ActinideLos Alamos National Research Laboratory Quarterly

3rd/4th quarter 2003

Highlights of the Plutonium Futures— The Science Conference 2003

N u c l e a r M a t e r i a l s R e s e a r c h a n d T e c h n o l o g y Actinide Research Quarterly

Actinide Research Quarterly is produced by Los Alamos National Laboratory

NMT Division Director In This Issue Steve Yarbro Scientific Advisors, 1 Plutonium Futures—The Science Conference 2003 Seaborg Institute David L. Clark Gordon D. Jarvinen 4 Plutonium: Whether boon or risk, both viewpoints must be respected Editor Meredith S. Coonley, IM-1 6 A plutonium potpourri Designer Susan L. Carlson, IM-1 9 Plutonium-238 metal as a multiphase system Contributing Writers Vin LoPresti, IM-1 12 Nanoscopic approaches Octavio Ramos, Jr., IM-1 Charmian Schaller, IM-1 to aquatic plutonium chemistry Contributing Photographer 17 Comparison of aqueous phase binding constants Mick Greenbank, NMT-3 with tetra- and hexavalent actinides Printing Coordination Lupe Archuleta, IM-4 20 Researchers use high-resolution inelastic x-ray scattering and a microbeam approach

23 Investigations of protactinium and americium metals provide important insights Address correspondence to Actinide Research Quarterly c/o Meredith Coonley 29 Researching the phytoremediation of plutonium Mail Stop E500 Los Alamos 35 Plenary speakers discuss National Laboratory Los Alamos, NM 87545 the many avenues of research

If you have questions, 38 Point/Counterpoint comments, suggestions, or contributions, please contact the ARQ staff at 39 Five views on opportunities and [email protected]. constraints for actinide research Phone (505) 667-0392 Fax (505) 665-7895 44 The conference in a nutshell

Actinide Research Quarterly highlights recent achievements and ongoing programs of the Nuclear Materials Technology (NMT) Division. We welcome your suggestions and contributions. ARQ can be read on the World Wide Web at: http://www.lanl.gov/orgs/nmt/nmtdo/AQarchive/AQhome/AQhome.html

LALP-03-067

Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

Plutonium Futures— The Science Conference 2003

ore than 1,000 tons of plutonium exist throughout the world in the form of used nuclear fuel, nuclear weapons M components, various nuclear inventories, legacy materials and wastes. It is clear that large inventories of plutonium must be pru- dently managed for many decades. A complex blend of global political, socioeconomic and technological challenges must be overcome to manage these inventories efficiently and safely. From a technological perspective, plutonium is one of the most complex elements in the Periodic Table. The metal exhib- its six solid allotropes at ambient pressure and its phases are Conference participants notoriously unstable with temperature, pressure, chemical received a specially additions and time. Plutonium sits near the middle of the designed coin (shown actinide series, which marks the emergence of 5f electrons at far right) encased with in the valence shell. Elements to the left of plutonium have a sample of Trinitite, a delocalized (bonding) electrons, while elements to the right greenish, glasslike of plutonium exhibit more localized (non-bonding) character. substance found only Plutonium is poised in the middle, and for the delta-phase at ground zero at Trinity metal, the electrons seem to be in a unique state of being neither Site in southern New fully bonding or localized, which leads to novel electronic interactions Mexico. The man-made and unusual physical and chemical behavior. The concept of localized mineral and was formed on July 16, 1945, when or delocalized 5f electrons also pervades the bonding descriptions of the desert sand was many of the plutonium molecules and compounds. An understanding melted by the intense of the electronic structure of the pure element and its compounds heat of the world’s first continues to challenge both theorists and experimentalists in all areas nuclear explosion and of plutonium science. then solidified. The coin The “Plutonium was designed by Los Futures—The Science Alamos' Jay Tracy. Conference”was es- tablished to increase awareness of the im- portance of pluto- nium research and facilitate communica- tion among its inter- national practitioners.

NuclearNuclear MaterialsMaterials Technology/Technology/LosLos AlamosAlamos NationalNational LaboratoryLaboratory 1 1 ActinideActinide ResearchResearch QuarterlyQuarterly

Plutonium Futures— The Science 2003

Moreover, we hope that this series of conferences will stimulate the next generation of scientists and students to study the fundamental proper- ties of plutonium. The 2003 conference, held in July in Albuquerque, N.M., was the third in this series and attracted more than 328 partici- pants from 63 institutions and 12 countries. We were also encouraged by the participation of 42 students, an increase over past years. More than 180 contributed presentations covered the latest results in pluto- nium condensed matter physics, materials science, compounds and complexes, environmental behavior, detection and analysis, separations and purification, nuclear fuel cycles, and waste isolation and disposal. In this issue of Actinide Research Quarterly, we present some highlights of the 2003 conference in the form of short articles from conference par- ticipants, interviews with poster presenters, and reports from ARQ staff. By increasing the international dialogue through conferences and publications, we hope to facilitate the developing renaissance in pluto- nium science that will enable efficient solutions to Cold War legacy problems and provide maximum benefits for society from the enormous energy potential of plutonium.

—Gordon D. Jarvinen David L. Clark

2 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

Structural Trends and Bonding of the 5f-Elements (UÐAm) Poster with the Oxoligand IO Ð session 3 highlights A. C. Bean,1 B. L. Scott,1 T. Albrecht-Schmitt,2 and W. Runde1

1Los Alamos National Laboratory and 2Auburn University

To assess the environmental impact of transuranium elements (neptunium, plutonium, and americium) on nuclear waste repositories, scientists must better understand the solid-state chemistry of transuranium compounds. Researchers are particularly interested in how these elements might react with fission-product radionuclides, such as iodine-129 in its oxidized and reduced forms, such as the iodide (IÐ) or iodate Ð (IO3 ) ions. Iodate is of particular interest for nuclear waste.

“We used a process known as hydrothermal synthesis (mineral synthesis in the presence of heated water) to produce single crystals, which we used to obtain crystal structures and Raman spectra of five novel transuranium iodates,” said Amanda Bean of Los Alamos. “Our end goal was to look at the fundamental chemistry of these iodates for the first time.”

The five transuranium iodates are as follows: NpO2(IO3)2(H2O), Amanda NpO2(IO3)2¥H2O, PuO2(IO3)2¥(H2O), and two forms of Am(IO3)3. Bean This research presents a vital first step in applying a synthetic approach to unraveling the structural properties of materials that contain the series of the light actinide elements. With such structural properties in hand, researchers will be better able to address issues related to long-term storage and disposition of nuclear materials.

—Octavio Ramos, Jr.

Nuclear Materials Technology/Los Alamos National Laboratory 1 Actinide Research Quarterly

Keynote Keynote speaker Sig Hecker Speaker Plutonium: Whether boon or risk, both viewpoints must be respected

“ lutonium evokes the entire gamut of enhancements in the security of nuclear mate- human emotions, from good to evil, rials and a reduction in the level of nuclear- Pfrom hope to despair, from the salva- materials trafficking. tion of humanity to its utter destruction.” On the flip side, Hecker raised concerns de- With typical dramatic flair, Los Alamos Senior riving from the “horizontal proliferation” of Fellow Sig Hecker cut the ribbon on the nuclear weapons in Asia, including radiologi- opening morning of the conference. cal-terrorism concerns following the events of Adding the obvious, that “there’s no 9/11/01, which “indicate that we do not live other element in the periodic table that in a very peaceful world.” He also commented shares such a burden,” Hecker then surged on the challenges of dealing with the legacy into a historical perspective. Its backdrop waste of the nuclear era, citing specific in- was the dual-use scenario of domestic stances in both the United States and Russia power and nuclear weapons—drawing and including the decommissioning and energy from an atomic nucleus that can be dismantlement of the latter’s nuclear tapped for “a factor of increase in millions submarine fleet. of energy production.” Never one to end on a sour note, Hecker Beginning with President Eisenhower’s noted the resurgence in nuclear-power re- 1953 Atoms for Peace initiative, Hecker out- search in the United States, mentioning both lined the period of intense international col- fuel-cycle initiatives and the positive connota- Sig Hecker laboration on the peaceful uses of atomic en- tions of “discussions about the construction of ergy that spanned the period from the 1955 nuclear power plants . . . for the first time in a First International Congress in Geneva couple of decades.” through the end of the Cold War. In Hecker’s Referring to opinions that plutonium is ei- view, the late 1980s marked a period of dimin- ther our greatest boon or our greatest risk, ished collaboration, while the first Plutonium Hecker cautioned that “we must respect both Futures Conference in 1997 symbolized the points of view.” He then rolled into the day’s “attempt to regenerate” the spirit of interna- events by declaring that “no matter what you tional collaboration in plutonium science. believe about the political challenges, scientifi- Hecker’s post-1997 review adopted a cally, plutonium is without question the most sociopolitical angle, citing the cooperative en- complex and fascinating element . . . and deavors whereby the United States and Russia that’s, after all, why we’re here this week.” continue to reduce their arsenals of nuclear Finally, in calling for a continued interna- weapons. He reviewed such initiatives as tional collaboration to solve scientific and burning excess weapons plutonium for peace- sociopolitical challenges, Hecker concluded ful energy production and blending-down by expressing the hope that “this conference Russia’s declared 500-ton excess of highly will, in particular, stimulate interest in the enriched uranium to low-enriched uranium. younger generation in carrying on the chal- Some of the latter has already been purchased lenges and all those things that we have yet by the United States for its light-water electric- to learn associated with plutonium and the ity-generating reactors. In addition, Hecker other actinides.” discussed what he characterized as significant —Vin LoPresti

4 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

Poster Theory of Positron Annihilation in Helium-Filled Bubbles in Plutonium Session Highlights P. A. Sterne and J. E. Pask

Lawrence Livermore National Laboratory

Scientists at Lawrence Livermore National Laboratory (LLNL) have developed a first- principles, finite-element-based method to calculate positron lifetimes for defects in metals. This method can treat system cell sizes of several thousand atoms, thus enabling researchers to model defects in plutonium that range in size from mono- vacancy to helium-filled bubbles of more than 1 nanometer in diameter.

“Our objective is to perform some theoretical calculations that can help us interpret positron annihilation experimental data,” said Phillip Sterne of LLNL. “Recent experiments at Livermore have identified two lifetime components in aged plutonium samples, one at around 182 picoseconds and another at about 350 to 400 picoseconds. But what do these values mean? What’s the interpretation? To understand what they mean, we’ve been doing first- principles calculations using a modeling technique that can help us interpret which defects are responsible for the lifetime components we are observing during our experiments.”

Sterne noted that positrons are great probes for defects. Positrons are attracted to the nanosized defects found in the alloy. As the positrons decay, they release energy in the form of gamma rays. Gamma rays produce a distinct signature that helps scientists identify the size, quantity, and type of defect in an alloy. Retired Los Alamos When helium atoms are introduced into vacancy clusters, the positron lifetime is Scientist reduced. The modeling studies indicate that the experimentally observed positron Robert lifetimes in aged plutonium metal are consistent with the presence of two to three Penneman helium atoms per vacancy. (left) and

—Octavio Ramos, Jr. Phillip Sterne

Nuclear Materials Technology/Los Alamos National Laboratory 5 Actinide Research Quarterly

Plutonium Tutorial Tutorial A plutonium potpourri

Challenges in plutonium IAEA activities in plutonium physics and chemistry nonproliferation and security eaborg Institute Director David Clark The tutorial’s second presentation, given by kicked off an approximately four-hour Graham Andrew of the International Atomic S plutonium tutorial designed to Energy Agency (IAEA), proved to be a perti- “stimulate the next generation of scientists and nent segue, particularly in light of recent inter- students.” He framed his discussion in the national events precipitating concern over most interesting way possible, that is, in terms nuclear terrorism. He reviewed how the of the metal’s chemical and metallurgical pe- agency functions and interacts with national culiarities. From the uniqueness of its oxida- states and emphasized the ongoing challenge tion-state behavior (whereby, under certain implicit in “checking whether what you mea- conditions, plutonium solutions can simulta- sure can possibly be what you’re told.” neously exhibit the presence of three or four Of particular interest were new analytical different oxidation states) to the different crys- tools developed to assay plutonium samples tal structures and physical properties of its al- for isotope ratio, lotropes (physical phases), the discussion accu- as well as to de- rately reflected the reason why so many scien- termine the con- tists worldwide are engaged in the study of a tent of minor single chemical element and its compounds. actinides such “I’m going to walk you through some of the as neptunium, interesting scientific challenges that will be ad- americium, and Dave Clark dressed at the conference,” Clark offered, and curium. Also of he quickly delivered on the promise, covering interest was the the areas of stockpile, materials, and environ- revelation that, mental stewardship. His focus was the phe- in addition to its nomenon of nuclear materials aging, and he central offices in used an example of an aged tanker that had Vienna, the recently broken apart off the coast of Spain to agency main- generally frame the issue’s importance. tains laborato- Clark did not ignore the fact that, as science ries at certain often does, a seemingly completely negative large nuclear- Graham Andrew characteristic has also been harnessed for posi- fuel reprocess- tive applications. Most notable was the use of ing plants such as the one at Rokkashamura, plutonium-238 as a heat and power source in Japan. This need was highlighted by the statis- space missions, particularly the Mars Rover tic that mixed-oxide (MOX) fuel requirements projects in which Los Alamos plutonium scien- for commercial light-water reactors comprise tists have been intimately involved. Addition- about 190 metric tons per year, compared with ally, Clark predicted that because of self-irra- only 24 tons per year currently being produced diation-based changes occurring in plutonium- by reprocessing. based superconductors over time, “plutonium Andrew also discussed the limitations of tra- will teach us quite a bit about superconductiv- ditional safeguards and the efforts to expand ity mechanisms.” safeguards by getting nations to “sign on to more wide-ranging access and forensic envi- ronmental sampling . . . from mines to nuclear waste” and via expanded inspector training to

