L. León Vintró

Tracking of Airborne Radionuclides from the Damaged Fukushima Dai-Ichi Nuclear Reactors by European Networks

Radioactive emissions into the atmosphere from the damaged reactors of the Fukushima Dai-ichi nuclear power plant (NPP) started on March 12th, 2011. Among the various radionuclides released, iodine-131 ((131)I) and cesium isotopes ((137)Cs and (134)Cs) were transported across the Pacific toward the North American continent and reached Europe despite dispersion and washout along the route of the contaminated air masses. In Europe, the first signs of the releases were detected 7 days later while the first peak of activity level was observed between March 28th and March 30th. Time variations over a 20-day period and spatial variations across more than 150 sampling locations in Europe made it possible to characterize the contaminated air masses. After the Chernobyl accident, only a few measurements of the gaseous (131)I fraction were conducted compared to the number of measurements for the particulate fraction. Several studies had already pointed out the importance of the gaseous (131)I and the large underestimation of the total (131)I airborne activity level, and subsequent calculations of inhalation dose, if neglected. The measurements made across Europe following the releases from the Fukushima NPP reactors have provided a significant amount of new data on the ratio of the gaseous (131)I fraction to total (131)I, both on a spatial scale and its temporal variation. It can be pointed out that during the Fukushima event, the (134)Cs to (137)Cs ratio proved to be different from that observed after the Chernobyl accident. The data set provided in this paper is the most comprehensive survey of the main relevant airborne radionuclides from the Fukushima reactors, measured across Europe. A rough estimate of the total (131)I inventory that has passed over Europe during this period was <1% of the released amount. According to the measurements, airborne activity levels remain of no concern for public health in Europe.

Recent observations on the physico-chemical speciation of plutonium in the Irish Sea and the western Mediterranean

Abstract Data on the physico-chemical speciation of Pu in the Irish Sea and the western Mediterranean, gathered in the course of research expeditions carried out in the period 1988–1993, are reviewed in detail in this paper. Measurements of the oxidation state distribution of 239,240 Pu (and 238 Pu) in filtered water sampled throughout the Irish Sea show little variation, geographically or temporally, with some 87 ± 6% in the oxidized, Pu(V), state overall. No distinction is observed between surface and bottom waters, reflecting both the shallow and the well-mixed nature of these waters. Interestingly, the 241 Pu(IV)/ 239,240 Pu(IV) ratio in filtered water from the north-eastern Irish Sea, close to the Sellafield source-term, is found to be significantly higher than the corresponding 241 Pu(V)/ 239,240 Pu(V) ratio, while the latter appears to be identical to the 241 Pu/ 239,240 Pu ratio in suspended particulate from the same zone. It is suggested that this distinction is of importance in the interpretation of the mechanisms responsible for the hold-up and dispersion of Pu in the near field. The percentage of Pu in colloidal form in open waters, as operationally defined by enhanced sorption on Al 2 O 3 , seldom exceeds 15%. There is some evidence of higher percentages in near-shore waters containing proportionately more Pu in the reduced, Pu(IV), state. Chemical speciation and enhanced sorption analyses on samples of ultrafiltered water confirm that a significant proportion of the Pu(IV) is in a colloidal form and that the size of the colloidal particles or aggregates involved (

Perturbation in the 240Pu239Pu global fallout ratio in local sediments following the nuclear accidents at Thule (Greenland) and Palomares (Spain)

It is well established that the main source of the plutonium found in marine sediments throughout the Northern Hemisphere is global stratospheric fallout, characterized by a typical 240Pu/239Pu atom ratio of approximately 0.18. Measurement of perturbations in this ratio at various sites which had been subjected to close-in fallout, mainly from surface-based testing (e.g. Bikini Atoll, Nevada test site, Mururoa Atoll), has confirmed the feasibility of using this ratio to distinguish plutonium from different fallout sources. In the present study, the 240Pu/239Pu ratio has been examined in samples of sediment (and soil) collected at Thule (Greenland) and Palomares (Spain), where accidents involving the release and dispersion of plutonium from fractured nuclear weapons occurred in 1968 and 1966, respectively. The 240Pu/239Pu ratio was measured by high-resolution alpha spectrometry and spectral deconvolution, and confirmed in the case of the most active samples by high-resolution X-ray spectrometry. Only samples which displayed plutonium heterogeneities, i.e. hot particles or concentrations well in excess (at least two orders of magnitude) of those expected from global fallout, were selected for analysis. The analytical results showed that at Thule the mean 240Pu/239Pu atom ratio was 0.033 +/- 0.004 (n = 4), while at Palomares the equivalent ratio appeared to be significantly higher at 0.056 +/- 0.003 (n = 4). Both ratios are indicative of low burn-up plutonium and are consistent with those reported for weapons-grade plutonium. It is noteworthy that the mean 238Pu/239Pu activity ratio in the Thule samples, at 0.0150 +/- 0.0017 (n = 4), was also lower than that measured in the Palomares samples, namely, 0.0275 +/- 0.0012 (n = 4). The 241Pu/239Pu ratios were similarly different. Finally, the data show, in contrast to Palomares, that not all of the samples from the Thule accident site were contaminated with plutonium of identical isotopic composition.

