anthropogenic actinides in the enviroment · 2011-05-19 · anthropogenic actinides in the...
TRANSCRIPT
Outline
• Anthropogenic Sources of Actinides
• Methods of Measurement
• Applications
• Plutonium isotopes in a Lake Erie Sediment profile
• 236U from global fall-out in a Carribean Coral Core
Reactor
Weapon
Diamond, H., P. R. Fields, et al. (1960). "Heavy Isotope Abundances in Mike Thermonuclear Device." Physical Review 119(6): 2000.
Ivy Mike
0
20
40
60
80
100
120
140
160
180
200
1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963
Yie
ld (
Mt)
year
Atmospheric Testing
USSR
USA
Plutonium
P.W. Krey, in “Transuranium Nuclides
in the Environment”
240Pu/239Pu 0.1760±0.0140
241Pu/239Pu 0.0016±0.0003 (June 2006)
242Pu/239Pu 0.0044±0.0011
~2000kg of 239Pu from the stratospheric
fall-out have been dispersed world-
wide
Production of 236U
(n,3n)
Main Production channel in thermonuclear devices is (n,3n) of 14MeV neutrons on 238U. (Sakaguchi et al)
Main Production in Reactors via (n,γ) of thermal neutrons on 235U
236U from global fall-out
236U/239Pu = 0.24±0.02
(typ.: 1012...1013 at236U/m2)
Sakaguchi et al., 2008
236U Inventory Natural:
• from 235U(n,)236U; nucleogenic or cosmogenic;
• 236U/U between 10-14 and 10-10
• ~30 kg exist in top 1000 m of earth crust
Anthropogenic:
• 106 kg were produced in nuclear reactors
• 103 kg were produced and globally dispersed in atmospheric testing => estimated oceanic 236U/U between 3·10-9 (dispersed in top 200m) and 2·10-10 (fully dispersed)
• second most abundant anthropogenic radio-nuclide
• difficult to measure
Techniques of Measurement
• Decay counting
• Thermal Ionisation Mass Spetrometry (TIMS)
• Inductively Coupled Plasma – MS (ICPMS)
• Accelerator Mass Spectrometry (AMS)
-Spectroscopy
•Mikroprecipitation with NdF3 onto membrane filter
•Elektroplating
•238Pu / 239(40)Pu
242Pu
239(40)Pu
238Pu
Liquid szintillation
• For emitter 241Pu
TIMS (thermal ionization mass spectrometry)
Re filament Triton TIMS
Pictures provided by S.
Richter, IRMM, Geel,
Begium.
ICP-MS
http://www.gvinstruments.co.uk/
IsoProbe (GV Instruments)
AMS
The Vienna Environmental Research Accelerator (VERA)
Detection efficiency and limits 239Pu
atoms/count Detection limit
AMS
(many laboratories) 104 X×104
ICP-MS
Ketterer 2008 103
Taylor et al. 2001 107 (for 10% uncertainty)
TIMS
Beasley et al. 1998 20 ~3 × 104 (“for precise ratio measurement”)
Abundance sensitivity for 236U/U true samples quoted
AMS
ANTARES Hotchkis et al. 2000 7×10-9 1×10-8
CAMS Buchholz et al. 2007 4×10-9 1×10-9
VERA Winkler et al. 2011 3×10-12 1×10-13
ANU Wilcken at al. 2008 1×10-12 1×10-13
ICP-MS
REIMEP-18 3 ×10-8
Buchholz et al. 2007 1.5 ×10-8 – 2×10-7 1.5×10-8
TIMS
Richter et al. 1999 2×10-10
Why AMS is more sensitive than MS
•Simplest conventional MS:
• Produce mono-energetic ions (fixed energy over charge, E/q)
• Separate masses with magnetic sector field (selects momentum over charge, p/q)
m/q is fixed
• High energy improves beam emittance
better ion optics
• Problems with molecular ions:
• conventional MS can hardly separate 235UH from 236U
destroy molecules in AMS during stripping
• Problems with "tails": no ion source is really mono-energetic, no separator is perfect
• Tails originate partly from interaction with residual gas
AMS increases energy to several MeV, this reduces the relevant cross sections.