6 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

help enhance each individual inspector’s rec- Colloid-facilitated ognition capabilities. This concern extended to transport of plutonium the proliferation resistance of future energy The afternoon’s final presenter was systems, in the sense of “making that nuclear Annie Kersting of Lawrence Livermore material less attractive to a proliferator.” This National Laboratory. “What we really IAEA initiative is, of course, particularly rel- want to understand is the life cycle of a evant since the events of 9/11/01, with the sub- colloid,” Kersting said, and she pro- sequent increased concerns about sabotage and ceeded to explain why this was a vital radiological terrorism (“dirty bombs”). goal to chemists trying to sort out the complexities of plutonium’s interaction The nuclear fuel cycle with the natural environment. and new initiatives Mobile particulates generally smaller In comparing the broad outlines of so-called than 1 micron, colloids originate from a “once-through,” “European/Japanese,” and diversity of environmental sources; and “advanced proliferation-resistant” nuclear fuel both organic (carbon-based) and inorganic Edward Arthur cycles, Edward Arthur’s presentation was emi- (mineral-based) colloids are ubiquitous in soil nently pragmatic. Arthur, former Los Alamos and water. Understanding colloids and their sig- acting office director for nuclear technology nificance in environmental transport of pluto- and applications, cited the requirement for re- nium and other actinides thus entails a combina- processing of spent fuel as the first step in any tion of geology, hydrology, and chemistry, as of the cycles. He then made it clear that the Kersting’s presentation illuminated at every turn. cost of spent-fuel reprocessing did not cur- As such, she discussed the need for field rently compare very favorably with the studies to firmly foot laboratory experiments reprocessed fuel’s electricity value. and theory in the local geology and hydrology He stressed that “front-end” operations such of sites such as Rocky Flats and Savannah as decladding, storage, and separations, and River. Such local geochemistry determines back-end waste-management components if colloids will or will not facilitate the collectively accounted “for 75 percent of the transport of plutonium or other actinides cost of a fuel reprocessing facility.” But he was through the environment. also unequivocal in his view that economic Kersting explained that these studies issues had to be “looked at from an overall have so far painted an incredibly complex system perspective.” landscape for actinide transport, since both Regardless of the obstacles, the U.S. Senate’s adsorption and desorption had to be as- FY04 budget includes a funding target of $78 sessed and because “measuring the con- million for the multi-laboratory Advanced Fuel centration of colloids in water is very diffi- Cycle Initiative and authorizes a study of a cult.” For example, “you may see pluto- demonstration hydrogen-producing nuclear nium sorb very strongly to say silica and power plant. Ultimately however, to achieve iron oxide, and then you go down a few the goals of Power for the Twenty-First Cen- meters, and it may desorb off that silica but tury, reprocessing must handle tens of thou- still stay on the iron oxide.” sands of tons of spent nuclear fuel. It must also Kersting summarized her talk by fram- deal with such factors as radiation, criticality, ing the overall issue as an imperative to and chemical hazards such as strong acids and understand the fundamental chemistry Annie Kersting solvents—what Arthur characterized as “major of actinide nanoparticles. technical issues still facing reprocessing.” —Vin LoPresti

Nuclear Materials Technology/Los Alamos National Laboratory 7 Actinide Research Quarterly

Poster Session Understanding and Predicting Plutonium Alloys Aging: A Coupled Highlights Experimental and Theoretical Approach N. Baclet,1 P. Pochet,1 Ph. Faure,1 C. Valot,1 L. Gosmain,1 Ch. Valot,2 J. L. Flament,3 and C. Berthier3

1CEAÐCentre de Valduc (France), 2LRRS, Université de Bourgogne (France), and 3CEAÐIle de France (France)

Defects in a plutonium alloy begin at the atomic level. Once created, such defects can diffuse, leading to changes in the physical properties of the alloy. Such changes in turn may affect the reliability and safety of a nuclear weapon.

To better understand and thus better predict aging effects of plutonium alloys, scientists in France have embarked on a multiscale approach that involves both theoretical and experimental components. The two-pronged challenge is to identify which aging parameters are relevant for modeling and develop experimental techniques that can measure such physical parameters.

One parameter relevant for modeling is the interatomic potential, which is the key to modeling displacement cascades using molecular dynamics. The Nathalie interatomic potential is intimately related to the elastic constants of the plutonium- gallium alloys of interest. Scientists in France are developing x-ray diffraction Baclet techniques to determine these values.

“We have tested this technique on pure copper and rhodium samples because the mechanical properties of these materials are similar to those of delta-plutonium,” said Nathalie Baclet of CEAÐCentre de Valduc. “Preliminary results are promising. We are now improving the technique so that it takes into account the material’s texture.”

—Octavio Ramos, Jr.

8 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

Contributed Plutonium-238 metal Papers as a multiphase system Sergey I. Gorbunov revious investigations at the Research A combination of the effects observed in Institute of Atomic Reactors of plutonium-238 metal can be explained by its P plutonium metal containing more continuous and intensive self-irradiation, than 80 percent plutonium-238 show a num- which results in co-existence of the known ber of physical properties in the metal that are plutonium crystal modifications in this metal considerably different from those found in or- at room temperature. To understand the na- dinary “low-radioactivity” plutonium-239. ture of these differences, samples of various For example, the hydrostatic densities of plu- ages were studied using x-ray diffraction to tonium-238 samples are always substantially reveal the crystal structure of the plutonium- lower than those of similarly treated samples 238 metal and to discern how specific alpha of plutonium-239 (by 1.1–2.3 grams per cubic radioactivity and self-irradiation (aging) 3 This article was centimeter [g/cm ] and more). More details affect this structure. contributed by Sergey can be found in an article in Radiochemistry I. Gorbunov of the (English), 2001, Vol. 43, pages 111-117. Results Federal State Unitary When a special thermomechanical treat- The metal samples each had different plu- Enterprise “State ment is used on plutonium-238 samples, the tonium-238 content and were not subjected to Scientific Centre of densities increase only for a short time. The compression loads. They were composed of the Russian densities decrease quickly after pressure is micron-thick layers of plutonium-238 con- FederationÐResearch released and the shapes of the samples densed at high temperature and high vacuum Institute of Atomic become distorted. onto flat tantalum substrates. Moreover, these Reactors” (FSUE Other differences between the two isotopes samples did not contain radiogenic helium in “SSC RF RIAR”), Russia. can be seen in the thermograms of plutonium- the initial state. 238 as the temperature is increased. The inten- The table below lists the properties of these sities of peaks of the alpha to beta, beta to samples. Samples 1, 2, and 3 were considered gamma, and gamma to delta (α→β, β→γ, γ→ δ) high-radioactivity metals, sample 4 was la- allotropic transformations are always substan- beled as medium radioactivity, and sample 5 tially lower than those of the corresponding was low radioactivity. plutonium-239 material. Plutonium-238 also The samples were prepared, kept, and has a temperature coefficient of electrical re- investigated under similar conditions. sistivity that is lower in the region of the al- All of them had a comparable purity and, pha-phase field as well as some other differ- according to the spectral analysis results, ences in the temperature dependence of did not contain certain impurities in the electrical resistance. quantities sufficient to stabilize the “tempera- ture” modifications of plutonium. The x-ray

Characteristics of plutonium samples Atomic fraction Alpha- Sample Mass, mg Thickness, µm of 238Pu, % radioactivity* 1 86.5 3.5 6Ð8 240 2 86.5 0.5 0.8Ð1.0 240 3 86.5 21 37Ð46 240 4 5.8 10 18Ð22 17 5 1.1 8 14Ð18 4 *With reference to 239Pu isotope

Nuclear Materials Technology/Los Alamos National Laboratory 9 Actinide Research Quarterly

Contributed Papers

analysis was performed within about a one- In samples 1, 2, and 3, a ratio of the total in- year aging timeframe of the samples at tensity of the (020) and (211) alpha-plutonium room temperature. diffraction peaks to the total intensity of the Changes Only the alpha-phase diffraction peaks were (111) delta-plutonium, (111) gamma-pluto- in the phase compositions of the observed in the “low-radioactivity” plutonium nium, and (101) delta-prime-plutonium peaks high-radioactivity x-ray patterns (sample 5) during the whole pe- was designated as criterion C of this process. A (1, 2) and “low- riod of the investigation (see table below). Indi- number of transformation stages can be found radioactivity” (5) vidual diffraction peaks of the “high-tempera- on the curves resulting from plotting the value plutonium samples ture” modifications (beta, gamma, delta, and, of criterion C versus the age of the sample during their aging. probably, delta-prime) were recorded in the since fabrication. The figure on the next page presents the curve for sample 3 as Allotropic plutonium modification** an example. A similar curve was Sample τ*, day αβ γ δ δ' ε also obtained for “mid-radioactiv- 1 ± + + + + Ð ity” sample 4. 20 + ± + + Ð Ð The peak-by-peak analysis of the 1 40 + ± + + + Ð x-ray patterns of the high-radioac- 60 + + + + + Ð tivity samples shows that the delta 98 + ± ± + Ð Ð phase in them disappears more 343 + ± − + Ð Ð slowly than the other “high-tem- 1 ± ± ± + Ð Ð perature” plutonium modifications 43 + + + + + + (see table at left). Nevertheless, at a 2 52 + + + + ± ± certain stage of aging these modifi- 101 + + + + ± − cations can recur, apparently, as in- 339 + Ð Ð + Ð Ð termediate phases in the (delta to + Ð Ð Ð Ð Ð alpha) transformation. So, in 5 1 343 + Ð Ð Ð Ð Ð samples 1 and 2 within 40- to 60- day aging, the maximum number of * Aging period the allotropic modifications is re- x-ray patterns of the “medium” specific alpha- corded (five and even six). At the end of long- ** ÇÐÈ no diffraction radioactivity plutonium (sample 4) along with term aging (about one year), only the alpha peaks of this lattice; the strong alpha-plutonium peaks. and delta phases are observed by x-ray analy- Ç±È individual peaks A unique phenomenon of the dynamic co- sis of the high-radioactivity samples, and no of this lattice; existence of four, five, and even six allotropic obvious signs of the “high-temperature” Ç+È sufficient peaks modifications of plutonium was observed in phases were revealed in the “mid-radioactiv- to calculate the samples 1, 2, and 3 of high-radioactivity metal. ity” sample. parameters of this During the long-term aging of high-radioac- Some peculiarities were revealed on the lattice tivity samples 1, 2, and 3 and “mid-radioactiv- curves showing the atomic volume changes of ity” sample 4, a slow process of changes in the delta-plutonium lattice in sample 3 and of their phase compositions was recorded: a rela- the alpha-plutonium lattice in sample 4. A vol- tive decrease in the content of the “ high-tem- ume increase at the beginning of self-irradia- perature” low-density modifications and an tion results in a “hole” (an abrupt decrease increase in the content of the high-density and subsequent recovery of this parameter) alpha-phase of plutonium. after some period of aging. The atomic vol- umes of the indicated lattices reduce slowly in the course of further aging of the samples.

10 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

Discussion Atomic fraction of helium, % In these experiments with thin layers of plu- 0 0.1 0.2 0.3 0.4 0.5 tonium-238 metal deposited on a metal sub- 101 strate, we have considerably increased the ra- III III IV V diation-induced self-damage rate of the metal, but we cannot similarly increase the annealing 100 rate of radiation defects. Therefore, these ex- periments do not cover the accelerated aging of bulk plutonium metal. A quantitative 10-1 change in the radiation-defects concentration leads to observable qualitative changes in the Criterion C metal properties, but this does not change 10-2 the nature of plutonium. An increase in the specific alpha radioactivity allows a deeper insight into the fundamental properties 10-3 of plutonium. 0 100 200 300 The analysis of the experimental results Aging period, day shows that the co-existence of the different The behavior of “fluctuons” was investi- allotropic modifications in high-radioactivity Criterion C versus gated theoretically by Russian theorist M.A. plutonium at room temperature is caused both aging period and Krivoglaz. A comparison of properties of the by intensive self-irradiation and the complex calculated content of fluctuons with those of plutonium-238 metal radiogenic helium in phase diagram of plutonium metal. Radiation- allows the so-called “unusual” behavior of this sample 3. induced vacancies and clusters of vacancies system to be explained. appear to play a large role in the formation An increase in the vacancy concentration of the “high-temperature” modifications. due to intensive plutonium self-damage leads The bonding 5f-orbitals in plutonium have to an appearance of the “high-temperature” a small overlap; therefore, a considerable relax- modifications already at room temperature. ation of the lattice can take place near the va- The crystal lattice around the large vacancy cancy and an area with an increased atomic clusters arising in the deceleration regions of volume (fluctuation of atomic volume) ap- recoils is under tension. It provides relative sta- pears. The potential energy of the bonding bility of the “high-temperature” phases. 5f electrons belonging to the atoms located To explain the slow change in the phase near the vacancy decreases, the electrons local- composition of high- and “mid-radioactivity” ize (they transfer to the fluctuation states), and plutonium during long-term aging as well as “fluctuons” (bound states of electrons and the complex multistage nature of this process, fluctuation) appear. researchers proposed another mechanism. This The bonds typical of one of the “high-tem- was related to accumulation of radiogenic he- perature” modifications arise inevitably be- lium and evolution of its forms in the metal, tween these atoms with localized 5f electrons. especially in the intragrain cavities. Helium Thus, vacancies in plutonium apparently are behavior also contributes to the changes in the nonlocal; they represent an ensemble of atoms atomic volumes of plutonium lattices under with a different bonding pattern forming in the self-irradiation. lattice as an entity.

