Anthropogenic Actinides in the Environment. Stephan Winkler

Transcription

1 Anthropogenic Actinides in the Environment Stephan Winkler

2 Outline Anthropogenic Sources of Actinides Methods of Measurement Applications Plutonium isotopes in a Lake Erie Sediment profile 236 U from global fall-out in a Carribean Coral Core

3 Reactor Weapon

4 Ivy Mike Diamond, H., P. R. Fields, et al. (1960). "Heavy Isotope Abundances in Mike Thermonuclear Device." Physical Review 119(6): 2000.

5 Yield (Mt) 200 Atmospheric Testing USSR USA year

6 Plutonium P.W. Krey, in Transuranium Nuclides in the Environment 240 Pu/ 239 Pu ± Pu/ 239 Pu ± (June 2006) 242 Pu/ 239 Pu ± ~2000kg of 239 Pu from the stratospheric fall-out have been dispersed worldwide

7 Production of 236 U (n,3n) Main Production channel in thermonuclear devices is (n,3n) of 14MeV neutrons on 238 U. (Sakaguchi et al) Main Production in Reactors via (n,γ) of thermal neutrons on 235 U

8 236 U from global fall-out 236 U/ 239 Pu = 0.24±0.02 (typ.: at 236 U/m 2 ) Sakaguchi et al., 2008

9 236 U Inventory Natural: from 235 U(n, ) 236 U; nucleogenic or cosmogenic; 236 U/U between and ~30 kg exist in top 1000 m of earth crust Anthropogenic: 10 6 kg were produced in nuclear reactors 10 3 kg were produced and globally dispersed in atmospheric testing => estimated oceanic 236 U/U between (dispersed in top 200m) and (fully dispersed) second most abundant anthropogenic radio-nuclide difficult to measure

10 Techniques of Measurement Decay counting Thermal Ionisation Mass Spetrometry (TIMS) Inductively Coupled Plasma MS (ICPMS) Accelerator Mass Spectrometry (AMS)

11 -Spectroscopy 239(40) Pu 242 Pu 238 Pu Mikroprecipitation with NdF 3 onto membrane filter Elektroplating 238 Pu / 239(40) Pu

12 For emitter 241 Pu Liquid szintillation

13 TIMS (thermal ionization mass spectrometry) Re filament Triton TIMS Pictures provided by S. Richter, IRMM, Geel, Begium.

14 ICP-MS IsoProbe (GV Instruments)

15 AMS

16 The Vienna Environmental Research Accelerator (VERA)

17 Detection efficiency and limits 239 Pu AMS atoms/count Detection limit (many laboratories) 10 4 X 10 4 ICP-MS Ketterer Taylor et al (for 10% uncertainty) TIMS Beasley et al ~ ( for precise ratio measurement )

18 Abundance sensitivity for 236 U/U AMS true samples quoted ANTARES Hotchkis et al CAMS Buchholz et al VERA Winkler et al ANU Wilcken at al ICP-MS REIMEP Buchholz et al TIMS Richter et al

19 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 235 UH from 236 U 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)

20 Applications

21 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) Relatively large sample sizes needed (3 to 500L)

22 Soil erosion and sediment transport S.E. Everett, S.G. Tims, G.J. Hancock, R. Bartley, L.K. Fifield, Comparison of Pu and 137 Cs as tracers of soil and sediment transport in a terrestrial environment J. Environ. Radioact. 99 (2008) X. Zhang and D.E. Walling. Characterizing Land Surface Erosion from Cesium-137 Profiles in Lake and Reservoir Sediments. J. Environ. Qual. 34 (2005)

23 239 Pu v 137 Cs Both bind readily to soil and sediment particles, but 239 Pu today is 14 times more abundant. An AMS measurement of 239 Pu from 3g processed sample takes 15 minutes (max)

24 239 Pu vs. 137 Cs Both bind readily to soil and sediment particles, but 239 Pu today is 14 times more abundant. An AMS measurement of 239 Pu from 3g processed sample takes 15 minutes (max) The same analysis done with 137 Cs requires 100g samples and 2 days of counting!

25 Nuclear Safeguards and Forensics

26 Characterizing Uranium Ores Non-anthropogenic 239 Pu and 236 U, 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 236 U/ 238 U in pristine U-rich waters. Chemical evolution of U in rocks and soils

27 Plutonium Isotopes in a Lake Erie Sediment Core and Vermillion River Range Soils

28 244 Pu as indicator for global fallout? Reactor Weapon

29 Lake Erie Sediment Core 15

30 Chronology of Core 15

31 Vermillion River Range Soil The was soil collected from pockets on an exposed granite surface N, 92.5 W 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 Bq per processed sample. The sediment core had only ~24mBq for a 3g sample of the fall-out peak

32 The ANU 14UD Tandem Accelerator

33 Results Sample Approx. Year Activity (mbq/g) 240 Pu/ 239 Pu 241 Pu/ 239 Pu Pu/ 239 Pu Pu counts 239 Pu Counts (1/10 of time for 244 Pu) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Pu/ 239 Pu (8.7±1.0) 10-5 (1.86±0.26) 10-4