Trends in plutonium, americium and radiocaesium accumulation and long-term bioavailability in the western Irish Sea mud basin

Abstract Time-series data on plutonium, americium and 137Cs concentrations in surficial sediments and seawater, gathered at selected locations in the western Irish Sea mud basin in the period 1988–97, are examined and modelled in an attempt to reconstruct the deposition history of Sellafield-sourced radionuclides in the basin and estimate representative mean availability times for these nuclides in this zone. Predictions using a semi-empirical modelling approach have been confirmed by the analysis of 210Pb-dated sediment cores retrieved from the basin, and interpreted in terms of the processes likely to control the transport of particle-reactive radionuclides from the northeastern Irish Sea to the western Irish Sea. The proposed interpretation has been supported by the analysis of transuranium concentrations and radionuclide ratios along two east–west transects from Sellafield, which has highlighted the importance of sediment remobilisation processes in the dispersion of particle-reactive radionuclides post-input. These processes have been incorporated in a low-resolution compartmental model of the Irish Sea and used to predict future 239,240Pu concentration trends in the western Irish Sea mud basin. Model predictions for 239,240Pu in the surface sediment and seawater compartments in this zone, validated by comparison with independent data sets, indicate that concentrations have already peaked and are beginning to decline slowly with mean availability times of ca. 100 years in both compartments. In the case of 137Cs, the decline is much more rapid, the corresponding availability times being at least an order of magnitude shorter.

Vertical and horizontal fluxes of plutonium and americium in the western mediterranean and the Strait of Gibraltar

New data on the vertical distributions of plutonium and americium in the waters of the western Mediterranean and the Strait of Gibraltar are examined in terms of the processes governing their delivery to, transport in and removal from the water column within the basin. Residence times for plutonium and americium in surface waters of approximately 15 and approximately 3 years, respectively, are deduced, and it is shown that by the mid 1990s only approximately 35% of the 239,240Pu and approximately 5% of the 241Am deposited as weapons fallout still resided in the water column. Present 239,240Pu inventories in the water column and the underlying sediments are estimated to be approximately 25 TBq and approximately 40 TBq, respectively, which reconcile well with the time-integrated fallout deposition in this zone, taken to be approximately 69 TBq. The data show that there are significant net outward fluxes of plutonium and americium from the basin through the Strait of Gibraltar at the present time. These appear to be compensated by net inward fluxes of similar magnitude through the Strait of Sicily. Thus, the time-integrated fallout deposition in the western basin can be accounted for satisfactorily in terms of present water column and sediment inventories. Enhanced scavenging on the continental shelves, as evidenced by the appreciably higher transuranic concentrations in shelf sediments, supports this contention.

Radionuclide impurities in proton-irradiated [18O]H2O for the production of 18F-: activities and distribution in the [18F]FDG synthesis process.

Proton- and neutron-induced activation products in the components of a high-pressure [(18)O]H(2)O target vessel used for the production of (18)F(-) in a medical cyclotron have been identified using high resolution gamma spectrometry. The activities leached from the target vessel into the [(18)O]H(2)O during irradiation, and the distribution of the identified radionuclide impurities in the various cartridges and solutions used in the [(18)F]FDG synthesis process have been measured and are discussed from the perspective of waste disposal. The results indicate that, at the energies and beam currents employed, only a few, relatively short-lived radionuclides are present in the irradiated [(18)O]H(2)O, and that the activities involved (<10 kBq in each case) are well below typical exemption limits. Activities of beta-emitting (3)H in irradiated [(18)O]H(2)O, produced via the (18)O(p,(3)H)(16)O reaction, have also been determined using liquid scintillation spectrometry. Measured activity concentrations, in the range 150-180 kBq g(-1), are consistent with those reported by other workers. Analyses of the synthesised [(18)F]FDG confirm the radiochemical purity of the product, both for (3)H and for gamma-emitting radionuclides in the energy range 25-1650 keV.