Nuclear Materials Technology/Los Alamos National Laboratory 11 Actinide Research Quarterly

Contributed Predicting long-term actinide mobility Papers Nanoscopic approaches to Thomas aquatic plutonium chemistry Fanghänel

he Institut für Nukleare Entsorgung Since different oxidation states exhibit differ- (INE) in Karlsrühe Research Center, ent chemical properties, a reliable speciation T Germany, focuses on the basic research (with regard to the oxidation states) is re- relevant to the assessment of prospective quired. Our work focuses on the tetravalent repositories for radioactive waste, in- plutonium ion, which, in aquatic solution, is cluding both technical and scientific known to be unstable even at low pH in dilute chemical aspects. The predictive model- concentrations, not only because of dispropor- ing of long-term actinide mobility in the tionation—the interconversion of its ionic spe- geosphere is contingent on basic knowl- cies—but also because of colloid formation. edge of the aquatic chemistry of ac- This instability of dilute plutonium(IV) has tinides. In this context, plutonium is made the appraisal of its thermodynamic solu- of special interest. bility in aquatic systems extremely difficult, Plutonium appears in spent fuel in resulting in a large number of controversial re- small amounts (about 1 percent), but af- sults. Thus, assessing the chemical reactions of ter the decay of short-lived fission prod- plutonium(IV) in dilute concentrations in the ucts, plutonium-239 represents the domi- low pH region requires sensitive speciation nant radioactive inventory for thousands and colloid characterization methods. of years. Our present work combines two different On the one hand, the solubility con- laser spectroscopic speciation approaches, pro- straints of plutonium have led to the per- viding the possibility of assessing the chemical ception that this element will be immobi- reactions of plutonium(IV) at concentrations of Thomas Fanghänel lized easily in the repository environment. On only a few micromoles per liter or even lower. of the Institut für the other hand, this particular property of low Nukleare Entsorgung, solubility induces the formation of colloids Karlsrühe Research Center, Germany, (tiny, nano-sized particles—either real colloids makes a point during or pseudo colloids), which may strongly en- his discussion of hance plutonium’s potential for mobility in the nanoscopic aquifer. These characteristic properties are of approaches to cardinal importance for our work. aquatic plutonium chemistry. Challenges Different oxidation states (typically III–VI) of plutonium can coexist in aqueous solution under the appropriate conditions, with the relative abundance of each oxidation state de- This article was pending on the chemical conditions such as contributed by pH, Eh, and ionic strength. Plutonium exhibits Clemens Walther, Claudia Bitea, a complicated redox behavior that permits Jong Il Yun, transformation of one oxidation state into Researchers from the Karlsruhe Research Center Jae Il Kim, Thomas another. For example, the collision of two Institut für Nukleare Entsorgung are using a Fanghänel, Christian plutonium(IV) ions generates one plutonium(III) “homebuilt” laser setup to study the colloidal transport M. Marquardt, Volker and one plutonium(V) ion, and the of plutonium in aqueous solutions. In the front row, Neck, and Alice plutonium(V) ion subsequently can be oxi- from left to right, are: Claudia Bitea and Alice Seibert. Seibert of the dized to a plutonium(VI) ion by an additional In the back row, from left to right, are: Christian Institut für Nukleare collision with a plutonium(IV) ion. Marquardt, Clemens Walther, and Jong Il Yun. Entsorgung, Karlsrühe Research Center.

12 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003 ]

-1 1013 light scattering Experimental approaches 1012 slope-6 We use two methods in our research: 1011 laser-induced breakdown detection (LIBD) 1010 9 for colloid quantification and laser-induced 10 REM/TEM/AFM photoacoustic spectroscopy (LPAS) for 108 7 chemical speciation. 10 106 LIBD is based on nonlinear interaction of 105 LIBD slope-2 colloids with a tightly focused laser beam (see 104 sidebar on page 14). This leads to the forma- 103 SPC tion of a hot, dense plasma, detected either 102 particle number concentration [ml optically (via light emission) or acoustically 10 100 1000 (via its expansion-generation shockwave). particle diameter [nm] Each plasma event corresponds to a single This figure shows the sensitivity of laser-induced colloid and, when compared with the number breakdown detection (LIBD) compared with of laser shots, this provides a measure of classical characterization methods. LIBD’s colloid concentration. sensitivity is enhanced by more than six orders of Compared with dynamic light scattering magnitude compared with dynamic light (the most frequently applied colloid detection scattering, and the accessible size range is technique), the sensitivity, particularly for col- considerably extended toward smaller colloids loids smaller than 50 nanometers, is enhanced when compared with the single-particle counter by more than six orders of magnitude. Com- (SPC). pared with the so-called single-particle counter, a static light-scattering device, the accessible size range is considerably extended (photothermal method) rather than the trans- toward smaller colloids. mitted light. A 10-nanosecond light pulse of a For particles of 20-nanometer diameter, for tunable dye-laser is guided through the instance, the sensitivity corresponds to detect- sample, and the energy of the absorbed pho- ing a pinhead one millimeter in diameter in a tons gives rise to a rapid temperature increase good-sized hotel swimming pool. with subsequent expansion and generation of The speciation methods are based on linear an acoustic shock wave. light absorption. Plutonium’s oxidation states Analogous to the LIBD method, this acoustic III, IV, V, and VI exhibit characteristic absorp- wave is detected by a piezo detector, but be- tion bands. The absorption strength is directly cause its energy content is rather low (less than proportional to the amount of species present. one nanojoule), the signal is electronically am- From the (visible) absorption spectra, a quanti- plified prior to data recording. The acoustic tative speciation is obtained, typically by UV- (and electronic) signal is linearly proportional visible (UV-Vis) spectroscopy, which measures to the absorption strength at the specific wave- the extinction (i.e., absorption plus scattering length of the laser beam. Absorption spectra losses) of white light wavelength resolved af- are obtained by scanning the laser over the A time evolution ter passing through the sample. desired spectral range. In addition to the in- of laser-induced plasma plumes However, this method is limited to measur- creased sensitivity down to the micromole-per- incited on single ing plutonium(IV) concentrations of approxi- liter range, LPAS (in contrast to UV-Vis) is not influenced by the presence of colloids, which colloids as observed mately 10 micromolar. LPAS lowers this limit by an ultrafast (200 by a factor of up to 100 by detecting the effects give rise to light scattering in the blue end of the spectrum. picoseconds) of the light absorbed by the plutonium ions charge-coupled device camera.

Nuclear Materials Technology/Los Alamos National Laboratory 13 Actinide Research Quarterly

Contributed Papers

hν hν hν hν CCD camera Plasma - + e M M+ + - e- e- + M e M e- Quartz cell Optical - + - e M + detection e M M+ e- e- e- Telescope Plasma Energy gain

u hν Laser beam Particle First electron Avalanche via Hot by multiphoton inverse plasma t ionization (MPI) bremsstrahlung Acoustic wave particle counting

The basics of LIBD

Laser-induced breakdown detection (LIBD) is based on plasma formation due to dielectric breakdown in the high-field region of a focused pulsed laser beam when a colloid is present. We commonly experience low- energy plasmas in everyday life, ranging from “flames” over neon lights to the blinding flash of lightning.

The plasma used for LIBD begins with a single (so-called “lucky”) electron, which is created by the high-electric field of the laser. In the language of quantum physics, this is equivalent to the nonresonant absorption of four to six photons (multiphoton ionization or MPI) within only some picoseconds. This process is very unlikely, and it takes on the order of 1015 photons to free one electron.

By interaction with the electric field, this electron is accelerated (by inverse bremsstrahlung) to energies sufficient to ionize neighboring atoms by means of collisions, resulting in a second electron (second “generation”), which undergoes the same process. Within typically only 30 such generations, a hot, dense plasma is created, which is observed by either its light emission or by detection of the acoustic shock wave generated by its rapid expansion.

Because each plasma event corresponds to one single colloid, the relative number of events per number of laser shots provides a measure of colloid concentration. The photon flux required for breakdown decreases with the increasing number of molecules inside the colloid. By making use of that dependence, the colloid size can be ascertained by varying the laser pulse energy.

14 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

A plutonium-242 solution of pure oxidation This mononuclear plutonium species un- state (IV) was prepared by electrochemical dergoes further colloid formation and is thus reduction in 0.5-molar hydrochloric acid to in equilibrium with colloids. Knowing the hy- 2+ plutonium(III), followed by careful re-oxida- drolysis constant of Pu(OH)2 from the litera- tion back to plutonium(IV). The oxidation ture, the solubility product (log Ksp = –59 at state was monitored by UV spectroscopy and zero ionic strength) of plutonium colloids the plutonium concentration was determined (presumably oxy- by liquid scintillation spectroscopy. The pH hydrate) can be de- was increased with an associated decrease in rived from our data. plutonium concentration by very slow, Since the amount of stepwise dilution with 0.5-molar sodium chlo- plutonium(IV) col- ride solution. The pH adjustment ultimately loids is relatively led to the formation of plutonium(IV) colloid small at a given pH of kernels, as observed by a well-defined, sharp colloid kernel forma- increase of the LIBD signal. Such colloid for- tion, there remains a mation is the most sensitive indication that the considerable quantity solubility limit has been exceeded. of (hydrolyzed) Macroscopically, the solubility limit is de- plutonium(IV) ionic fined as the concentration at which the total species undergoing amount of a substance is no longer present in disproportionation ionic form but instead forms a precipitate. with time. This reac- Here, our precipitate takes the form of col- tion is directly observed by LPAS for Laser-induced loids, which are so small that they remain plutonium(IV) and (III), at a plutonium con- photoacoustic suspended because of Brownian motion. centration within the region of colloid forma- spectroscopy (LPAS) Analogous experiments were conducted at tion above the solubility curve. detects the oxidation different concentrations of the plutonium The disproportionation reaction can be states of plutonium
ions in solution by stock solution. With decreasing plutonium(IV) written as 3Pu(IV) + 2H2O 2Pu(III) + Pu(VI) concentration, the pH of colloid kernel forma- + 4H+, which requires that, for three consumed means of light tion increased. plutonium(IV) ions, two plutonium(III) ions and absorption. This one plutonium(VI) ion must be formed. The ratio figure shows plutonium(IV) of plutonium(IV) decrease and plutonium(III)/ Evaluation undergoing Depending on pH, the tetravalent pluto- plutonium(VI) increase should scale as 3:2:1. disproportionation nium ion is more or less hydrolyzed, meaning Our LPAS measurement however, corre- with time. The signal that it is surrounded by up to four hydroxide sponds to a ratio of 1, indicating a decrease on the left at 470 (OH–) molecules. Only in very acidic condi- of plutonium(IV) that is too low by at least nanometer (nm) is tions (a pH less than zero), does a non-hydro- 50 percent. due to the electronic lyzed Pu4+ aquo-ion exist in solution. The hy- To explain this discrepancy, we must keep in absorption of drolysis does not change the spectral proper- mind that plutonium(IV) colloids are in equilib- plutonium(IV). This ties of Pu4+, and so it cannot be detected di- rium with the plutonium(IV) ionic signal decreases rectly by LPAS. But from the crossing points of species, and that the plutonium(IV) tied up with time, while the colloid formation—log of plutonium(IV) con- in colloids is spectroscopically invisible in the plutonium(III) signal at 605nm increases centration versus pH, we obtain the solubility LPAS experiment. by the same amount. curve with slope = –2, indicating that the 2+ plutonium(IV) dihydroxo complex Pu(OH)2 represents the dominant species.

Nuclear Materials Technology/Los Alamos National Laboratory 15 Actinide Research Quarterly

Contributed Papers

Over the course of 70 hours, as plutonium(IV) Diluting a plutonium-242 -3 ionic species are consumed by the disproportion- Dilution with solution with 0.5-molar + [Pu]tot ation reaction, the fraction of (spectroscopically + 0.5 M NaCl sodium chloride results o o [PuIV] invisible) plutonium(IV) present as colloids redis- -4 + in a decrease in + solved, and this addition to the spectroscopically plutonium concentration + + observable plutonium(IV) fraction was sufficient log[Pu] ++ -5 Onset of colloid and an increase in pH + to account for the discrepancy. Once this was formation (top), until the solubility + taken into account, the true decrease of is exceeded and -6 plutonium(IV) could be calculated, and satisfied plutonium(IV) colloid the required 3:2 ratio of the reaction. This was kernels are formed 0.06 confirmed by LIBD measurements, where the col- (crossing point). The formation of the colloid loid dissolution with time was observed directly. kernels is shown by a 0.04 At lower concentrations (below 10 micromolar well-defined, sharp (E=2.5mJ) total plutonium concentration), LPAS is no longer increase of the laser- capable of detecting small amounts of BDP 0.02 induced breakdown BDP=1% plutonium(IV) colloids present in the solution. detection (LIBD) signal However, such minute concentration of colloids 0 (bottom). 0 0.5 1.0 1.5 2.0 2.5 (less than one micromolar) can still be detected by LIBD, and the dissolution with -log[H+] time has been demonstrated for solutions of one micromolar total plutonium. A combination of LIBD and LPAS facilitates a sound assessment of chemical reactions of plutonium(IV) involved at a rim of its solubility- -3.5 The solubility product constrained pH for the total plutonium(IV) con- of plutonium colloids + centration down to micromoles per liter and for can be derived from the its colloids, about 100 times less (down to a par- -4.0 experimental data. For ticle concentration of 10 nanomoles per liter). decreasing plutonium + The present experiment shows how compli- concentration, the -4.5 cated the thermodynamic assessment of crossing point of colloid plutonium(IV) solubility is. For this reason, there formation is shifted to is a wide scattering of the plutonium(IV) solubil- higher pH. From the log[Pu] -5.0 + ity product published in the literature—either for slope = Ð2, we infer the its oxide or for its hydroxide, with the differences plutonium(IV) to be present as dihydroxo- [Pu] slope-2 being a few orders of magnitude. Our research -5.5 + tot complex, Pu(OH) 2+, in + has demonstrated how the use of novel spectro- 2 [PuIV] equilibrium with scopic approaches can reduce the uncertainties that have, up to now, hindered the assessment of colloids. -6.0 0 0.5 1.0 1.5 2.0 2.5 plutonium(IV) solubility. -log[H+]

