcampaign by collecting superficial soils from several zones ofthe Palomares region. The first objective was to investigate thepresence of weapon-grade Pu remaining from the accident.The results obtained in the performance of this study formthe core of this paper. Some evidence supporting the existenceof this contamination was investigated by determining the241Am and 239 þ 240Pu inventories in the collected soilsamples, and by analysing their associated 238Pu/239 þ 240Puactivity ratios. However, in addition, the screening of someof the soil samples with higher 241Am levels was carried outby 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 characterisationof possible ‘‘hot particles’’ is also reported in this paper.

488 M.C. Jimenez-Ramos et al. / Environmental Pollution 142 (2006) 487e492

2. Materials and methods

2.1. Site characteristics and sampling

The village of Palomares is located in the south-east of Spain (37 � 15# N,

1 � 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 area of about 230 hectares (Iranzo

et al., 1987).

The landscape of this region can be considered as semi-arid, with very lit-

tle tree cover, scarce associated vegetation and temporal small streams that are

dry most of the year. The soil in the area has a silica-carbonaceous nature,

where SiO2, AlO2, CaO and (CO3)2� are its main components.

In order to perform this work, several superficial soil samples were col-

lected in October 2001 from a zone located to the west of Palomares village

(Fig. 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.

2.2. Determination of 239 þ 240Pu inventoriesand 238Pu/239 þ 240Pu activity ratios in soil

2.2.1. Alpha-particle spectrometry

In order to perform the determination of Pu-isotope alpha-emitters using

alpha-particle spectrometry, a radiochemical procedure was applied to each

soil sample to isolate and deposit these nuclides in thin layers. This radio-

chemical procedure is fully described in Vioque et al. (2002).

The detection system used for the measurements was an alpha spectrome-

ter 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 50e80% were

obtained.

2.3. Isolation, identification and characterisationof the ‘‘hot particles’’

2.3.1. 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.

The presence of Pu ‘‘hot particles’’ in one of the several aliquots corre-

sponding 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 divi-

sion. Therefore, through successive divisions of the sample and their measure-

ments 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 re-

verse electrode germanium (ReGe) detector, with a relative photo-peak effi-

ciency 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.

2.3.2. 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 adhe-

sive 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 micro-

scope Philips XL-30 with secondary and backscattering electron detectors was

used.

Specifically, for the identification of possible Pu ‘‘hot particles’’, the back-

scattered secondary electron image mode (BSE) was used due to the high

EL PEÑÓN HILL

A10

A7A8

A9

A6

A3 A2 A1

UNCULTIVATED AREA PALOMARESCHILDREN SCHOOL

CEMETERY

Sampling stations

Fig. 1. Palomares sampling site. In the bottom right-hand corner of the figure, the location of Palomares village is highlighted. The exact location of the sampling

stations are marked.

489M.C. Jimenez-Ramos et al. / Environmental Pollution 142 (2006) 487e492

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 topo-

graphy and size, while to obtain qualitative information about the composition

of its superficial layers, an energy dispersive X-ray spectrometer (EDX) inte-

grated with the electron microscopy system, was used for pointing X-ray

analyses.

3. Results and discussion

3.1. 239 þ 240Pu soil inventories and associated238Pu/239 þ 240Pu activity ratios

In Table 1 the 239 þ 240Pu inventories (Bq/m2) determined inthe superficial soil samples collected in the vicinity of the vil-lage of Palomares in October 2001 are presented. Samplingsites are indicated in Fig. 1. The majority of these sites accord-ing to the indications given in Iranzo et al. (1987), were selectedat the surroundings of the places where wreckage from theaircraft accident was found, and in the vicinity of the locationwhere one of the bombs exploded. At some of the samplingpoints, replicate determinations in different aliquots werecarried out so as to analyse the homogeneity in the Pucontamination.

By observing the results it is quite clear to conclude that themost contaminated Pu spots are those located nearest to thevillage of Palomares (A1, A2, A3). The 239 þ 240Pu levelsfound in these sampling points are at least two orders of mag-nitude higher than those which would have been expected atthe Palomares latitude if the only source of plutonium hadbeen the atmospheric fallout, which are in the range 30e50 Bq/m2 (Hardy et al., 1973; UNSCEAR, 1982). This givesa clear indication of the presence of some traces of the de-stroyed weapons more than 30 years after the accident andconfirms previous evidence based on 241Am determinationsby gamma-ray spectrometry in the same soil samples.

The levels of Pu soil contamination decrease when the dis-tance from the village increases. In fact, taking into accountthat only the uppermost 5 cm layer at each sampling pointhas been collected, the inventories in samples A8, A9 andA10 are compatible with the expected and previously givenvalues when the Pu origin is weapon test fallout.

