Journal of Radioanalytical and Nuclear Chemistry, Vol. 248, No. 3 (2001) 623–627
Uranium and plutonium containing particles in a sea sediment sample
from Thule, Greenland
M. Moring,1* T. K. Ikäheimonen,1 R. Pöllänen,1 E. Ilus,1 S. Klemola,1 J. Juhanoja,2 M. Eriksson3
1
STUK – Radiation and Nuclear Safety Authority, P.O. Box 14, FIN-00881 Helsinki, Finland
2
Electron Microscopy, P.O. Box 56, FIN-00014 University of Helsinki, Finland
3
Risø National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark
(Received December 13, 2000)
Particles composed of radioactive materials and probably originating from US nuclear weapons were identified in sea sediment samples collected
from Thule, Greenland, in 1997. The weapons were destroyed close to the Thule Air Base in 1968 in an aeroplane crash, which dispersed
radioactive materials in the environment. The presence of particulate radioactive materials in the sediment samples was revealed by combining
gamma-spectrometry and autoradiography. Isolation and separation of a radioactive particle from a bulk sample were performed using
autoradiography, phosphor plate imaging and scanning electron microscopy. Using X-ray microanalysis as well as alpha and beta activity analysis,
U and weapons-grade Pu were detected in the granular, brittle particle.
Introduction
In January 1968, a B52 bomber from the US
Strategic Air Command caught fire and crashed on the
sea ice on Bylot Sound, about 12 km off the Thule Air
Base in Northwest Greenland. The plane carried four
nuclear weapons and part of the plutonium was dispersed
over a few square kilometres of the ice in an explosive
fire. During the following months an extensive cleaning
campaign was organized in the area and the remains of
the wreck plus large amounts of contaminated ice were
shipped to the US. The underlying sea sediments (water
depth about 185 m) received a fraction of the weapons
material, especially when the ice melted in the next
summer.
On the basis of previous surveys in 1968, 1970,
1974, 1979, 1984 and 1991 it has been estimated that the
amount of remaining radioactive material in the seabed
of Bylot Sound is approximately 1.4 TBq (~0.5 kg)
239+240Pu, 0.025 TBq 238Pu, 4.6 TBq 241Pu and
0.07 TBq 241Am.1–7 At the site of the accident the
amount of 239+240Pu in the sediments was
59000 Bq.m–2.8 The latest expedition was launched to
the site in 1997. In total about 1000 samples from
bottom sediments, seawater and biota were taken during
the cruise.
In the present study, the presence of radioactive
particles in the sediment samples is shown. One
radioactive particle was successfully isolated from a bulk
sample. Different analysis techniques were used to
characterise the properties of the particle.
Experimental
Sampling
The sampling was carried out on board a Greenland
fisheries investigation vessel. Most sediment samples
were taken with a Finnish ‘Gemini Twin Corer’9
delivering two parallel cores, 8 cm in diameter each.
Immediately after the sampling, the cores were extruded
from the coring tubes and sliced into 1-cm thick slices
with the precision sectioning apparatus of the Gemini
Corer. In most cases, the parallel slices from the two
cores were combined into one sample. Before analysis
the sediment samples were freeze-dried and
homogenized.
The particle considered in this paper was separated
from a sediment core taken from the site of the accident.
The sediment at this location is not ideal for sampling,
because it consists of relatively hard clay mixed with
sand and stones. In addition, benthic animals cause
significant bioturbation of the sediment layers.
Consequently, no clear vertical stratification of the
radionuclides in the uppermost 0–16 cm sediment layers
was detected.
Particle separation
Radioactive particles were identified in several slices
of the sediment core. The particle considered here was in
the uppermost 0–1 cm slice. The slice was repeatedly
* E-mail: [email protected]
0236–5731/2001/USD 17.00
© 2001 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht
M. MORING et al.: URANIUM AND PLUTONIUM CONTAINING PARTICLES IN A SEA SEDIMENT SAMPLE
Fig. 1. Detail (~10×10 cm2) of the X-ray film used in autoradiography of the 1-g sediment subsample.