16 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003 Bicyclic and acyclic diamides Comparison of aqueous phase Contributed binding constants with tetra- Papers and hexavalent actinides Sergei Sinkov iamides have been studied exten- One of the possible approaches to quantify sively as agents for selective the magnitude of ligand-to-metal binding is D extraction of trivalent f-block metal the application of ultraviolet-visible (UV-Vis) ions from aqueous solutions. They form the optical absorbance spectroscopy to monitor basis for separation processes designed to par- spectral changes in- tition the minor actinides from spent nuclear duced by 1a in the fuel for subsequent transmutation (with con- absorption spectra of comitant closure of the nuclear fuel cycle) or actinide ions in aque- immobilization (for disposal). ous solution. The One possible drawback associated with the spectral information current generation of diamide extractants con- obtained by varying cerns the relatively modest actinide distribu- the ligand-to-metal tion coefficients obtained even at high (greater ratio not only allows than 1 molar) extractant concentrations. In a estimation of the recently published report in the Journal of the number of light-ab- American Chemical Society, we described the sorbing species in extraction characteristics for a new type of solution, but can be diamide molecule. This new diamide (struc- used for measurement ture 1b, below) is one million to 10 million of the binding (or times more efficient at extracting trivalent lan- formation) constants thanides and actinides from aqueous solution of a ligand with than previously examined diamides such as metal ions. tetraalkylmalonamides. The following sections compare the Sergei Sinkov of It was assumed that this extraction en- formation constants for the dimethyl Pacific Northwest hancement reflects superior ligand-metal bicyclic diamide (1a) and N,N,N’,N’- National Laboratory binding of the bicyclic diamide as compared tetramethylmalonamide (TMMA), a related (PNNL) presented to acyclic analogs such as tetraalkylmal- non-cyclic compound, with plutonium(IV), results of research on the development onamides. A fuller understanding of such plutonium(VI) and uranium(VI) as measured a new diamide structure-function aspects of actinide extrac- in the aqueous phase at 1.0-molar ionic ligand with some tion by amides is needed for the intelligent strength (HNO3). truly amazing design of extractants with superior character- extraction istics to those currently available. Results with plutonium(IV) capabilities. In this experiment, we conducted a spec- Compared to trophotometric titration by exposing a single conventional portion of a plutonium-239, -240(IV) stock diamide ligands, the solution (better than 99.5 percent valence new ligand molecule A new bicyclic is one million to 10 diamide ligand O purity) to successive introduction and dis- O solution of either solid crystals (in the case million times more architecture efficient at removing of 1a) or tiny aliquots of a concentrated optimized for R R f-block metals from bidentate N stock solution (in the case of TMMA). Nei- process solutions. lanthanide/ N ther precipitation nor perturbations in the actinide binding. oxidation state of plutonium(IV) were ob- R = methyl (1a), served with either ligand throughout the R = octyl (1b). titration procedure at a metal concentra- tion of 8 millimolar (mM).

Nuclear Materials Technology/Los Alamos National Laboratory 17 Actinide Research Quarterly

Contributed Papers

A three-dimensional As with plutonium(IV), the 1a ligand structure of the new produced pronounced spectral changes bicyclic diamide. in the plutonium(VI) spectrum, with Carbon atoms are black, nitrogen two, new, well-resolved peaks at 840.6 atoms are blue, nm and 846.3 nm emerging with even a hydrogen atoms are moderate excess of ligand. After reach- light grey, and ing a ligand-to-metal molar ratio of 80, all oxygen atoms are three peaks of plutonium(VI) gradually dis- red. The vectors on appeared. Moreover, several new spectral fea- each oxygen atom, tures emerged as shown in the figure on page which indicate the 26. For example, the visible range of the spec- direction required for trum showed new major peak maxima at 497 optimal interaction nm and 684 nm. This new spectral signature with a metal ion, clearly indicated a reduction of plutonium(VI) converge in the computer-designed to plutonium(IV) in the presence of 1a. structure, With 1a, the most significant spectral A careful redox speciation analysis of the in contrast to changes observed with increasing ligand con- plutonium(VI) and (IV) spectra showed that conventional centration were found in the region of the this plutonium(VI) reduction was accompa- malonamide with main plutonium(IV) peak (a 476→497 nanom- nied by the formation of plutonium(V)—a diverging vectors. eter [nm] shift) and in the red part of the weak spectral feature of variable intensity at plutonium(IV) spectrum (a 660→684 nm shift 568 nm. Once we accounted for the presence and a 2.4-times intensification of the absorp- of plutonium(IV) and (V), we used the nonlin- tion band). ear spectra processing routine known as Although the spectral changes for TMMA SQUAD to process the portion of the spectral were similar, we found that such changes were set in which plutonium(VI) remains predomi- less pronounced and not identical to those nant in terms of concentration. The formation observed with 1a. Moreover, these changes oc- constants refined from this analysis are shown curred at much higher ligand-to-metal ratios. in the table on page 19. Using the Singular Value Decomposition In contrast to 1a, the complexation of procedure, we found no less than four plutonium(VI) with TMMA did not induce complexed species for the plutonium(IV)-1a any redox reaction (even after an overnight system and three major complexed species contact time). Complexation effects were sig- (and perhaps even a fourth minor species) for nificantly weaker, with the 1:2 complex seen the TMMA-plutonium(IV) system. not as a separate peak but rather as a weak shoulder in the 842–846 nm range. Results with plutonium(VI) Not only was the most intense and sharp Results with uranium(VI) absorption peak of plutonium(VI) at 833.6 nm Both ligands induced significant spectral 2+ monitored for expected complexation effects, changes in the UO2 spectrum, accompanied but a wider range (330–950 nm) was examined by a bathochromic shift and intensification of to evaluate possible changes in the oxidation the absorption bands. Subsequent analysis in- state of the plutonium (initially better than dicated that 1a again acted as a much stronger 99.5 percent hexavalent plutonium).

18 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

0.7 Pu(VI) Pu(VI) 0.6 #01-07 0.6 7 additions binding agent than did TMMA. In both cases of the ligand 0.5 #08-16 no spectral evidence was found for formation Pu(VI)L kinetics at L/Pu = const 0.4 0.5 Pu(VI)L2 of higher complexes (1:3 and 1:4 stoichiometry). 0.3

0.2 Conclusion 0.4 Optical absorbance 0.1 Pu(VI)L

Using UV-visible spectroscopy, we have 0.0 quantified binding constants for both bicyclic -0.1 Pu(VI)L2 0.3 0246810121416 and acyclic diamide ligands with plutonium(IV), Spectrum # plutonium(VI), and uranium(VI) in acidic Pu(IV)L aqueous solution. Optical absorbance 4 0.2 This work has yielded three key results. 16 First, we showed that alternating the diamide 16 structure from acyclic to bicyclic increased 0.1 08 568 nm (Pu(V)) the overall formation constants with 08 plutonium(IV) by as much as seven orders of 0.0 magnitude. Second, we found that comparing actinide(VI)-ligand binding affinities reveals 480 520 560 600 640 680 720 760 800 840 880 enhanced binding to uranium(VI) versus Wavelength, nanometer (nm) plutonium(VI). And third, the plutonium(VI), The spectrophotometric titration of plutonium(VI) by 1a in 1.0-molar HNO3. but not uranium(VI), in the bicyclic diamide Spectra shown in dark pink correspond to seven successive additions triggers reduction of plutonium(VI) to of the ligand up to L/M =80. Red spectra are kinetics of the plutonium(V) and (IV). plutonium(VI)L+plutonium(VI)L2 reduction to plutonium(IV)L4. Black trace We are already at work evaluating the shows the reference spectrum of plutonium(IV) with no ligand present formation constants of TMMA and 1a with at the same acidity and metal concentration as the initial spectrum of actinides in the +3 and +5 oxidation states. plutonium(VI). The kinetic series spectra 08, 09, 10, 11, 12, 13, 14, 15, and 16 were taken at zero minutes, 10 minutes, 20 minutes, 30 minutes, 40 minutes, 52 minutes, 63 minutes, 150 minutes, and 24.5 hours, respectively, after the last addition of the ligand.

A summary of all the refined formation Complexant Species logβij constants resulting from this work. This article was MiLj Pu(IV) Pu(VI) U(VI) Conditional formation constants of contributed by Sergei plutonium(IV), plutonium(VI), and 1:1 4.42 (0.08) 1a 2.36 (0.04) 2.85 (0.02) Sinkov, Brian Rapko, uranium(VI) complexes with 1a and and Gregg Lumetta 1:2 7.95 (0.14) 3.34 (0.07) 4.61 (0.05) tetramethylmalonamide in 1.0-molar of Pacific Northwest HNO at 23±0.5o C. Values in brackets 3 1:3 10.36 (0.13) - - National Laboratory represent one sigma standard (PNNL); and James 1:4 11.48 (0.14) - - deviations. Hutchison and Bevin 1:1 2.56 (0.08) 0.44 (0.01) 1.00 (0.01) TMMA Parks of the Department of 1:2 4.08 (0.11) -0.03 (0.03) 0.97 (0.03) Chemistry, University 1:3 5.01 (0.14) - - of Oregon, Eugene.

1:4 4.50 (0.38) - -

Nuclear Materials Technology/Los Alamos National Laboratory 19 Actinide Research Quarterly

Landmark experiment opens up new research opportunities in actinide science Contributed Researchers use high-resolution inelastic x-ray Papers scattering and a microbeam approach to determine Joe Wong the first full phonon dispersion curves ever measured for any plutonium-bearing materials

Editor’s note: The six scientists from Lawrence energy resolution. With the advent of highly Livermore National Laboratory (LLNL) who co- brilliant x-ray sources and high-performance authored this article have garnered an award for focusing optics, samples with volumes as their groundbreaking research. The team recently small as one ten-thousandth of a cubic milli- received a Science and Technology Award, the high- meter can now be studied. These capabilities est honor granted by LLNL for science and technol- have opened up new experimental opportuni- ogy accomplishments. ties for materials that are only available in small quantities, as is the case for many ac- nderstanding the physical basis for tinide systems. The research is covered in de- the intriguing properties of pluto- tail in a paper published in August in Science U nium materials such as force con- magazine (see Science, stants, sound velocities, elasticity, phase stabil- Volume 301, page 1078 [2003]). ity, and thermodynamic properties critically Our samples were large-grain polycrystal- hinges on the ability to produce high-quality line specimens prepared from a plutonium- experimental data. Of these, phonon disper- gallium alloy containing about 0.6 percent sion curves (PDCs) are key to the elucidation by weight of gallium produced by a strain- of many of these physical phenomena. enhanced recrystallization technique. To ex- A phonon is a lattice vibration, the acoustic tract the phonon energies, the spectra were fit- equivalent of a photon, the basic quanta of ted by convolving the experimentally deter- This article was light. PDCs in plutonium and its alloys have mined resolution profile with a model function contributed by Joe defied experimental determinations for the consisting of a Lorentzian for the elastic contri- Wong, Daniel Farber, past forty years because of the inability to bution and a pair of Lorentzians, constrained Florent Occelli, Adam grow the large single crystals (at least a few by the thermal phonon population factor, for J. Schwartz, Mark cubic millimeters in volume) necessary for in- the inelastic part. Wall, and Carl Boro of elastic neutron scattering (INS) measurements. The resulting PDCs along the three principal Lawrence Livermore Another obstacle was the high thermal-neu- symmetry directions in the face-centered cubic National Laboratory; tron absorption cross section of the most (fcc) lattice are displayed in the figure below, Michael Krisch of the common isotope, plutonium-239. European Synchrotron Furthermore, only recently have Radiation Facility, theoretical computations of pluto- Grenoble, France; and Tai-C. Chiang and nium PDCs begun to overcome the Ruqing Xu, of the difficulties in treating the f electrons Department of Physics accurately within the standard first- and Frederick Seitz principles methods. Thus, the PDCs Materials Research for plutonium-bearing systems have Laboratory, University remained essentially unknown ex- of Illinois at Urbana- perimentally and theoretically. Champaign. The experimental difficulties asso- ciated with INS are circumvented in our experimental approach by em- (meV) Phonon Energy ploying inelastic x-ray scattering (IXS) with milli-electron volt (meV)

20 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

together with a fit using a standard Born-von Kármán (B-vK) force constant model. An ad- equate fit to the experimental data is obtained if interactions up to the fourth-nearest neigh- bors are included and the calculated phonon density of states derived from the B-vK model is also shown. The dashed curves are recent dynamical mean field theory (DMFT) results. The elastic moduli calculated from slopes of the experimental phonon dispersion curves near the G point are: C = 35.3 ±1.4 giga pascal ± 11 ± (GPa), C12 = 25.5 1.5 GPa and C44 = 30.53 1.1 GPa. These values are in excellent agreement with those of the only other measurement on a values in other “unusual” fcc metals such as Joe Wong and his similar alloy (1 percent atomic weight gallium) gamma-cerium: 2.8, lanthanum: 4.1, and colleagues used the using ultrasonic techniques as well as with thorium: 3.6. high-resolution those recently calculated from a combined Published results support the conclusion inelastic x-ray DMFT and linear response theory for pure that delta-plutonium–gallium is the most scattering beam line delta-plutonium. at the European elastically anisotropic fcc metal known. The small difference between C and C is Synchrotron 11 44 Furthermore, we observe a large deviation, very unusual for a metal with a face-centered Radiation Facility in D, from the Cauchy criterion. Our measure- cubic structure. The shear moduli C and C’ = Grenoble, France, 44 ment yields D = (C –C )/C = 0.9, which (C - C )/2 differ by a factor of six. This is in 44 12 12 (shown above) to 11 12 differs significantly from zero and implies contrast to that of a “normal” fcc metal such as obtain the first-ever that the interatomic forces have a strong experimentally aluminum: 1.2, and is significantly higher than noncentral component. determined phonon The most striking feature of the experimen- dispersion curves of a tal PDCs is a soft-mode behavior for the TA plutonium alloy. ESRF branch along [111]. A similar feature (but oc- is a third-generation curring at about twice the energy and at a synchrotron, and the only facility with a Phonon dispersions along high-symmetry higher crystal momentum toward the L point) beam line capable of directions in a face-centered cubic plutonium- is also seen in a recent DMFT calculation for gallium alloy containing about 0.6 percent by measuring delta-plutonium. The cubic fcc crystal struc- weight of gallium. The experimental data are plutonium’s lattice shown as circles. Along the [0ξξ] direction, there ture of delta-plutonium can be viewed as being vibration energy. are two transverse branches [011]<01-1>(T ) composed of hexagonally close-packed atomic 1 planes stacked along the [111] direction with a and [011]<100> (T2). Note the softening of the TA[ξξξ] branch toward the L point. The lattice specific stacking arrangement. The soft trans- parameter of our samples is a = 0.4621 verse mode at L suggests that each atomic nanometer. The solid curves are the fourth- plane could easily slide relative to its immedi- nearest neighbor Born-von Kármán model fit. ate neighboring atomic planes to form new The derived phonon density of states, stacking arrangements. normalized to three states per atom, is plotted in the right panel. The dashed curves are calculated dispersions for pure delta-plutonium based on dynamical mean field theory (DMFT).