Moreover, the fingerprint of the Palomares accident in thesoil samples with higher Pu inventories was additionally

Table 1239 þ 240Pu inventories (Bq/m2), determined by alpha-particle spectrometry,

in aliquots of the superficial soil samples collected in the Palomares region

Sampling point 239 þ 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

investigated by analysing 238Pu/239 þ 240Pu activity ratios.The obtained values, compiled in Table 2, are clearly compat-ible with the possible weapon-grade origin of the Pu found inthem, and are clearly lower than would be expected if theorigin of the Pu contamination were the weapon-test fallout andthe destruction in the stratosphere of the SNAP-9A satellite (inthis case, 238Pu/239 þ 240Pu activity ratio values in the range0.03e0.04 corrected to the date of our measurements, wouldbe expected (Hardy et al., 1973; Bunzl and Kracke, 1988)).This fact, together with the associated inventory levels, rein-forces the idea that a vast proportion of the Pu associatedwith the most contaminated samples originates from the Pudispersed in the Palomares accident.

On the other hand, the inhomogeneity of Pu concentrationsamong the aliquots of samples A2 and A3 made us considerthe presence of ‘‘hot particles’’. This fact was first confirmedby taking each one of the most contaminated soil samples(A1, A2 and A3), and by dividing them after homogenisationinto six aliquots for 241Am determination by Ge gamma-rayspectrometry. In spite of the homogenisation steps, quite dif-ferent concentration levels of 241Am were found in some casesin several of the aliquots corresponding to the same superficialsoil sample. However, definite evidence of the presence of‘‘hot particles’’ was obtained by performing 241Am measure-ments of successive divisions of a given aliquot from sampleA2 which presents high levels of this radionuclide. The finalresult of this exercise was that about 90% of the 241Am orig-inally present in the initial aliquot analysed (about 50 g) wasconfined in a mass amount lower than 1 mg, obviouslya ‘‘hot particle’’.

The unambiguous verification of the presence of a consider-able fraction of the Pu in the Palomares area as ‘‘hot particles’’is extremely important for the establishment of an appropriatestrategy for a proper and accurate estimation of the remainingPu inventory. Furthermore, knowledge about the structural andphysical characteristics of the Pu source-term in the area is es-sential to assess the short- and long-term consequences of thecontamination, since it can have a great influence on the mo-bility and biological uptake of the transuranic elements in theaffected ecosystem with subsequent implications for dose andenvironmental impact assessment (Salbu et al., 1998).

3.2. ‘‘Hot particles’’ in Palomares soils

The special characteristic of the transuranic contaminationsource in the case of the Palomares accident (the presence ofa considerable fraction of the contamination in particulate

Table 2238Pu/239 þ 240Pu activity ratios found in the aliquots of the Palomares soil

samples which show higher levels of contamination

Sample 239 þ 240Pu (Bq/m2) 238Pu/239 þ 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

490 M.C. Jimenez-Ramos et al. / Environmental Pollution 142 (2006) 487e492

form) has been confirmed through the identification and char-acterisation of some of these particles. To this end, after gam-ma-ray spectrometry screening, several techniques (SEM,XRMA) were applied, as previously stated, to a few milli-grams of soil samples where its presence was suspected. Asan example, Fig. 2a, which corresponds to a BSE image ob-tained by SEM (magnification of 674), shows the situationof one hot particle found in a soil sample which had been pre-viously confined to a few milligrams. This identification is car-ried out by observing the areas with strong detection signals(light areas) which indicate the presence of elements withhigh Z. The same hot particle, also identified by the applica-tion of the BSE mode, but with more magnification in the im-age, is shown in Fig. 2b (magnification of 1374). Similar ‘‘hotparticles’’ have been isolated and identified in other Palomaressoil aliquots.

Once the candidate ‘‘hot particle’’ was identified, informa-tion about the composition in its upper surface layer was ob-tained by the application of the micro X-ray spectrometrytechnique. The original objective was to confirm that the iden-tified particle was mainly formed by plutonium, and not byother elements with high atomic numbers which can producea similar image in the BSE mode.

In Fig. 3a, the X-ray spectrum corresponding to a point ofthe ‘‘hot particle’’ is shown. From this spectrum the presence

Fig. 2. (a) Back-scattered secondary electron (BSE) image of one isolated

hot particle from Palomares. (b) The same as in (a), but under higher

magnification.

of Pu and also of U as components of the ‘‘hot particle’’ can bededuced, since although the M peaks of these two elementscannot be resolved (3e4 keV), their characteristics major Lpeaks (well resolved between them) are observed in the 12e18 keV energy region. The peaks appearing at lower energiesthan those of Pu and U M lines correspond to lighter elements(Si, Al, .) normally present in background soil adhered to theparticle. Moreover, the comparison of the spectrum shown inFig. 3a with that shown in Fig. 3b from a background soil par-ticle from the same sample confirms the clear abundance of Puand U in the ‘‘hot particle’’.