• add more, redundant separators to cut off tails (also in conventional MS, e.g. WARP filter)
Applications
Ocean transport
C.-K. Kim, C.-S. Kim, B.U. Chang, S.W. Choi, C.S. Chung, G.H. Hong, K. Hirose, Y. Igarashi,
Plutonium isotopes in seas around the Korean Peninsula, Sci. Total Environ. 318 (2004)
197–209.
Relatively large sample sizes needed (3 to 500L)
S.E. Everett, S.G. Tims, G.J. Hancock, R.
Bartley, L.K. Fifield,
Comparison of Pu and 137Cs as tracers of soil
and sediment transport in a terrestrial
environment
J. Environ. Radioact. 99 (2008) 383–393.
X. Zhang and D.E. Walling. Characterizing
Land Surface Erosion from Cesium-137
Profiles in Lake and Reservoir Sediments.
J. Environ. Qual. 34 (2005) 514-523
Soil erosion and sediment transport
• Both bind readily to soil and sediment particles, but 239Pu today is 14 times more abundant.
• An AMS measurement of 239Pu from 3g processed sample takes 15 minutes (max)
239Pu v 137Cs
• Both bind readily to soil and sediment particles, but 239Pu today is 14 times more abundant.
• An AMS measurement of 239Pu from 3g processed sample takes 15 minutes (max)
• The same analysis done with 137Cs requires 100g samples and 2 days of counting!
239Pu vs. 137Cs
Nuclear Safeguards and Forensics
Characterizing Uranium Ores
• Non-anthropogenic 239Pu and 236U, although low-levels. Production via neutron capture from s.f. and (alpha,n) on light elements. Can be used to fingerprint ore bodies. (e.g. K. Wilcken, et al, 2008)
• A potential exploration tool: measurement of 236U/238U in pristine U-rich waters.
• Chemical evolution of U in rocks and soils
Plutonium Isotopes in a Lake Erie Sediment Core and Vermillion River
Range Soils
244Pu as indicator for global fallout?
Reactor
Weapon
Lake Erie Sediment Core 15
Chronology of Core 15
48.2° N, 92.5° W
Vermillion River Range Soil
The was soil collected from pockets on an exposed granite surface.
These pockets served as traps for run-off from large areas of the granite surface, thereby concentrating atmospherically deposited radionuclides over a wide area into the pockets.
While there is no stratigraphy, this offered high Pu content - 5-10Bq per processed sample. The sediment core had only ~24mBq for a 3g sample of the fall-out peak
The ANU 14UD Tandem Accelerator
Results Sample Approx.
Year
Activity
(mBq/g)
240Pu/239Pu 241Pu/239Pu
·10-3
242Pu/239Pu
·10-3
244Pu
counts
239Pu Counts
(1/10 of time
for 244Pu)
244Pu/239Pu
1 2004
(8.7±1.0)·10-5
8 1997 7.21 ± 3.66
23 1983 1.57 ± 0.50 3.99 ± 0.83
37 1972 3.48 ± 0.70 0 1158
41 1968 3.67±0.02 0.183±0.005 5.38 ± 0.72 3 2790
42 1967 4.59±0.07 0.180±0.002 4.06 ± 0.44 1 3211
43 1966 5.53±0.06 0.184±0.008 4.32 ± 0.35 2 12914
44 1965 6.60±0.08 0.189±0.006 0.89 ± 0.27 4.27 ± 0.37 11 22106
45 1964 8.50±0.28 0.182±0.006 2.58 ± 0.41 3.90 ± 0.27 38 31820
46 1963 6.17±0.02 0.158±0.002 3.50 ± 0.42 15 10400
47 1962 5.31±0.06 0.138±0.004 1.39 ± 0.35 2.52 ± 0.53 10 7448
48 1961 4.43±0.06 0.140±0.004 3.19 ± 0.69 0 934
(1.86±0.26)·10-4
49 1960 4.77±0.10 0.130±0.002 1.09 ± 0.23 2.46 ± 0.87 11 6695
50 1959 4.34±0.10 0.160±0.001 5.52 ± 0.62 1 2461
51 1958 1.18 ± 0.14 6.15 ± 0.83 40 17836
52 1957 0.39±0.03 0.154±0.035 0.77 ± 0.34 3.95 ± 0.82
53 1956 0.60 ± 0.60 1.80 ± 0.90
Results
Sample 240Pu/239Pu (ICP-MS)
244Pu/239Pu (AMS)
244Pu counts
Vermillion 1 0.188 (9.24 ± 0.55)·10−5 920
Vermillion 2 (7.59 ± 0.47)·10−5 746
