P.I. Mitchell

Preliminary estimates of cumulative caesium and plutonium deposition in the Irish terrestrial environment

A number of soil and peat cores, sampled throughout Ireland from November 1987 to March 1988, have been analyzed for fallout caesium and plutonium. Specifically,134Cs and137Cs levels were measured by high resolution gamma spectrometry while238Pu and239,240Pu were determined after chemical extraction by alpha spectrometry. The inventories of these nuclides at the locations sampled have been calculated from the resulting profiles and, by extrapolation, preliminary estimates made of the cumulative deposition throughout Ireland. The relative contribution from weapons testing in the atmosphere and the Chernobyl accident to the total caesium inventory have been determined by the134Cs/137Cs isotopic ratio. As expected, no plutonium of Chernobyl origin was detected in surface layers. The cumulative238Pu/239,240Pu ratio measured in Irish soils was found to be consistant with mid-latitude ratios reported by others.

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.

Variations in the concentration of plutonium, strontium-90 and total alpha-emitters in human teeth collected within the British Isles.

Concentrations of plutonium-239, plutonium-240, strontium-90 and total alpha-emitters have been measured in childrens teeth collected throughout Great Britain and Ireland. The concentrations of plutonium and strontium-90 were measured in batched samples, each containing approximately 50 teeth, using low-background radiochemical methods. The concentrations of total alpha-emitters were determined in single teeth using alpha-sensitive plastic track detectors. The results showed that the average concentrations of total alpha-emitters and strontium-90 were approximately one to three orders of magnitude greater than the equivalent concentrations of plutonium-239,240. Regression analyses indicated that the concentrations of plutonium, but not strontium-90 or total alpha-emitters, decreased with increasing distance from the Sellafield nuclear fuel reprocessing plant-suggesting that this plant is a source of plutonium contamination in the wider population of the British Isles. Nevertheless, the measured absolute concentrations of plutonium (mean = 5 +/- 4 mBq kg-1 ash wt.) were so low that they are considered to present an insignificant radiological hazard.

Vertical distribution of radiocaesium, plutonium and americium in the Catalan Sea (northwestern Mediterranean)

Caesium-137, 239,240Pu and 241Am concentration profiles (0–1000m) have been determined in unfiltered large volume water samples collected from the Catalan Sea (northwestern Mediterranean). Results showed that radiocaesium concentration decreases quickly through the water column while the transuranic concentration increases with depth, showing a faster migration to the bottom layers. Comparing our results with those reported by other authors (1975–1980), radiocaesium input from Chernobyl releases has been identified through the profile. In addition, transuranic concentrations have decreased considerably in the different layers of the profile. Integrated activities through the water column were 2.6 ± 0.2kBq m−2 for 137Cs, 23±3Bq m−2 for 239,240Pu and 1.5 ± 0.5Bq m−2 for 241 Am. According to the estimated inventory and the integrated fall-out in the Mediterranean area, about 56% of 137Cs, 27% of 239,240Pu and 4% of 241 Am which was deposited over the Mediterranean Sea by generalized fallout remains at present in the first 1000m of the water column. Vertical distribution of 239,240Pu137Cs and 241Am137Cs activity ratios showed a clear separation between transuranics and radiocaesium through the water profile. 241Am239, 240Pu activity ratio revealed faster sinking of americium relative to plutonium.

Studies of the vertical distribution of134Cs,137Cs,238Pu,239, 240Pu,241Pu,241Am and210Pb in ombrogenous mires at mid-latitudes

Six peat cores taken from three unmodified blanket and raised bogs in Ireland were sectioned and analyzed for a range of radionuclides including134Cs,137Cs,238Pu,239Pu,241Am, and210Pb.134Cs and137Cs were measured by high resolution gamma-spectrometry, while the transuranium nuclides were determined after chemical separation by alpha-spectrometry.241Pu, present on the electroplated discs together with Pu(α), was measured directly by low-level liquid scintillation counting. Core chronologies were established by measuring the unsupported210Pb component using low energy photon spectrometry (LEPS). From the resulting profiles, relaxation depths and migration rates for the above mentioned radionuclides have been determined and differences in the values of these parameters interpreted.238Pu/239,240Pu and241Pu/239,240Pu ratios have been examined carefully and are discussed in some detail. Finally, the contribution from Chernobyl to the total radiocaesium inventory in each core has been established using the134Cs/137Cs ratio observed in the initial fallout from Chernobyl.

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.