Nuclear Materials Technology/Los Alamos National Laboratory 21 Actinide Research Quarterly

Contributed Papers

Pure delta-plutonium, on lowering tempera- As evident by the data presented in the fig- ture, transforms into the gamma phase, which ure on the previous page, our IXS experiment has a face-centered orthorhombic structure. validates the main qualitative predictions of a This structure can be described in terms of a recent DMFT calculation for delta-plutonium stack of slightly distorted hexagonally packed in terms of a low shear elastic modulus C’, a

atomic layers, and indeed is the type of struc- Kohn-like anomaly in the T1[011] branch, and a ture that could be easily obtained by layer par- large softening of the T[111] modes. Such ex- allel shear in the fcc phase. perimental-theoretical agreements give cre- Thus, the soft mode at L is likely a key fea- dence to the DMFT approach for the theoreti- ture associated with the delta-to-gamma tran- cal treatment of 5f-electron systems. However, sition, even though this transition is sup- while there is good qualitative agreement be- pressed by gallium stabilization in the present tween theory and experiment, quantitative dif- case. This notion is indeed substantiated by ferences are evident. These differences provide the fact that the first interplanar force constant the benchmark for refined theoretical treat- (derived from the B-vK model) of the T[111] ments and further experiments with pluto- mode is more than an order of magnitude nium and other 5f elements. smaller than those of the transverse modes in the other two directions. Acknowledgements The small amount of gallium in the sample This work was performed under the aus- is sufficient to allow the system to bypass the pices of the Department of Energy by the Uni- gamma and beta phases and make a transition versity of California, Lawrence Livermore Na- directly to a monoclinic alpha-prime phase at tional Laboratory under Contract No. W-7405- about -110oC. The alpha-prime phase, despite Eng-48 and the Department of Energy by the its apparent complexity, is also a slightly dis- University of Illinois Frederick Seitz Materials torted hexagonal close-packed structure, and Research Laboratory under Grant No. is easily accessible from the fcc phase through DEFG02-91ER45439. We are thankful to the same shear mechanism. Francesco Sette, European Synchrotron Radia- A similar soft-mode behavior has been ob- tion Facility (ESRF) for his support and en- served in gamma-cerium and lanthanum. In couragement in this project, and to Paul those cases, the transition leads, instead, to a Berkvens and Patrick Colomp, also with ESRF, double-hexagonal-close-packed structure, but for their advice and technical assistance. the same general concept of layer parallel shear applies.

22 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003 International team of researchers employs high pressure to probe bonding in actinide metals Contributed Investigations of protactinium and Papers americium metals provide important insights into actinide bonding concepts Richard Haire The science question The three divalent f-element metals— ressure, like temperature, is an im- europium, ytterbium, and einsteinium— portant variable in the chemistry, are special cases, and result in part due to Pphysics and materials science of ma- their higher f-electron promotion energies. terials. It has been found that the f electrons of With the some of the early lanthanide met- the actinide elements—the so- als, it is interesting to note called 5f-electron series of ele- that after acquiring 4f-elec- ments—are also greatly af- tron character in their bond- fected by pressure. At atmo- ing by applying high pres- This paper was contributed by spheric pressure, protactinium, sures, they adopt some of the Richard Haire, uranium, neptunium, and plu- same low-symmetry struc- Chemical Sciences tonium have itinerant 5f elec- tures exhibited by the protac- Division, Oak Ridge trons (electrons involved to tinium-plutonium metals that National Laboratory; varying degrees in the metallic have itinerant 5f electrons. Steve Heathman bonding), and they exhibit un- This immediately raises the and Monique Idiri, usual low-symmetry structures question of whether applying European and properties. In contrast, the pressure on the transpluto- Commission, transplutonium metals nor- nium metals, which have lo- Joint Research mally have localized 5f elec- calized 5f electrons, can bring Centre, Institute trons, display symmetrical about significant changes in for Transuranium Elements; Tristan structures, and their bonding their bonding. Le Bihan, European consists of three non-f-electron As these actinides have Synchrotron Oak Ridge’s Dick Haire conduction electrons (i.e., more spatially extended Radiation Facility; discussed research by an “trivalent metals”). In this re- f electrons than their lan- and Andreas international team on the gard they mimic the lanthanide thanide homologs, it should Lindbaum, Vienna use of high pressure to metals. be easier to force their 5f elec- University of probe actinide bonding. One obvious difference be- trons to become bonding by Technology, Institute tween these f elements across applying pressure—that is, for Solid State the series is seen in the variation of their they may be more sensitive to the affects of Physics. atomic volumes at normal pressure, as shown pressure than their lanthanide counterparts. in the figure at right. The similarity of the trivalent, localized 4f- and 5f-electron met- als’ volumes contrasts Atomic volumes with the behavior ex- of the f-electron hibited by the four elements reflect itinerant 5f-electron differences in metals (i.e., protac- bonding present for tinium-plutonium). the elements. The The latter have much smaller volumes for smaller atomic vol- the protactinium- umes in addition to plutonium metals their lower crystal reflect their itinerant 5f-electrons. symmetries.

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Contributed Papers

Selecting the f-electrons for study An international team of scientists from Oak Studies of the actinides can be difficult, Ridge National Laboratory, the European Insti- given their varying levels of radioactivity and tute of Transuranium Elements, the European availability. Of interest to researchers is the po- Synchrotron Radiation Facility and the Vienna tential for forcing increased bonding in the ac- University of Technology carried out the studies. tinides by applying pressure. For example, can The researchers examined the structural be- pressure force divalent einsteinium metal (ele- haviors of the protactinium and americium up ment 99) to acquire a third bonding electron? to pressures as high as 130 giga pascals (GPa) Basically, can pressure force one element to using different types of diamond anvil cells to mimic the bonding, properties, and structures generate static pressures on the metals through of another—a sort of modern alchemy? different transmitting media (silicon oil or liq- Two actinides were selected for recent ex- uid nitrogen). Pressure calibrations employed perimental studies to probe these questions. ruby fluorescence, and/or copper or platinum One study involved americium and a more metals via their known equation of states. recent study involved protactinium. (Reports Modern diffraction techniques employed mul- have been published in Physics Review Letters, tiple wavelengths of high-intensity radiation Vol. 85, 2961; Phys. Rev.B, 63, 214101, (2001) from the synchrotron and charge-coupled de- and 67, 134101, 2003.) The behavior of these vice (CCD) detectors. two metals under pressure is of special inter- est; the studies looked at the actinide elements What was found experimentally? that “initiate” and “complete” the “dip” seen The research found that the tetragonal struc- in the actinide volume curve in the figure on ture of protactinium at atmospheric pressure, the previous page. where itinerant 5f-electron behavior in the Protactinium has the distinction of being the bonding already exists, converts to an ortho- first actinide metal with a 5f electron (actinium rhombic structure at about 77 GPa that is iso- and thorium have d-electrons), and it already structural with that of alpha-uranium. has partially itinerant 5f electrons. In contrast, At this pressure, the protactinium’s atomic americium is the first element in the series volume is reduced to about 62 percent of that having fully localized 5f electrons at atmo- at atmospheric pressure. Accompanying this spheric pressure, following the four preceding first-order transformation from the protac- actinides that have itinerant 5f electrons. tinium-I (Pa-I) to protactinium-II (Pa-II) crystal These recent studies were conducted to de- structure is a small relative volume collapse termine if pressure can force additional 5f-elec- of 0.8 percent. It is postulated that structural tron bonding in protactinium—making it more transformation with pressure (the Pa-I to Pa-II like uranium—and if pressure can force the transformation) reflects an acquisition of ad- onset of 5f-electron bonding in americium— ditional 5f-electron character in the metallic making it behave like the protactinium– bonding and alters its basic properties. plutonium metals, or specifically, its near neighbor plutonium.

24 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

With americium under pressure, it was de- An important difference between the two termined that the delocalization of its 5f elec- metals is that americium starts out having trons occurs in two steps, with the progressive fully delocalized 5f electrons, while the 5f elec- formation of lower-symmetry crystal struc- trons of protactinium already exhibit some 5f- tures (i.e., the third and fourth structures). electron itinerancy at atmospheric pressure. Overall there are three phases changes up to Both acquire additional 5f character in their 100 GPa; the first change from double hexago- bonding with pressure. This initial difference nal close-packed (dhcp) to face-centered cubic is reflected in specific behavior and properties (fcc) represents only a second-order phase under pressure. Comparisons between the change. In the first transition involving delo- compression and structural behaviors of am- calization (third phase, Am-III), americium ericium, uranium, and a 50 atomic percent acquires the structure of gamma-plutonium— americium-curium alloy with pressure are an orthorhombic structure (space group shown in the figure on page 26. Fddd)—at about 10 GPa, this structure The more compressible nature and overall is accepted as reflecting itinerant volume reduction of americium metal is due 5f-electron character. to it starting as a fully localized 5f-electron, This first-order transformation (Am-II to trivalent metal, whereas uranium and protac- Am-III) is accompanied by a small but signifi- tinium initially have itinerant 5f electrons. The cant relative volume collapse of about 2 per- figure on page 27 also shows that the slope of cent, which signifies that a change in bonding the compressibility curves for alpha-uranium, occurs. It is followed by formation of a fourth Pa-II, Am -IV, and the 50 atomic percent phase (Am-IV), a primitive orthorhombic (Am,Cm)-IV structures (i.e., in the high- structure (space group Pnma) at about 18 GPa pressure regions where they exist) are all and exists up to 100 GPa. The latter is closely very similar, as they should be, given that related to a Cmcm structure, the alpha- all have either an alpha-uranium or a closely uranium structure and the Pa-II structure); related structure. this structure is also accepted as reflecting the The latest published calculations based on presence of itinerant 5f electrons in the metal- theory for the behavior of americium under lic bonding. Accompanying the formation of pressure were in agreement with this experi- Am-IV is a more significant relative volume mental work. In addition, Per Söderlind collapse of about 7 percent. (LLNL) and Olof Eriksson of the University Thus, pressure has forced the delocalization of Uppsala, Sweden, have also used a calcula- of the 5f electrons in americium after its tional approach involving Gibb’s free energies atomic volume and interatomic distances are to predict a series of structural transforma- reduced considerably (at 100 GPa the volume tions for protactinium metal under pressure is about 46 percent of that at atmospheric (see Physics Review B, Vol. 56, 10719, 1997). pressure). Under pressure, the Am-II phase becomes like plutonium (its near neighbor), and at higher pressures, americium then adopts the alpha-uranium structure.

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Contributed Papers

Additional Aspects Several other pieces of important informa- tion can be acquired from such experimental pressure studies (i.e., the bulk modulus and its pressure derivative acquired by applying the Birch-Murnaghan equation of state). In a simple picture, the bulk modulus indicates the “stiffness” of a structure’s lattice—the more rigid the structure, the less compressible it is, and the more likely contains a greater degree of bonding. Bulk modulli and pressure deriva- tives are given in the figure at left for ameri- cium, uranium, protactinium, and the ameri- cium-curium alloy. Note that the modulus for americium is about 29 GPa (derived from the Am-I phase), while those for alpha-uranium and the Pa-I structure are more than 100 GPa. It can be informative to compare the com- pression behaviors of the different metals, the pressures at which structural transformations A comparison of the Our recent experimental work on protac- occur, and the overall picture of the relative compression tinium confirmed their prediction that an al- volume behaviors. behavior of uranium, pha-uranium structure would be formed un- The close relationship between the body- protactinium, der pressure from the body-centered tetrago- centered tetragonal structure of protactinium americium, and a 50 nal form, with the differences between experi- (Pa-I) and the orthorhombic Cmcm structure atomic percent of alpha-uranium can be seen in the figures at americium-curium ment and theory being only at which pressure alloy is shown. Bulk and at what relative atomic volume the Pa-I to right. The main difference between the struc- modulli and pressure Pa-II transition occurs. However, the story for tures occurs from a buckling of the “chains” in derivatives extracted protactinium remains incomplete, as theory the latter due to the “y” parameter of the 4c from the data are predicts that additional structural transforma- sites of Cmcm. When the Pa-I structure also given. (Adapted tions will occur at much higher pressures changes to the Pa-II structure, the “y” from the Journal of (presently difficult to reach experimentally). parameter of the Pa-II structure differs by Condensed Matter It will be a difficult but interesting challenge less than 0.02 from that of alpha-uranium. Physics, Vol. 15, to try to reach these ultra high pressures to S2297, 2003.) confirm the theoretical predictions.

26 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

The Am-IV structure also shows a close simi- larity to that of alpha-uranium; it is only neces- sary to shift the position parameter “z” of the Pnma structure 4c sites to zero from “z=0.10.” The three structures in the top of the figure at right show the close relationship that exists be- tween the four phases. The structural sequence for americium metal under pressure is shown in the lower part of that figure. The first two transformation processes basically involve changes in the stacking sequence of planes; and the formation of the Am-IV structure requires an additional “buckling” of the hexagonal planes.

Special facets of americium and protactinium Important changes in the role of the 5f elec- trons in the actinides can be extracted from perturbations in their atomic volumes and the structures displayed under pressure. The work on americium and protactinium metals under pressure have shown that pressures increases the itinerant (bonding) nature of their 5f elec- trons. The onset in (americium) or increase in (protactinium) delocalized 5f-electron character reduces the atomic volumes of the metals, as well as producing structures of lower symme- try. In contrast, heating plutonium reduces the 5f-electron contribution and increases its atomic volume (over that due to expansion). Using a simple picture, plutonium becomes more like the localized 5f-electron metals upon heating and americium becomes more like plu- tonium under pressure. Both changes reflect Structures of selected actinides under pressure. The similarity of the the changing influence of the 5f electrons in protactinium-II and americium-IV structure to that of the alpha-uranium the elements’ bonding. structure is evident. The lower segment shows the structural sequence of americium under pressure to 100 giga pascals.

