Characterization of U/Pu particles originating from the nuclear weapon accidents at Palomares, Spain, 1966 and Thule, Greenland, 1968

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Abstract

Following the USAF B-52 bomber accidents at Palomares, Spain in 1966 and at Thule, Greenland in 1968, radioactive particles containing uranium (U) and plutonium (Pu) were dispersed into the environment. To improve long-term environmental impact assessments for the contaminated ecosystems, particles from the two sites have been isolated and characterized with respect to properties influencing particle weathering rates. Low 239Pu/235U (0.62–0.78) and 240Pu/239Pu (0.055–0.061) atom ratios in individual particles from both sites obtained by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) show that the particles contain highly enriched U and weapon-grade Pu. Furthermore, results from electron microscopy with Energy Dispersive X-ray analysis (EDX) and synchrotron radiation (SR) based micrometer-scale X-ray fluorescence (μ-XRF) 2D mapping demonstrated that U and Pu coexist throughout the 1–50 μm sized particles, while surface heterogeneities were observed in EDX line scans. SR-based micrometer-scale X-ray Absorption Near Edge Structure Spectroscopy (μ-XANES) showed that the particles consisted of an oxide mixture of U (predominately UO2 with the presence of U3O8) and Pu ((III)/(IV), (IV)/(V) or (III), (IV) and (V)). Neither metallic U or Pu nor uranyl or Pu(VI) could be observed. Characteristics such as elemental distributions, morphology and oxidation states are remarkably similar for the Palomares and Thule particles, reflecting that they originate from similar source and release scenarios. Thus, these particle characteristics are more dependent on the original material from which the particles are derived (source) and the formation of particles (release scenario) than the environmental conditions to which the particles have been exposed since the late 1960s.

Introduction

On the morning of the 17th of January 1966, a US Air Force B-52 bomber carrying four 1.5 Mt thermonuclear bombs collided with a KC-135 fuel tanker aircraft during a refuelling operation over Southern Spain. Both aircrafts caught fire at an altitude of 8500 m above the village of Palomares and subsequently exploded (Iranzo et al., 1987). The nuclear weapons were released from the B-52 bomber and two of the bombs detonated conventionally upon impact on land. The explosion and subsequent fire (nuclear fuel partially burned) caused the dispersion of particles containing Pu and U (Lind et al., 2004, Jiménez-Ramos et al., 2006) over a terrestrial area of about 2.3 km2 (Iranzo et al., 1987) situated close to the Mediterranean. A similar accident occurred at Thule, Greenland, 1968, where a B-52 bomber carrying four thermonuclear bombs caught fire and crashed on the ice at Bylot Sound. Due to explosive fire, particles containing Pu and U (Eriksson, 2002, Lind et al., 2005) were dispersed over the ice within a distance of a few kilometres and some of the fissile material has been located within the sediments of Bylot Sound (Aarkrog, 1971).

To assess the environmental impact of radioactive particles released from weapon-grade material and deposited in ecosystems, detailed information is required on particle characteristics such as size, composition, morphological structure and oxidation states of matrix elements such as U and Pu, influencing particle weathering rates and subsequent mobilization of radionuclides from particles present in soil–water and sediment–water systems (Salbu, 2000b). Previous studies of radioactive particles released from different nuclear sources under different release conditions, such as reactor accidents involving explosions or fires, have demonstrated that the particle characteristics are source and release scenario dependent (Salbu, 2000a). A key property influencing the radioecological behaviour of U and Pu is their oxidation state (Silva and Nitsche, 2001, Choppin, 2003). Uranium chemistry predicts that particle weathering rate (y 1) increases with the oxidation state for U and should be higher for U3O8 particles than for UO2 (Kashparov et al., 1999). Although experimental solubility data reported for Pu oxides/hydroxides scatter widely (Runde et al., 2002), the solubility products of Pu(OH)3 [K°sp = 10 26.2 ± 0.8, (Felmy et al., 1989)], tetravalent Pu [Pu(OH)4, K°sp = 10 58.7 ± 0.9 (Knopp et al., 1999); PuO2, K°sp = 10 63.8 ± 1 (Kim and Kanellakopulos, 1989)] and pentavalent PuO2OH [K°sp = 10 9.3, (Zaitseva et al., 1968)], indicate that particle weathering rates will strongly depend on the oxidation state of Pu. Although information on a limited number of particles cannot be considered to be representative for the whole contaminated area, the information on solid-state speciation including oxidation states will be essential in environmental impact assessments. Once U and Pu are in a solution, their overall mobility is governed by the environmental conditions (e.g., soil–water pH and concentration of interacting agents, redox conditions and microbial activity).

