Curium and the Transactinides

Transcription

1 Curium and the Transactinides Dr Clint Sharrad Centre for Radiochemistry Research School of Chemical Engineering and Analytical Science Research Centre for Radwaste and Decommissioning Dalton Nuclear Institute The University of Manchester

2

3 Marie Curie No involvement in the discovery of curium or the transactinides.

4 Who first discovered Cm? Glenn T. Albert Ralph A. Seaborg Ghiorso James Nobel prize for Chemistry 1951 Discovered 10 elements Discovered 12 elements Expert in developing radiation detection instrumentation

5 G. T. Seaborg, R. A. James and A. Ghiorso, National Nuclear Energy Series, 1949, 14B,

6 Was anyone else involved in the discovery of curium??? Stanley G. Thompson Submitted Ph.D. thesis entitled Nuclear and Chemical Properties of Americium and Curium in 1948

7 Why Curium? G. T. Seaborg, R. A. James and A. Ghiorso, National Nuclear Energy Series, 1949, 14B,

8 Why Curium? Vasili Samarsky- Bykhovets Johan Gadolin Lanthanides Actinides Transactinides Marie & Pierre Curie Albert Einstein Enrico Fermi Dmitri Mendelev Alfred Noble Ernest Lawrence Cn Copernicium (285) Ernest Rutherford Glenn T. Seaborg Niels Bohr Lise Meitner Wilhelm Roentgen Nicolaus Copernicus

9 1850 Timeline 1859 Pierre Curie born 1867 Maria Skłodowska born 1891 Maria Skłodowskamoves to Paris to study chemistry at the Sarbonne 1895 Maria Skłodowska marries Pierre Curie 1898 Curie s publish discovery of Po and Ra 1903 Curie s awarded Nobel prize for physics (with Becquerel) 1906 Death of Pierre Curie 1911 Marie Curie awarded Nobel prize for chemistry 1912 Glenn Seaborg born 1915 Albert Ghiorso born Death of Marie Curie; Seaborg awarded B.Sc Seaborg awarded PhD; Ghiorso awarded B.Sc Start of WWII 1940 Discovery of plutonium (Seaborg et al.) 1942 Manhattan project established 1944 Discovery of curium and americium 1949 Discovery of berkelium; 1950 Discovery of californium 1951 Seaborg awarded Nobel prize for chemistry (with McMillan)

10 Any link between Marie Curie and Seaborg et al.?

11 How was Cm first made? 60 inch cyclotron at Berkeley 239 Pu + He Cm + n 239 Pu + He Cm + 3n Pu(NO 3 ) 4 solutions allowed to evaporate onto a grooved Pt plate. Mild ignition formed PuO 2. Predicted redox properties of Cm were exploited to separate from Pu. Proof of the presence of Cm by analysing α particle energies. Also made by neutron irradiation of Am samples.

12 Where is Cm found?

13 Curium facts Curium isotopes from 238 Cm to 251 Cm. Most common isotopes are 244 Cm, 246 Cm and 248 Cm. Typically formed by neutron capture. Most Cm isotopes have a higher specific activity than 239 Pu. Predominantly α emitters. Chemical properties are similar to the lanthanides.

14 D. L. Clark, The Chemical Complexities of Plutonium, Los Alamos Science, 2000, 26. Redox properties Curium vs Light actinides

15 Curium redox properties Cm(III) oxidation state is very stable. - as predicted by Seaborg. - due to half-filled (5f 7 ) configuration. Redox potentials for the Cm(IV/III) couple are not known but oxidation to Cm 4+ only occurs with the strongest oxidising agents and conditions. Curium spectroscopy Solutions of Cm(III) are normally colourless but concentrated solutions can have a green appearance. Weak f-f transitions observed. Strong flourescence at ~600 nm after appropriate excitation. - Used to probe Cm solution speciation. W. T. Carnall, P. R. Fields, D. C. Stewart and T. K. Keenan, J. Inorg. Nucl.Chem., 1958, 6, 213.