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Contributed Papers

Future experiments einsteinium’s 5f electrons to delocalize, per- A number of experiments are envisioned to haps displaying the behavior observed with This work was further explore the actinide metals under pres- americium metal under pressure. One can supported in part sure to better understand the science that is also easily envision numerous other studies by the European Commission and occurring. Certainly, studies under higher of actinide alloys and/or compounds by the Division of pressures are desired to explore the potential under pressure. Chemical Sciences, for an onset of new interactions that may Geoscience and drive the formation of new structures and Conclusion Bioscience, OBES, alter the bonding. Studies under pressure can provide impor- USDOE, under contract DEACOR- There is also the need to study actinide ele- tant insights into the 5f-electron behaviors and 00OR22725 with ments with higher Z values using these im- their changing roles. It appears that orthor- Oak Ridge National proved experimental techniques, and both cu- hombic structure-types (i.e., alpha-uranium or Laboratory, managed rium and californium metals are two potential the closely related Pnma structure-type) are by UT-Battelle, LLC. candidates. Their behavior under pressure will encountered under pressures in the region of The authors wish to acknowledge beam be affected by the withdrawal of their 5f elec- 100 GPa. Whether this type of structure is a time provided at the trons from their Fermi edges, due to the in- dominant high-pressure structure in this re- ESRF (ID30 beam creased nuclear charge. The latter should make gion remains to be determined. Theory pre- line) in Grenoble, it more difficult to force the delocalization of dicts more symmetrical structures may again France, where the their 5f electrons, and should require higher be encountered at even higher pressures, due diffraction studies were performed, pressures to bring about. to an increase in dominance of electrostatic and the efforts of Einsteinium is the highest actinide for interactions and Born-Mayer repulsions. personnel at the which such pressure experiments can be envi- These studies with americium and protac- ESRF. One author, sioned, given the quantities of actinides ex- tinium metals will not only help other re- Andreas Lindbaum pected to be available. The behavior of this searchers to better understand the bonding wishes to thank the Austrian Academy metal under pressure would be very interest- and electronic structure of two elements, but of Sciences (APART ing to study, but it is also an exceptionally dif- also that of other actinides. They should enable 10739) and the ficult material to study because of the very the development of important systematics con- Austrian Science short half-life and intense radiation of its most cerning the actinides. The experimental data Fund (P14932) for available isotope (einsteinium-253). should also serve as a platform from which financial support. Special thanks also An attempt to study this metal under pres- one may evaluate and fine-tune theoretical go to Gerald Lander sure was unsuccessful, due to crystal destruc- predictions, thereby enhancing the overall un- (European Institute tion by its self-irradiation. A special interest in derstanding of these complex actinide metals. for Transuranium einsteinium’s behavior under pressure is that, In addition, such research may contribute to an Elements) for in principle, this divalent metal could be improved understanding of other elements in discussions and his strong interest in forced by pressure to become a trivalent metal the Periodic Table. promoting these like the americium through californium ele- experimental efforts. ments at atmospheric pressure. With addi- tional pressure, it may be possible to force

28 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

Process may have advantages over traditional cleanup methods Contributed Researching the Papers phytoremediation of plutonium Christy Ruggerio hytoremediation—using plants to Phytoextraction takes the process a step fur- clean up the environment—relies on ther; it relies on metal-accumulating plants to the fact that plants are essentially actually take up metals into their aboveground This article was P contributed by Christy solar-driven pumps, transpiring water from parts, thereby removing toxic metals from soils their roots through their leaves. In the process, and water. Metal-loaded plants are harvested, Ruggiero, Elise Deladurantaye, they can degrade organic chemicals and stabi- dried, ashed, composted, or stored. The vol- Brandy Duran, and ume of resulting waste is generally a fraction of lize contaminants in their root zone, or even Stephen Stout of take up contaminants into their leaves, thus waste produced by many current, more inva- Structural Inorganic allowing them to be removed from the soil. sive remediation technologies, and associated Chemistry (C-SIC); Phytoremediation has distinct advantages costs are much less. and Scott Twary of over traditional cleanup methods: it is safe, Most research on metal phytoextraction has the Bioscience relatively inexpensive, usually simple to focused on finding naturally occurring plants Division Szilard implement with existing agricultural practices, that can take up a large amount of metal com- Resource (B-3). and has a positive public image. pared to their biomass (called hyper-

Elise Deladurantaye, a student in the Structural Inorganic Chemistry Group (C-SIC), checks to see if barley plants have enough nutrient solution to grow for one more day before being harvested to determine their uranium uptake levels. The barley seeds are first sprouted on a small water-soaked mat and then transferred to a nutrient-rich growth solution, where they grew hydroponically in a controlled-atmosphere growth chamber in a Chemistry Division laboratory. The bottles containing the plant solutions are equipped with an air bubbler inlet to make sure the growth solution doesn’t stagnate and a HEPA-filtered outlet. The plants grow in the nutrient solution with either no iron, normal iron levels, or high iron levels (well-fertilized). After adjusting to the iron levels for one week, a solution of uranium is added to the nutrient solution and the plants grow in (and take up) the uranium. The plants are harvested by clipping off the stems and the roots and letting them completely dry in small vials. The dry plant parts are digested in acid and then analyzed for uranium content using ICP (Inductively-coupled Plasma atomic emission) spectroscopy and compared to uranium concentration standards.

Phytoremediation of metals has been particu- accumulators), and then developing these larly successful. A few techniques are being plant species through genetic alteration for explored commercially, including improved field performance, and for a wider phytostabilization and phytoextraction. range of metal uptake. Phytostabilization relies on the plants to de- Work with hyperaccumulators has focused crease contaminant migration by decreasing on commercially produced “heavy” metal con- wind and water erosion of soils, while the taminants—such as lead, mercury, chromium, plants pull soluble species into their root cadmium, zinc, copper, arsenic, and nickel— zone where metals can be sorbed onto roots rather than radionuclides such as plutonium, and stabilized.

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Contributed Papers

thorium, and uranium. These groups of metals Phytotech Inc., a metal phytoremediation are chemically very different, so it is unlikely company, published work in “Environmental that any known hyperaccumulators will be Science and Technology,” [1998, 32(13), 2004] able to hyperaccumulate plutonium or other showing that added citrate can increase plant actinides. Researchers have even speculated uptake of uranium more than 1,000-fold. Unfortunately, adding such chelators can Graminaceous backfire. Even though they increase metal plants produce solubility, the rate of metal translocation into and release the plant roots may still be limited. This can phytosiderophores, lead to large amounts of solubilized metals a family of that are not able to get into the plant, leading nonprotein amino to increased soil migration and leaching to acid that enhances ground waters. Luckily, there may be a better solubility and uptake way to “trick” plants into taking up pluto- of iron and possibly nium, or at least trapping it in the plant other metals from root zone. the environment. Phytosiderophores Plutonium behaves similarly to iron, a nec- are derivatives of essary nutrient in biological systems. Both mugineic acid. iron(III) and plutonium(IV) readily hydrolyze, Shown here is have rich redox chemistry, have high coordi- the mugineic that no hyperaccumulator will ever be found nation numbers, have similar charge-to-radius acid family of for the actinides. ratio, and often have predictably similar bind- phytosiderophores, Fortunately for actinide phytoremediation, ing affinities for the same chelating ligands. nicotianamine (a true hyperaccumulators are not absolutely Both have extremely low solubility in the en- phytosiderophore necessary. A plant with high biomass but a vironment. Successful development of many biosynthetic lower percent of metal uptake will remove plutonium complexation and decontamination precursor), EDTA, agents has relied on this iron-plutonium simi- and citric acid. the same amount of metal from the soil as a plant that has low biomass (such as most larity by exploiting iron chelating ligands. hyperaccumulators) but a higher percent of We hope to be able to use plant iron-uptake metal uptake. systems to trick plants to take up large Maximum efficiency is still achieved by in- amounts of plutonium, or at least trap it creasing the percent of metal uptake into the in the plant root zone. plant. This can be done by increasing the Graminaceous plants (grasses such as metal solubility, increasing the ability of the barley, oat, and wheat) produce and release metal to cross the root membrane, and increas- phytosiderophores, a family of nonprotein ing the amount of metal translocated into the amino acid that enhances solubility and higher plant parts. uptake of iron and possibly other metals Increasing metal solubility alone has from the environment (see figure at left). been shown to dramatically improve Phytosiderophores, all derivatives of phytoextraction efficiency. Chelators such as mugineic acid, form stable complexes with EDTA or citrate are added to soils to improve numerous metals. metal solubility and increase plant uptake, a Phytosiderophores have been shown to process called chelate-enhanced, or induced, enhance soil mobility of copper, iron, manga- phytoextraction. For example, researchers at nese, and zinc as much as microbial

30 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

siderophores and more Researchers have than some synthetic used ultraviolet- chelators. Some visible (UV-Vis) spectroscopy to researchers have examine the binding shown that of plutonium(IV) to phytosiderophores are mugineic acid (MA) up to 100 times more or EDTA over a large efficient than anthro- pH range. pogenic or bacterial iron chelators for iron uptake into graminaceous plants, presumably because they are the chelate form recognized by the plant roots. They are notably similar to EDTA in structure and metal-binding chemis- try. Both have multi- carboxylate chelating groups and form extremely strong complexes with iron and at which phytosiderophores solubilize pluto- other transition metal ions. nium hydroxides in comparison to EDTA and We have examined the binding of the resistance of plutonium-phytosiderophore plutonium(IV) to mugineic acid over a large complexes to microbial degradation. pH range using ultraviolet-visible (UV-Vis) To investigate whether we can use this iron- spectroscopy (see figure above). Mugineic acid uptake system for plutonium and other ac- prevents hydrolysis and precipitation of tinides uptake into these plants, we need to plutonium(IV) to a pH greater than 10; spectra know if plants recognize and take up actinide- Pictured are of plutonium(IV)-EDTA and plutonium(IV)- proposed structures mugineic acid are nearly identical at all but the of the plutonium- highest pH. In this pH range, plutonium(IV)- EDTA complex as EDTA is believed to be a hydroxo rather than the pH increases, and possible aquo complex, as it is under acidic pH. analogous structures The plutonium-mugineic acid complex may for the plutonium- not undergo this change, or it may occur at an mugineic acid even higher pH. Possible structures of the complex. Data plutonium(IV)EDTA and plutonium(IV)- suggest that mugineic acid complex are shown in the figure mugineic acid may at right. These data suggest that mugineic acid be as powerful a will be as powerful a ligand as EDTA for keep- ligand as EDTA for ing plutonium species soluble in the environ- keeping plutonium ment. We are currently directly testing the rate species soluble in the environment.

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Contributed Papers

Initial tests with 500 2.5 uranium show some pH6, Citrate pH6, No Citrate interesting results. pH5, Citrate Barley plants were 400 pH5, No Citrate 2.0 grown in hydroponic solutions with 1,000 parts per million 300 1.5 uranium with or without citrate; without added iron, 200 1.0 with 7.5 micromoles added iron (“normal”), and with 100 micromoles of 100 0.5

added iron (“high”); dry weight mg uranium/g root and at a pH of 5 and mg uranium/g shoot dry weight 6. Error bars show 0 0 one standard No Fe Normal Fe High Fe No Fe Normal Fe High Fe deviation among the three to four plants grown at each siderophore complexes or whether they are Our initial tests with uranium show some condition. trapped in the plant root zone. Alternatively, interesting results. Barley plants grown in hy- we need to know if phytosiderophores droponic solutions with 1,000 parts per mil- create soluble species that aren’t taken up lion uranium with or without citrate have low or broken down, and therefore contribute to uranium uptake into plant shoots (typically plutonium migration. 0.2–3.5 milligrams per gram plant dry To test if phytosiderophores can aid in ac- weight), with no consistent response to plant tinide uptake, we are examining the effects of iron status, presence of citrate, or pH (pH 5 or plant iron status. Iron-starved plants produce 6), although on average, uptake with citrate more phytosiderophores than iron-saturated was slightly higher (see figure above). This ones, and have dramatically higher uptakes of was surprising because we expected citrate zinc, nickel, manganese, copper, and even cad- and decreased pH to cause significantly mium (under some growth conditions), than higher shoot uptake because of their ability to do high-iron nutritional plants. keep uranium soluble. We did not expect the We are growing both a phytosiderophore- iron status of barley plants to have a major producing plant (barley) and a non-phyto- effect on uranium uptake into shoots because, siderophore-producing “hyperaccumulator” although the phytosiderophore could contrib- species (Indian mustard) under variable iron ute to uranium solubility, it is unlikely the conditions, and with actinides added as vari- iron(III)-phytosiderophore uptake system ous chelated forms (phytosiderophore-che- in plants would recognize and take up a lated, citric acid-chelated, and unchelated) to uranium(VI) phytosiderophore complex. see how this affects uptake into the plant.