34 Results Sample 240 Pu/ 239 Pu 244 Pu/ 239 Pu 244 Pu counts (ICP-MS) (AMS) Vermillion (9.24 ± 0.55) Vermillion 2 (7.59 ± 0.47) Vermillion (7.42 ± 0.45)

35 Mixing: 241 Pu/ 239 Pu vs 240 Pu/ 239 Pu 3.0E-03 "Ivy Mike" 2.5E-03 post-moratorium 2.0E-03 Lake Erie 241 Pu/ 239 Pu 1.5E E E-04 pre-moratorium fission device global average (P.W. Krey) nw testing Vermillion Range 0.0E Pu/ 239 Pu

36 Mixing: 242 Pu/ 239 Pu vs 240 Pu/ 239 Pu 2.0E-02 "Ivy Mike" 242 Pu/ 239 Pu 1.5E E E-03 pre-moratorium fission device global average (P.W. Krey) post-moratorium Lake Erie nw testing Vermillion Range 0.0E Pu/ 239 Pu

37 Mixing: 244 Pu/ 239 Pu vs 240 Pu/ 239 Pu 1.2E E-03 "Ivy Mike" 8.0E-04 Lake Erie 244 Pu/ 239 Pu 6.0E E-04 pre-moratorium post- moratorium nw testing Vermillion Range 2.0E-04 fission device 0.0E Pu/ 239 Pu

38 244 Pu as indicator for global fallout? 244 Pu is a good indicator and even allows us to distinguish USSR and USA test series, but

39 244 Pu as indicator for global fallout? 1.E+00 1.E-01 A Pu/ 239 Pu isotopic ratio 1.E-02 1.E-03 1.E-04 1.E-05 1.E-06 1.E-07 Eye guide global fallout Salzburg soil Sellafield sea sediment "Ivy Mike" bomb test Sellafield upper limit 1.E isotope mass number A P. Steier, et al

40 244 Pu and 241 Pu Both can be used to distinguish global fall-out and a mixture of more recent sources 241 Pu is easier accessible to a number of techniques and is sufficient if details of the fall-out don t matter 244 Pu provides a time marker within the fallout pulse, potentially of interest to earth science studies

41 Measurement of 236 U/ 238 U in corals as a proxy for anthropogenic and pre-anthropogenic 236 U in ocean waters

42 236 U as oceanic tracer U behaves conservatively in ocean water under oxic conditions the residence time being ~0.5My. Unlike 129 I or 14 C, atmosphere-ocean exchange plays no role. There are bound to be locally distinct 236 U/U signatures due to rainfall patterns and highly localized releases from reactor accidents and reprocessing plants (e.g. Sellafield)

43 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

44 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 anthropogenic 236 U

45 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 anthropogenic 236 U very likely this is no longer possible from actual seawater today

46 Sample Collection Montastraea faveolata Colony forming species The Reef Environment

47 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)

48 Coral Stratigraphy

49

50 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%H 2 O 2 + 1% NaOH to hydrolyse residual organics HNO 3 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 coprecipitated with Fe(OH) 3 followed by a standard UTEVA column extraction method.

51 2.0E-09 HMF-1 1.5E U/ 238 U 1.0E E E year

52 2.0E-09 HMF-1 1.5E U/ 238 U 1.0E E E year

53 Pre-anthropogenic 236 U Samples Samples covering years 1943 to 1905 were measured so far (20-70µg U each) The lowest measured 236 U/U ratio is 3.3± , which is still far above expected levels However, this was just a first test and significantly more material is available to attack this limit.

54 Pre-nuclear age samples 1.0E U/ 238 U 1.0E-11 HMF-1 1.0E Year

55 Detection limit for small samples 1.0E E U/ 238 U 1.0E E E E-13 HMF-1 nuclear age HMF-1 pre-nuclear Vienna-KkU dilution Procedural Blank 1.0E E E E E E E E U 5+ current

56 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 % 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

57 Limit on pre-nuclear 236 U/U So far we can limit (90% C.L.) pre-nuclear 236 U/U in the oceans as < 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

58 236 U Conclusions The 236 U bomb-pulse seems to be wellpreserved 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 236 U an ocean tracer that may have wider application

59 Summary The tracing of anthropogenic actinides is a growing field with new application being added as techniques improve The potential of minor actinides and 236 U is being unlocked only now In both areas advanced AMS systems like VERA are virtually without serious competition with regard to detection limit

60 Acknowledgments Scientific Collaborators Peter Steier Keith Fifield Steve Tims Jessica Carilli Mike Ketterer

61 Acknowledgments University of Vienna Australian National University Northern Arizona University Scripps Institute of Oceanography Australian Nuclear Science and Technology organisation

62 Thank You for Your Attention! AMS-12, Wellington, NZ, 2011