16 Curium separations Most common methods for actinide separations are by ionexchange or solvent extraction processes. How was Cm separated from Puwhen it was first made? 1) Bombarded PuO 2 dissolved in H 2 SO 4 and heated to dryness. 2) Residue dissolved in HNO 3 with any remaining insoluble oxide dissolved by heating with a small amount of HF. 3) Pu oxidised to Pu(VI) in HNO 3 (or Cr 2 O 2-7 ). 4) Addition of fluoride precipitates insoluble CmF 3 (and LnF 3 present as fission products) while the Pu remains soluble. 5) The fluoride precipitate redissolved and procedure repeated until all Pu removed. Higher FP concentration in Cm fraction accepted, as the αactivity from Cm could still be examined.

17 Methods for the extraction of U and Pu from spent nuclear fuel have been established (e.g. PUREX). Separation usually achieved by exploiting the different chemical properties of light actinides vs FPs. Why separate curium? Research interests. Waste management timeframes for storage/disposal. Separate other nuclides: - Americium used in smoke detectors. -Lanthanides potential worldwide shortages.

18 How is curium separated from lanthanides? With great difficulty!! General process: - transfer of a charged metal complex (or ion) from a polar aqueous phase to an immiscible phase (different solvation properties). Dictated by: - phase transfer properties of the species present. - the relative affinity of the counterphasefor the species to be separated. For curium/lanthanide separations need to exploit the subtle differences in ionic radii/covalency/polarisability. Main difference between ion-exchange and solvent extraction methods: - Solvation of a hydrophobic complex in solvent extraction. -Resin acts as a second aqueous phase in ion exchange.

19 Example separation processes Most processes attempt to separate both Am and Cm from lanthanides. TRAMEX Tertiary Amine Extraction n-octyl and n-decyl tertiary amines in diethylbenzene Various mixtures used in all 3 extraction steps. Separation of Cm from Am achieved by exploiting accessible higher oxidation states of Am. Developed at Oak Ridge National Lab (1961). Pilot plant purified ~1.5 kg of 244 Cm. W. E. Prout, H. E. Henry, H. P. Holcomb, W. J. Jenkins, DP-1302, 1972.

20 TALSPEAK Trivalent Actinide-Lanthanide Separation by Phosporous Reagent Extraction from Aqueous Complexes O O O O HO OH N N N DTPA OH OH OHO O HO OH Lactic acid B. Weaver and F. Kappelmann, F., ORNL- 3559, HDEHP DTPA is known as a hold back reagent as the complexes it forms with Cm and Am stay in the aqueous phase. The lanthanides are extracted into the organic phase.

21 Where to from here for curium/lanthanide separations? Better understanding of the chemistry that underpins separations processes is required. Molecular speciation, binding affinities, mass transfer kinetics, role of phase transfer catalysts, solubilities, ph dependency, ionic strength etc. TALSPEAK O O HO OH O OH N N N O OH OHO O OH What is the role of lactate? Buffer, Complexant or both? HO Using soft donor ligands for preferential binding of trivalent actinides over lanthanides exploits the slightly greater covalency exhibited in actinide binding vs lanthanides. continuing development of novel extractants.

22 Uses for Curium Space batteries in satellites or crewless space probes Cm produces 3 W/g of heat energy. Used to characterise lunar soil.

23 Transactinides Cn Copernicium (285) Most of the transactinides can be formed using 248 Cm. Obtained by hot fusion reactions with 18 O, 19 F, 22 Ne, 26 Mg, 34 S and 48 Ca projectiles. Need particle accelerators that provide heavy ion beam currents of ~ particles per second. Particle accelerator at Dubna laboratories.

24 Properties of the Transactinides All transactinide isotopes are radioactive. Half-lives less than 3 min; typically between 30 s and 0.5 ms. Some isotopes can only be formed a single atom at a time. Providing proof of existence is extremely difficult and has, at times, been controversial. Single atom chemistry Initial characterisation usually by measuring radioactive decay. - many transactinidesconfirmed by detecting αemission to known α-decaying daughters and granddaughters. Single atom experiments need to be repeated many times to get statistically valid results. Development of chemical procedures with fast process times and reproducible (usually automated) methods. Gas phase - thermochromatographic separations. - Aqueous chemistry rapid HPLC and liquid-liquid extractions.

25 Why?

26 We must not forget that when radium was discovered no one knew that it would prove useful in hospitals. The work was one of pure science. And this is a proof that scientific work must not be considered from the point of view of the direct usefulness of it. It must be done for itself, for the beauty of science, and then there is always the chance that a scientific discovery may become, like radium, a benefit for humanity.