The presence of uranium in the core of the destroyed Palo-mares weapons is not surprising, although the possibility thatthese weapons were of a plutonium type is infeasible. In fact,the same conclusion was obtained in the analysis of ‘‘hot par-ticles’’ produced in a similar accident, that of Thule (Greenland),which occurred in 1968. The measurements of single,isolated ‘‘hot particles’’ from Thule also show that the de-stroyed weapons contained, in addition to Pu, very significantquantities of U (Moring et al., 2001; Eriksson, 2002).

Unfortunately, in the analysed Palomares ‘‘hot particle’’ noprecise information about the element ratio (U/Pu) can be in-ferred from the X-ray spectra obtained. Although the L X-raylines of U and Pu are separated and well defined allowing theirunambiguous identification, in order to evaluate such a ratio itis necessary to consider that the cross-section for L-shell X-rayproduction is 1.37 times higher for U than for Pu and that theelectron L-shell binding energy is greater for Pu than for ura-nium (Eriksson, 2002). As a consequence, when the incidentelectrons lose energy in their passage through the particle,more electrons can interact with uranium than with plutoniumwhich results in a biased element ratio, very difficult to cor-rect. In other words, the emissions of U and Pu X-rays changewith depth into the particle in a manner which would result ina U/Pu X-ray ratio that is dependent on the thickness and evenon the coating. Nevertheless, it is clear from a semiquantitativeanalysis of the X-ray spectra that U and Pu are the predomi-nant elements in the ‘‘hot particle’’ analysed.

Additional information, in particular about the shape of theidentified ‘‘hot particle’’, was obtained by applying the scan-ning electron microscopy technique in conventional mode(secondary electron, SE, mode). One picture of the ‘‘hot par-ticle’’ obtained through the detection of secondary electronsis shown in Fig. 4. Several SE micrographs of this particlewere taken at various tilt angles (between 0 � and 50 �) toobtain the maximum information about its morphology. Theobjective was to estimate the volume of the particle.

Since the particle shape was irregular, the volume could notbe determined accurately, however our estimation was that ofabout 5000 mm3. In this sense, it is worth noting its dimen-sions, in accordance with previous results found in the Palo-mares area, which indicated the presence of a considerableproportion of the plutonium in fractions with large grainsize (Iranzo et al., 1991). This finding is potentially of veryhigh importance, since if a major fraction of the contamina-tion in the Palomares exits as particles with these dimensions,it considerably diminishes the potential doses due to

491M.C. Jimenez-Ramos et al. / Environmental Pollution 142 (2006) 487e492

Fig. 3. (a) X-ray spectrum obtained by impinging electrons onto a point of the surface of the hot particle identified in Fig. 2. To maximise 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.

inhalation which can be received by the population living orworking in the zone. The isolated particle has a dimensionthat prevents its resuspension by the prevailing winds in thearea.

Fig. 4. Secondary electron (SE) image corresponding to the hot particle

identified in Fig. 3. Tilt angle ¼ 0 �.

Finally, further information about the radioactive content ofthe previously identified and characterised particle (Figs. 2and 3) has been obtained through its direct measurement byalpha-particle spectrometry (thick target alpha-spectrometry).A content of approximately 60 Bq of 239 þ 240Pu in the‘‘hot particle’’ was inferred from this determination. However,the accuracy of this last figure should be taken with caution,since in order to maintain the integrity of the particle for thefuture application of additional microanalytical techniques,no radiochemical isolation and deposition into thin layers ofthe Pu isotopes present in the particle were applied.

4. Conclusions

The presence of remaining Pu contamination in a terrestrialzone affected by the well-known 1966 Palomares accident hasbeen confirmed through the determination of 239 þ 240Pu inven-tories in superficial soil samples and through the analysis oftheir associated 238Pu/239 þ 240Pu activity ratios. A considerablefraction of this contamination is present as ‘‘hot particles’’,some of which have been isolated, identified and characterised

492 M.C. Jimenez-Ramos et al. / Environmental Pollution 142 (2006) 487e492

in our lab. Through these ‘‘hot particle’’ studies it was foundthat not only did the weapons in Palomares contain significantamounts of plutonium, but they also contained uranium.

Acknowledgements

This work has been financed through the EU 5th Frame-work programme, project FIGE-CT2000-0108 (ADVANCE).The help of the staff from Electron Microscopy Central Ser-vice, University of Seville, was greatly appreciated. Specialthanks are also given to the members of our research groupwho collaborated in the soil sampling campaign.

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