Oxidation-state distribution of plutonium in surface and subsurface waters at Thule, northwest Greenland

The speciation of plutonium in Arctic waters sampled on the northwest Greenland shelf in August 1997 is discussed in this paper. Specifically, we report the results of analyses carried out on seawater sampled (a) close to the Thule air base where, in 1968, a US military aircraft carrying four nuclear weapons crashed on sea ice, releasing kilogram quantities of plutonium to the snow pack and underlying seabed sediments, and (b) at a reference station (Upernavik) located approximately 400 km to the south. The data show that most of the plutonium in the dissolved phase at Thule is in the form of Pu(V, VI) (mean: 68+/-6%; n = 6), with little if any distinction apparent between surface and bottom waters. Further, the oxidation state distribution at stations close to the accident site is similar to that measured at Upernavik, remote from this site. It is also similar to the distribution observed in shelf waters at mid-latitudes, suggesting that the underlying processes controlling plutonium speciation are insensitive to temperature over the range 0-25 degrees C. Measurements using tangential-flow ultrafiltration indicate that virtually all of the plutonium (including the fraction in a reduced chemical form) is present as fully dissolved species. Most of this plutonium would seem to be of weapons fallout origin, as the mean 238Pu/239,240Pu activity ratio in the water column (dissolved phase) at Thule (0.06+/-0.02; n = 10) is similar to the global fallout ratio at this latitude (approximately 0.04). Thus, there is little evidence of weapons-grade plutonium in the water column at Thule at the present time.

Determination of the 240Pu/239Pu atom ratio in low activity environmental samples by alpha spectrometry and spectral deconvolution

Abstract Accurate measurement of the 240 Pu/ 239 Pu ratio in environmental samples is of importance as it may provide evidence for the definitive identification of a particular source-term. Usually, the measurement is performed by mass spectrometry, unless the activities involved are relatively high. In such cases, as the published literature shows, it is feasible to determine this ratio using high resolution alpha spectrometry and appropriate algorithms to deconvolute the partially resolved 239,240 Pu multiplet. In this paper, a simple technique, based on commercially-available software developed for gamma spectra analysis (MicroSAMPO®), is described by which this complex multiplet can be resolved at the much lower activities typical of many environmental samples. In our approach, it is not necessary to make any alterations to the normal alpha spectrometric set-up (including energy dispersion), other than to improve collimation. The instrumental function is defined for each spectrum by fitting a modified gaussian with exponential tails to the comparatively well-resolved 242 Pu “doublet” (used as tracer) and, if present, the 238 Pu “doublet”. The fitted peaks are used to create an energy calibration file with which, using published energy data, the positions (in channels) of the component peaks of the multiplet are predicted. These positions are not altered subsequently when MicroSAMPOs interactive multiplet analysis facility is used to quantify the relative spectral intensities of the components. Before calculating the 240 Pu/ 239 Pu ratio, it is advisable to correct for coincidence summing of alpha particles and conversion electrons. The technique has been applied to the determination of the 240 Pu/ 239 Pu ratio in a set of environmental samples, most of which were supplied by IAEA-MEL under their laboratory intercomparison programme. Subsequently, replicate samples were analysed independently using thermal ionisation mass spectrometry. The agreement between the two sets of data was most satisfactory. Further validation of this deconvolution technique was provided by the good agreement between the measured alpha-emission probabilities for the component peaks in the 239, 240 Pu multiplet and published values.

Actinide analysis of a depleted uranium penetrator from a 1999 target site in southern Serbia.

Following the detection of 236U in depleted uranium (DU) ammunition used during the Balkans conflict in the 1990s, concern has been expressed about the possibility that other nuclides from the nuclear fuel cycle and, in particular, transuranium nuclides, might be present in this type of ammunition. In this paper, we report the results of uranium and plutonium analyses carried out on a depleted uranium penetrator recovered from a target site in southern Serbia. Our data show the depleted nature of the uranium and confirm the presence of trace amounts of plutonium in the penetrator. The activity concentration of (239+240)PU, at 45.4+/-0.7 Bq kg(-1), is the highest reported to date for any penetrator recovered from the Balkans. This concentration, however, is comparable to that expected to be present naturally in uranium ores and, from a radiological perspective, would only give rise to a very small increase in dose to exposed persons compared to that from the DU itself.

210Pb-dating of a lake sediment core from Lough Carra (Co. Mayo, western Ireland): use of paleolimnological data for chronology validation below the 210Pb dating horizon

The chronologies and sediment accumulation rates for a lake sediment sequence from Lough Carra (Co. Mayo, western Ireland) were established by applying the constant initial concentration (CIC) and constant rate of supply (CRS) hypotheses to the measured (210)Pb(excess) profile. The resulting chronologies were validated using the artificial fallout radionuclides (137)Cs and (241)Am, which provide independent chronostratigraphic markers for the second half of the 20th century. The validity of extrapolating the derived CIC and CRS dates below the (210)Pb dating horizon using average sedimentation rates was investigated using supplementary paleolimnological information and historical data. Our data confirm that such an extrapolation is well justified at sites characterised by relatively stable sedimentation conditions.