32 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

Barley plant roots have very high sorption If plutonium-phytosiderophore complexes of uranium from solution, as has been ob- are recognized and translocated by the served with other plant species. We observed phytosiderophore uptake system in plants, values from 4 milligrams uranium to more plutonium solubilization may be balanced than 600 milligrams uranium per gram dry with uptake, eliminating a problem of chelate- root weight—one to two orders of magnitude induced phytoextraction. more than in shoots (see figure on page 32). Phytosiderophores may also improve trans- Interestingly, we did observe a significant location into higher plant parts. Their biosyn- difference in root uranium binding based thetic precursor, nicotinamine, is ubiquitous in on growth conditions. plants and is believed to affect cation (copper, Barley roots have significantly more ura- iron, zinc, and manganese) mobilization and nium bound without citrate present than with distribution in plants, presumably citrate, regardless of pH and iron status. This having a direct influence on plant metal could simply be due to increased precipitation uptake and translocation. of uranium onto the roots without citrate. Even if efficient uptake into the higher parts However, without citrate present, roots bound of the plant cannot be achieved, we will deter- significantly more uranium at pH 5 than at pH mine how phytosiderophores will affect 6. This may be caused by such rapid precipita- phytostabilization. Phytosiderophores will tion of uranium from solution at pH 6 that it is chelate actinides. These chelated actinides unavailable for interaction with the plant could be fluxed to the root zone of the plant, as roots. And at pH 5, roots of plants with excess are other metal-phytosiderophore complexes, iron bound more uranium than roots of plants and could have increased precipitation onto grown without iron. This could have many the plant roots. Concentrating contaminants causes. There could be increased microorgan- and fixing them onto the plant roots may help ism presence on the “high-iron” roots increas- prevent migration and improve the efficiency ing sorption of metals, or there could be in- of phytostabilization. Alternately, creased solubilization of uranium on the roots phytosiderophores could chelate actinides and by chelators, such as phytosiderophores, when prevent precipitation onto plant roots, thereby plants are iron starved. increasing the risk of contaminant migration. We will soon test plutonium uptake into We think this research holds enormous po- plants under these same conditions, where we tential. Grasses grow anywhere and every- expect iron status and phytosiderophores to where, from arid and semiarid sites, like the make significant differences. We now know Nevada Test Site and the Rocky Flats Environ- phytosiderophores are good chelators for mental Technology Site, to wet and subtropical plutonium(IV), and have a good chance of be- sites like Oak Ridge National Laboratory and ing recognized by the iron-uptake system of the Savannah River Site. the plant because of the similarities between iron and plutonium.

Nuclear Materials Technology/Los Alamos National Laboratory 33 Actinide Research Quarterly

Plant uptake of plutonium

Plant uptake of actinides is well documented, These studies also indicate plutonium is particularly from past research on human health transported as an anionic organic complex impacts of nuclear weapons use and testing. that is different than the form supplied to root- We know far more about plutonium uptake into bathing solutions. In hydroponic studies in plants from these past studies soybeans using plutonium(IV) nitrate, xylem Root Rhizosphere than we know about other exudates analysis of samples with plutonium actinides. Relative plant added both in vivo and in vitro (xylem exudates fragment uptake availability for collected then plutonium added) using anion actinides is neptunium and cation exchange columns and thin-layer Fe(III) PS > uranium > curium > electrophoresis showed that both iron and americium > plutonium, plutonium are present primarily as organic acid which is a reflection of complexes (instead of amino acid or peptide). their solubility under environmental Nickel(II) and cadmium(II) were present primarily Fe Fe(III) conditions. with components of the amino acid/peptide fraction, again showing the differing uptake Soil Many variables have and transport systems for hard and soft metals. microbes been shown to affect The forms of plutonium transported in the xylem plutonium uptake into appear to change with plant age, and the Fe(III)-PS plants: plutonium form amounts of plutonium parallel essential ion and concentration, soil concentrations. PS chemistry, soil type, number Fe(III) of successive plantings, plant The evidence suggests that plutonium may track species, and plant surface area the normal plant ligands used for metal uptake (when fallout and resuspended soil are major and growth. Indeed, it has been suggested that Grasses produce accumulation contributors). after absorption in the plant, trace contaminants phytosiderphores are translocated, metabolized, and stored (PS) to acquire the As with most metals, it seems that the generally analogously to nutrient elements. iron (Fe) they need. predominant factor in plutonium uptake In support of this, nutrient elements, especially Could these from soil by plants depends on chelation of iron, zinc, and copper, that compete for reaction phytosiderophores plutonium to increase plutonium solubility. sites and organic ligands affect the chemical also be used to trick Application of synthetic chelators (EDTA and form of plutonium in the plant. plants into taking up DPTA), to either soil or hydroponic solutions, plutonium? has been shown to increase uptake of plutonium Phytosiderophore-mediated uptake could explain up to 1,300 times—resulting in a 20-fold some of the observations made on plutonium concentration. Ranking of various ligands uptake into grasses. and counterions for enhancing uptake of plutonium has been shown to be nitrate < acetate < glycolate < oxalate < citrate < EDTA < DPTA, which is a reflection of ligand affinity for plutonium. Pu removed Plutonium is translocated from Grasses from soil the plant roots into the higher The plutonium parts of the plants. Plants reduce Erosion and phytoremediation Wind and water Soil migration plutonium(VI) upon uptake. erosion amendments, prevented process is illustrated. additional Various studies have shown Pu chelators plutonium is present only as Pu Pu Pu Pu Pu e Pu plutonium(IV) (greater than 90 c s, Pu du Pu Pu ore Pu ro h s p p id Pu Pu Pu percent) in plant xylem, even if nts ro c Phytoremediation: la e , a P sid rs to to Pu uptake supplied as plutonium(VI). This y la ph e Pu concentrated Phytostabilization: ch reduction process may be similar and stabilized Pu association to the purported requirement for Phytosiderophore- reduction of iron(III) to iron(II) chelated plutonium prior to root membrane Mobile/soluble/chelated ade chelators translocation. Sorbed and "nonmobile" Plants degr plutonium plutonium (PuO2) pogenic and natural) Chelators (anthro Migration

3434 NuclearNuclear Materials Materials Technology/ Technology/LosLos Alamos Alamos National National Laboratory Laboratory 3rd/4th quarter 2003

The future of plutonium science Plenary Plenary speakers discuss Speakers the many avenues of research

Condensed Matter Physics negative pressure to a thin deposited film of parallel institution to Los Alamos plutonium to illustrate what he described as a in the sense of its broad diversity “pseudo phase change” in delta-phase pluto- A of nuclear-materials research, the nium. While he admitted that the exact inter- European Union’s Institute of Transuranium pretation was still controversial, he also en- Elements (ITU) was ably represented by thused, “these experi- Gerry Lander at the conference’s first ments are very beautiful, plenary presentation. and they give us a nice Lander was enthusiastic in elaborating handle on the electronic some novel ITU approaches to the actinide structure of plutonium.” electronic structure conundrum. He described Continuing on this a body of work that employed high pressure theme of understanding to alter the atomic volume in actinides. electronic structure, Displaying compressibility curves for ura- Lander described the dis- nium, americium, and an americium-curium covery of superconduc- alloy, Lander presented evidence for a tivity in plutonium com- transition from localized to bonding behavior pounds as “the most ex- in americium’s 5f electrons when the element citing thing in actinide was compressed to 50 percent of its science for the last fifty atomic volume. years.” He was animated He subsequently described a clever way of about the ITU–Los performing the reverse experiment, applying Alamos partnership, describing it as a collabo- Gerry Lander ration in which the two organizations contrib- uted complementary skills and were continu- ing to work very closely together.

Actinide Compounds and Complexes Los Alamos staff member Wolfgang Runde of the Isotope and Nuclear Chemistry Group (C-INC) offered a primer in the selected chemistry of actinide compounds, particularly those relevant to modern methods of pluto- nium purification related to pit manufacturing. Despite what he characterized as “intensive study of transuranic minerals,” Runde empha- sized that single-crystal structures of pluto- nium compounds—particularly those of plutonium(V) and (VI)—remain rare. Because of their importance in plutonium processing and purifications, and in long-term nuclear waste management, his presentation focused on the oxalates, hydroxides, and silicates of plutonium, which despite their importance, Wolfgang Runde are poorly characterized on a structural level.

Nuclear Materials Technology/Los Alamos National Laboratory 35 Actinide Research Quarterly

Plenary Speakers

His talk covered aspects of Reis’ vision emphasized nuclear power. both the preparation of such With the construction of a system of next- compounds and the insights generation nuclear power plants, electricity acquired into their molecular could become safe, plentiful, affordable, and structure. environmentally friendly. This extensive In addition to the wealth of growth of nuclear power would require coun- specific information presented tries to work together to settle regional con- in this talk, it also conveyed an flicts and curtail international terrorism and overall sense of the interesting rogue states. Wars would have to be mini- and complicated chemistry mized if not eliminated. that keeps actinide researchers Key tasks for the United States include ex- engaged in trying to compre- tending licenses of existing nuclear power hend the myriad of bonding plants and opening the Yucca Mountain re- Vic Reis and behaviors that Theresa Fryberger actinides display in both laboratory and pository, Reis said. The United States govern- natural environments. ment also must help industry with costs asso- ciated with construction of new plants, con- The Nuclear Fuel Cycle tinue the development of the advanced fuel On Dec. 8, 1953, President Dwight cycle initiative, and design a fourth-generation st nd Eisenhower delivered his “Atoms for Peace” nuclear reactor. (See ARQ 1 /2 quarter 2003.) speech before the United Nations. Eisenhower’s goal was to apply “the miracu- Materials Science and Plutonium lous inventiveness of man” to change “the Properties fearful atomic dilemma” into something that One of the more interesting characteristics could benefit humanity. of plutonium is how it ages. In essence, pluto- nium ages from “inside out” and “outside in,” which means that it not only changes its own properties, but also has the potential to change the properties of surrounding materials. “The three most important aging effects in plutonium are the radiogenic decay of the various plutonium isotopes, the possible thermodynamic insta- bility of the plutonium alloy itself, and the Fifty years later, Vic Reis postulated that corrosion of perhaps it was time to initiate what he called plutonium’s surface an “Atoms for Peace II.” Before he outlined the during both storage possible vision for such an endeavor, he noted and function,” said Joe that this new vision would also be for fifty Martz of Los Alamos’ years, which is equivalent to the lifetime of one Materials Science and nuclear power plant. Reis, of Science Applica- Technology (MST) Di- tions International Corp., is a former DOE vision. “These aging assistant secretary for defense programs. effects accumulate slowly over decades.

Joe Martz

36 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

Furthermore, such effects may not take place in remediation decisions for specific problems at a linear fashion.” specific DOE sites; (but) we have far to go To ensure the performance, safety, and reli- before we can describe the complex interactions ability of the U.S. nuclear weapons stockpile, and processes that are important in transport.” researchers must be able to predict when aging In the future, she said, scientists must use effects negatively influence a weapon’s perfor- the available DOE computing capabilities and mance. To accomplish this goal, scientists must insert mechanistic and chemical information understand the properties of plutonium into models; use interdisciplinary throughout its “lifetime,” from the time it is teams to do comprehensive, long- manufactured to the time that it is “retired term field studies that include model- from service.” ing in design and interpretation; and “Virtually all conditions in plutonium are develop improved characterization ripe to age-related damage,” Martz concluded. tools. “Yet, we have found no first-order effects after several decades. Hence, we feel reasonably Detection and Analysis comfortable that the lifetimes of components Christopher Puxley of the Atomic will last at least several tens of years.” Weapons Establishment at Aldermaston in the United Kingdom Actinides in the Environment brought scientists up to date on the Teresa Fryberger, director of the DOE’s application of vibrational spectros- Division of Environment Remediation Sci- copy to actinide analysis. ences, Office of Biological and Environmental Vibrational spectroscopy can be Research, Office of Science, told her audience: used for in situ nondestructive analy- “It is clear that we won’t be able to remove all sis of both bulk and trace actinide contamination at all sites. We must understand compounds—notably for the detec- what contaminants are mobile and under tion and analysis of typical species what conditions.” and unexpected contaminants that Such understanding, she said, will make it occur on the surface of actinides. possible to develop science-based risk evalua- Puxley made three points about tions to produce better decision-making; using fiber-optic analysis on actinides. First, Christopher Puxley more-effective remediation and containment fiber-optic midinfrared analysis is applicable to strategies; and a solid understanding of what, organic materials and their degradation prod- when, and where to monitor at sites in long- ucts. Its applicability to inorganic compounds term stewardship. is limited to oxyanion identification, low-mo- Current actinide research in the DOE weap- lecular-weight atoms in materials (e.g., hy- ons complex includes study of uranium migra- drides), and low-molecular-weight, multiply- tion under the tank farms at Hanford Site bonded anions. in Washington; plutonium transport at Rocky Second, fiber-optic Raman analysis is appli- Flats Closure Project in Colorado; plutonium cable to both inorganic and organic materials. sources and mobility at Savannah River Site in It is extremely useful for the identification of South Carolina; and microbial effects on ac- inorganic compounds but is difficult to apply tinide speciation and mobility. to compounds that are black. “We have learned a great deal about the And third, because of its high sensitivity and molecular interaction of actinides in the sub- small spot size, fiber-optic Raman spectroscopy surface,” Fryberger said. “We have been able is a valuable technique to detect contamination to provide scientific data that has impacted and surface species on bulk materials.

—Vin LoPresti, Octavio Ramos, Jr., and Charmian Schaller

Nuclear Materials Technology/Los Alamos National Laboratory 37 Actinide Research Quarterly

Helen Caldicott and Sig Hecker Point/Counterpoint Point Counterpoint Helen Caldicott, founder of Physicians for Social Helen Caldicott’s views on nuclear weapons did not go Responsibility, sketched out a very dark picture of plutonium unchallenged. Sig Hecker, Los Alamos Senior Fellow, rose and nuclear weapons in a speech that opened the “Actinides from the audience to present “another point of view.” in the Environment and Life Sciences” portion of the conference. Hecker, a plutonium metallurgist, said, “You brought us back to reality because the last few days, we were having great fun…. I She told her audience of plutonium appreciate your point of view. I happen not to agree with it.” scientists—many of them from the And, he said, the viewpoint of plutonium scientists is “not less nuclear weapons laboratories—that, noble in terms of service to “Plutonium is incredibly carcinogenic.” humanity” than Caldicott’s. Sufficient exposure, she said, kills cells, and the cells that survive are He said that if one looks at the “likely to have mutations.” The resulting statistics, about a million people genomic progression of instability can died as a result of wars in the 1500s continue for more than 30 generations. and 1600s. In the first half of the 20th century, 87 million people died in Caldicott, an Australian pediatrician wars. “Soon,” Hecker said, “we with a specialty in cystic fibrosis, said, would have found a way to wipe “We find out what radiation does ourselves off the face of the Earth.” by doing decent epidemiological However, in the last half of the 20th Helen Caldicott studies,” but not enough century there was a dramatic drop in epidemiological studies are being Sig Hecker the number of deaths. “In science, done at autopsy, partly because of their expense. we call that a discontinuity . . . a discontinuity in the death of mankind,” said Hecker. This was She mentioned new work being done by Alexander Miller an apparent reference to Caldicott’s lack of scientific data in at the Armed Forces Radiobiology Research Institute in her discussion. Bethesda, Maryland, John Little at the Harvard School of 100

Public Health, and Eric Wright at the University of Dundee “Nuclear weapons got mankind 87M in Scotland on “the bystander effect”—the development of to think differently,” Hecker said. 75 genetic mutations by cells on the periphery of tissue exposed “I view it as a good thing that to radiation. occurred during the buildup of 50 nuclear weapons.” The question She said that plutonium in the particulate form that is less than now, he said, is, “How does one Millions of People 25 20M five microns in diameter can be inhaled. It can settle into the manage significantly reducing 6.2M 6.4M 17M lower part of the lungs, where it can cause mutations. the number of nuclear weapons 0 1.5M 1500s 1600s 1700s About five tons of plutonium was released into the atmosphere without the number of deaths 1800s 1900 1949 during weapons testing, she said, and one result is that, “We sneaking up once again?” to to Death due to war 1948 1990 are increasing genetic disease by the contamination of the atmosphere with plutonium,” and contamination lives on in Caldicott said, “I know you say you developed nuclear power the genes. to maintain the peace, and it was a good idea, but you don’t know” whether those weapons will eventually be used. She Plutonium, which has a 24,000-year half-life, was first made said she believes that unless mankind abolishes nuclear for nuclear weapons. Caldicott said she knew some of the weapons, “there will be a nuclear war” somewhere, someday early scientists who worked on the bomb, but, “I became because of the “fallibility of human minds.” terribly alarmed about what they did.” Jon Schwantes of Lawrence Berkeley National Laboratory And she concluded, “I’m talking to you as a physician, as a asked Caldicott whether she had developed any figures on the pediatrician … You must stop building nuclear weapons! … I transport of plutonium particles and their resulting danger as don’t know if you’ve got 10 more years.” compared to the transport and resulting danger of smoke particles.