Vermillion 3 0.194 (7.42 ± 0.45)·10−5 802
Mixing: 241Pu/239Pu vs 240Pu/239Pu
pre-moratorium
post-moratorium
0.0E+00
5.0E-04
1.0E-03
1.5E-03
2.0E-03
2.5E-03
3.0E-03
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
24
1P
u/2
39P
u
240Pu/239Pu
Lake Erie
nw testing
VermillionRangefission device
"Ivy Mike"
global average (P.W. Krey)
Mixing: 242Pu/239Pu vs 240Pu/239Pu
pre-moratorium
post-moratorium
0.0E+00
5.0E-03
1.0E-02
1.5E-02
2.0E-02
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
24
2P
u/2
39P
u
240Pu/239Pu
Lake Erie
nw testing
VermillionRange
fission device
"Ivy Mike"
global average (P.W. Krey)
Mixing: 244Pu/239Pu vs 240Pu/239Pu
pre-moratorium post- moratorium
0.0E+00
2.0E-04
4.0E-04
6.0E-04
8.0E-04
1.0E-03
1.2E-03
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
24
4P
u/2
39P
u
240Pu/239Pu
Lake Erie
nw testing
VermillionRange
"Ivy Mike"
fission device
244Pu as indicator for global fallout?
• 244Pu is a good indicator and even allows us to distinguish USSR and USA test series, but…
244Pu as indicator for global fallout?
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
239 240 241 242 243 244 245
isotope mass number A
AP
u/2
39P
u is
oto
pic
ra
tio
Eye guide global fallout
Salzburg soil
Sellafield sea sediment
"Ivy Mike" bomb test
Sellafield upper limit
P. Steier, et al
244Pu and 241Pu
• Both can be used to distinguish global fall-out and a mixture of more recent sources
• 241Pu is easier accessible to a number of techniques and is sufficient if details of the fall-out don’t matter
• 244Pu provides a time marker within the fall-out pulse, potentially of interest to earth science studies
Measurement of 236U/238U in corals as a proxy for anthropogenic and pre-anthropogenic 236U in ocean
waters
236U as oceanic tracer
• U behaves conservatively in ocean water under oxic conditions – the residence time being ~0.5My.
• Unlike 129I or 14C, atmosphere-ocean exchange plays no role.
• There are bound to be locally distinct 236U/U signatures due to rainfall patterns and highly localized releases from reactor accidents and reprocessing plants (e.g. Sellafield)
U in Corals
• Corals build uranium into their aragonite skeletons (~3ppm)
• This uranium represents the uranium in seawater at the time of growth
• Some corals display well-defined yearly banding
U in Corals
• Corals build uranium into their aragonite skeletons (~3ppm)
• This uranium represents the uranium in seawater at the time of growth
• Some corals display well-defined yearly banding
Determine the year-by-year 236U bomb-pulse in ocean waters
Get hold of U from seawater not yet affected by anthro-pogenic 236U
U in Corals
• Corals build uranium into their aragonite skeletons (~3ppm)
• This uranium represents the uranium in seawater at the time of growth
• Some corals display well-defined yearly banding
Determine the year-by-year 236U bomb-pulse in ocean waters
Get hold of U from seawater not yet affected by anthro-pogenic 236U – very likely this is no longer possible from actual seawater today
Montastraea faveolata Colony forming species
Sample Collection
The Reef Environment
Sampling Location • Coral Core HMF-1 was collected in January 2007 at the Turneffe Atoll
(17°18'25" N, 87°48'04" W) • The depth was 19ft and the total length of the core is 112cm • It has been previously used as reference for a study on contaminant
trace elements on cores from other sites in the Carribean.