The black dots show the presence of low energy beta- or alpha-active particles. The exposure time was 21 d
divided into smaller subsamples and further divisions
were performed on the subsample that contained the
largest amount of 241Am. Finally, when the mass of
the subsample was approximately 1 g, it was spread
thinly between two sheets of Mylar and an X-ray
sensitive film (Kodak BioMax MR) was placed on top.
This package was put into a lightproof container with
photomultiplicative inner surfaces. The location of the
active particles was determined from the spots on the
film (Fig. 1). The radioactive particle analyzed in the
present paper together with a number of background
particles was extracted from the subsample onto
adhesive tape. Further separation of the active particle
was performed by transferring parts of the sample from
one adhesive tape to another, or by slicing the sample
(tape) with a surgeons knife under a microscope.
To determine the location of the particle on the tape,
X-ray sensitive photographic plates were used. These
plates are commonly used in dental X-ray imaging. The
advantages of this method are short exposure time (even
by a factor of 100 shorter compared to X-ray film) and
no need for chemical development. These imaging plates
are useful only for small-area samples because their
sensitive area is 3×4 cm2. Several consecutive separation
and measurement procedures were needed in order to get
a sample that included few enough particles to be further
analyzed with a scanning electron microscope.
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Gamma-ray spectrometry
Low-background, high-resolution, n-type HPGe
detectors of relative efficiency 20–28% were used in the
gamma-ray analysis of the sediment samples. The
samples were measured in plastic beakers of 21 mm in
diameter. The height of the 0–1 cm slice (dried sediment
sample) was 10 mm and the density 0.7 g.cm–3. Gamma99 software10 was used in the analyses. Semiempirical
methods and the computer code DECCA11 were used in
efficiency calibrations.
Scanning electron microscopy (SEM)
and X-ray microanalysis
The sample on the adhesive tape was fixed on an
aluminum specimen stub. The specimen was coated with
carbon (thickness about 20 nm) and examined in SEM
(Zeiss DSM 962). When searching for the Pu/Uparticles, backscattered electrons (BSE) were used
because of the high contrast between elements with high
and low atomic number. Secondary electrons were used
in topographic imaging.
A thin-windowed Link Isis energy dispersive
spectrometer (EDS) was used for pointing X-ray
analyses and X-ray maps. As the Pu and U M-lines are
overlapping, the L-lines were used. To maximize the
ionization of U and Pu L-lines an acceleration voltage of
30 kV was applied.
M. MORING et al.: URANIUM AND PLUTONIUM CONTAINING PARTICLES IN A SEA SEDIMENT SAMPLE
Alpha- and beta-spectrometry
An alpha-spectrometric measurement of the
undigested particle was performed with a Canberra Quad
system with a PIPS alpha-detector. The sample on the
adhesive tape was placed in an alpha-chamber at a
distance of 1 mm from the detector. The results were
calculated manually.
The particle was digested with wet-ashing using
strong hydrochloric and nitric acids at 300 °C. Before
ashing the 242Pu and 243Am tracers were added. The
fractions of U, Pu and Am were separated from the
solution with the extraction chromatographic resins
UTEVA and TRU (Eichrom, Inc. USA).12 The Pu
fraction was divided into three aliquots. The first Pu
aliquot and the Am fraction were electrodeposited onto
platinum (Pu) and stainless steel (Am) discs and
measured with alpha-spectrometry. A Canberra Alpha
Analyst system with low background PIPS detectors and
the Genie 2000-PC programme were used in the
analyses.
The second Pu aliquot was used for liquid
scintillation counting of 241Pu beta-activity. A low
background liquid scintillation counter (Wallac,
Quantulus 1220) was used in the analysis. The counter
has an anticoincidence detector and a pulse-shape
analyser that allows discrimination between alpha- and
beta-pulses. The sample was counted twice, first for
beta-efficiency determination and secondly for
alpha/beta measurement.13 The U fraction and the third
Pu aliquot were saved for future analyses.