She said the risk of plutonium to workers is “quite high,” and, “The amount of plutonium in the world is huge.” She asked, “How can you quantify something that lasts half a million years?” Schwantes said, “You can do risk assessment.” The exchange continued, and Schwantes concluded, “If there’s no chance of the plutonium coming to your lungs, then there’s no risk….” —Charmian Schaller

38 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

Panel discussion Panel Five views on opportunities and Discussion constraints for actinide research

onday evening’s panel discussion And, you know, there are people who think . . . brought together five preeminent that a milligram of uranium will kill a whole Mnames in actinide science: province in France. You can’t somehow get Graham Andrew, of the International Atomic across to these people that there’s uranium all Energy Agency (IAEA); Theresa Fryberger, over the place. head of the DOE’s Division of Environment But the transport now, especially Remediation Science; Gerry Lander of the post-September 2001, has become more Institute of Transuranium Elements (ITU) in difficult. There’s been a total crackdown. Karlsrühe, Germany; Senior Los Alamos And that just means collaboration be- Fellow and former director Sig Hecker; and tween the labs is cut again. So there’s Vic Reis of Science Applications International this huge tendency to reduce things, to Corp. (SAIC). The discussion was moderated cut us off from the outside. by Los Alamos’ Ed Arthur. Let me just say a bit about reactor fu- ture. And all the countries, in my view, Gerry Lander: There are tremendously ex- are not collaborating enough. I think citing prospects for actinide science . . . but there is good commercial reason for there are also some fundamental problems that. There is, of course, a lot of money that we, as the actinide people, should ad- in nuclear fuel . . . and so there is a lack dress. The first is: how do we as institutes that of collaboration, which is driven partly have special capabilities share those capabili- by money. But in the end, if there is not ties with people on the outside? collaboration, we all go down together. I The pressure’s on us from the institutional would like to see a more-open forum on aspect to reduce the number of places, to re- discussing the pros and cons of those fuels Gerry Lander duce the flexibility at those places. And at the rather than this sort of internal battling that same time, we have to reach out to other seems to be going on all the time. people, because otherwise, this field is going I am enormously distressed by the huge cuts to get smaller and smaller in the funding eye. that we are seeing in our neighboring institu- Somebody, today, made the comment that age tions, and the whole atmosphere that we see in profile at this conference is much better . . . many European countries with a working much younger. Bringing in students and nuclear power system—a constant reduction. showing them things can be done is another It’s not just because of the waste legacy, but crucial aspect that we have to be fully also because of the whole question of how the aware of. third world fits in . . . for power and for raising The other thing that has personally frus- the standards of living. trated me is an institutional problem again. Transport of our materials has become an ab- solute nightmare. In the old days we used to carry around bits of uranium. I never went anywhere without bits of uranium in my suitcase. Now that’s absolutely forbidden.

Nuclear Materials Technology/Los Alamos National Laboratory 39 Actinide Research Quarterly

Panel Discussion

Vic Reis: The future of civilization really much of the politics will . . . I won’t say take depends upon how actinides, and specifically care of itself . . . but it will be a lot easier. plutonium, are managed over the next fifty years. It’s that important; I don’t think there’s Theresa Fryberger: If you look at the anything more important. I come at this from cleanup efforts of the weapons complex in the both the perspective of national security and United States, a lot of people would say that global climate change. And if you think this has not been as successful as it might have about the major things that can occur over been. And part of the reason is because they the next fifty years, how one deals with ac- don’t know where contaminants will go; they tinides, plutonium in particular, will be key don’t know how they behave; they cannot pre- to much of that. dict risk. I worked twenty years ago in the Office of If we’re hoping to make a place for clean Science and Technology Policy in the White nuclear energy or safe deposition of waste, we House. I was really impressed with the as- need to understand this. We’ve studied the tronomy community, because what as- transport of contaminants for several decades, tronomy did then—and I suspect, still does— particularly for is that they argue a lot among themselves. Yucca Moun- But they then present a unified research tain. And agenda, and that makes life a lot easier for people ask: the people in the Office of Management and “Well, why Budget and those people in the National Sci- don’t we know Vic Reis ence Foundation who have to put the policy more?” One together for Congress. reason is that, What I’ve learned over the past four years until about a since I’ve left the warm confines of the nuclear decade ago, we weapons complex and moved out in to the didn’t have the broader world is that it’s very hard to get a ability to do unified research agenda. things like spe- I suspect that we have a good many of the ciation and ac- entire community right here at this meeting. tual environ- The good news, right, is that there aren’t very mental many of you. The bad news is that you can’t samples that seem to figure out what to do. At least by the we’re now able time it reaches the Department of Energy and to focus on. Theresa Fryberger Congress, it’s so complicated that the interplay So we really between the politics and the research commu- couldn’t understand at the molecular level nity frequently gets destroyed. But the re- what the real chemistry was. We’ve made search community ought to be able to figure... great progress over the last decade, but we out what that agenda is; and if they do that, need to continue to make more.

40 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

So where do I think we need to go in envi- Unfortunately, the problems of proliferation ronmental sciences in the near future? Scaling and terrorism threaten to set back all our ef- is an issue; we need to get beyond the molecu- forts for decades. There’s a great need for in- lar level. We need to make more progress, both ternational collaboration. Meanwhile, temporal and spatial. We need better character- there’s the Patriot Act . . . supposed to ization techniques in the field . . . and we need be keeping terrorists out, and is keep- to focus science a little bit differently. We need ing academicians out instead. long-term comprehensive field studies that are Russia is a huge environmental conducted by teams—dedicated teams of scien- laboratory, and we should be over tists from various disciplines. And we need to there taking advantage of things use modeling, both for designing experiments they’ve done wrong. It illustrates that as well as to model systems. it’s almost impossible, today, to get experimental work done. Sig Hecker: The near future? It’s the f elec- trons that are exciting and that’s one of the ex- Graham Andrew: What I’ve been hearing Sig Hecker citing challenges—just exactly what are the from my colleagues . . . and I agree . . . is that and Vic Reis f electrons doing? And that’s going to be heard the basic science is driven by both strategic a number of times: that plutonium sits right at and policy requirements. We must make a that knife edge between the very different kind decision early on and go from basic science to of behavior—bonding or localized . . . you the next step. Yet we can’t pursue all options. know, participating in the process or being An example of crucial decision-making is chemically inert. And we still have lots of years the area of waste. Unless we resolve repository of exciting research to try to soak that up. and waste-management problems, we won’t To me, actinides mean new materials, new be proliferating weapons, but rather, behavior, and new thinking. Personally, what’s Yucca Mountains. been of greatest interest . . . what’s so interest- Beyond plutonium, we need to improve our ing and fascinating is plutonium’s instability. capabilities in minor actinide science. Minor We saw today the enormous gyrations that they actinide science needs to be taken forward in find as a function of temperature with all the the next ten years; we need new forensics. Do six allotropes. So it’s unstable with temperature we have the same understanding of, for ex- but it’s also very unstable with pressure. Then ample, solution chemistry for the minor ac- with the six allotropes—when you start squeez- tinides as we do for plutonium? ing on them—those high volume states start to We need much better international coopera- squeeze out, and then, with any actinide, you tion with respect to fuel-cycle options. For ex- may end up with interesting results in the crys- ample, do we want to proliferate reprocessing tal structure. But if you get the right chemical plants or coalesce those efforts into, for ex- additions like gallium and aluminum, you can ample, having three or four such facilities stabilize nice metallic crystal structures. around the world? Most important, we need to be concerned about capturing the knowledge base that al- ready exists . . . dealing with capturing the knowledge of people leaving the field. Else, we risk squandering what is an irre- placeable resource.

Nuclear Materials Technology/Los Alamos National Laboratory 41 Actinide Research Quarterly

Poster Session Synthesis of New Plutonium Main-Group Metal Materials Highlights Peter K. Dorhout,1 Ryan F. Hess,2 Kent D. Abney,2 and Steven D. Conradson2

1Colorado State University and 2Los Alamos National Laboratory

“This project focuses on understanding the electron density around plutonium in a variety of different matrices,” explained Peter Dorhout, a professor at Colorado State University. “We prepared a new compound this past year that has plutonium in a selenium matrix. The selenium chan share electron density in a ‘balancing’ fashion. That is, depending upon what the plutonium metal ‘wants,’ the selenium can either accept electron density or push electronic density onto the metal.”

This new compound has the formula

KPu3Se8-x. To determine the electron density of this new compound, researchers used x-ray absorption near- edge structure (XANES), which provides oxidation-state information about the absorbing, or target, atom. The key result from this work was that the absorption edges of such heavy chalcogenide systems appear close to the value Peter found for plutonium metal (1Ð2 electron volts). This result implies that the electronic Dorhout environment around plutonium in this compound is very similar to that found in the metal.

“You really have to understand the electron density around plutonium with the particular ligands before you can make any bold assertions about what kinds of oxidation states you’re going to find,” said Dorhout. “Most studies have been done in oxides and aqueous solutions, but the real environment isn’t always just oxygen or water. So, I think that by digging real deeply into oxidation chemistry, we found some more applicable aspects to our work with respect to how we can better interpret XANES spectroscopy.”

—Octavio Ramos, Jr.

42 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

Poster The Localized and Itinerant Nature of 5f Electrons in Pu and Pu Compounds Session J. J. Joyce, J. M. Wills, T. Durakiewicz, M. T. Butterfield, E. Guziewicz, Highlights J. L. Sarrao, L. A. Morales, D. P. Moore, and A. J. Arko

Los Alamos National Laboratory

Scientists are using photoelectron spectroscopy and a computational mixed-level model to determine the electronic structure of plutonium metal and several plutonium compounds, including materials that have magnetic and superconducting transitions. “We are trying to understand the electronic structure of plutonium in a broader context,” said John Joyce of Los Alamos. “To do this, we are looking at plutonium from many different crystal structures and with many different ligands.”

By examining a large number of alloys and compounds, these workers have observed the development of a common electronic structure based on the dual nature of plutonium 5f electrons: localized or itinerant.

“From a programmatic standpoint, we’re interested in developing an atomic-scale model that shows how oxides and hydrides work with plutonium. John Such work is relevant to life sciences and long-term storage issues.” Joyce

By applying both experimental (photoelectron spectroscopy) and computational (mixed-level model) techniques to describe the electronic structure of a broad range of plutonium materials, researchers at Los Alamos anticipate a much better understanding of the electronic properties of plutonium.

—Octavio Ramos, Jr.

Nuclear Materials Technology/Los Alamos National Laboratory 43 Actinide Research Quarterly

Plutonium Futures conference

Gerd Rosenblatt provided two postscripts for his audience: The next plutonium conference will be the “Actinides 2005 International Conference,” July 3-8, 2005, in Manchester, England. To keep up with developments, those interested in the conference should visit the Web site (www.actinides2005.com). And the International Union of Pure and Applied Chemistry and the International Union of Pure and Applied Physics ruled in August that GSI in Gerd Rosenblatt Darmstadt, Germany, (a heavy ion research center funded by the Federal Government The conference in a nutshell of Germany and the state of Hesse) has proved the existence The “Plutonium Futures—The Science Conference 2003” of another element— closed with a tour-de-force summary of the entire five day number 110. The program by Gerd Rosenblatt, one of the founders of the element, which conference. Rosenblatt, of Lawrence Berkeley National was discovered in Laboratory, said he listened to all forty-five talks and looked 1994, will be known at all 125 posters. “The scientific quality of the papers as “darmstadtium” (presentations and posters) has just gone up and up and up,” and have the chemical Rosenblatt said. On the other hand, he commented, the average symbol Ds. Rosenblatt age of those attending the conference has gone down said that GSI has also notably—perhaps as much as 10 years—a fact that gives discovered element Rosenblatt “a lot of hope” for the future of plutonium science.” 111. But, he said, no Such diverse participation is vital for the field, he said. “This other suggested has been an exciting meeting, and we’re making exciting elements yet seem progress,” Rosenblatt concluded, but he reminded his sufficiently reliable audience of the call by Dr. Helen Caldicott, founder of to be approved. Physicians for Social Responsibility, for an end to all nuclear weapons. “We have to think about what we do and why and our responsibilities to our fellow human beings,” he said.

44 Nuclear Materials Technology/Los Alamos National Laboratory 3rd/4th quarter 2003

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