From the fact that these particles have persisted in the environment for decades it is obvious that the weathering rates have been rather slow. On the other hand, recent reports (García-Tenorio et al., 2004, Espinosa et al., 2005) suggest that Pu in Palomares soils is becoming more mobilised and potentially bioavailable as a result of natural weathering and agricultural practices (e.g. irrigation and fertilization). Pu extracted from contaminated Palomares soils (4 g) by distilled H2O in 24 h increased by two orders of magnitude from 1986 to 2001 and up to 14% of Pu in soil (4 g) was extracted by cow urine in 24 h (Espinosa et al., 2005). To understand and estimate the impact of such particle transformation processes, information on the solid-state speciation of the particle matrix elements is needed. The Palomares and Thule accidents provide a unique possibility to study long-term actinide behaviour in the environment including radioactive particle weathering. The source terms and release scenarios are similar, whereas the affected ecosystems and environmental compartments (i.e. temperate, semi-arid terrestrial vs. arctic, marine benthic) in which the particles have resided are very different.

In the present work, individual radioactive particles extracted from contaminated soil and sediment samples collected at Palomares and Thule (Lind et al., 2005) have been characterized with respect to size, 2D elemental composition and distribution, morphology and oxidation states as well as isotopic composition. Non-destructive analyses were performed using γ-spectrometry, scanning electron microscopy (SEM) and Environmental Scanning Electron Microscope (ESEM) in combination with Energy Dispersive X-ray analysis (SEM–EDX and ESEM–EDX) as well as synchrotron radiation (SR) based microscopic techniques. The SR-based microscopic techniques were especially developed for characterization of particles containing U (Salbu et al., 2003, Lind et al., 2005). SR-based micro-X-ray fluorescence (μ-SRXRF) provides information on the elemental composition of the particles, while SR-based micro-X-ray absorption near edge structure (μ-XANES) spectroscopy provides information on oxidation states of U and Pu. Finally, ICP-MS analyses of dissolved individual particles reveal the isotopic composition of the actinides. Although the number of analysed particles is limited, the study on individual particles should provide detailed information on the phenomenon of accidentally released radioactive particles in the environment per se.

Section snippets

Palomares

Two 10 cm soil cores were collected in 2001 from a contaminated area, located to the west of the Palomares village (Fig. 1). Each sample was air dried and divided into 6 aliquots to be measured by gamma spectrometry. Elevated activity concentrations of 241Am (Eγ = 59.5 keV) indicating the presence of 241Pu were found in one aliquot from each of the two cores. From these aliquots a few soil grains were isolated from the matrix by performing successive divisions and measuring the subsamples by

γ-spectrometry and ICP-MS

Based on γ-measurements of individual particles the 241Am activities ranged from 0.2–13.5 Bq/particle for Palomares (n = 5) and 1.0–2.7 Bq/particle for Thule (n = 3) particles (Table 1). ICP-MS data showed the 239 + 240Pu activity and 240Pu/239Pu atom ratio of particle PB to be 42.6 ± 5.9 Bq (1 SD) and 0.061 ± 0.006, respectively. This atom ratio is in agreement with Chamizo et al. (2006), using Accelerator Mass Spectrometry to determine a mean 240Pu/239Pu atom ratio of 0.0657 ± 0.0006 in soils containing

Conclusions

The present work demonstrates that particles originating from the Palomares and Thule accidents contain a mixture of enriched U and weapon-grade Pu material. The 239Pu/235U ratio is a factor of 1.3 higher for the measured Palomares particle than for the particle from Thule. SEM–EDX mapping and SR-based μ-XRF mapping indicated that U and Pu were homogeneously distributed in the particles. However, results from ESEM–EDX line scan analyses with ∼ 1 μm resolution revealed that U and Pu are

Acknowledgements

We gratefully acknowledge the support provided by the European Commission (contracts no. IHP-Contract HPRI-CT-1999–00040/2001–00140 and FIGE-CT-2000–00108), IAEA (CRP, “Radiochemical, Chemical and Physical Characterization of Radioactive Particles in the Environment”) and the Norwegian Research Council (project no. 141479/720). The authors are indebted to Gerald Falkenberg (HASYLAB BL) for beamline assistance with the μ-XANES measurements, Melissa A. Denecke (Forschungszentrum Karlsruhe) for

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