Results
The 241Am activity of the sediment slice as
determined with gamma-spectrometry was 1.6 Bq.
In the beginning of the manual separation process,
the activity of the separated particle was 0.4 Bq, which is
a quarter of the total 241Am activity of the slice. In a
later gamma-ray analysis, the particle activity was only
0.19 Bq, indicating that the particle was fragmented
during the separation process.
In the backscattering mode of SEM fragments of a
particle containing large amounts of Pu and U were
detected among background particles composed of
lighter elements (Fig. 2). In elemental analysis mostly Pu
and U were found in the fragments (Fig. 3) although
other elements, such as Na, Al, Si, Cl and Fe were also
present. The Pu/U mass ratio was measured at several
points of the particle and ranged from 0.1 to 0.2. The
variation of the mass ratio indicated that the distribution
of U and Pu within the particle was slightly
inhomogeneous. However, no clear distribution pattern
was visible in the element maps.
Table 1. Summary of the analysis results of the fragmented particle (Fig. 2). The activities with uncertainties
at 1σ confidence level refer to 31 March 2000
Method of analysis
241
Am activity
239+240
Pu activity
238
Pu
241
Pu
235
activity
activity
U activity
235
U mass
Total mass
Pu/U ratio
Bq
Bq
Bq
Bq
Bq
Bq
Bq
ng
ng
%
Gamma-spectrometry
Radiochemistry + alpha-spectrometry
Alpha-spectrometry of the entire particle
Radiochemistry + alpha-spectrometry
Radiochemistry + alpha-spectrometry
Radiochemistry + beta-spectrometry
Gamma-spectrometry
Gamma-spectrometry
SEM size estimation
SEM
Result
0.19
0.12
1.1
1.41
0.034
1.6
<0.0025
<30
<20
10–20
Uncertainty
0.01
0.005
0.9–1.5*
0.03
0.002
0.2
Counting time
3.7 d
21 h
5h
21 h
21 h
6.6+10 h**
3.7 d
3.7 d
* Lower and upper limits.
**6.6 h beta-efficiency determination and 10 h alpha/beta measurement.
Fig. 2. A backscattered electron mode (BSE) picture of the fragmented particle. In the BSE mode of SEM heavy elements, such as U and Pu,
appear as light areas (left). The magnification to the right show the granular structure of the particle (secondary electron mode)
625
M. MORING et al.: URANIUM AND PLUTONIUM CONTAINING PARTICLES IN A SEA SEDIMENT SAMPLE
Fig. 3. An X-ray spectrum taken from the fragmented particle. In the spectrum the major L peaks of U and Pu are clearly resolved (11–18 keV
region) whereas the M peaks (3–4 keV) cannot be resolved. The Na, Si and Cl found in the analysis is probably due to normal sea salt and
sediment. Also the Al and Fe may be of natural origin
Fig. 4. Alpha-spectra measured from the three-dimensional particle (thick line) and massless samples (Pu and Am fractions). The values are
corrected with the chemical yield. 242Pu and 243Am where used as tracers. The measurements are not made with the same apparatus
The fragments were photographed also in a 60° tilt
angle allowing a size estimation of the original particle.
If the fragments consisted of solid UO2 with 10% of
239PuO , their total weight would be less than 20 ng and
2
their activity of 239Pu would be less than 4 Bq. This can
be regarded as an overestimate because of the granular
structure of the fragments and because lighter elements
were present in the particle. No 235U was detected in the
gamma-ray analysis. The detection limit of 235U was
0.0025 Bq using a low-background Canberra BEGe
detector (BE5030). This corresponds to 31 ng of pure
235U, which is approximately two times larger than the
mass of the particle (Table 1).