Jessica E. Carilli, et al. Marine Pollution Bulletin 58 (2009) 1835–1842
Coral Stratigraphy
Cleaning and Sample Preparation
• Removal of discoloured pieces and contamination from sawing
• 2x ultra-sonic washes in deionised bi-distilled water • For the pre-nuclear samples:
Ultrasonic wash with 30%H2O2 + 1% NaOH to hydrolyse residual organics
HNO3 clean in ultrasonic (loss of ~5% of the coral skeleton This was also done for some of nuclear-age samples, to
test for adsorbed hot particles (no evidence for that)
After cleaning corals are dissolved in HCl and co-precipitated with Fe(OH)3 followed by a standard UTEVA column extraction method.
0.0E+00
5.0E-10
1.0E-09
1.5E-09
2.0E-09
1940 1950 1960 1970 1980 1990 2000 2010
23
6U
/23
8U
year
HMF-1
0.0E+00
5.0E-10
1.0E-09
1.5E-09
2.0E-09
1940 1950 1960 1970 1980 1990 2000 2010
23
6U
/23
8U
year
HMF-1
Pre-anthropogenic 236U Samples
• Samples covering years 1943 to 1905 were measured so far (20-70µg U each)
• The lowest measured 236U/U ratio is 3.3±0.6·10-12, which is still far above expected levels
However, this was just a first test and significantly more material is available to attack this limit.
Pre-nuclear age samples
1.0E-12
1.0E-11
1.0E-10
1.0E-09
1900 1910 1920 1930 1940 1950
23
6U
/23
8U
Year
HMF-1
Detection limit for small samples
1.0E-14
1.0E-13
1.0E-12
1.0E-11
1.0E-10
1.0E-09
1.0E-08
1.0E-07
1.0E-13 1.0E-12 1.0E-11 1.0E-10 1.0E-09 1.0E-08 1.0E-07
23
6U
/23
8U
238U5+ current
HMF-1 nuclear age
HMF-1 pre-nuclear
Vienna-KkU dilution
Procedural Blank
The Fe Carrier Problem
• Currently at least half of the background can be attributed to the Fe carrier
• Different Fe carriers were tried (Spectrosol, Goodfellow 99.99+% Fe)
• Cleaning with UTEVA improved results but there still is some U at the 3-10ppm levels left
• First test with pre-nuclear iron lacks statistics at this point for first conclusions
Limit on pre-nuclear 236U/U
• So far we can limit (90% C.L.) pre-nuclear 236U/U in the oceans as <4·10-12
• This we expected for certain beforehand
• However, from the experience in this project so far there is a good chance we improve on this limit over the next few months
236U Conclusions
• The 236U bomb-pulse seems to be well-preserved in corals.
• The response time seems to be at most 1 year - which is shorter than generally assumed
• The expected northern-southern hemisphere difference, rainfall dependent fall-out patterns, and accidental releases will make 236U an ocean tracer that may have wider application
Summary
• The tracing of anthropogenic actinides is a growing field with new application being added as techniques improve
• The potential of minor actinides and 236U is being unlocked only now
• In both areas advanced AMS systems like VERA are virtually without serious competition with regard to detection limit
Acknowledgments
Scientific Collaborators
Peter Steier
Keith Fifield
Steve Tims
Jessica Carilli
Mike Ketterer
Acknowledgments
University of Vienna
Australian National University
Northern Arizona University
Scripps Institute of Oceanography
Australian Nuclear Science and Technology organisation