Nuclide identification from the alpha-spectrum of an
entire (three-dimensional) particle is not always possible,
because of the self-absorption of alpha-particles in the
radioactive particle itself.14 In the present case, however,
626
clear edges at the 241Am and 239+240Pu energies are
visible because of the small thickness of the particle
fragments (Fig. 4). The ratio of the count rates at these
edges can be used to estimate the 239+240Pu activity from
the 241Am activity determined by gamma-ray analysis. In
addition, the total count rate from the particle can be
used to determine a lower limit for the total alphaactivity. The estimated 239+240Pu activity of the
fragmented particle was 1.1 Bq (Table 1). The
(over)estimated Pu activity of 4 Bq from the SEM
analyses and the measured 1.1 Bq for the same particle
are in reasonable agreement when the granular structure
of the particle and the uncertainties in the size and Pu/U
ratio estimations are taken into account.
The combined 239+240Pu activity of the particle as
determined by radiochemical means was 1.4 Bq and the
241Am activity was 0.12 Bq. The Pu results are in good
M. MORING et al.: URANIUM AND PLUTONIUM CONTAINING PARTICLES IN A SEA SEDIMENT SAMPLE
agreement with the alpha-spectrometry measurement of
the entire particle. However, the activity of 241Am is
somewhat lower compared to the value obtained in
gamma-ray analyses (0.19 Bq, Table 1). This may
indicate that 241Am dissolved incompletely in the sample
treatment.14
The 241Pu beta-activity of the particle was 1.6 Bq.
Corrected to the time of the accident, i.e., January 1968,
the 241Pu/(239+240)Pu activity ratio was 5.3±0.7. The
241Am/241Pu activity ratio in March 2000 was 0.12±
0.02. From these results the time since 241Am was
extracted is estimated to be 32.6±3 y and the time for the
extraction is then August 1967. From Pu discs originally
analysed in 1968 and re-analysed in 1981 (241Am
ingrowth)
AARKROG
et
al.4
estimated
the
241Pu/(239+240)Pu ratio to be 3.3±0.4 in January 1968.
They estimated the time of 241Am extraction to be
February 1960 (±4 y).4 The difference in the time
estimates might partly be explained by the differences of
the solubility of Pu and Am in saltwater conditions, as
the time between the samplings was 16 years.
Discussion and conclusions
We have demonstrated the use of particle analysis
methods not commonly performed in analytical
laboratories. Analysis of individual particles is expensive
compared to bulk analysis because a major part of the
work has to be done manually. When there is a good
possibility of finding radioactive particles in a sample,
this extra cost is often justified by the amount of
additional information achievable. The most profound
benefits usually lie in the possibility to study the particle
structure and composition, including non-radioactive
elements. This may enable estimation of the properties
of the material from which the particles originated and
conclusions on the formation and history of the particles.
In the present case, we demonstrated the presence of
particles containing both U and Pu in the debris from the
Thule plane crash. Furthermore, by eliminating natural
background U and Ra we got a relevant detection limit
for 235U for a single micron-sized particle with gammaspectrometry. This would not have been achieved
without separation of the particle from the bulk sample.
In the analysed particle, the 238Pu/239+240Pu and
241Pu/239+240Pu ratios are low, as can be expected for
weapons grade Pu.
Future efforts in analysing radioactive particles from
the Thule Air Base would clearly benefit from the use of
other measurement methods such as ICPMS or SIMS.
They would give us the isotopic composition of U and,
perhaps, help us resolve the isotopes 239Pu and 240Pu.
Bulk analysis of the Thule sediment sample by mass
spectrometry could not yield correct results, because the
natural U content in the sediment would distort them.
Especially in the work of non-proliferation and
monitoring of compliance with the Comprehensive Test
Ban Treaty (CTBT), the ability to determine the origin
of radioactive particles would be of great importance. In
the CTBTO regime, particle analyses would probably be
required to trace and demonstrate a clandestine nuclear
weapons test that did not work properly (a fizzle).
*
The Thule – 1997 sampling expedition was financed by Dancea
(Danish Co-operation for the Environment in the Arctic). The Captain
and the crew of the Greenland Fisheries Investigation Vessel “Adolf
Jensen” are acknowledged for their generous help during the
expedition. We are also grateful to Dr. Henning DAHLGAARD and
Dr. Tom RYAN for their co-operation in the sampling.
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